Embryology - User contributions [en-gb]

User:Z3418989

2014-10-29T00:53:03Z

Z3418989: /* Lab Attendance */

{{StudentPage2014}}

==Lab Attendance==
===Lab 1 --[[User:Z8600021|Mark Hill]] ([[User talk:Z8600021|talk]]) 12:50, 6 August 2014 (EST)===

reference 1

[http://www.ncbi.nlm.nih.gov/pubmed/25084016 PMID2508416]

<pubmed>25084016</pubmed>

reference 2

[http://www.ncbi.nlm.nih.gov/pubmed/25101180]

<pubmed>25101180</pubmed>

reference 3

[http://www.ncbi.nlm.nih.gov/pubmed/25100708]

<pubmed>25100708</pubmed>

===Lab 2 --[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:22, 13 August 2014 (EST)===
===Lab 3 --[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:40, 20 August 2014 (EST)===
===Lab 4--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:51, 27 August 2014 (EST)===
===Lab 5----[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:47, 3 September 2014 (EST)===
===Lab 6--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:03, 10 September 2014 (EST)===
===Lab 7--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:23, 17 September 2014 (EST)===
===Lab 8--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:36, 24 September 2014 (EST)===
===Lab 9--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:18, 8 October 2014 (EST)===
===Lab 10--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:10, 15 October 2014 (EST)===
===Lab11 was there but forgot to do attendance===
===Lab12--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:51, 29 October 2014 (EST)

=Lab Assessment 1=

==Research article 1==

[http://www.ncbi.nlm.nih.gov/pubmed/25100710 Altered Protein Expression Profiles in Umbilical Veins: Insights into Vascular Dysfunctions of the Children Born after In Vitro Fertilization.]

Summary

IVF children have been noticed to have cardiovascular problems and remodeling. However not much is known of how IVF treatment could cause these cardiovascular problems and this is the main concern of this article. The scientists’ previous studies have led them to discover that ART like IVF may cause differentially expressed proteins (DEPs) in the IVF placenta.

The umbilical veins and cord blood from 45 IVD and 48 naturally conceived (CV) newborns was collected and tissue samples were collected and then put to undergo in vitro fertilization under various conditions. E_2 cord blood levels was also examined. Using a randomizing program, 3 IVF and 3 NC umbilical veins were selected for proteomic analysis by the iTRAQ, a proteomic analysis technology. First the 6 umbilical vein sample proteins were extracted, separated with chromatography and identified using different methods like a mass spectrometer and the MASCOT search engine (Gao et al, 2014).

Also the E_2 in human umbilical vein endothelial cells (HUVECs) and cord blood was measured.
For further validation of proteomic results PCR and Western blotting analysis was conducted on 11 and 4 umbilical veins respectively (Gao et al, 2014).

Results showed 47 DEPs (20 up-regulated; 27 down-regulated) were found in IVF newborns in comparison to NC newborns. Q-PCR and Western blotting done showed validated results as proteins lumican, nestin and PTGDS were up-regulated and vimentin was down regulated as observed with proteomic results. The bioinformatics analysis conducted showed that umbilical vein DEPs had a connection with development of many systems including cardiovascular system development and carbon metabolism (Gao et al, 2014).
This study indicates there is different in expression of proteins in IVF-newborns compared to NC newborns and that DEPs might correlate with IVF-related cardiovascular issues (Gao et al, 2014).

==Research article 2==

[http://www.ncbi.nlm.nih.gov/pubmed/24888396 Improvement in in vitro fertilization rate, decrease in reactive oxygen species and spermatozoa death incidence in rams by dietary fish oil.]

Summary

This experiment aims to investigates, in rams, the effects of fish oil on level of reactive oxygen species (ROS), spermatozoa death incidence and in vitro fertilization (IVF) (Behzad, 2014).

9 Rams were randomly selected and split into control (5) and fish oil (4) groups. A diet was administered of essentially 0% fish oil to the control groups and 2.5% to the fish oil group and other things such as equal amounts of vitamin E. After 21 days semen from both groups was collated via an artificial vagina. Semen continued to be collected weekly following this. This continued on for 70 days during breeding season. (Behzad, 2014).
Every week after the initial 21 days ROS level and spermatozoa death incidence was measured via flow cytometry. However during only the first (day 21) and last (day 70) weeks of sperm collection sperm was analyzed using a sperm analyzer program called CASA and swimming up technique was used to prepare sperm for IVF. (Behzad, 2014).

The results showed a greater volume, concentration and sperm motility in fish oil groups. They found higher fertilization rates in fish oil groups; 56% compared to 49%. In third week of samples O_2 and spermatozoa death incidence was lower in fish oil groups (Behzad, 2014).

Therefore this articulate argues that dietary omega-3 which is found in fish oil could be used to increase fertilization rates in vitro fertilization.

--[[User:Z8600021|Mark Hill]] These are both good articles and summaries (5/5)

=Lab Assessment 2=

[[File:Promising System for Selecting Healthy In Vitro Fertilized Embryos in Cattle.png|800px]]

Promising System for Selecting Healthy In Vitro Fertilized Embryos in Cattle<ref><pubmed>22590579</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3348877/figure/pone-0036627-g001/]</ref>

<references/>

--[[User:Z8600021|Mark Hill]] The file and associated reference and copyright information is correct. My only comment would be to reformat the image to a smaller size, this is a large image file (1 Mb) . (5/5)

=Lab Assessment 3=

Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

--[[User:Z8600021|Mark Hill]] These are useful references, you have included the reference title as a single sentence, you should have included why you had selected these references for your section. (4/5)

=Lab Assessment 4=

==cord stem cell therapy==

Induction of Highly Functional Hepatocytes from Human Umbilical Cord Mesenchymal Stem Cells by HNF4a Transduction

In this article they describe how human umbilical mesenchymal stem cells were turned into hepatocyte/liver like cells. Using plasmid transfection of these cells Hepatocyte nuclear factor 4 alpha (HNFα) was overexpressed. HNFα is known to be crucial in liver development and hepatic differentiation. Then the expression of different proteins and genes were observed by Western blotting and RT-PCR methods.
What they found was that the liver like cells with overexpressed HNFα caused hepatic specific proteins, genes, liver enriched transcription facors and the Wnt/β-Catenin pathway (Hang et al, 2014); all of which play a role in hepatic development.
Hepatic specific proteins like ALB and AFB were observed to be expressed. Liver enriched transcription factors control expression genes involved in liver function. After HNFα transfection they found that HNF6, CEBP/ α and HNF3 β were overexpressed. Hang et al (2014) postulated that HNFα might enhance hepatic differentiation via liver enriched factors. And as expected during hepatic differentiation Hang et al observed the Wnt/β-Catenin pathway to be inactivated by HNFα administration.
The results show that human umbilical mesenchymal stem cells can be used to form hepatic like cells and HNFα further to activate important genes for hepatic differentiation. These findings provide the experimental basis for clinical procedures like liver generation after a hepatectomy and liver transplantation.

Induction of Highly Functional Hepatocytes from Human Umbilical Cord Mesenchymal Stem Cells by HNF4α Transduction.<ref><pubmed>25137413</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/25137413]</ref>

<references/>

Human umbilical cord mesenchymal stromal cells suppress MHC class II expression on rat vascular endothelium and prolong survival time of cardiac allograft

Umbilical cord mesechymal stem cells (UC-MSCs) are known to have immunomodulatory effects and this is the basis of this investigation.
UC-MSCs were taken from human umbilical cords. MHC class II transactivator gene construct (CIITA) was introduced to form a transgenic rat line. After the addition of MHC class II to the vascular endothelium, they were analyzed by immunological staining. UC-MSCs were introduced to one group of transgenic rats. Survival time of the cardiac allograft of the transgenic groups with the UC-MSCs were compared with the group without the UC-MSCs.
What they found was that with repeated infusion of UC-MSCs that the cardiac allograft survival time increased.
Ying et al (2014) go onto describe that basically UC-MSCs reduced MHC class II expression on vascular endothelium of transplanted hearts and increased regulatory anti-inflammatory cytokines like IL10, transforming growth factor (TGF)-β1 and suppressed proinflammatory cytokines like IL2, IFN-γ. This had the combined effect of increasing the survival time of the rat cardiac allograft. Why this is therapeutically relevant is that it shows great potential of use of umbilical cord mesenchymal stem cells for organ transplantation potential (Ying et al, 2014).

Human umbilical cord mesenchymal stromal cells suppress MHC class II expression on rat vascular endothelium and prolong survival time of cardiac allograft.<ref><pubmed>25126177</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/25126177]</ref>

<references/>

==Vascular "shunts" in embryo==

There are three major shunts; foramen ovale, ductus arteriosus and ductus venosus; two shunts direct pulmonary blood to systemic circulation and the third connects the vena cava and umbilical vein. These shunts close following birth and third shunt becomes non-functional once the umbilical cord is cut.
Foramen Ovale is an interatrial septum opening. After birth with the closing of this shunt the fossa ovalis remains in its remnant place. Foramen Ovale is between the right and left atrium and allows between them and has a valve to prevent backflow (during fetal period).
Ductus arteriosus helps in supporting the fetal lung and is a small muscular vessel joining the pulmonary trunk and aorta which allows blood from pulmonary trunk to go to the aorta. Pressure drop in lungs after first birth causes smooth muscle in ductus arteriosus to constrict and eventually degenerate. What is left is connective tissue remnant known as ligamentum arteriosum.
Ductus venosus transports oxygenated blood from the placenta to the fetus’ heart. The degenerated remnant of ductus venosus is known as ligamentum venosum.

=Lab Assessment 5=

==Meckel's Diverticulum==

Meckel’s diverticulum
Meckel’s diverticulum is caused by the vitelline duct not being destroyed, resulting in a blind pouch of the intestine <ref><pubmed>15729078</pubmed></ref>. This pouch is in the lower part of the small intestine, the ileum, and presents itself at birth. <ref>http://www.nlm.nih.gov/medlineplus/ency/article/000234.htm</ref>
The vitelline duct transports nutrients from the yolk sac to the fetus. Normally around week 5 to 7 of embryonic development the vitelline duct begins to narrow and eventually gets obliterated. When the vitelline duct doesn’t get obliterated a number of conditions may arise, however 97% of times it is a Meckel’s diverticulum <ref>http://emedicine.medscape.com/article/931229-overview#showall</ref> and it occurs in approximately 2% of the population <ref><pubmed>4755212</pubmed></ref>. In fact Meckel’s diverticulum is the most common abnormality of gastrointestinal tract development.
Meckel’s diverticulum contains all layers of the small intestine and is on the anti-mesenteric border of the ileum; the side of the small intestine where the vitelline sac/yolk used to be attached and the border opposite to where the blood and nerve supply is provided <ref><pubmed>25006469</pubmed></ref>. Meckel’s diverticulum has its own unique blood supply from the vitelline artery.

Meckel’s diverticulum is normally asymptomatic but if there are symptoms most of the time it presents itself as the presence of stomach or pancreas tissue at the edge of the diverticulum. Meckel’s diverticulum can cause bleeding, inflammation, rupture or blockage. It can even cause intussusception which is the movement of the upstream intestine in the downstream intestine.<ref>http://www.eapsa.org/Meckel_s_Diverticulum/4294.htm</ref> This may clinically present itself as intestinal bleeding which results in bloody stools which is present more in children younger than 5. Intestinal blockage may occur in early months of life. Inflammation which can occur presents itself similarly to appendicitis and is treated by surgical removal. <ref>http://www.eapsa.org/Meckel_s_Diverticulum/4294.htm</ref>

==References==

<references/>

=Lab Assessment 7=

==Research Article==

Tbx1 is a gene that is important in pharyngeal apparatus development and is required for derivatives such as the thyroid. Tbx1 acts on mesoderm which lies next to the thyroid and this in turn controls thyroid size and early primordium. Tbx1 also known to regulate Fgf8 in the mesoderm. The article thus describes the investigation of the possibility that Fgf8 is a mediator of Tbx1 mediated reactions between mesoderm and the thyroid and that the Tbx1-Fgf8 pathway is important in early thyroid development <ref name="PMID19389367"><pubmed>19389367</pubmed></ref>. Lania et al (2009) describe the function of Tbx1 to be affecting how many cells in the primordium and affects thyroid at primordium formation or prior to it <ref name="PMID19389367"/>. They found the primordium of Tbx1 mutants to grow but never reach the normal size. Lania et al (2009) test their hypothesis of the Tbx1-Fgf8 pathway by stopping expression of Fgf8 in Tbx1 mutant mice, and they found that there was thyroid hypoplasia. They also found this with Tbx1 deficient mice. Lania et al (2009) describe that Fgf8 cDNA when expressed in Tbx1 domain in Tbx1 mutant mice had a therapeutic effect on a thyroidal primordiam size deficiencies <ref name="PMID19389367"/>. There association between Tbx1 and Fgf8 is confirmed in this experiment and they hypothesize that the Tbx1-Fgf8 controls primordium growth by controlling proliferation of endodermal thyroid progenitor cells <ref name="PMID19389367"/>.

<references/>

==layers and tissue involved with Tooth development==

Teeth in the embryo are derived from the ectoderm and the mesoderm. Specifically the ectoderm of the first pharyngeal arch, mesoderm and the neural crest ectomesenchyme. The dental lamina is a thin ectodermal layer which proliferates to form two horse shoe shaped structure. Extoderm forms the ameloblasts which participates in enamel formation<ref name="PMID12640730"><pubmed>12640730</pubmed></ref>. The enamel organs which are cellular aggregation forms swellings on the dental lamina which eventually is where the tooth will form and will affect the size and structure of the crown of the tooth as well as the adjacent mesoderm structures known as the dermal papillae <ref>http://www.britannica.com/EBchecked/topic/1512077/tooth-germ</ref>. The dermal papillae gets enclosed by some of the enamel organ whilst some is left unenclosed, which eventually forms a sac known as the follicular sac. Each of these structures have different fates; the enamel organ differentiates to form the enamel cap; the dental papillae form the dentine and pulp chamber; follicular sac forms the periodontal membrane <ref>http://www.britannica.com/EBchecked/topic/1512077/tooth-germ</ref>. These structures form specialized teeth cells like the odontoblasts, ameloblasts and cementoblasts. The neural crest ectomesenchyme form the odontoblasts. Odontoblasts contribute to the outer dental pulp and makes dentin which is calcified tissue which surfaces structures like the enamel and pulp<ref name="PMID12640730"/>.

<references/>

=Lab Assessment 8=

For the testis to develop the XY chromosome must be present. Primordial Germ Cells (PGCs) in early gastrulation migrate through primitive streak. These migrated PGCs then congregate in-between the hindgut and yolk sac. They then migrate to the germinal ridge which now begins the stage of development of the testis. The gonadal ridge is formed by the proliferation of epithelium and mesenchyme of mesothelium on its medial side. The PGCs are originally found with endodermal cells of umbilical vesicle. The dorsal part which gets involved with the embryo and the PGCs migrate into the underlying mesenchyme. In Week 4-5 the intermediate mesoderm forms the pronephros near the pharyngeal arches. The pronephros disintegrates and forms the mesonephros or intermediate kidney. The mesonephros has two mesonephric ducts which open up into the cloacal cavity. The mesonephros extends caudally towards the hind gut. The anterior portion of the hind gut separates itself to form the urogenital sinus into which the mesonephric ducts initially open up into. Superior part of the primitive urogenital sinus forms the bladder. The inferior part of the primitive urogential sinus is where either male or female gonad structures develop from.

SRY gene on Y chromosome expresses testis determining factor (TDF). In week 8 TDF differentiates sertoli cells which express Mullerian duct inhibitory factor (MDIF) which degenerates the paramesonephric duct. In week 9 by the action of TDF, leydig cells for them to express testosterone which differentiates mesonephric ducts. The mesonephric ducts associated with medullary sex chords forms the rete testis by condensing and anastomising in the indifferent gonad. The mesonephric duct associated with gonad forms the ductus deferens and also forms the vas deferens. Tunica albuginea also develops which forms the connective tissue over the testis, which forms when the seminiferous cords connection with the epithelium is lost. Medullary sex cords form seminiferous tubules. The developing testis suspends itself by the mesorchium.

Hill, M.A. (2014) Embryology ANAT2341 Lab 8 - Sex Determination

Remnant of the Wolffian Body in the Male

[[File: Keith1902 fig082.jpg|300px]]

=Lab Assessment 9=

Group 1

You guys have a good contents list and cover the main topics under respiratory fetal development. However subheadings “Current Research, Models and Findings”, “Historic Findings” and “ Abnormalities” can be made into separate subsections, instead of under Lung Development Stages as 2.2, 2.3, 2.4 and can be categorized as 3,4,5 on the contents list. This can be easily changed. Also one reference list at the end of the whole page for all the sections would be good.
The introduction gives a good overview of respiratory development. It might be good if in the introduction it outlines the focuses of the page, in particular that you guys will be focusing on fetal development. There is a very good use of a table to describe lung development stages. The table and information provided shows good understanding of overall lung development. Possibly more information could be given on lung development and fetal development. Maybe the molecular pathways involved could be mentioned. Lung development diagrams would be even more useful in conveying the message.
The Current Research, Models and Findings has good information. It is simple and clearly conveyed and easy to grasp. It was good that you guys discussed the current understanding of morphogenesis, with recent findings about FGF10 and FGFR2. Possibly these sort of molecular pathways involved could be discussed further as I would assume much research in that area would be happening. Possibly animal models and human models could be discussed in this topic.
Historic findings is very comprehensive with diagrams and also a very good use of dot points in chronological order to describe the sequentially the historic findings in the context of respiratory development. The dot points are also easy to read and understand. The youtube link also under the references for historic findings is also a good tool for learning and explaining. The abnormalities section is very comprehensive with descriptions of many abnormalities. There could be more diagrams included as can be done well when describing abnormalities. It is a good opportunity to use diagrams and maybe putting images of abnormal vs normal lungs would be a good way to help teach at peer level the abnormalities that form.
There is a good use of diagrams throughout the page. The first 2 diagrams of the lung histology could be explained or described a little further. For example if there is a difference and similarities between slides a, b,c or d in the first diagram. References and in text citations are done well. However the references for all the subsections could be kept in one References subsection at the end of the page. This would make navigating the page more easier.
There are elements of teaching within the page. For example the table explaining the stages of development; dot point for historic findings; diagrams; youtube link on respiratory development. More teaching elements could be introduced with different explanations and more interesting examples. This could be nicely incorporated into the recent findings topic and abnormalities.
But on the whole your page is really good and it is clear that much research has been done. If you keep going the way you guys are going it should turn out really good.

Group 2

They key topics under renal are listed clearly in the contents box. The subsections seem to cover all the relevant topics related to renal. The introduction is useful in that broadly explains what the page is mainly about and gives context. They have also ordered the sections well; introduction, historic findings, developmental timeline… references.
Currently there is no information under ‘Historic findings’ and this information will be added I trust. The developmental timeline is good and succinct. However more information could be added to it but it is understood its ok if it doesn’t have much information because the other sections like “Kidney”, “Ureter” and “Urethra” sections cover it in more detail.
The current research model section is good and discusses use of animal models and their use in some current research. The Kidney, Urethra and Ureter development research is extensive with lots of information and some diagrams which is useful in explaining. The diagrams used are useful, in particular the nephrogenesis diagram. It is useful and relevant to what is being discussed. Also the diagram for anatomical position of the Kidney is useful in explaining. It would be hard to clearly convey such a pictorial concept without such a diagram. The MRI diagram of renal agenesis is interesting and useful also in describing renal agenesis abnormality.
One image under the Urethra section doesn’t have a description unlike the other images, it could be added if you think it is necessary.
Throughout the page the content is cited and referenced. There are separate references list for abnormalities which could be added to the main reference list. Multiple reference lists can be collated into one. Also there a number of references under ‘Polycystic Kidney disease’ which if added to main reference list would be good. Also there are some repeats in the main reference list, in particular the paper ‘The number of fetal nephron progenitor cells limits ureteric branching and adult nephron endowment’. This could be fixed by referring to the how to reference page provided; https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial
Teaching at a peer level was accomplished with the many useful diagrams as mentioned before. More teaching elements could be added to the page, like a video link or tables. These would be helpful in trying to explain development of renal structures.
Your group’s page is really good and if you keep adding more information and fix up the references it would make even better.

Group 3

You have covered the key topics in relation to GIT. There is a good progression of topics, beginning with a GIT system overview and moving into more specific foregut, midgut and hindgut explanations. There are no subheadings under Hindgut however in the content box, the subheadings found in the hindgut section could be listed in the content box. In the explanation of the organs there is mentioning of the earlier embryonic weeks of GIT development. This may be important to set up the basis on which the fetal development begins. The page could add an introduction section to mention mention that your page focuses on fetal period of development, just for the knowledge of readers so that they know the page focuses on fetal development. The GIT system overview can be included under the introduction.
There was a good use of diagrams, in particular the hand drawn diagrams of midgut herniation and retraction of Midgut. This diagram shows a good understanding of gut formation and is really helpful in explaining it too peer level audiences. Also the diagram of large omphacele in the deformities also shows good understanding. However diagrams would really help wen explaining the Foregut organs like the oesophagus, stomach, liver etc. Reading the text is pretty heavy and pictures and videos would really help in supplementing the text. Images would also be really good for structures like Peyer’s Patches and Interstitial cells of Cajal.
The timeline shows a good overview knowledge of gut formation and is useful for readers to refer to keep in context when reading the more detailed descriptions further on in the page. The other topics substantially cover all the other topics to a level around teaching level. Further deeper research can be done in particular to do with ‘recent findings’. However group you guys has not included a ‘historic findings’ section which I know you guys will do before the deadline. You guys have discussed recent findings but haven’t discussed current research models. Also there could be more information on the recent findings. The ‘anorectal deformities’ and ‘cloacal extrophy’ descriptions could be added to the main deformities section and it could be emphasized that it is a hindgut deformity.
References and citations are done correctly. The link in the recent findings subsection could also should be added to the main reference subsection and removed from the recent findings subsection. The citation number hyperlinks are meant to be put at the end of paragraphs or sentences instead of at the front of them as was done at the start of the ‘Oesophagus’ subsection and in ‘Stomach’.
In foregut, midgut and hindgut subsections there were a lot of text but there wasn’t enough in text citations within the text. Instead of putting the citation number hyperlinks at the top under the headings they could be included in the text or at the end of the texts. There are little errors like in Hindgut section, under Cloaca partitioning, ‘esenchyme’ was written instead of ‘mesenchyme’.
Otherwise your page is really good and comprehensive. Too go beyond the normal teach level of information you guys could add more information to historic findings and recent findings. A good recent findings section will give a good contemporary twist to your page, too keep the readers interested.

Group 4

Your group have excellent topics that cover the genital topic extensively. I feel it was a good approach to discuss the background to genital development in the ‘System Development’ section. It sets a good basis for the rest of the page which is focusing on fetal development. There could be some mentioning that the page is primarily focusing on fetal development for viewers who might read it in the future. There is information missing on about femal genital development in the first table, ‘System development’ section. Likewise there are some information missing on male genital development for example in the current findings. I’m sure you guys will add that information as the assignment progresses.
There is a good use of diagrams. The first diagram in the ‘System Development’ maybe needs a description. Under current models there is a diagram which seems not to be working. This can be easily fixed by referencing to the manual on editing as you guys would have already known. Otherwise there are a lot of really good hand drawn diagrams throughout the page which are helpful and show a good knowledge of the concepts. There are some references in the ‘System Development’ section which could be added to the main reference list. Likewise in other sections there are small references lists which could be added to the main reference list for easy reading through of topics. There must be a lot citations for current research, maybe the in text number links can be added if future readers want to know the original source.
The page shows an extensive amount research and it is clear that the group has done a lot of work. There is an element of teaching at a peer with the good diagrams as previously discussed. Research beyond the level of teaching is also evident and this can be further explored with the remaining time left for the assignment.
Overall good job guys! Keep going

Group 5

You have covered the main topics. I very much like how you have simply listed the relevant topics. It is very useful how you have put in the introduction what the page is mainly focusing on and that it is focusing on fetal development. This is very useful for readers that may come across your website in the future in giving context. Your page has a particularly good use of tables. The first table with the weeks, description and phase diagrams is very good. It really helps in understanding. And I can say that it is good method of explaining the fetal skin development to peers. It is also innovative and gives the reader a comprehensive understanding of the topic. It shows that the group understands the topic as they can express it so simply and effectively. Again the combination of images and descriptive tables for the teeth section is very useful. There is an image in the historic findings section where it has not worked. As you guys must it can be fixed by referring to the referencing manual on the website (https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial ). The recent findings sections use of coloured boxes is a good visual change. It is helpful for reading and attracting towards this section.
It is also evident that your group has done a lot of research and I your group has gone past the normal teaching level knowledge. Possibly more information could be added to Historic Findings section. There are a few minor things like spelling of Mammillary as ‘Mamailliary’ in the Historic Findings section. There are a lot of references and in text citations which is good. However there are separate reference lists for each section. This could be modified by putting them all together into a main reference list. This can easily be done before the dead line.
Overall this page is awesome!

Group 6

You have covered the main topics by listing the endocrine organs. However the sections are lacking some key information which I assume you will add later. The introduction is empty and it would be very helpful it outlined what the page was about and what the page was focusing in terms of endocrine development. Also historic findings and current research models and findings haven’t been addressed yet. It is present to a small extent in some endocrine organ descriptions. By identifying these topics the page could be greatly contributed too. The page has good use of timeline for all the organ descriptions. However the timelines need to be expanded on with more detail.
Maybe a better use of headings is possible, where subheadings under each sections can be made. For example under hypothalamus the following subheadings can be added and used; historic findings, recent models and findings, hypothalamus development during fetal period, abnormalities occurring during fetal period. These topics are covered in some sections, but if subsection headings were made, it would be much more easy to read and navigate through. As there is a lot of organs to cover this may be useful.
Diagrams and tables could really help fill the page up and help in giving a more comprehensive coverage of the topic. Some tables aren’t fully filled up, for example the table under ‘hypothalamus’ and ‘Associated Abnormalities’. The filled up tables which are in the pancreas and adrenal gland really help these sections and if added and fully filled up for other sections could really add to the page. There is a helpful use of dot points within the page which helps make the material readable and structured, particularly in thymus, pancreas and gonad development.
Some sections don’t have adequate information on fetal development for example the Hypothalamus and Pituitary sections. There seems to be a teaching level of knowledge being displayed. Deeper research could be done to further enhance the project and fulfil project aims. Also more tools for helping peers understand the topic could be used, for example diagrams and hand drawn diagrams, video links etc.
References are done and there is a lot of in text citations. Some sections like the ‘Pineal Gland’ and ‘Hypothalamus’ section has no in text citations, which need to be added. The official references section is empty. If all the references from each of the sections could be added to the main reference section it would be great for the page. This can easily be done by referring to the how to reference page on the website; https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial.

Group 7

The introduction is helpful in introducing the CNS. However the introduction is a good opportunity to outline what the page will be focusing on about the CNS, for example that it is focusing on fetal development. More could be added to the introduction for it mention briefly other things like recent findings, historic findings and fetal development introduction. In the content box ‘Brain’ and ‘Spinal Cord are in bold, it would be good to make it normal. Most of the key topics were addressed on your page. However you guys should add historic findings if you get time. It is part of the criteria and it would be good for your page.
The table under Brain development is really good and it is simple and easy to follow. If it were possible, if appropriate images were put into the table it would make the table really good. You guys have a lot of different articles for research models and findings, but as you are probably already going to do, would be good to explain each of them. Some sections are empty like the ‘Meninges Development’ which I’m sure you guys will get too before the dead line.
There was a good use of diagrams. The first diagram is particularly useful. It is a good pictorial representation of the CNS development. It is a good medium to try explaining it effectively to peers. In the ‘Brain Development section (-) was used to demarcate points. And in the ‘Development during fetal period’ dot points where used instead. It might be a good idea to use the dot points throughout the page for consistency.
It is evident that you guys have done a considerable amount of information. Some more research wouldn’t hurt so that you guys can go beyond normal teaching level descriptions. Different teaching tools for peers might be a good idea, or some sort of way to make the page more interactive or captivating. For example hand drawn diagrams or video links.
The references are done well but there are some references throughout the page which can be added to the main reference section. Overall it was good project guys all the best.

=Lab Assessment 10=

Retinal determination (RD) network are fundamental to the development of the eye. RD network consists of regulated transcriptional factors which control many genes including eyes absent (eya) which plays a role in retinal development. <ref name="PMID25057928"><pubmed>25057928</pubmed></ref>
Karandikar et al (2014) found 2 control regions that control eya expression and are important in anterior to the MF (eye-IAM) and in photoreceptors (eya-PSE). <ref name="PMID25057928"/>
The morphogenetic furrow (MF) is a boundary that moves located imaginal disc. In front of the MF a lot of cellular processes and cell cycle events take place in front of the MF. The conserved transcriptional factor co-factor is expressed in this area in front of MF. Karandikar et al (2014) is known to be important in eye development as eya1 and eya2 mutations results in eye loss in adult flies. Research prior to this show that Eya plays a role in forming Sine oculis (So) homeodomain transcription factor. <ref name="PMID25057928"/>

Eya is expressed throughout different stages of retinal development from the morphogenetic furrow (MF) to photoreceptor cells. Deletion of 2 regulatory regions retinal differention anterior and posterior to MF. Deletion of eya-IAM results in decrease in retinal field size. Whilst deletion of eya-PSE results in detriment to cone and pigment cell morphology. Experiment found that Cut, cone cell marker, and Ci which regulates the Hedgehog pathway are activated by eya gene. <ref name="PMID25057928"/>

Eya genomic rescue (〖eya〗^GR) is the construct insert the deleted eya-IAM and eya-PSE . To check the deletions PCR was performed on DNA. Using immunohistochemistry of the third instar eye disc, pupil eye disc and adult eyes Karandikar et al (2014) could observe the mutations effects on eye development. <ref name="PMID25057928"/>
〖eya〗^∆IAM clone doesn’t fully stop retinal development but reduces levels of eya. Lowered levels of eya showed slowed G1 arrest. This was expected from previous studies as eya plays a role in G1 arrest of retinal progenitors. Eya in its communication with So affects ato expression which is involved with retinal development. <ref name="PMID25057928"/>

It was observed that Eya and So regulate Cut. Cut is normally expressed in cone cells of the forming retina. The So-Eya complex was found to activate and repress Cut expression and was thus found to be important in cone cell development. As Iz was found to be a target of So, so it was proposed that So-Eya complex may control Cut expression through Iz. Eya also downregulates 〖Ci〗^Act. 〖Ci〗^Act is found behind MF and is a nuclear effector of the hedgehog pathway. Eya was also observed as previously mentioned to affect cone and pigment cell differention. It was known previously that decreased Ci posterior to MF results in increased pigment cells whilst reduced eya posterior to MF causes cone cell abnormalities. This led to knowledge that cone cell loss affect disrupting pigment cell development. <ref name="PMID25057928"/>
[[Sensory - Vision Development]]

<references/>

=Lab Assessment 11=

The article draws attention to induced pluripotent stem cells’ great potential in regenerative medicine. The possibility of decellularization and recellularization the lung is an advent in bioengineering. Induced pluripotent stem cells or embryonic stem cells are a possible cell source of recellularization. <ref name="PMID25347858"><pubmed>25347858</pubmed></ref>
Nkx2.1 is a transcription factor important for lung, thyroid and forebrain development. Especially in the lungs in the embryonic period as a progenitor for the development of the lung. <ref name="PMID20152174"><pubmed>20152174</pubmed></ref> and earliest progenitor marker from the endoderm for lung development. Nkx2.1 also expresses proteins like Foxa2 and Sox2 <ref name="PMID22482505"><pubmed>22482505</pubmed></ref>
This article delves into the effects of oxygen tension on mouse iPSC and ESC to form Nkx2.1+lung/thyroid progenitor cells. The experiment followed procedure as described by Longmire et al (2012) to develop Nkx2.1. <ref name="PMID25347858"/>
EBs was observed to first be formed. EBs are important as they are a preliminary to the later important differentiation stages. In both 20% and 5% oxygen tension in the beginning and at 3 days they both similar expressions of EBs. However as time progressed and around 5 days later in both ESCs and iPSCs there were lower expressions of EB at oxygen tension of 20% than at oxygen tension of 5%. Similarly it was found that EB adhesion was higher at 5% oxygen tension than 20% oxygen tension. Therefore in summary the lower 5% oxygen enhanced EB formation. <ref name="PMID25347858"/>
After the formation of EBs, formation of definitive endoderm cells is important in lung formation. By giving activin to EBs for 3 days the effects were measured. Foxa2 and Sox17 are transcription facts indicative of definitive endoderm and were found in both 20 and 5% oxygen tensions in ESC but were lower at 20% oxygen in iPSCs. Analysis of Nkx2.1and Foxa2, Pax8 and Oct4 by quantitative PCR was performed. Varying oxygen tension to 20% and a low 5% the effects were observed and it was found that low oxygen tension improved the progenitor formative capabilities. Using gene expression analysis it was found that in comparison to ESCs and iPSCs in 20% Oxygen tension and ESCs and iPSCs in 5% oxygen tension, Nkx2.1 and Foxa2 were expressed more. This was observed on day 12 in the developmental process and was observed in both ESCs and iPSCs. In coherence with this Oct4 gene was downregulated at 5% oxygen tension when compared with at 20% oxygen tension. <ref name="PMID25347858"/>
Immunofluorescence showed the expression of both Nkx2.1 and Foxa which was expected. Garreta et al (2014) use a immunofluorescence on all Foxa2, Nkx2.1 and Pax8 confirmed previous results and showed increased formation of all three progenitors in both ESC and IPSCs. The reason this is of great consequence is because it draws attention to induced pluripotent stem cells’ great potential in regenerative medicine. <ref name="PMID25347858"/>

<references/>

User:Z3418989

2014-10-29T00:51:31Z

Z3418989: /* Lab Attendance */

{{StudentPage2014}}

==Lab Attendance==
===Lab 1 --[[User:Z8600021|Mark Hill]] ([[User talk:Z8600021|talk]]) 12:50, 6 August 2014 (EST)===

reference 1

[http://www.ncbi.nlm.nih.gov/pubmed/25084016 PMID2508416]

<pubmed>25084016</pubmed>

reference 2

[http://www.ncbi.nlm.nih.gov/pubmed/25101180]

<pubmed>25101180</pubmed>

reference 3

[http://www.ncbi.nlm.nih.gov/pubmed/25100708]

<pubmed>25100708</pubmed>

===Lab 2 --[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:22, 13 August 2014 (EST)===
===Lab 3 --[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:40, 20 August 2014 (EST)===
===Lab 4--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:51, 27 August 2014 (EST)===
===Lab 5----[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:47, 3 September 2014 (EST)===
===Lab 6--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:03, 10 September 2014 (EST)===
===Lab 7--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:23, 17 September 2014 (EST)===
===Lab 8--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:36, 24 September 2014 (EST)===
===Lab 9--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:18, 8 October 2014 (EST)===
===Lab 10--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:10, 15 October 2014 (EST)===
===Lab11 was there but forgot to do attendance
===Lab12--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:51, 29 October 2014 (EST)

=Lab Assessment 1=

==Research article 1==

[http://www.ncbi.nlm.nih.gov/pubmed/25100710 Altered Protein Expression Profiles in Umbilical Veins: Insights into Vascular Dysfunctions of the Children Born after In Vitro Fertilization.]

Summary

IVF children have been noticed to have cardiovascular problems and remodeling. However not much is known of how IVF treatment could cause these cardiovascular problems and this is the main concern of this article. The scientists’ previous studies have led them to discover that ART like IVF may cause differentially expressed proteins (DEPs) in the IVF placenta.

The umbilical veins and cord blood from 45 IVD and 48 naturally conceived (CV) newborns was collected and tissue samples were collected and then put to undergo in vitro fertilization under various conditions. E_2 cord blood levels was also examined. Using a randomizing program, 3 IVF and 3 NC umbilical veins were selected for proteomic analysis by the iTRAQ, a proteomic analysis technology. First the 6 umbilical vein sample proteins were extracted, separated with chromatography and identified using different methods like a mass spectrometer and the MASCOT search engine (Gao et al, 2014).

Also the E_2 in human umbilical vein endothelial cells (HUVECs) and cord blood was measured.
For further validation of proteomic results PCR and Western blotting analysis was conducted on 11 and 4 umbilical veins respectively (Gao et al, 2014).

Results showed 47 DEPs (20 up-regulated; 27 down-regulated) were found in IVF newborns in comparison to NC newborns. Q-PCR and Western blotting done showed validated results as proteins lumican, nestin and PTGDS were up-regulated and vimentin was down regulated as observed with proteomic results. The bioinformatics analysis conducted showed that umbilical vein DEPs had a connection with development of many systems including cardiovascular system development and carbon metabolism (Gao et al, 2014).
This study indicates there is different in expression of proteins in IVF-newborns compared to NC newborns and that DEPs might correlate with IVF-related cardiovascular issues (Gao et al, 2014).

==Research article 2==

[http://www.ncbi.nlm.nih.gov/pubmed/24888396 Improvement in in vitro fertilization rate, decrease in reactive oxygen species and spermatozoa death incidence in rams by dietary fish oil.]

Summary

This experiment aims to investigates, in rams, the effects of fish oil on level of reactive oxygen species (ROS), spermatozoa death incidence and in vitro fertilization (IVF) (Behzad, 2014).

9 Rams were randomly selected and split into control (5) and fish oil (4) groups. A diet was administered of essentially 0% fish oil to the control groups and 2.5% to the fish oil group and other things such as equal amounts of vitamin E. After 21 days semen from both groups was collated via an artificial vagina. Semen continued to be collected weekly following this. This continued on for 70 days during breeding season. (Behzad, 2014).
Every week after the initial 21 days ROS level and spermatozoa death incidence was measured via flow cytometry. However during only the first (day 21) and last (day 70) weeks of sperm collection sperm was analyzed using a sperm analyzer program called CASA and swimming up technique was used to prepare sperm for IVF. (Behzad, 2014).

The results showed a greater volume, concentration and sperm motility in fish oil groups. They found higher fertilization rates in fish oil groups; 56% compared to 49%. In third week of samples O_2 and spermatozoa death incidence was lower in fish oil groups (Behzad, 2014).

Therefore this articulate argues that dietary omega-3 which is found in fish oil could be used to increase fertilization rates in vitro fertilization.

--[[User:Z8600021|Mark Hill]] These are both good articles and summaries (5/5)

=Lab Assessment 2=

[[File:Promising System for Selecting Healthy In Vitro Fertilized Embryos in Cattle.png|800px]]

Promising System for Selecting Healthy In Vitro Fertilized Embryos in Cattle<ref><pubmed>22590579</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3348877/figure/pone-0036627-g001/]</ref>

<references/>

--[[User:Z8600021|Mark Hill]] The file and associated reference and copyright information is correct. My only comment would be to reformat the image to a smaller size, this is a large image file (1 Mb) . (5/5)

=Lab Assessment 3=

Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

--[[User:Z8600021|Mark Hill]] These are useful references, you have included the reference title as a single sentence, you should have included why you had selected these references for your section. (4/5)

=Lab Assessment 4=

==cord stem cell therapy==

Induction of Highly Functional Hepatocytes from Human Umbilical Cord Mesenchymal Stem Cells by HNF4a Transduction

In this article they describe how human umbilical mesenchymal stem cells were turned into hepatocyte/liver like cells. Using plasmid transfection of these cells Hepatocyte nuclear factor 4 alpha (HNFα) was overexpressed. HNFα is known to be crucial in liver development and hepatic differentiation. Then the expression of different proteins and genes were observed by Western blotting and RT-PCR methods.
What they found was that the liver like cells with overexpressed HNFα caused hepatic specific proteins, genes, liver enriched transcription facors and the Wnt/β-Catenin pathway (Hang et al, 2014); all of which play a role in hepatic development.
Hepatic specific proteins like ALB and AFB were observed to be expressed. Liver enriched transcription factors control expression genes involved in liver function. After HNFα transfection they found that HNF6, CEBP/ α and HNF3 β were overexpressed. Hang et al (2014) postulated that HNFα might enhance hepatic differentiation via liver enriched factors. And as expected during hepatic differentiation Hang et al observed the Wnt/β-Catenin pathway to be inactivated by HNFα administration.
The results show that human umbilical mesenchymal stem cells can be used to form hepatic like cells and HNFα further to activate important genes for hepatic differentiation. These findings provide the experimental basis for clinical procedures like liver generation after a hepatectomy and liver transplantation.

Induction of Highly Functional Hepatocytes from Human Umbilical Cord Mesenchymal Stem Cells by HNF4α Transduction.<ref><pubmed>25137413</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/25137413]</ref>

<references/>

Human umbilical cord mesenchymal stromal cells suppress MHC class II expression on rat vascular endothelium and prolong survival time of cardiac allograft

Umbilical cord mesechymal stem cells (UC-MSCs) are known to have immunomodulatory effects and this is the basis of this investigation.
UC-MSCs were taken from human umbilical cords. MHC class II transactivator gene construct (CIITA) was introduced to form a transgenic rat line. After the addition of MHC class II to the vascular endothelium, they were analyzed by immunological staining. UC-MSCs were introduced to one group of transgenic rats. Survival time of the cardiac allograft of the transgenic groups with the UC-MSCs were compared with the group without the UC-MSCs.
What they found was that with repeated infusion of UC-MSCs that the cardiac allograft survival time increased.
Ying et al (2014) go onto describe that basically UC-MSCs reduced MHC class II expression on vascular endothelium of transplanted hearts and increased regulatory anti-inflammatory cytokines like IL10, transforming growth factor (TGF)-β1 and suppressed proinflammatory cytokines like IL2, IFN-γ. This had the combined effect of increasing the survival time of the rat cardiac allograft. Why this is therapeutically relevant is that it shows great potential of use of umbilical cord mesenchymal stem cells for organ transplantation potential (Ying et al, 2014).

Human umbilical cord mesenchymal stromal cells suppress MHC class II expression on rat vascular endothelium and prolong survival time of cardiac allograft.<ref><pubmed>25126177</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/25126177]</ref>

<references/>

==Vascular "shunts" in embryo==

There are three major shunts; foramen ovale, ductus arteriosus and ductus venosus; two shunts direct pulmonary blood to systemic circulation and the third connects the vena cava and umbilical vein. These shunts close following birth and third shunt becomes non-functional once the umbilical cord is cut.
Foramen Ovale is an interatrial septum opening. After birth with the closing of this shunt the fossa ovalis remains in its remnant place. Foramen Ovale is between the right and left atrium and allows between them and has a valve to prevent backflow (during fetal period).
Ductus arteriosus helps in supporting the fetal lung and is a small muscular vessel joining the pulmonary trunk and aorta which allows blood from pulmonary trunk to go to the aorta. Pressure drop in lungs after first birth causes smooth muscle in ductus arteriosus to constrict and eventually degenerate. What is left is connective tissue remnant known as ligamentum arteriosum.
Ductus venosus transports oxygenated blood from the placenta to the fetus’ heart. The degenerated remnant of ductus venosus is known as ligamentum venosum.

=Lab Assessment 5=

==Meckel's Diverticulum==

Meckel’s diverticulum
Meckel’s diverticulum is caused by the vitelline duct not being destroyed, resulting in a blind pouch of the intestine <ref><pubmed>15729078</pubmed></ref>. This pouch is in the lower part of the small intestine, the ileum, and presents itself at birth. <ref>http://www.nlm.nih.gov/medlineplus/ency/article/000234.htm</ref>
The vitelline duct transports nutrients from the yolk sac to the fetus. Normally around week 5 to 7 of embryonic development the vitelline duct begins to narrow and eventually gets obliterated. When the vitelline duct doesn’t get obliterated a number of conditions may arise, however 97% of times it is a Meckel’s diverticulum <ref>http://emedicine.medscape.com/article/931229-overview#showall</ref> and it occurs in approximately 2% of the population <ref><pubmed>4755212</pubmed></ref>. In fact Meckel’s diverticulum is the most common abnormality of gastrointestinal tract development.
Meckel’s diverticulum contains all layers of the small intestine and is on the anti-mesenteric border of the ileum; the side of the small intestine where the vitelline sac/yolk used to be attached and the border opposite to where the blood and nerve supply is provided <ref><pubmed>25006469</pubmed></ref>. Meckel’s diverticulum has its own unique blood supply from the vitelline artery.

Meckel’s diverticulum is normally asymptomatic but if there are symptoms most of the time it presents itself as the presence of stomach or pancreas tissue at the edge of the diverticulum. Meckel’s diverticulum can cause bleeding, inflammation, rupture or blockage. It can even cause intussusception which is the movement of the upstream intestine in the downstream intestine.<ref>http://www.eapsa.org/Meckel_s_Diverticulum/4294.htm</ref> This may clinically present itself as intestinal bleeding which results in bloody stools which is present more in children younger than 5. Intestinal blockage may occur in early months of life. Inflammation which can occur presents itself similarly to appendicitis and is treated by surgical removal. <ref>http://www.eapsa.org/Meckel_s_Diverticulum/4294.htm</ref>

==References==

<references/>

=Lab Assessment 7=

==Research Article==

Tbx1 is a gene that is important in pharyngeal apparatus development and is required for derivatives such as the thyroid. Tbx1 acts on mesoderm which lies next to the thyroid and this in turn controls thyroid size and early primordium. Tbx1 also known to regulate Fgf8 in the mesoderm. The article thus describes the investigation of the possibility that Fgf8 is a mediator of Tbx1 mediated reactions between mesoderm and the thyroid and that the Tbx1-Fgf8 pathway is important in early thyroid development <ref name="PMID19389367"><pubmed>19389367</pubmed></ref>. Lania et al (2009) describe the function of Tbx1 to be affecting how many cells in the primordium and affects thyroid at primordium formation or prior to it <ref name="PMID19389367"/>. They found the primordium of Tbx1 mutants to grow but never reach the normal size. Lania et al (2009) test their hypothesis of the Tbx1-Fgf8 pathway by stopping expression of Fgf8 in Tbx1 mutant mice, and they found that there was thyroid hypoplasia. They also found this with Tbx1 deficient mice. Lania et al (2009) describe that Fgf8 cDNA when expressed in Tbx1 domain in Tbx1 mutant mice had a therapeutic effect on a thyroidal primordiam size deficiencies <ref name="PMID19389367"/>. There association between Tbx1 and Fgf8 is confirmed in this experiment and they hypothesize that the Tbx1-Fgf8 controls primordium growth by controlling proliferation of endodermal thyroid progenitor cells <ref name="PMID19389367"/>.

<references/>

==layers and tissue involved with Tooth development==

Teeth in the embryo are derived from the ectoderm and the mesoderm. Specifically the ectoderm of the first pharyngeal arch, mesoderm and the neural crest ectomesenchyme. The dental lamina is a thin ectodermal layer which proliferates to form two horse shoe shaped structure. Extoderm forms the ameloblasts which participates in enamel formation<ref name="PMID12640730"><pubmed>12640730</pubmed></ref>. The enamel organs which are cellular aggregation forms swellings on the dental lamina which eventually is where the tooth will form and will affect the size and structure of the crown of the tooth as well as the adjacent mesoderm structures known as the dermal papillae <ref>http://www.britannica.com/EBchecked/topic/1512077/tooth-germ</ref>. The dermal papillae gets enclosed by some of the enamel organ whilst some is left unenclosed, which eventually forms a sac known as the follicular sac. Each of these structures have different fates; the enamel organ differentiates to form the enamel cap; the dental papillae form the dentine and pulp chamber; follicular sac forms the periodontal membrane <ref>http://www.britannica.com/EBchecked/topic/1512077/tooth-germ</ref>. These structures form specialized teeth cells like the odontoblasts, ameloblasts and cementoblasts. The neural crest ectomesenchyme form the odontoblasts. Odontoblasts contribute to the outer dental pulp and makes dentin which is calcified tissue which surfaces structures like the enamel and pulp<ref name="PMID12640730"/>.

<references/>

=Lab Assessment 8=

For the testis to develop the XY chromosome must be present. Primordial Germ Cells (PGCs) in early gastrulation migrate through primitive streak. These migrated PGCs then congregate in-between the hindgut and yolk sac. They then migrate to the germinal ridge which now begins the stage of development of the testis. The gonadal ridge is formed by the proliferation of epithelium and mesenchyme of mesothelium on its medial side. The PGCs are originally found with endodermal cells of umbilical vesicle. The dorsal part which gets involved with the embryo and the PGCs migrate into the underlying mesenchyme. In Week 4-5 the intermediate mesoderm forms the pronephros near the pharyngeal arches. The pronephros disintegrates and forms the mesonephros or intermediate kidney. The mesonephros has two mesonephric ducts which open up into the cloacal cavity. The mesonephros extends caudally towards the hind gut. The anterior portion of the hind gut separates itself to form the urogenital sinus into which the mesonephric ducts initially open up into. Superior part of the primitive urogenital sinus forms the bladder. The inferior part of the primitive urogential sinus is where either male or female gonad structures develop from.

SRY gene on Y chromosome expresses testis determining factor (TDF). In week 8 TDF differentiates sertoli cells which express Mullerian duct inhibitory factor (MDIF) which degenerates the paramesonephric duct. In week 9 by the action of TDF, leydig cells for them to express testosterone which differentiates mesonephric ducts. The mesonephric ducts associated with medullary sex chords forms the rete testis by condensing and anastomising in the indifferent gonad. The mesonephric duct associated with gonad forms the ductus deferens and also forms the vas deferens. Tunica albuginea also develops which forms the connective tissue over the testis, which forms when the seminiferous cords connection with the epithelium is lost. Medullary sex cords form seminiferous tubules. The developing testis suspends itself by the mesorchium.

Hill, M.A. (2014) Embryology ANAT2341 Lab 8 - Sex Determination

Remnant of the Wolffian Body in the Male

[[File: Keith1902 fig082.jpg|300px]]

=Lab Assessment 9=

Group 1

You guys have a good contents list and cover the main topics under respiratory fetal development. However subheadings “Current Research, Models and Findings”, “Historic Findings” and “ Abnormalities” can be made into separate subsections, instead of under Lung Development Stages as 2.2, 2.3, 2.4 and can be categorized as 3,4,5 on the contents list. This can be easily changed. Also one reference list at the end of the whole page for all the sections would be good.
The introduction gives a good overview of respiratory development. It might be good if in the introduction it outlines the focuses of the page, in particular that you guys will be focusing on fetal development. There is a very good use of a table to describe lung development stages. The table and information provided shows good understanding of overall lung development. Possibly more information could be given on lung development and fetal development. Maybe the molecular pathways involved could be mentioned. Lung development diagrams would be even more useful in conveying the message.
The Current Research, Models and Findings has good information. It is simple and clearly conveyed and easy to grasp. It was good that you guys discussed the current understanding of morphogenesis, with recent findings about FGF10 and FGFR2. Possibly these sort of molecular pathways involved could be discussed further as I would assume much research in that area would be happening. Possibly animal models and human models could be discussed in this topic.
Historic findings is very comprehensive with diagrams and also a very good use of dot points in chronological order to describe the sequentially the historic findings in the context of respiratory development. The dot points are also easy to read and understand. The youtube link also under the references for historic findings is also a good tool for learning and explaining. The abnormalities section is very comprehensive with descriptions of many abnormalities. There could be more diagrams included as can be done well when describing abnormalities. It is a good opportunity to use diagrams and maybe putting images of abnormal vs normal lungs would be a good way to help teach at peer level the abnormalities that form.
There is a good use of diagrams throughout the page. The first 2 diagrams of the lung histology could be explained or described a little further. For example if there is a difference and similarities between slides a, b,c or d in the first diagram. References and in text citations are done well. However the references for all the subsections could be kept in one References subsection at the end of the page. This would make navigating the page more easier.
There are elements of teaching within the page. For example the table explaining the stages of development; dot point for historic findings; diagrams; youtube link on respiratory development. More teaching elements could be introduced with different explanations and more interesting examples. This could be nicely incorporated into the recent findings topic and abnormalities.
But on the whole your page is really good and it is clear that much research has been done. If you keep going the way you guys are going it should turn out really good.

Group 2

They key topics under renal are listed clearly in the contents box. The subsections seem to cover all the relevant topics related to renal. The introduction is useful in that broadly explains what the page is mainly about and gives context. They have also ordered the sections well; introduction, historic findings, developmental timeline… references.
Currently there is no information under ‘Historic findings’ and this information will be added I trust. The developmental timeline is good and succinct. However more information could be added to it but it is understood its ok if it doesn’t have much information because the other sections like “Kidney”, “Ureter” and “Urethra” sections cover it in more detail.
The current research model section is good and discusses use of animal models and their use in some current research. The Kidney, Urethra and Ureter development research is extensive with lots of information and some diagrams which is useful in explaining. The diagrams used are useful, in particular the nephrogenesis diagram. It is useful and relevant to what is being discussed. Also the diagram for anatomical position of the Kidney is useful in explaining. It would be hard to clearly convey such a pictorial concept without such a diagram. The MRI diagram of renal agenesis is interesting and useful also in describing renal agenesis abnormality.
One image under the Urethra section doesn’t have a description unlike the other images, it could be added if you think it is necessary.
Throughout the page the content is cited and referenced. There are separate references list for abnormalities which could be added to the main reference list. Multiple reference lists can be collated into one. Also there a number of references under ‘Polycystic Kidney disease’ which if added to main reference list would be good. Also there are some repeats in the main reference list, in particular the paper ‘The number of fetal nephron progenitor cells limits ureteric branching and adult nephron endowment’. This could be fixed by referring to the how to reference page provided; https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial
Teaching at a peer level was accomplished with the many useful diagrams as mentioned before. More teaching elements could be added to the page, like a video link or tables. These would be helpful in trying to explain development of renal structures.
Your group’s page is really good and if you keep adding more information and fix up the references it would make even better.

Group 3

You have covered the key topics in relation to GIT. There is a good progression of topics, beginning with a GIT system overview and moving into more specific foregut, midgut and hindgut explanations. There are no subheadings under Hindgut however in the content box, the subheadings found in the hindgut section could be listed in the content box. In the explanation of the organs there is mentioning of the earlier embryonic weeks of GIT development. This may be important to set up the basis on which the fetal development begins. The page could add an introduction section to mention mention that your page focuses on fetal period of development, just for the knowledge of readers so that they know the page focuses on fetal development. The GIT system overview can be included under the introduction.
There was a good use of diagrams, in particular the hand drawn diagrams of midgut herniation and retraction of Midgut. This diagram shows a good understanding of gut formation and is really helpful in explaining it too peer level audiences. Also the diagram of large omphacele in the deformities also shows good understanding. However diagrams would really help wen explaining the Foregut organs like the oesophagus, stomach, liver etc. Reading the text is pretty heavy and pictures and videos would really help in supplementing the text. Images would also be really good for structures like Peyer’s Patches and Interstitial cells of Cajal.
The timeline shows a good overview knowledge of gut formation and is useful for readers to refer to keep in context when reading the more detailed descriptions further on in the page. The other topics substantially cover all the other topics to a level around teaching level. Further deeper research can be done in particular to do with ‘recent findings’. However group you guys has not included a ‘historic findings’ section which I know you guys will do before the deadline. You guys have discussed recent findings but haven’t discussed current research models. Also there could be more information on the recent findings. The ‘anorectal deformities’ and ‘cloacal extrophy’ descriptions could be added to the main deformities section and it could be emphasized that it is a hindgut deformity.
References and citations are done correctly. The link in the recent findings subsection could also should be added to the main reference subsection and removed from the recent findings subsection. The citation number hyperlinks are meant to be put at the end of paragraphs or sentences instead of at the front of them as was done at the start of the ‘Oesophagus’ subsection and in ‘Stomach’.
In foregut, midgut and hindgut subsections there were a lot of text but there wasn’t enough in text citations within the text. Instead of putting the citation number hyperlinks at the top under the headings they could be included in the text or at the end of the texts. There are little errors like in Hindgut section, under Cloaca partitioning, ‘esenchyme’ was written instead of ‘mesenchyme’.
Otherwise your page is really good and comprehensive. Too go beyond the normal teach level of information you guys could add more information to historic findings and recent findings. A good recent findings section will give a good contemporary twist to your page, too keep the readers interested.

Group 4

Your group have excellent topics that cover the genital topic extensively. I feel it was a good approach to discuss the background to genital development in the ‘System Development’ section. It sets a good basis for the rest of the page which is focusing on fetal development. There could be some mentioning that the page is primarily focusing on fetal development for viewers who might read it in the future. There is information missing on about femal genital development in the first table, ‘System development’ section. Likewise there are some information missing on male genital development for example in the current findings. I’m sure you guys will add that information as the assignment progresses.
There is a good use of diagrams. The first diagram in the ‘System Development’ maybe needs a description. Under current models there is a diagram which seems not to be working. This can be easily fixed by referencing to the manual on editing as you guys would have already known. Otherwise there are a lot of really good hand drawn diagrams throughout the page which are helpful and show a good knowledge of the concepts. There are some references in the ‘System Development’ section which could be added to the main reference list. Likewise in other sections there are small references lists which could be added to the main reference list for easy reading through of topics. There must be a lot citations for current research, maybe the in text number links can be added if future readers want to know the original source.
The page shows an extensive amount research and it is clear that the group has done a lot of work. There is an element of teaching at a peer with the good diagrams as previously discussed. Research beyond the level of teaching is also evident and this can be further explored with the remaining time left for the assignment.
Overall good job guys! Keep going

Group 5

You have covered the main topics. I very much like how you have simply listed the relevant topics. It is very useful how you have put in the introduction what the page is mainly focusing on and that it is focusing on fetal development. This is very useful for readers that may come across your website in the future in giving context. Your page has a particularly good use of tables. The first table with the weeks, description and phase diagrams is very good. It really helps in understanding. And I can say that it is good method of explaining the fetal skin development to peers. It is also innovative and gives the reader a comprehensive understanding of the topic. It shows that the group understands the topic as they can express it so simply and effectively. Again the combination of images and descriptive tables for the teeth section is very useful. There is an image in the historic findings section where it has not worked. As you guys must it can be fixed by referring to the referencing manual on the website (https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial ). The recent findings sections use of coloured boxes is a good visual change. It is helpful for reading and attracting towards this section.
It is also evident that your group has done a lot of research and I your group has gone past the normal teaching level knowledge. Possibly more information could be added to Historic Findings section. There are a few minor things like spelling of Mammillary as ‘Mamailliary’ in the Historic Findings section. There are a lot of references and in text citations which is good. However there are separate reference lists for each section. This could be modified by putting them all together into a main reference list. This can easily be done before the dead line.
Overall this page is awesome!

Group 6

You have covered the main topics by listing the endocrine organs. However the sections are lacking some key information which I assume you will add later. The introduction is empty and it would be very helpful it outlined what the page was about and what the page was focusing in terms of endocrine development. Also historic findings and current research models and findings haven’t been addressed yet. It is present to a small extent in some endocrine organ descriptions. By identifying these topics the page could be greatly contributed too. The page has good use of timeline for all the organ descriptions. However the timelines need to be expanded on with more detail.
Maybe a better use of headings is possible, where subheadings under each sections can be made. For example under hypothalamus the following subheadings can be added and used; historic findings, recent models and findings, hypothalamus development during fetal period, abnormalities occurring during fetal period. These topics are covered in some sections, but if subsection headings were made, it would be much more easy to read and navigate through. As there is a lot of organs to cover this may be useful.
Diagrams and tables could really help fill the page up and help in giving a more comprehensive coverage of the topic. Some tables aren’t fully filled up, for example the table under ‘hypothalamus’ and ‘Associated Abnormalities’. The filled up tables which are in the pancreas and adrenal gland really help these sections and if added and fully filled up for other sections could really add to the page. There is a helpful use of dot points within the page which helps make the material readable and structured, particularly in thymus, pancreas and gonad development.
Some sections don’t have adequate information on fetal development for example the Hypothalamus and Pituitary sections. There seems to be a teaching level of knowledge being displayed. Deeper research could be done to further enhance the project and fulfil project aims. Also more tools for helping peers understand the topic could be used, for example diagrams and hand drawn diagrams, video links etc.
References are done and there is a lot of in text citations. Some sections like the ‘Pineal Gland’ and ‘Hypothalamus’ section has no in text citations, which need to be added. The official references section is empty. If all the references from each of the sections could be added to the main reference section it would be great for the page. This can easily be done by referring to the how to reference page on the website; https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial.

Group 7

The introduction is helpful in introducing the CNS. However the introduction is a good opportunity to outline what the page will be focusing on about the CNS, for example that it is focusing on fetal development. More could be added to the introduction for it mention briefly other things like recent findings, historic findings and fetal development introduction. In the content box ‘Brain’ and ‘Spinal Cord are in bold, it would be good to make it normal. Most of the key topics were addressed on your page. However you guys should add historic findings if you get time. It is part of the criteria and it would be good for your page.
The table under Brain development is really good and it is simple and easy to follow. If it were possible, if appropriate images were put into the table it would make the table really good. You guys have a lot of different articles for research models and findings, but as you are probably already going to do, would be good to explain each of them. Some sections are empty like the ‘Meninges Development’ which I’m sure you guys will get too before the dead line.
There was a good use of diagrams. The first diagram is particularly useful. It is a good pictorial representation of the CNS development. It is a good medium to try explaining it effectively to peers. In the ‘Brain Development section (-) was used to demarcate points. And in the ‘Development during fetal period’ dot points where used instead. It might be a good idea to use the dot points throughout the page for consistency.
It is evident that you guys have done a considerable amount of information. Some more research wouldn’t hurt so that you guys can go beyond normal teaching level descriptions. Different teaching tools for peers might be a good idea, or some sort of way to make the page more interactive or captivating. For example hand drawn diagrams or video links.
The references are done well but there are some references throughout the page which can be added to the main reference section. Overall it was good project guys all the best.

=Lab Assessment 10=

Retinal determination (RD) network are fundamental to the development of the eye. RD network consists of regulated transcriptional factors which control many genes including eyes absent (eya) which plays a role in retinal development. <ref name="PMID25057928"><pubmed>25057928</pubmed></ref>
Karandikar et al (2014) found 2 control regions that control eya expression and are important in anterior to the MF (eye-IAM) and in photoreceptors (eya-PSE). <ref name="PMID25057928"/>
The morphogenetic furrow (MF) is a boundary that moves located imaginal disc. In front of the MF a lot of cellular processes and cell cycle events take place in front of the MF. The conserved transcriptional factor co-factor is expressed in this area in front of MF. Karandikar et al (2014) is known to be important in eye development as eya1 and eya2 mutations results in eye loss in adult flies. Research prior to this show that Eya plays a role in forming Sine oculis (So) homeodomain transcription factor. <ref name="PMID25057928"/>

Eya is expressed throughout different stages of retinal development from the morphogenetic furrow (MF) to photoreceptor cells. Deletion of 2 regulatory regions retinal differention anterior and posterior to MF. Deletion of eya-IAM results in decrease in retinal field size. Whilst deletion of eya-PSE results in detriment to cone and pigment cell morphology. Experiment found that Cut, cone cell marker, and Ci which regulates the Hedgehog pathway are activated by eya gene. <ref name="PMID25057928"/>

Eya genomic rescue (〖eya〗^GR) is the construct insert the deleted eya-IAM and eya-PSE . To check the deletions PCR was performed on DNA. Using immunohistochemistry of the third instar eye disc, pupil eye disc and adult eyes Karandikar et al (2014) could observe the mutations effects on eye development. <ref name="PMID25057928"/>
〖eya〗^∆IAM clone doesn’t fully stop retinal development but reduces levels of eya. Lowered levels of eya showed slowed G1 arrest. This was expected from previous studies as eya plays a role in G1 arrest of retinal progenitors. Eya in its communication with So affects ato expression which is involved with retinal development. <ref name="PMID25057928"/>

It was observed that Eya and So regulate Cut. Cut is normally expressed in cone cells of the forming retina. The So-Eya complex was found to activate and repress Cut expression and was thus found to be important in cone cell development. As Iz was found to be a target of So, so it was proposed that So-Eya complex may control Cut expression through Iz. Eya also downregulates 〖Ci〗^Act. 〖Ci〗^Act is found behind MF and is a nuclear effector of the hedgehog pathway. Eya was also observed as previously mentioned to affect cone and pigment cell differention. It was known previously that decreased Ci posterior to MF results in increased pigment cells whilst reduced eya posterior to MF causes cone cell abnormalities. This led to knowledge that cone cell loss affect disrupting pigment cell development. <ref name="PMID25057928"/>
[[Sensory - Vision Development]]

<references/>

=Lab Assessment 11=

The article draws attention to induced pluripotent stem cells’ great potential in regenerative medicine. The possibility of decellularization and recellularization the lung is an advent in bioengineering. Induced pluripotent stem cells or embryonic stem cells are a possible cell source of recellularization. <ref name="PMID25347858"><pubmed>25347858</pubmed></ref>
Nkx2.1 is a transcription factor important for lung, thyroid and forebrain development. Especially in the lungs in the embryonic period as a progenitor for the development of the lung. <ref name="PMID20152174"><pubmed>20152174</pubmed></ref> and earliest progenitor marker from the endoderm for lung development. Nkx2.1 also expresses proteins like Foxa2 and Sox2 <ref name="PMID22482505"><pubmed>22482505</pubmed></ref>
This article delves into the effects of oxygen tension on mouse iPSC and ESC to form Nkx2.1+lung/thyroid progenitor cells. The experiment followed procedure as described by Longmire et al (2012) to develop Nkx2.1. <ref name="PMID25347858"/>
EBs was observed to first be formed. EBs are important as they are a preliminary to the later important differentiation stages. In both 20% and 5% oxygen tension in the beginning and at 3 days they both similar expressions of EBs. However as time progressed and around 5 days later in both ESCs and iPSCs there were lower expressions of EB at oxygen tension of 20% than at oxygen tension of 5%. Similarly it was found that EB adhesion was higher at 5% oxygen tension than 20% oxygen tension. Therefore in summary the lower 5% oxygen enhanced EB formation. <ref name="PMID25347858"/>
After the formation of EBs, formation of definitive endoderm cells is important in lung formation. By giving activin to EBs for 3 days the effects were measured. Foxa2 and Sox17 are transcription facts indicative of definitive endoderm and were found in both 20 and 5% oxygen tensions in ESC but were lower at 20% oxygen in iPSCs. Analysis of Nkx2.1and Foxa2, Pax8 and Oct4 by quantitative PCR was performed. Varying oxygen tension to 20% and a low 5% the effects were observed and it was found that low oxygen tension improved the progenitor formative capabilities. Using gene expression analysis it was found that in comparison to ESCs and iPSCs in 20% Oxygen tension and ESCs and iPSCs in 5% oxygen tension, Nkx2.1 and Foxa2 were expressed more. This was observed on day 12 in the developmental process and was observed in both ESCs and iPSCs. In coherence with this Oct4 gene was downregulated at 5% oxygen tension when compared with at 20% oxygen tension. <ref name="PMID25347858"/>
Immunofluorescence showed the expression of both Nkx2.1 and Foxa which was expected. Garreta et al (2014) use a immunofluorescence on all Foxa2, Nkx2.1 and Pax8 confirmed previous results and showed increased formation of all three progenitors in both ESC and IPSCs. The reason this is of great consequence is because it draws attention to induced pluripotent stem cells’ great potential in regenerative medicine. <ref name="PMID25347858"/>

<references/>

User:Z3418989

2014-10-28T22:46:36Z

Z3418989: /* Lab Assessment 10 */

{{StudentPage2014}}

==Lab Attendance==
===Lab 1 --[[User:Z8600021|Mark Hill]] ([[User talk:Z8600021|talk]]) 12:50, 6 August 2014 (EST)===

reference 1

[http://www.ncbi.nlm.nih.gov/pubmed/25084016 PMID2508416]

<pubmed>25084016</pubmed>

reference 2

[http://www.ncbi.nlm.nih.gov/pubmed/25101180]

<pubmed>25101180</pubmed>

reference 3

[http://www.ncbi.nlm.nih.gov/pubmed/25100708]

<pubmed>25100708</pubmed>

===Lab 2 --[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:22, 13 August 2014 (EST)===
===Lab 3 --[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:40, 20 August 2014 (EST)===
===Lab 4--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:51, 27 August 2014 (EST)===
===Lab 5----[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:47, 3 September 2014 (EST)===
===Lab 6--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:03, 10 September 2014 (EST)===
===Lab 7--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:23, 17 September 2014 (EST)===
===Lab 8--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 11:36, 24 September 2014 (EST)===
===Lab 9--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:18, 8 October 2014 (EST)===
===Lab 10--[[User:Z3418989|Z3418989]] ([[User talk:Z3418989|talk]]) 12:10, 15 October 2014 (EST)

=Lab Assessment 1=

==Research article 1==

[http://www.ncbi.nlm.nih.gov/pubmed/25100710 Altered Protein Expression Profiles in Umbilical Veins: Insights into Vascular Dysfunctions of the Children Born after In Vitro Fertilization.]

Summary

IVF children have been noticed to have cardiovascular problems and remodeling. However not much is known of how IVF treatment could cause these cardiovascular problems and this is the main concern of this article. The scientists’ previous studies have led them to discover that ART like IVF may cause differentially expressed proteins (DEPs) in the IVF placenta.

The umbilical veins and cord blood from 45 IVD and 48 naturally conceived (CV) newborns was collected and tissue samples were collected and then put to undergo in vitro fertilization under various conditions. E_2 cord blood levels was also examined. Using a randomizing program, 3 IVF and 3 NC umbilical veins were selected for proteomic analysis by the iTRAQ, a proteomic analysis technology. First the 6 umbilical vein sample proteins were extracted, separated with chromatography and identified using different methods like a mass spectrometer and the MASCOT search engine (Gao et al, 2014).

Also the E_2 in human umbilical vein endothelial cells (HUVECs) and cord blood was measured.
For further validation of proteomic results PCR and Western blotting analysis was conducted on 11 and 4 umbilical veins respectively (Gao et al, 2014).

Results showed 47 DEPs (20 up-regulated; 27 down-regulated) were found in IVF newborns in comparison to NC newborns. Q-PCR and Western blotting done showed validated results as proteins lumican, nestin and PTGDS were up-regulated and vimentin was down regulated as observed with proteomic results. The bioinformatics analysis conducted showed that umbilical vein DEPs had a connection with development of many systems including cardiovascular system development and carbon metabolism (Gao et al, 2014).
This study indicates there is different in expression of proteins in IVF-newborns compared to NC newborns and that DEPs might correlate with IVF-related cardiovascular issues (Gao et al, 2014).

==Research article 2==

[http://www.ncbi.nlm.nih.gov/pubmed/24888396 Improvement in in vitro fertilization rate, decrease in reactive oxygen species and spermatozoa death incidence in rams by dietary fish oil.]

Summary

This experiment aims to investigates, in rams, the effects of fish oil on level of reactive oxygen species (ROS), spermatozoa death incidence and in vitro fertilization (IVF) (Behzad, 2014).

9 Rams were randomly selected and split into control (5) and fish oil (4) groups. A diet was administered of essentially 0% fish oil to the control groups and 2.5% to the fish oil group and other things such as equal amounts of vitamin E. After 21 days semen from both groups was collated via an artificial vagina. Semen continued to be collected weekly following this. This continued on for 70 days during breeding season. (Behzad, 2014).
Every week after the initial 21 days ROS level and spermatozoa death incidence was measured via flow cytometry. However during only the first (day 21) and last (day 70) weeks of sperm collection sperm was analyzed using a sperm analyzer program called CASA and swimming up technique was used to prepare sperm for IVF. (Behzad, 2014).

The results showed a greater volume, concentration and sperm motility in fish oil groups. They found higher fertilization rates in fish oil groups; 56% compared to 49%. In third week of samples O_2 and spermatozoa death incidence was lower in fish oil groups (Behzad, 2014).

Therefore this articulate argues that dietary omega-3 which is found in fish oil could be used to increase fertilization rates in vitro fertilization.

--[[User:Z8600021|Mark Hill]] These are both good articles and summaries (5/5)

=Lab Assessment 2=

[[File:Promising System for Selecting Healthy In Vitro Fertilized Embryos in Cattle.png|800px]]

Promising System for Selecting Healthy In Vitro Fertilized Embryos in Cattle<ref><pubmed>22590579</pubmed>| [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3348877/figure/pone-0036627-g001/]</ref>

<references/>

--[[User:Z8600021|Mark Hill]] The file and associated reference and copyright information is correct. My only comment would be to reformat the image to a smaller size, this is a large image file (1 Mb) . (5/5)

=Lab Assessment 3=

Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

--[[User:Z8600021|Mark Hill]] These are useful references, you have included the reference title as a single sentence, you should have included why you had selected these references for your section. (4/5)

=Lab Assessment 4=

==cord stem cell therapy==

Induction of Highly Functional Hepatocytes from Human Umbilical Cord Mesenchymal Stem Cells by HNF4a Transduction

In this article they describe how human umbilical mesenchymal stem cells were turned into hepatocyte/liver like cells. Using plasmid transfection of these cells Hepatocyte nuclear factor 4 alpha (HNFα) was overexpressed. HNFα is known to be crucial in liver development and hepatic differentiation. Then the expression of different proteins and genes were observed by Western blotting and RT-PCR methods.
What they found was that the liver like cells with overexpressed HNFα caused hepatic specific proteins, genes, liver enriched transcription facors and the Wnt/β-Catenin pathway (Hang et al, 2014); all of which play a role in hepatic development.
Hepatic specific proteins like ALB and AFB were observed to be expressed. Liver enriched transcription factors control expression genes involved in liver function. After HNFα transfection they found that HNF6, CEBP/ α and HNF3 β were overexpressed. Hang et al (2014) postulated that HNFα might enhance hepatic differentiation via liver enriched factors. And as expected during hepatic differentiation Hang et al observed the Wnt/β-Catenin pathway to be inactivated by HNFα administration.
The results show that human umbilical mesenchymal stem cells can be used to form hepatic like cells and HNFα further to activate important genes for hepatic differentiation. These findings provide the experimental basis for clinical procedures like liver generation after a hepatectomy and liver transplantation.

Induction of Highly Functional Hepatocytes from Human Umbilical Cord Mesenchymal Stem Cells by HNF4α Transduction.<ref><pubmed>25137413</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/25137413]</ref>

<references/>

Human umbilical cord mesenchymal stromal cells suppress MHC class II expression on rat vascular endothelium and prolong survival time of cardiac allograft

Umbilical cord mesechymal stem cells (UC-MSCs) are known to have immunomodulatory effects and this is the basis of this investigation.
UC-MSCs were taken from human umbilical cords. MHC class II transactivator gene construct (CIITA) was introduced to form a transgenic rat line. After the addition of MHC class II to the vascular endothelium, they were analyzed by immunological staining. UC-MSCs were introduced to one group of transgenic rats. Survival time of the cardiac allograft of the transgenic groups with the UC-MSCs were compared with the group without the UC-MSCs.
What they found was that with repeated infusion of UC-MSCs that the cardiac allograft survival time increased.
Ying et al (2014) go onto describe that basically UC-MSCs reduced MHC class II expression on vascular endothelium of transplanted hearts and increased regulatory anti-inflammatory cytokines like IL10, transforming growth factor (TGF)-β1 and suppressed proinflammatory cytokines like IL2, IFN-γ. This had the combined effect of increasing the survival time of the rat cardiac allograft. Why this is therapeutically relevant is that it shows great potential of use of umbilical cord mesenchymal stem cells for organ transplantation potential (Ying et al, 2014).

Human umbilical cord mesenchymal stromal cells suppress MHC class II expression on rat vascular endothelium and prolong survival time of cardiac allograft.<ref><pubmed>25126177</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/25126177]</ref>

<references/>

==Vascular "shunts" in embryo==

There are three major shunts; foramen ovale, ductus arteriosus and ductus venosus; two shunts direct pulmonary blood to systemic circulation and the third connects the vena cava and umbilical vein. These shunts close following birth and third shunt becomes non-functional once the umbilical cord is cut.
Foramen Ovale is an interatrial septum opening. After birth with the closing of this shunt the fossa ovalis remains in its remnant place. Foramen Ovale is between the right and left atrium and allows between them and has a valve to prevent backflow (during fetal period).
Ductus arteriosus helps in supporting the fetal lung and is a small muscular vessel joining the pulmonary trunk and aorta which allows blood from pulmonary trunk to go to the aorta. Pressure drop in lungs after first birth causes smooth muscle in ductus arteriosus to constrict and eventually degenerate. What is left is connective tissue remnant known as ligamentum arteriosum.
Ductus venosus transports oxygenated blood from the placenta to the fetus’ heart. The degenerated remnant of ductus venosus is known as ligamentum venosum.

=Lab Assessment 5=

==Meckel's Diverticulum==

Meckel’s diverticulum
Meckel’s diverticulum is caused by the vitelline duct not being destroyed, resulting in a blind pouch of the intestine <ref><pubmed>15729078</pubmed></ref>. This pouch is in the lower part of the small intestine, the ileum, and presents itself at birth. <ref>http://www.nlm.nih.gov/medlineplus/ency/article/000234.htm</ref>
The vitelline duct transports nutrients from the yolk sac to the fetus. Normally around week 5 to 7 of embryonic development the vitelline duct begins to narrow and eventually gets obliterated. When the vitelline duct doesn’t get obliterated a number of conditions may arise, however 97% of times it is a Meckel’s diverticulum <ref>http://emedicine.medscape.com/article/931229-overview#showall</ref> and it occurs in approximately 2% of the population <ref><pubmed>4755212</pubmed></ref>. In fact Meckel’s diverticulum is the most common abnormality of gastrointestinal tract development.
Meckel’s diverticulum contains all layers of the small intestine and is on the anti-mesenteric border of the ileum; the side of the small intestine where the vitelline sac/yolk used to be attached and the border opposite to where the blood and nerve supply is provided <ref><pubmed>25006469</pubmed></ref>. Meckel’s diverticulum has its own unique blood supply from the vitelline artery.

Meckel’s diverticulum is normally asymptomatic but if there are symptoms most of the time it presents itself as the presence of stomach or pancreas tissue at the edge of the diverticulum. Meckel’s diverticulum can cause bleeding, inflammation, rupture or blockage. It can even cause intussusception which is the movement of the upstream intestine in the downstream intestine.<ref>http://www.eapsa.org/Meckel_s_Diverticulum/4294.htm</ref> This may clinically present itself as intestinal bleeding which results in bloody stools which is present more in children younger than 5. Intestinal blockage may occur in early months of life. Inflammation which can occur presents itself similarly to appendicitis and is treated by surgical removal. <ref>http://www.eapsa.org/Meckel_s_Diverticulum/4294.htm</ref>

==References==

<references/>

=Lab Assessment 7=

==Research Article==

Tbx1 is a gene that is important in pharyngeal apparatus development and is required for derivatives such as the thyroid. Tbx1 acts on mesoderm which lies next to the thyroid and this in turn controls thyroid size and early primordium. Tbx1 also known to regulate Fgf8 in the mesoderm. The article thus describes the investigation of the possibility that Fgf8 is a mediator of Tbx1 mediated reactions between mesoderm and the thyroid and that the Tbx1-Fgf8 pathway is important in early thyroid development <ref name="PMID19389367"><pubmed>19389367</pubmed></ref>. Lania et al (2009) describe the function of Tbx1 to be affecting how many cells in the primordium and affects thyroid at primordium formation or prior to it <ref name="PMID19389367"/>. They found the primordium of Tbx1 mutants to grow but never reach the normal size. Lania et al (2009) test their hypothesis of the Tbx1-Fgf8 pathway by stopping expression of Fgf8 in Tbx1 mutant mice, and they found that there was thyroid hypoplasia. They also found this with Tbx1 deficient mice. Lania et al (2009) describe that Fgf8 cDNA when expressed in Tbx1 domain in Tbx1 mutant mice had a therapeutic effect on a thyroidal primordiam size deficiencies <ref name="PMID19389367"/>. There association between Tbx1 and Fgf8 is confirmed in this experiment and they hypothesize that the Tbx1-Fgf8 controls primordium growth by controlling proliferation of endodermal thyroid progenitor cells <ref name="PMID19389367"/>.

<references/>

==layers and tissue involved with Tooth development==

Teeth in the embryo are derived from the ectoderm and the mesoderm. Specifically the ectoderm of the first pharyngeal arch, mesoderm and the neural crest ectomesenchyme. The dental lamina is a thin ectodermal layer which proliferates to form two horse shoe shaped structure. Extoderm forms the ameloblasts which participates in enamel formation<ref name="PMID12640730"><pubmed>12640730</pubmed></ref>. The enamel organs which are cellular aggregation forms swellings on the dental lamina which eventually is where the tooth will form and will affect the size and structure of the crown of the tooth as well as the adjacent mesoderm structures known as the dermal papillae <ref>http://www.britannica.com/EBchecked/topic/1512077/tooth-germ</ref>. The dermal papillae gets enclosed by some of the enamel organ whilst some is left unenclosed, which eventually forms a sac known as the follicular sac. Each of these structures have different fates; the enamel organ differentiates to form the enamel cap; the dental papillae form the dentine and pulp chamber; follicular sac forms the periodontal membrane <ref>http://www.britannica.com/EBchecked/topic/1512077/tooth-germ</ref>. These structures form specialized teeth cells like the odontoblasts, ameloblasts and cementoblasts. The neural crest ectomesenchyme form the odontoblasts. Odontoblasts contribute to the outer dental pulp and makes dentin which is calcified tissue which surfaces structures like the enamel and pulp<ref name="PMID12640730"/>.

<references/>

=Lab Assessment 8=

For the testis to develop the XY chromosome must be present. Primordial Germ Cells (PGCs) in early gastrulation migrate through primitive streak. These migrated PGCs then congregate in-between the hindgut and yolk sac. They then migrate to the germinal ridge which now begins the stage of development of the testis. The gonadal ridge is formed by the proliferation of epithelium and mesenchyme of mesothelium on its medial side. The PGCs are originally found with endodermal cells of umbilical vesicle. The dorsal part which gets involved with the embryo and the PGCs migrate into the underlying mesenchyme. In Week 4-5 the intermediate mesoderm forms the pronephros near the pharyngeal arches. The pronephros disintegrates and forms the mesonephros or intermediate kidney. The mesonephros has two mesonephric ducts which open up into the cloacal cavity. The mesonephros extends caudally towards the hind gut. The anterior portion of the hind gut separates itself to form the urogenital sinus into which the mesonephric ducts initially open up into. Superior part of the primitive urogenital sinus forms the bladder. The inferior part of the primitive urogential sinus is where either male or female gonad structures develop from.

SRY gene on Y chromosome expresses testis determining factor (TDF). In week 8 TDF differentiates sertoli cells which express Mullerian duct inhibitory factor (MDIF) which degenerates the paramesonephric duct. In week 9 by the action of TDF, leydig cells for them to express testosterone which differentiates mesonephric ducts. The mesonephric ducts associated with medullary sex chords forms the rete testis by condensing and anastomising in the indifferent gonad. The mesonephric duct associated with gonad forms the ductus deferens and also forms the vas deferens. Tunica albuginea also develops which forms the connective tissue over the testis, which forms when the seminiferous cords connection with the epithelium is lost. Medullary sex cords form seminiferous tubules. The developing testis suspends itself by the mesorchium.

Hill, M.A. (2014) Embryology ANAT2341 Lab 8 - Sex Determination

Remnant of the Wolffian Body in the Male

[[File: Keith1902 fig082.jpg|300px]]

=Lab Assessment 9=

Group 1

You guys have a good contents list and cover the main topics under respiratory fetal development. However subheadings “Current Research, Models and Findings”, “Historic Findings” and “ Abnormalities” can be made into separate subsections, instead of under Lung Development Stages as 2.2, 2.3, 2.4 and can be categorized as 3,4,5 on the contents list. This can be easily changed. Also one reference list at the end of the whole page for all the sections would be good.
The introduction gives a good overview of respiratory development. It might be good if in the introduction it outlines the focuses of the page, in particular that you guys will be focusing on fetal development. There is a very good use of a table to describe lung development stages. The table and information provided shows good understanding of overall lung development. Possibly more information could be given on lung development and fetal development. Maybe the molecular pathways involved could be mentioned. Lung development diagrams would be even more useful in conveying the message.
The Current Research, Models and Findings has good information. It is simple and clearly conveyed and easy to grasp. It was good that you guys discussed the current understanding of morphogenesis, with recent findings about FGF10 and FGFR2. Possibly these sort of molecular pathways involved could be discussed further as I would assume much research in that area would be happening. Possibly animal models and human models could be discussed in this topic.
Historic findings is very comprehensive with diagrams and also a very good use of dot points in chronological order to describe the sequentially the historic findings in the context of respiratory development. The dot points are also easy to read and understand. The youtube link also under the references for historic findings is also a good tool for learning and explaining. The abnormalities section is very comprehensive with descriptions of many abnormalities. There could be more diagrams included as can be done well when describing abnormalities. It is a good opportunity to use diagrams and maybe putting images of abnormal vs normal lungs would be a good way to help teach at peer level the abnormalities that form.
There is a good use of diagrams throughout the page. The first 2 diagrams of the lung histology could be explained or described a little further. For example if there is a difference and similarities between slides a, b,c or d in the first diagram. References and in text citations are done well. However the references for all the subsections could be kept in one References subsection at the end of the page. This would make navigating the page more easier.
There are elements of teaching within the page. For example the table explaining the stages of development; dot point for historic findings; diagrams; youtube link on respiratory development. More teaching elements could be introduced with different explanations and more interesting examples. This could be nicely incorporated into the recent findings topic and abnormalities.
But on the whole your page is really good and it is clear that much research has been done. If you keep going the way you guys are going it should turn out really good.

Group 2

They key topics under renal are listed clearly in the contents box. The subsections seem to cover all the relevant topics related to renal. The introduction is useful in that broadly explains what the page is mainly about and gives context. They have also ordered the sections well; introduction, historic findings, developmental timeline… references.
Currently there is no information under ‘Historic findings’ and this information will be added I trust. The developmental timeline is good and succinct. However more information could be added to it but it is understood its ok if it doesn’t have much information because the other sections like “Kidney”, “Ureter” and “Urethra” sections cover it in more detail.
The current research model section is good and discusses use of animal models and their use in some current research. The Kidney, Urethra and Ureter development research is extensive with lots of information and some diagrams which is useful in explaining. The diagrams used are useful, in particular the nephrogenesis diagram. It is useful and relevant to what is being discussed. Also the diagram for anatomical position of the Kidney is useful in explaining. It would be hard to clearly convey such a pictorial concept without such a diagram. The MRI diagram of renal agenesis is interesting and useful also in describing renal agenesis abnormality.
One image under the Urethra section doesn’t have a description unlike the other images, it could be added if you think it is necessary.
Throughout the page the content is cited and referenced. There are separate references list for abnormalities which could be added to the main reference list. Multiple reference lists can be collated into one. Also there a number of references under ‘Polycystic Kidney disease’ which if added to main reference list would be good. Also there are some repeats in the main reference list, in particular the paper ‘The number of fetal nephron progenitor cells limits ureteric branching and adult nephron endowment’. This could be fixed by referring to the how to reference page provided; https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial
Teaching at a peer level was accomplished with the many useful diagrams as mentioned before. More teaching elements could be added to the page, like a video link or tables. These would be helpful in trying to explain development of renal structures.
Your group’s page is really good and if you keep adding more information and fix up the references it would make even better.

Group 3

You have covered the key topics in relation to GIT. There is a good progression of topics, beginning with a GIT system overview and moving into more specific foregut, midgut and hindgut explanations. There are no subheadings under Hindgut however in the content box, the subheadings found in the hindgut section could be listed in the content box. In the explanation of the organs there is mentioning of the earlier embryonic weeks of GIT development. This may be important to set up the basis on which the fetal development begins. The page could add an introduction section to mention mention that your page focuses on fetal period of development, just for the knowledge of readers so that they know the page focuses on fetal development. The GIT system overview can be included under the introduction.
There was a good use of diagrams, in particular the hand drawn diagrams of midgut herniation and retraction of Midgut. This diagram shows a good understanding of gut formation and is really helpful in explaining it too peer level audiences. Also the diagram of large omphacele in the deformities also shows good understanding. However diagrams would really help wen explaining the Foregut organs like the oesophagus, stomach, liver etc. Reading the text is pretty heavy and pictures and videos would really help in supplementing the text. Images would also be really good for structures like Peyer’s Patches and Interstitial cells of Cajal.
The timeline shows a good overview knowledge of gut formation and is useful for readers to refer to keep in context when reading the more detailed descriptions further on in the page. The other topics substantially cover all the other topics to a level around teaching level. Further deeper research can be done in particular to do with ‘recent findings’. However group you guys has not included a ‘historic findings’ section which I know you guys will do before the deadline. You guys have discussed recent findings but haven’t discussed current research models. Also there could be more information on the recent findings. The ‘anorectal deformities’ and ‘cloacal extrophy’ descriptions could be added to the main deformities section and it could be emphasized that it is a hindgut deformity.
References and citations are done correctly. The link in the recent findings subsection could also should be added to the main reference subsection and removed from the recent findings subsection. The citation number hyperlinks are meant to be put at the end of paragraphs or sentences instead of at the front of them as was done at the start of the ‘Oesophagus’ subsection and in ‘Stomach’.
In foregut, midgut and hindgut subsections there were a lot of text but there wasn’t enough in text citations within the text. Instead of putting the citation number hyperlinks at the top under the headings they could be included in the text or at the end of the texts. There are little errors like in Hindgut section, under Cloaca partitioning, ‘esenchyme’ was written instead of ‘mesenchyme’.
Otherwise your page is really good and comprehensive. Too go beyond the normal teach level of information you guys could add more information to historic findings and recent findings. A good recent findings section will give a good contemporary twist to your page, too keep the readers interested.

Group 4

Your group have excellent topics that cover the genital topic extensively. I feel it was a good approach to discuss the background to genital development in the ‘System Development’ section. It sets a good basis for the rest of the page which is focusing on fetal development. There could be some mentioning that the page is primarily focusing on fetal development for viewers who might read it in the future. There is information missing on about femal genital development in the first table, ‘System development’ section. Likewise there are some information missing on male genital development for example in the current findings. I’m sure you guys will add that information as the assignment progresses.
There is a good use of diagrams. The first diagram in the ‘System Development’ maybe needs a description. Under current models there is a diagram which seems not to be working. This can be easily fixed by referencing to the manual on editing as you guys would have already known. Otherwise there are a lot of really good hand drawn diagrams throughout the page which are helpful and show a good knowledge of the concepts. There are some references in the ‘System Development’ section which could be added to the main reference list. Likewise in other sections there are small references lists which could be added to the main reference list for easy reading through of topics. There must be a lot citations for current research, maybe the in text number links can be added if future readers want to know the original source.
The page shows an extensive amount research and it is clear that the group has done a lot of work. There is an element of teaching at a peer with the good diagrams as previously discussed. Research beyond the level of teaching is also evident and this can be further explored with the remaining time left for the assignment.
Overall good job guys! Keep going

Group 5

You have covered the main topics. I very much like how you have simply listed the relevant topics. It is very useful how you have put in the introduction what the page is mainly focusing on and that it is focusing on fetal development. This is very useful for readers that may come across your website in the future in giving context. Your page has a particularly good use of tables. The first table with the weeks, description and phase diagrams is very good. It really helps in understanding. And I can say that it is good method of explaining the fetal skin development to peers. It is also innovative and gives the reader a comprehensive understanding of the topic. It shows that the group understands the topic as they can express it so simply and effectively. Again the combination of images and descriptive tables for the teeth section is very useful. There is an image in the historic findings section where it has not worked. As you guys must it can be fixed by referring to the referencing manual on the website (https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial ). The recent findings sections use of coloured boxes is a good visual change. It is helpful for reading and attracting towards this section.
It is also evident that your group has done a lot of research and I your group has gone past the normal teaching level knowledge. Possibly more information could be added to Historic Findings section. There are a few minor things like spelling of Mammillary as ‘Mamailliary’ in the Historic Findings section. There are a lot of references and in text citations which is good. However there are separate reference lists for each section. This could be modified by putting them all together into a main reference list. This can easily be done before the dead line.
Overall this page is awesome!

Group 6

You have covered the main topics by listing the endocrine organs. However the sections are lacking some key information which I assume you will add later. The introduction is empty and it would be very helpful it outlined what the page was about and what the page was focusing in terms of endocrine development. Also historic findings and current research models and findings haven’t been addressed yet. It is present to a small extent in some endocrine organ descriptions. By identifying these topics the page could be greatly contributed too. The page has good use of timeline for all the organ descriptions. However the timelines need to be expanded on with more detail.
Maybe a better use of headings is possible, where subheadings under each sections can be made. For example under hypothalamus the following subheadings can be added and used; historic findings, recent models and findings, hypothalamus development during fetal period, abnormalities occurring during fetal period. These topics are covered in some sections, but if subsection headings were made, it would be much more easy to read and navigate through. As there is a lot of organs to cover this may be useful.
Diagrams and tables could really help fill the page up and help in giving a more comprehensive coverage of the topic. Some tables aren’t fully filled up, for example the table under ‘hypothalamus’ and ‘Associated Abnormalities’. The filled up tables which are in the pancreas and adrenal gland really help these sections and if added and fully filled up for other sections could really add to the page. There is a helpful use of dot points within the page which helps make the material readable and structured, particularly in thymus, pancreas and gonad development.
Some sections don’t have adequate information on fetal development for example the Hypothalamus and Pituitary sections. There seems to be a teaching level of knowledge being displayed. Deeper research could be done to further enhance the project and fulfil project aims. Also more tools for helping peers understand the topic could be used, for example diagrams and hand drawn diagrams, video links etc.
References are done and there is a lot of in text citations. Some sections like the ‘Pineal Gland’ and ‘Hypothalamus’ section has no in text citations, which need to be added. The official references section is empty. If all the references from each of the sections could be added to the main reference section it would be great for the page. This can easily be done by referring to the how to reference page on the website; https://embryology.med.unsw.edu.au/embryology/index.php/Help:Reference_Tutorial.

Group 7

The introduction is helpful in introducing the CNS. However the introduction is a good opportunity to outline what the page will be focusing on about the CNS, for example that it is focusing on fetal development. More could be added to the introduction for it mention briefly other things like recent findings, historic findings and fetal development introduction. In the content box ‘Brain’ and ‘Spinal Cord are in bold, it would be good to make it normal. Most of the key topics were addressed on your page. However you guys should add historic findings if you get time. It is part of the criteria and it would be good for your page.
The table under Brain development is really good and it is simple and easy to follow. If it were possible, if appropriate images were put into the table it would make the table really good. You guys have a lot of different articles for research models and findings, but as you are probably already going to do, would be good to explain each of them. Some sections are empty like the ‘Meninges Development’ which I’m sure you guys will get too before the dead line.
There was a good use of diagrams. The first diagram is particularly useful. It is a good pictorial representation of the CNS development. It is a good medium to try explaining it effectively to peers. In the ‘Brain Development section (-) was used to demarcate points. And in the ‘Development during fetal period’ dot points where used instead. It might be a good idea to use the dot points throughout the page for consistency.
It is evident that you guys have done a considerable amount of information. Some more research wouldn’t hurt so that you guys can go beyond normal teaching level descriptions. Different teaching tools for peers might be a good idea, or some sort of way to make the page more interactive or captivating. For example hand drawn diagrams or video links.
The references are done well but there are some references throughout the page which can be added to the main reference section. Overall it was good project guys all the best.

=Lab Assessment 10=

Retinal determination (RD) network are fundamental to the development of the eye. RD network consists of regulated transcriptional factors which control many genes including eyes absent (eya) which plays a role in retinal development. <ref name="PMID25057928"><pubmed>25057928</pubmed></ref>
Karandikar et al (2014) found 2 control regions that control eya expression and are important in anterior to the MF (eye-IAM) and in photoreceptors (eya-PSE). <ref name="PMID25057928"/>
The morphogenetic furrow (MF) is a boundary that moves located imaginal disc. In front of the MF a lot of cellular processes and cell cycle events take place in front of the MF. The conserved transcriptional factor co-factor is expressed in this area in front of MF. Karandikar et al (2014) is known to be important in eye development as eya1 and eya2 mutations results in eye loss in adult flies. Research prior to this show that Eya plays a role in forming Sine oculis (So) homeodomain transcription factor. <ref name="PMID25057928"/>

Eya is expressed throughout different stages of retinal development from the morphogenetic furrow (MF) to photoreceptor cells. Deletion of 2 regulatory regions retinal differention anterior and posterior to MF. Deletion of eya-IAM results in decrease in retinal field size. Whilst deletion of eya-PSE results in detriment to cone and pigment cell morphology. Experiment found that Cut, cone cell marker, and Ci which regulates the Hedgehog pathway are activated by eya gene. <ref name="PMID25057928"/>

Eya genomic rescue (〖eya〗^GR) is the construct insert the deleted eya-IAM and eya-PSE . To check the deletions PCR was performed on DNA. Using immunohistochemistry of the third instar eye disc, pupil eye disc and adult eyes Karandikar et al (2014) could observe the mutations effects on eye development. <ref name="PMID25057928"/>
〖eya〗^∆IAM clone doesn’t fully stop retinal development but reduces levels of eya. Lowered levels of eya showed slowed G1 arrest. This was expected from previous studies as eya plays a role in G1 arrest of retinal progenitors. Eya in its communication with So affects ato expression which is involved with retinal development. <ref name="PMID25057928"/>

It was observed that Eya and So regulate Cut. Cut is normally expressed in cone cells of the forming retina. The So-Eya complex was found to activate and repress Cut expression and was thus found to be important in cone cell development. As Iz was found to be a target of So, so it was proposed that So-Eya complex may control Cut expression through Iz. Eya also downregulates 〖Ci〗^Act. 〖Ci〗^Act is found behind MF and is a nuclear effector of the hedgehog pathway. Eya was also observed as previously mentioned to affect cone and pigment cell differention. It was known previously that decreased Ci posterior to MF results in increased pigment cells whilst reduced eya posterior to MF causes cone cell abnormalities. This led to knowledge that cone cell loss affect disrupting pigment cell development. <ref name="PMID25057928"/>
[[Sensory - Vision Development]]

<references/>

=Lab Assessment 11=

The article draws attention to induced pluripotent stem cells’ great potential in regenerative medicine. The possibility of decellularization and recellularization the lung is an advent in bioengineering. Induced pluripotent stem cells or embryonic stem cells are a possible cell source of recellularization. <ref name="PMID25347858"><pubmed>25347858</pubmed></ref>
Nkx2.1 is a transcription factor important for lung, thyroid and forebrain development. Especially in the lungs in the embryonic period as a progenitor for the development of the lung. <ref name="PMID20152174"><pubmed>20152174</pubmed></ref> and earliest progenitor marker from the endoderm for lung development. Nkx2.1 also expresses proteins like Foxa2 and Sox2 <ref name="PMID22482505"><pubmed>22482505</pubmed></ref>
This article delves into the effects of oxygen tension on mouse iPSC and ESC to form Nkx2.1+lung/thyroid progenitor cells. The experiment followed procedure as described by Longmire et al (2012) to develop Nkx2.1. <ref name="PMID25347858"/>
EBs was observed to first be formed. EBs are important as they are a preliminary to the later important differentiation stages. In both 20% and 5% oxygen tension in the beginning and at 3 days they both similar expressions of EBs. However as time progressed and around 5 days later in both ESCs and iPSCs there were lower expressions of EB at oxygen tension of 20% than at oxygen tension of 5%. Similarly it was found that EB adhesion was higher at 5% oxygen tension than 20% oxygen tension. Therefore in summary the lower 5% oxygen enhanced EB formation. <ref name="PMID25347858"/>
After the formation of EBs, formation of definitive endoderm cells is important in lung formation. By giving activin to EBs for 3 days the effects were measured. Foxa2 and Sox17 are transcription facts indicative of definitive endoderm and were found in both 20 and 5% oxygen tensions in ESC but were lower at 20% oxygen in iPSCs. Analysis of Nkx2.1and Foxa2, Pax8 and Oct4 by quantitative PCR was performed. Varying oxygen tension to 20% and a low 5% the effects were observed and it was found that low oxygen tension improved the progenitor formative capabilities. Using gene expression analysis it was found that in comparison to ESCs and iPSCs in 20% Oxygen tension and ESCs and iPSCs in 5% oxygen tension, Nkx2.1 and Foxa2 were expressed more. This was observed on day 12 in the developmental process and was observed in both ESCs and iPSCs. In coherence with this Oct4 gene was downregulated at 5% oxygen tension when compared with at 20% oxygen tension. <ref name="PMID25347858"/>
Immunofluorescence showed the expression of both Nkx2.1 and Foxa which was expected. Garreta et al (2014) use a immunofluorescence on all Foxa2, Nkx2.1 and Pax8 confirmed previous results and showed increased formation of all three progenitors in both ESC and IPSCs. The reason this is of great consequence is because it draws attention to induced pluripotent stem cells’ great potential in regenerative medicine. <ref name="PMID25347858"/>

<references/>

2014 Group Project 8 part 2

2014-10-24T09:07:30Z

Z3418989: /* Research Models */

==Current Research and Findings==

===Research Models===
Current research models used to explore fetal myogensis include both human and animal models. Below is a list of a few different models;

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon. The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.<ref name="PMID19899117"><pubmed>19899117</pubmed></ref>

Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.<ref name="PMID19899117"/>

Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.<ref name="PMID19899117"/>

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head.<ref name="PMID24913107"><pubmed>24913107</pubmed></ref>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"><pubmed>25239622</pubmed></ref>
Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>
However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

Pannexins are known to be present within skeletal muscle. Pannexins were found in mRNA of skeletal muscle <ref><pubmed>15028292</pubmed></ref>. And Panx1 in sarcolemma of rodent skeletal muscle <ref name="PMID23321639"><pubmed>23321639</pubmed></ref>
Pannexins have also been observed to play certain roles in contraction as studies showed that Panx channel blockers reduced ATP release from electrical stimulation of myotubes <ref name="PMID23321639"/><ref name="PMID23583931"><pubmed>23583931</pubmed></ref> The role however of pannexins in cell differentiation and proliferation is unknown and what Langlois et al (2014) investigate in this experiment. <ref name="PMID25239622"/>
HSMM (primary Human Skeletal Muscle Myoblasts) was obtained from post quadriceps and psoas major. And SkMC (primary Human Skeletal Muscle cells) was got from upper arm or leg. <ref name="PMID25239622"/> Western Blot analysis confirmed the presence of Panx1, Panx2 and Panx3 in the human, rat and mouse skeletal muscle tissue samples. Pan1 and Panx3 was detected in the tissue samples but Panx2 wasn’t. Panx1 and Panx2 stained differently within skeletal muscle and hinted that they had different functions. <ref name="PMID25239622"/> It is known that in the fetal stage there is lots of proliferation whilst in the adult stages there is decreased proliferation. Decreased proliferation was indicated by decreased proliferating cell nuclear antigen (PCNA). Whilst increased differentiation resulted in increased myosin heavy chains (MHC). <ref name="PMID25239622"/> Panx1 of higher molecular weight is the main pannexin found in skeletal muscle, more so in fetal than adult cells. Lower molecular forms of both Panx1 and Panx3 are more increased in adult forms and the higher weight forms are more increased in fetal forms. This shift possibly suggests that Panx1 and Panx3 plays a role in skeletal muscle development. <ref name="PMID25239622"/> Langlois et al (2014) also found that Panx1 and Panx3 had a role to play in the sophisticated process of myogenesis, in particular myoblast proliferation and differentiation. Panx1 was low in in undifferentiated skeletal muscle cells and myoblasts but promoted myoblast differentiation and were thereafter more abundant. Panx3 of low molecular weight (~ 43 kDa) acted differently to higher weight species (~ 70 kDa). Expression of low molecular weight Panx3 promoted myoblast differentiation and repressed proliferation. Whilst expression of high molecular weight Panx3 coincided with proliferation and subsided with differentiation of myoblasts. Low molecular weight Panx3 were low in differentiated and undifferentiated HSMM but was expressed in skeletal muscle tissue which may indicate that lower molecular weight Panx3 plays a role more later on in differentiation. <ref name="PMID25239622"/> Langlois et al (2014) used 2 Panx3 shRNAs to reduce Panx3 expression and they observed an inhibition of Proliferation whilst not initiating differentiation. Other impacts on Panx were studied such as their glycosylation, phosphorylation and being sialylated are important in their functioning as membrane channels. Post-transcriptional modifications and molecular interactions of Panx are thus important in regulating Panx channel function and thus important in myoblast differentiation and proliferation. <ref name="PMID25239622"/> All evidences led to the conclusion that both Panx1 and Panx3 are expressed in skeletal muscle cells and participate in proliferation and differentiation of myoblast muscle cells.

===Dll1 signaling and craniofacial myogenesis===

Craniofacial skeletal muscle is derived from cranial mesoderm and trunk skeletal muscle is derived from somatic mesoderm. Hence craniofacial muscles are developmentally contrasting from trunk muscle. It is known that Notch signalling plays a role in myogenesis in trunk muscles. However Notch signalling role in myogenesis in craniofacial muscles has not been investigated that much, and is the focus of Czajkowski et al (2014) experiment. <ref name="PMID 25220152"><pubmed> 25220152</pubmed></ref>

The normal myogenic regulatory factors (MyoD, Myf5, Mrf4) act in myogenesis and all three are necessary for myogenesis to occur. However there are other regulatory factors which act upstream to affect this standard myogenic pathway. This is another area where trunk and cranial muscle differs in regulation as Pax3/7 act upstream of the normal myogenic regulatory factors. However in craniofacial muscle development Pitx2 and Tbx1 play this role.<ref name="PMID25220152"><pubmed>25220152</pubmed></ref>

The role of Notch signalling is not well known for craniofacial muscles. In mice Notch signalling is normally the result of a ligand such as DII1 or DII3 binding to a Notch receptor1-4. Subsequently this is followed by translocation into the nucleus and activation of target genes. In the context of myogenesis Notch signalling was observed repress MyoD and induce MyoR expression in C2C12 cells. <ref name="PMID25220152"/>

Fetal Skeletal muscle contains a renewing pool of progenitor cells. Notch signalling results in the regulation of myogenic differentiation. Therefore when notch signalling is lost or mutated uncontrolled myogenic differentiation results and the formation of muscle. Things that repress notch signalling like DII1 and Rbpj impair muscle growth. Therefore mutations in DII1 and Rbpj results in muscle growth. Therefore Notch signalling plays role in removing MyoD and compensating for mutations in DII1 or Rbpj, and maintaining progenitor muscle pool. Notch signalling therefore suppresses myogenic differentiation in trunk muscles. But not much is known of its role in craniofacial muscles. <ref name="PMID25220152"/>

Czajkowski et al (2014) found that in myogenic progenitor cells and muscle growth are deficient during early fetal development. Czajkowski et al (2014) observed that in the absence of these myogenic progenitor cells supernumerary myoblasts temporarily come and express MyoD. The overexpression of MyoD is in this case detrimental to the limited craniofacial progenitor cell in the mutant mice. Therefore the mutation and repression of MyoD which Czajkowski et al (2014) propose that Notch facilitates, allows for the craniofacial myogenesis. <ref name="PMID25220152"/>

Czajkowski et al (2014) finds out that Notch signalling acts in a similar as it does in the trunk muscles in the craniofacial muscles. It does this by regulating progenitor cell differentiation by controlling MyoD expression. The difference however between Notch signalling between craniofacial and trunk muscles is in its other functions such as homng of satellite cells and the independence of Pax7 expression from DII1 signalling in craniofacial muscles. <ref name="PMID25220152"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8 part 2

2014-10-24T09:05:31Z

Z3418989: /* Research Models */

==Current Research and Findings==

===Research Models===
Current research models used to explore fetal myogensis include both human and animal models. Below is a list of a few different models

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon. The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.<ref name="PMID19899117"><pubmed>19899117</pubmed></ref>

Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.<ref name="PMID19899117"/>

Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.<ref name="PMID19899117"/>

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head.<ref name="PMID24913107"><pubmed>24913107</pubmed></ref>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"><pubmed>25239622</pubmed></ref>
Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>
However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

Pannexins are known to be present within skeletal muscle. Pannexins were found in mRNA of skeletal muscle <ref><pubmed>15028292</pubmed></ref>. And Panx1 in sarcolemma of rodent skeletal muscle <ref name="PMID23321639"><pubmed>23321639</pubmed></ref>
Pannexins have also been observed to play certain roles in contraction as studies showed that Panx channel blockers reduced ATP release from electrical stimulation of myotubes <ref name="PMID23321639"/><ref name="PMID23583931"><pubmed>23583931</pubmed></ref> The role however of pannexins in cell differentiation and proliferation is unknown and what Langlois et al (2014) investigate in this experiment. <ref name="PMID25239622"/>
HSMM (primary Human Skeletal Muscle Myoblasts) was obtained from post quadriceps and psoas major. And SkMC (primary Human Skeletal Muscle cells) was got from upper arm or leg. <ref name="PMID25239622"/> Western Blot analysis confirmed the presence of Panx1, Panx2 and Panx3 in the human, rat and mouse skeletal muscle tissue samples. Pan1 and Panx3 was detected in the tissue samples but Panx2 wasn’t. Panx1 and Panx2 stained differently within skeletal muscle and hinted that they had different functions. <ref name="PMID25239622"/> It is known that in the fetal stage there is lots of proliferation whilst in the adult stages there is decreased proliferation. Decreased proliferation was indicated by decreased proliferating cell nuclear antigen (PCNA). Whilst increased differentiation resulted in increased myosin heavy chains (MHC). <ref name="PMID25239622"/> Panx1 of higher molecular weight is the main pannexin found in skeletal muscle, more so in fetal than adult cells. Lower molecular forms of both Panx1 and Panx3 are more increased in adult forms and the higher weight forms are more increased in fetal forms. This shift possibly suggests that Panx1 and Panx3 plays a role in skeletal muscle development. <ref name="PMID25239622"/> Langlois et al (2014) also found that Panx1 and Panx3 had a role to play in the sophisticated process of myogenesis, in particular myoblast proliferation and differentiation. Panx1 was low in in undifferentiated skeletal muscle cells and myoblasts but promoted myoblast differentiation and were thereafter more abundant. Panx3 of low molecular weight (~ 43 kDa) acted differently to higher weight species (~ 70 kDa). Expression of low molecular weight Panx3 promoted myoblast differentiation and repressed proliferation. Whilst expression of high molecular weight Panx3 coincided with proliferation and subsided with differentiation of myoblasts. Low molecular weight Panx3 were low in differentiated and undifferentiated HSMM but was expressed in skeletal muscle tissue which may indicate that lower molecular weight Panx3 plays a role more later on in differentiation. <ref name="PMID25239622"/> Langlois et al (2014) used 2 Panx3 shRNAs to reduce Panx3 expression and they observed an inhibition of Proliferation whilst not initiating differentiation. Other impacts on Panx were studied such as their glycosylation, phosphorylation and being sialylated are important in their functioning as membrane channels. Post-transcriptional modifications and molecular interactions of Panx are thus important in regulating Panx channel function and thus important in myoblast differentiation and proliferation. <ref name="PMID25239622"/> All evidences led to the conclusion that both Panx1 and Panx3 are expressed in skeletal muscle cells and participate in proliferation and differentiation of myoblast muscle cells.

===Dll1 signaling and craniofacial myogenesis===

Craniofacial skeletal muscle is derived from cranial mesoderm and trunk skeletal muscle is derived from somatic mesoderm. Hence craniofacial muscles are developmentally contrasting from trunk muscle. It is known that Notch signalling plays a role in myogenesis in trunk muscles. However Notch signalling role in myogenesis in craniofacial muscles has not been investigated that much, and is the focus of Czajkowski et al (2014) experiment. <ref name="PMID 25220152"><pubmed> 25220152</pubmed></ref>

The normal myogenic regulatory factors (MyoD, Myf5, Mrf4) act in myogenesis and all three are necessary for myogenesis to occur. However there are other regulatory factors which act upstream to affect this standard myogenic pathway. This is another area where trunk and cranial muscle differs in regulation as Pax3/7 act upstream of the normal myogenic regulatory factors. However in craniofacial muscle development Pitx2 and Tbx1 play this role.<ref name="PMID25220152"><pubmed>25220152</pubmed></ref>

The role of Notch signalling is not well known for craniofacial muscles. In mice Notch signalling is normally the result of a ligand such as DII1 or DII3 binding to a Notch receptor1-4. Subsequently this is followed by translocation into the nucleus and activation of target genes. In the context of myogenesis Notch signalling was observed repress MyoD and induce MyoR expression in C2C12 cells. <ref name="PMID25220152"/>

Fetal Skeletal muscle contains a renewing pool of progenitor cells. Notch signalling results in the regulation of myogenic differentiation. Therefore when notch signalling is lost or mutated uncontrolled myogenic differentiation results and the formation of muscle. Things that repress notch signalling like DII1 and Rbpj impair muscle growth. Therefore mutations in DII1 and Rbpj results in muscle growth. Therefore Notch signalling plays role in removing MyoD and compensating for mutations in DII1 or Rbpj, and maintaining progenitor muscle pool. Notch signalling therefore suppresses myogenic differentiation in trunk muscles. But not much is known of its role in craniofacial muscles. <ref name="PMID25220152"/>

Czajkowski et al (2014) found that in myogenic progenitor cells and muscle growth are deficient during early fetal development. Czajkowski et al (2014) observed that in the absence of these myogenic progenitor cells supernumerary myoblasts temporarily come and express MyoD. The overexpression of MyoD is in this case detrimental to the limited craniofacial progenitor cell in the mutant mice. Therefore the mutation and repression of MyoD which Czajkowski et al (2014) propose that Notch facilitates, allows for the craniofacial myogenesis. <ref name="PMID25220152"/>

Czajkowski et al (2014) finds out that Notch signalling acts in a similar as it does in the trunk muscles in the craniofacial muscles. It does this by regulating progenitor cell differentiation by controlling MyoD expression. The difference however between Notch signalling between craniofacial and trunk muscles is in its other functions such as homng of satellite cells and the independence of Pax7 expression from DII1 signalling in craniofacial muscles. <ref name="PMID25220152"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T09:03:26Z

Z3418989:

2014 Group Project 8 part 2

2014-10-24T08:57:03Z

Z3418989: Created page with "==Current Research and Findings== ===Research Models=== *Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Ske..."

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon. The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.<ref name="PMID19899117"><pubmed>19899117</pubmed></ref>

Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.<ref name="PMID19899117"/>

Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.<ref name="PMID19899117"/>

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head.<ref name="PMID24913107"><pubmed>24913107</pubmed></ref>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"><pubmed>25239622</pubmed></ref>
Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>
However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

Pannexins are known to be present within skeletal muscle. Pannexins were found in mRNA of skeletal muscle <ref><pubmed>15028292</pubmed></ref>. And Panx1 in sarcolemma of rodent skeletal muscle <ref name="PMID23321639"><pubmed>23321639</pubmed></ref>
Pannexins have also been observed to play certain roles in contraction as studies showed that Panx channel blockers reduced ATP release from electrical stimulation of myotubes <ref name="PMID23321639"/><ref name="PMID23583931"><pubmed>23583931</pubmed></ref> The role however of pannexins in cell differentiation and proliferation is unknown and what Langlois et al (2014) investigate in this experiment. <ref name="PMID25239622"/>
HSMM (primary Human Skeletal Muscle Myoblasts) was obtained from post quadriceps and psoas major. And SkMC (primary Human Skeletal Muscle cells) was got from upper arm or leg. <ref name="PMID25239622"/> Western Blot analysis confirmed the presence of Panx1, Panx2 and Panx3 in the human, rat and mouse skeletal muscle tissue samples. Pan1 and Panx3 was detected in the tissue samples but Panx2 wasn’t. Panx1 and Panx2 stained differently within skeletal muscle and hinted that they had different functions. <ref name="PMID25239622"/> It is known that in the fetal stage there is lots of proliferation whilst in the adult stages there is decreased proliferation. Decreased proliferation was indicated by decreased proliferating cell nuclear antigen (PCNA). Whilst increased differentiation resulted in increased myosin heavy chains (MHC). <ref name="PMID25239622"/> Panx1 of higher molecular weight is the main pannexin found in skeletal muscle, more so in fetal than adult cells. Lower molecular forms of both Panx1 and Panx3 are more increased in adult forms and the higher weight forms are more increased in fetal forms. This shift possibly suggests that Panx1 and Panx3 plays a role in skeletal muscle development. <ref name="PMID25239622"/> Langlois et al (2014) also found that Panx1 and Panx3 had a role to play in the sophisticated process of myogenesis, in particular myoblast proliferation and differentiation. Panx1 was low in in undifferentiated skeletal muscle cells and myoblasts but promoted myoblast differentiation and were thereafter more abundant. Panx3 of low molecular weight (~ 43 kDa) acted differently to higher weight species (~ 70 kDa). Expression of low molecular weight Panx3 promoted myoblast differentiation and repressed proliferation. Whilst expression of high molecular weight Panx3 coincided with proliferation and subsided with differentiation of myoblasts. Low molecular weight Panx3 were low in differentiated and undifferentiated HSMM but was expressed in skeletal muscle tissue which may indicate that lower molecular weight Panx3 plays a role more later on in differentiation. <ref name="PMID25239622"/> Langlois et al (2014) used 2 Panx3 shRNAs to reduce Panx3 expression and they observed an inhibition of Proliferation whilst not initiating differentiation. Other impacts on Panx were studied such as their glycosylation, phosphorylation and being sialylated are important in their functioning as membrane channels. Post-transcriptional modifications and molecular interactions of Panx are thus important in regulating Panx channel function and thus important in myoblast differentiation and proliferation. <ref name="PMID25239622"/> All evidences led to the conclusion that both Panx1 and Panx3 are expressed in skeletal muscle cells and participate in proliferation and differentiation of myoblast muscle cells.

===Dll1 signaling and craniofacial myogenesis===

Craniofacial skeletal muscle is derived from cranial mesoderm and trunk skeletal muscle is derived from somatic mesoderm. Hence craniofacial muscles are developmentally contrasting from trunk muscle. It is known that Notch signalling plays a role in myogenesis in trunk muscles. However Notch signalling role in myogenesis in craniofacial muscles has not been investigated that much, and is the focus of Czajkowski et al (2014) experiment. <ref name="PMID 25220152"><pubmed> 25220152</pubmed></ref>

The normal myogenic regulatory factors (MyoD, Myf5, Mrf4) act in myogenesis and all three are necessary for myogenesis to occur. However there are other regulatory factors which act upstream to affect this standard myogenic pathway. This is another area where trunk and cranial muscle differs in regulation as Pax3/7 act upstream of the normal myogenic regulatory factors. However in craniofacial muscle development Pitx2 and Tbx1 play this role.<ref name="PMID25220152"><pubmed>25220152</pubmed></ref>

The role of Notch signalling is not well known for craniofacial muscles. In mice Notch signalling is normally the result of a ligand such as DII1 or DII3 binding to a Notch receptor1-4. Subsequently this is followed by translocation into the nucleus and activation of target genes. In the context of myogenesis Notch signalling was observed repress MyoD and induce MyoR expression in C2C12 cells. <ref name="PMID25220152"/>

Fetal Skeletal muscle contains a renewing pool of progenitor cells. Notch signalling results in the regulation of myogenic differentiation. Therefore when notch signalling is lost or mutated uncontrolled myogenic differentiation results and the formation of muscle. Things that repress notch signalling like DII1 and Rbpj impair muscle growth. Therefore mutations in DII1 and Rbpj results in muscle growth. Therefore Notch signalling plays role in removing MyoD and compensating for mutations in DII1 or Rbpj, and maintaining progenitor muscle pool. Notch signalling therefore suppresses myogenic differentiation in trunk muscles. But not much is known of its role in craniofacial muscles. <ref name="PMID25220152"/>

Czajkowski et al (2014) found that in myogenic progenitor cells and muscle growth are deficient during early fetal development. Czajkowski et al (2014) observed that in the absence of these myogenic progenitor cells supernumerary myoblasts temporarily come and express MyoD. The overexpression of MyoD is in this case detrimental to the limited craniofacial progenitor cell in the mutant mice. Therefore the mutation and repression of MyoD which Czajkowski et al (2014) propose that Notch facilitates, allows for the craniofacial myogenesis. <ref name="PMID25220152"/>

Czajkowski et al (2014) finds out that Notch signalling acts in a similar as it does in the trunk muscles in the craniofacial muscles. It does this by regulating progenitor cell differentiation by controlling MyoD expression. The difference however between Notch signalling between craniofacial and trunk muscles is in its other functions such as homng of satellite cells and the independence of Pax7 expression from DII1 signalling in craniofacial muscles. <ref name="PMID25220152"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T06:45:42Z

Z3418989: /* Stapedius */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon. The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.<ref name="PMID19899117"><pubmed>19899117</pubmed></ref>

Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.<ref name="PMID19899117"/>

Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.<ref name="PMID19899117"/>

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"/>
Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>
However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

Pannexins are known to be present within skeletal muscle. Pannexins were found in mRNA of skeletal muscle <ref><pubmed>15028292</pubmed></ref>. And Panx1 in sarcolemma of rodent skeletal muscle <ref name="PMID23321639"><pubmed>23321639</pubmed></ref>
Pannexins have also been observed to play certain roles in contraction as studies showed that Panx channel blockers reduced ATP release from electrical stimulation of myotubes <ref name="PMID23321639"/> <ref name="PMID23583931"/> .The role however of pannexins in cell differentiation and proliferation is unknown and what Langlois et al (2014) investigate in this experiment. <ref name="PMID25239622"/>
HSMM (primary Human Skeletal Muscle Myoblasts) was obtained from post quadriceps and psoas major. And SkMC (primary Human Skeletal Muscle cells) was got from upper arm or leg. <ref name="PMID25239622"/> Western Blot analysis confirmed the presence of Panx1, Panx2 and Panx3 in the human, rat and mouse skeletal muscle tissue samples. Pan1 and Panx3 was detected in the tissue samples but Panx2 wasn’t. Panx1 and Panx2 stained differently within skeletal muscle and hinted that they had different functions. <ref name="PMID25239622"/> It is known that in the fetal stage there is lots of proliferation whilst in the adult stages there is decreased proliferation. Decreased proliferation was indicated by decreased proliferating cell nuclear antigen (PCNA). Whilst increased differentiation resulted in increased myosin heavy chains (MHC). <ref name="PMID25239622"/> Panx1 of higher molecular weight is the main pannexin found in skeletal muscle, more so in fetal than adult cells. Lower molecular forms of both Panx1 and Panx3 are more increased in adult forms and the higher weight forms are more increased in fetal forms. This shift possibly suggests that Panx1 and Panx3 plays a role in skeletal muscle development. <ref name="PMID25239622"/> Langlois et al (2014) also found that Panx1 and Panx3 had a role to play in the sophisticated process of myogenesis, in particular myoblast proliferation and differentiation. Panx1 was low in in undifferentiated skeletal muscle cells and myoblasts but promoted myoblast differentiation and were thereafter more abundant. Panx3 of low molecular weight (~ 43 kDa) acted differently to higher weight species (~ 70 kDa). Expression of low molecular weight Panx3 promoted myoblast differentiation and repressed proliferation. Whilst expression of high molecular weight Panx3 coincided with proliferation and subsided with differentiation of myoblasts. Low molecular weight Panx3 were low in differentiated and undifferentiated HSMM but was expressed in skeletal muscle tissue which may indicate that lower molecular weight Panx3 plays a role more later on in differentiation. <ref name="PMID25239622"/> Langlois et al (2014) used 2 Panx3 shRNAs to reduce Panx3 expression and they observed an inhibition of Proliferation whilst not initiating differentiation. Other impacts on Panx were studied such as their glycosylation, phosphorylation and being sialylated are important in their functioning as membrane channels. Post-transcriptional modifications and molecular interactions of Panx are thus important in regulating Panx channel function and thus important in myoblast differentiation and proliferation. <ref name="PMID25239622"/> All evidences led to the conclusion that both Panx1 and Panx3 are expressed in skeletal muscle cells and participate in proliferation and differentiation of myoblast muscle cells.

===Dll1 signaling and craniofacial myogenesis===

Craniofacial skeletal muscle is derived from cranial mesoderm and trunk skeletal muscle is derived from somatic mesoderm. Hence craniofacial muscles are developmentally contrasting from trunk muscle. It is known that Notch signalling plays a role in myogenesis in trunk muscles. However Notch signalling role in myogenesis in craniofacial muscles has not been investigated that much, and is the focus of Czajkowski et al (2014) experiment. <ref name="PMID 25220152"><pubmed> 25220152</pubmed></ref>

The normal myogenic regulatory factors (MyoD, Myf5, Mrf4) act in myogenesis and all three are necessary for myogenesis to occur. However there are other regulatory factors which act upstream to affect this standard myogenic pathway. This is another area where trunk and cranial muscle differs in regulation as Pax3/7 act upstream of the normal myogenic regulatory factors. However in craniofacial muscle development Pitx2 and Tbx1 play this role. <ref name="PMID25220152"/>

The role of Notch signalling is not well known for craniofacial muscles. In mice Notch signalling is normally the result of a ligand such as DII1 or DII3 binding to a Notch receptor1-4. Subsequently this is followed by translocation into the nucleus and activation of target genes. In the context of myogenesis Notch signalling was observed repress MyoD and induce MyoR expression in C2C12 cells. <ref name="PMID25220152"/>

Fetal Skeletal muscle contains a renewing pool of progenitor cells. Notch signalling results in the regulation of myogenic differentiation. Therefore when notch signalling is lost or mutated uncontrolled myogenic differentiation results and the formation of muscle. Things that repress notch signalling like DII1 and Rbpj impair muscle growth. Therefore mutations in DII1 and Rbpj results in muscle growth. Therefore Notch signalling plays role in removing MyoD and compensating for mutations in DII1 or Rbpj, and maintaining progenitor muscle pool. Notch signalling therefore suppresses myogenic differentiation in trunk muscles. But not much is known of its role in craniofacial muscles. <ref name="PMID25220152"/>

Czajkowski et al (2014) found that in myogenic progenitor cells and muscle growth are deficient during early fetal development. Czajkowski et al (2014) observed that in the absence of these myogenic progenitor cells supernumerary myoblasts temporarily come and express MyoD. The overexpression of MyoD is in this case detrimental to the limited craniofacial progenitor cell in the mutant mice. Therefore the mutation and repression of MyoD which Czajkowski et al (2014) propose that Notch facilitates, allows for the craniofacial myogenesis. <ref name="PMID25220152"/>

Czajkowski et al (2014) finds out that Notch signalling acts in a similar as it does in the trunk muscles in the craniofacial muscles. It does this by regulating progenitor cell differentiation by controlling MyoD expression. The difference however between Notch signalling between craniofacial and trunk muscles is in its other functions such as homng of satellite cells and the independence of Pax7 expression from DII1 signalling in craniofacial muscles. <ref name="PMID25220152"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T06:03:00Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"/>
Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>
However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

Pannexins are known to be present within skeletal muscle. Pannexins were found in mRNA of skeletal muscle <ref><pubmed>15028292</pubmed></ref>. And Panx1 in sarcolemma of rodent skeletal muscle <ref name="PMID23321639"><pubmed>23321639</pubmed></ref>
Pannexins have also been observed to play certain roles in contraction as studies showed that Panx channel blockers reduced ATP release from electrical stimulation of myotubes <ref name="PMID23321639"/> <ref name="PMID23583931"/> .The role however of pannexins in cell differentiation and proliferation is unknown and what Langlois et al (2014) investigate in this experiment. <ref name="PMID25239622"/>
HSMM (primary Human Skeletal Muscle Myoblasts) was obtained from post quadriceps and psoas major. And SkMC (primary Human Skeletal Muscle cells) was got from upper arm or leg. <ref name="PMID25239622"/> Western Blot analysis confirmed the presence of Panx1, Panx2 and Panx3 in the human, rat and mouse skeletal muscle tissue samples. Pan1 and Panx3 was detected in the tissue samples but Panx2 wasn’t. Panx1 and Panx2 stained differently within skeletal muscle and hinted that they had different functions. <ref name="PMID25239622"/> It is known that in the fetal stage there is lots of proliferation whilst in the adult stages there is decreased proliferation. Decreased proliferation was indicated by decreased proliferating cell nuclear antigen (PCNA). Whilst increased differentiation resulted in increased myosin heavy chains (MHC). <ref name="PMID25239622"/> Panx1 of higher molecular weight is the main pannexin found in skeletal muscle, more so in fetal than adult cells. Lower molecular forms of both Panx1 and Panx3 are more increased in adult forms and the higher weight forms are more increased in fetal forms. This shift possibly suggests that Panx1 and Panx3 plays a role in skeletal muscle development. <ref name="PMID25239622"/> Langlois et al (2014) also found that Panx1 and Panx3 had a role to play in the sophisticated process of myogenesis, in particular myoblast proliferation and differentiation. Panx1 was low in in undifferentiated skeletal muscle cells and myoblasts but promoted myoblast differentiation and were thereafter more abundant. Panx3 of low molecular weight (~ 43 kDa) acted differently to higher weight species (~ 70 kDa). Expression of low molecular weight Panx3 promoted myoblast differentiation and repressed proliferation. Whilst expression of high molecular weight Panx3 coincided with proliferation and subsided with differentiation of myoblasts. Low molecular weight Panx3 were low in differentiated and undifferentiated HSMM but was expressed in skeletal muscle tissue which may indicate that lower molecular weight Panx3 plays a role more later on in differentiation. <ref name="PMID25239622"/> Langlois et al (2014) used 2 Panx3 shRNAs to reduce Panx3 expression and they observed an inhibition of Proliferation whilst not initiating differentiation. Other impacts on Panx were studied such as their glycosylation, phosphorylation and being sialylated are important in their functioning as membrane channels. Post-transcriptional modifications and molecular interactions of Panx are thus important in regulating Panx channel function and thus important in myoblast differentiation and proliferation. <ref name="PMID25239622"/> All evidences led to the conclusion that both Panx1 and Panx3 are expressed in skeletal muscle cells and participate in proliferation and differentiation of myoblast muscle cells.

===Dll1 signaling and craniofacial myogenesis===

Craniofacial skeletal muscle is derived from cranial mesoderm and trunk skeletal muscle is derived from somatic mesoderm. Hence craniofacial muscles are developmentally contrasting from trunk muscle. It is known that Notch signalling plays a role in myogenesis in trunk muscles. However Notch signalling role in myogenesis in craniofacial muscles has not been investigated that much, and is the focus of Czajkowski et al (2014) experiment. <ref name="PMID 25220152"><pubmed> 25220152</pubmed></ref>

The normal myogenic regulatory factors (MyoD, Myf5, Mrf4) act in myogenesis and all three are necessary for myogenesis to occur. However there are other regulatory factors which act upstream to affect this standard myogenic pathway. This is another area where trunk and cranial muscle differs in regulation as Pax3/7 act upstream of the normal myogenic regulatory factors. However in craniofacial muscle development Pitx2 and Tbx1 play this role. <ref name="PMID25220152"/>

The role of Notch signalling is not well known for craniofacial muscles. In mice Notch signalling is normally the result of a ligand such as DII1 or DII3 binding to a Notch receptor1-4. Subsequently this is followed by translocation into the nucleus and activation of target genes. In the context of myogenesis Notch signalling was observed repress MyoD and induce MyoR expression in C2C12 cells. <ref name="PMID25220152"/>

Fetal Skeletal muscle contains a renewing pool of progenitor cells. Notch signalling results in the regulation of myogenic differentiation. Therefore when notch signalling is lost or mutated uncontrolled myogenic differentiation results and the formation of muscle. Things that repress notch signalling like DII1 and Rbpj impair muscle growth. Therefore mutations in DII1 and Rbpj results in muscle growth. Therefore Notch signalling plays role in removing MyoD and compensating for mutations in DII1 or Rbpj, and maintaining progenitor muscle pool. Notch signalling therefore suppresses myogenic differentiation in trunk muscles. But not much is known of its role in craniofacial muscles. <ref name="PMID25220152"/>

Czajkowski et al (2014) found that in myogenic progenitor cells and muscle growth are deficient during early fetal development. Czajkowski et al (2014) observed that in the absence of these myogenic progenitor cells supernumerary myoblasts temporarily come and express MyoD. The overexpression of MyoD is in this case detrimental to the limited craniofacial progenitor cell in the mutant mice. Therefore the mutation and repression of MyoD which Czajkowski et al (2014) propose that Notch facilitates, allows for the craniofacial myogenesis. <ref name="PMID25220152"/>

Czajkowski et al (2014) finds out that Notch signalling acts in a similar as it does in the trunk muscles in the craniofacial muscles. It does this by regulating progenitor cell differentiation by controlling MyoD expression. The difference however between Notch signalling between craniofacial and trunk muscles is in its other functions such as homng of satellite cells and the independence of Pax7 expression from DII1 signalling in craniofacial muscles. <ref name="PMID25220152"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T05:14:22Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"/>
Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>
However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

Pannexins are known to be present within skeletal muscle. Pannexins were found in mRNA of skeletal muscle <ref><pubmed>15028292</pubmed></ref>. And Panx1 in sarcolemma of rodent skeletal muscle <ref name="PMID23321639"><pubmed>23321639</pubmed></ref>
Pannexins have also been observed to play certain roles in contraction as studies showed that Panx channel blockers reduced ATP release from electrical stimulation of myotubes <ref name="PMID23321639"/> <ref name="PMID23583931"/> .The role however of pannexins in cell differentiation and proliferation is unknown and what Langlois et al (2014) investigate in this experiment. <ref name="PMID25239622"/>

HSMM (primary Human Skeletal Muscle Myoblasts) was obtained from post quadriceps and psoas major. And SkMC (primary Human Skeletal Muscle cells) was got from upper arm or leg. <ref name="PMID25239622"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T04:39:19Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"/>
Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>
However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

Pannexins are known to be present within skeletal muscle. Pannexins were found in mRNA of skeletal muscle <ref><pubmed>15028292</pubmed></ref>. And Panx1 in sarcolemma of rodent skeletal muscle <ref name="PMID23321639"><pubmed>23321639</pubmed></ref>
Pannexins have also been observed to play certain roles in contraction as studies showed that Panx channel blockers reduced ATP release from electrical stimulation of myotubes <ref name="PMID23321639"/> <ref name="PMID23583931"/> .The role however of pannexins in cell differentiation and proliferation is unknown and what Langlois et al (2014) investigate in this experiment. <ref name="PMID25239622"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:59:52Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"/>

Panx3 is known for fewer functions such as carcinogenesis and osteoprogenitor cell proliferation. <ref><pubmed>22947051</pubmed></ref> <ref><pubmed>24338011</pubmed></ref>

However recent studies have shown the potential role of Panx1 and Panx3 in skeletal muscle development.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:51:19Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref name="PMID25239622"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:44:26Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.<ref><pubmed>25239622</pubmed></ref>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:35:11Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and inflammatory responses.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:30:24Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and
inflammatory responses.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:28:49Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation and
inflammatory responses.<ref><pubmed>16682648</pubmed></ref>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:17:31Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation <ref><pubmed>16682648</pubmed></ref> and
inflammatory responses.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:13:37Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation <ref><pubmed>16682648</pubmed></ref> and
inflammatory responses.<ref name="PMID25239622"/>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:09:34Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles, a few of which are vasodilation <ref><pubmed>16682648</pubmed></ref> and
inflammatory responses <ref><pubmed>17036048</pubmed>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T03:03:51Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

Panx1 and Panx3 are part of the pannexin channel protein family. Panx1 is known to play many roles such as vasodilation <ref><pubmed>16682648</pubmed></ref>,

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T02:53:51Z

Z3418989: /* Current Research and Findings */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.<ref><pubmed>25239622</pubmed></ref>

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-24T02:28:30Z

Z3418989: /* Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

Langlois et al (2014) investigate the role of these Panx1 and Panx3 in the skeletal muscle of rodents and humans and their expression in the skeletal muscle of fetal and adult life.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==

<references/>

2014 Group Project 8

2014-10-23T23:02:22Z

Z3418989: /* Current Research and Findings */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

===Pannexin 1 and Pannexin 3 and Myoblast differentiation and proliferation===

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T22:55:31Z

Z3418989: /* Current Research and Findings */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

===Two studies on Triceps Brachii and Biceps Brachii and their parameters===

Grzonkowska et al (2014) studied 30 fetuses from the age of 12-29 weeks and observed that with an increase in fetal age there was an increase in fetal triceps brachii parameters. <ref name="PMID 24913107"><pubmed>24913107</pubmed></ref>
Similarly Szpinda et al (2013) studied the anatomical parameters of biceps brachii of 30 fetuses aged 17-30 weeks. <ref name="PMID23468258"><pubmed> 23468258</pubmed></ref>

{| style="width:100%"
|bgcolor="CEDFF2"|''' Triceps Brachii '''
|bgcolor="F5FAFF"|''' Biceps Brachii '''
|-
|bgcolor=" CEDFF2"|
*Triceps has three heads; long, lateral and medial head. <ref name="PMID24913107"/>
* The long head was observed to be the longest with a mean length of 43.36mm when compared with 37.06mm and 33.24 for lateral and medial heads respectively. <ref name="PMID24913107"/>
* The lateral heads muscle mean width was the greatest at 5.34mm with the long head and medial head’s mean width observed to be 3.74mm and 4.42mm respectively. <ref name="PMID24913107"/>
* The formation of the widest belly as the lateral head’s and the thinnest is the long head’s muscle belly. <ref name="PMID24913107"/>
* no variability due to sex or laterality. <ref name="PMID24913107"/>
* proportionate increase with fetal age of these parameters. <ref name="PMID24913107"/>
| bgcolor=" F5FAFF "|
*Biceps have two heads; long and short heads. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and short head’s mean length was 5.93mm. <ref name="PMID23468258"/>
* The long head of biceps brachii’s mean length was 5.68mm and its mean width at mid-length was 0.60mm. Whilst the short head had a mean width at mid-length of 0.65 and mean width at widest part of 0.72. <ref name="PMID23468258"/>
* Therefore in the fetus the belly of the long head is shorter and thinner than the short head’s. <ref name="PMID23468258"/>
* No variability due to sex or laterality. <ref name="PMID23468258"/>
* Proportionate increase with fetal age of these parameters. <ref name="PMID23468258"/>
* Szpinda et al (2013) observed a linear pattern of growth of biceps brachii. For example the long head of the biceps brachii’s length from 3.26-8.84mm which Szpinda et al (2013) found to follow the linear relationship; y = –0.801 + 0.276 × Age (R^2 = 0.591). <ref name="PMID23468258"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

The importance of these studies is that they measured the parameters of these developing muscles which were not done before. And they found that the individual components of these muscles developed proportionately as they are seen when they are fully developed.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T22:24:18Z

Z3418989: /* Current Research and Findings */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Research Models===

*Rats and Mice as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref name="PMID 25239622"><pubmed> 25239622</pubmed></ref>

*Pigs as Yang et al (2014) use in their experiment in comparing lean and obese pigs’ genes and muscle development<ref name="PMID 25229314"><pubmed> 25229314</pubmed></ref>

*Sheep as Duckett et al (2014) use in their experiment on the effects of ergot alkaloids on fetal growth in sheep. <ref name="PMID 25191653"><pubmed> 25191653</pubmed></ref>

*Humans as Langlois et al (2014) use in as Langlois et al (2014) use in their experiment to do with Pannexin 1 and Pannexin 3 Channels and Skeletal muscle. <ref><pubmed>25239622</pubmed></ref>

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T15:33:30Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

File:MYOGENESIS video.mp4

2014-10-23T15:30:23Z

Z3418989: Z3418989 uploaded a new version of "File:MYOGENESIS video.mp4"

Myogenesis

References

Hill, M.A. (2014) Embryology Lecture - Musculoskeletal Development. Retrieved October 23, 2014, from https://php.med.unsw.edu.au/embryology/index.php?title=Lecture_-_Musculoskeletal_Development

2014 Group Project 8

2014-10-23T15:25:19Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">Image:MYOGENESIS_MOVIE.flv</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T15:20:16Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_MOVIE.flv</html5media>
|}

[[Image:MYOGENESIS_MOVIE.flv]]

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T15:01:09Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_MOVIE.flv</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

File:MYOGENESIS video.mp4

2014-10-23T14:51:43Z

Z3418989: Z3418989 uploaded a new version of "File:MYOGENESIS video.mp4"

2014 Group Project 8

2014-10-23T14:33:29Z

Z3418989: /* Intermediate tendon of Human Digastricus and Omohyoideus */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles and their differences and similarities.

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T14:27:25Z

Z3418989: /* Intermediate tendon of Human Digastricus and Omohyoideus */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T14:23:45Z

Z3418989: /* Intermediate tendon of Human Digastricus and Omohyoideus */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID21538565"><pubmed>21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T14:20:32Z

Z3418989: /* Intermediate tendon of Human Digastricus and Omohyoideus */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"|
*2 bellies of muscle with an intermediate tendon<ref name="PMID 21538565 "><pubmed> 21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "|
* 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

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Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T14:16:44Z

Z3418989: /* Intermediate tendon of Human Digastricus and Omohyoideus */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

{| style="width:100%"
|bgcolor="CEDFF2"|'''Digastricus'''
|bgcolor="F5FAFF"|'''Omohyoideus'''
|-
|bgcolor=" CEDFF2"| *2 bellies of muscle with an intermediate tendon<ref name="PMID 21538565 "><pubmed> 21538565 </pubmed></ref>
* Week 7-9 posterior belly develops intermediate tendon with a bulb like terminal part and anterior belly doesn’t. <ref name="PMID21538565"/>
* Anterior belly moves towards and attaches to terminal bulb part of posterior belly’s intermediate tendon. <ref name="PMID21538565"/>
* Anterior belly hypothesized not to attach to cartilage and attach to intermediate tendon because of stylohyoideus, vascular arteries and hypoglossal nerve which blocked it from doing this. <ref name="PMID21538565"/>
| bgcolor=" F5FAFF "| * 2 bellies of muscle with an intermediate tendon. <ref name="PMID21538565"/>
* Week 7-9 omohyoideus takes a straight and superior-inferior path and is a single muscle belly. <ref name="PMID21538565"/>
* Omohyoideus becomes angulated by the lateral expansion of the clavicle and the shoulder. <ref name="PMID21538565"/>
* Omohyoid is tightly fitted in-between sternocleidomastoid and scalene muscles. <ref name="PMID21538565"/>
* Omohyoideus reaches greatest thickness (0.5mm) and intermediate tendon develops secondarily. <ref name="PMID21538565"/>
* Week 18-20 muscle fibers of intermediate tendon are converted to collagen fibers. <ref name="PMID21538565"/>
|-
|- style="height:10px"
| style="width:50%"|
|}

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs<ref name="PMID 14090530"><pubmed> 14090530</pubmed></ref>. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref> <ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations<ref name="PMID 10051637"><pubmed> 10051637</pubmed></ref>. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production<ref name="PMID 15236405"><pubmed> 15236405</pubmed></ref>.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

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Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

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Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

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Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

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2014 Group Project 8

2014-10-23T13:08:21Z

Z3418989: /* Background Early Embryonic development */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg|frame|right|350x250px]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png|frame|left|350x250px]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

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2014 Group Project 8

2014-10-23T13:04:27Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|middle|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T12:51:22Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Following the formation of myofibers, growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T12:42:08Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|858x248px|Structure of striated muscle]]

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T12:39:25Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|right|middle|858x248px|Structure of striated muscle]]

[[Image:Myogenesis_molecular.jpg|middle|858x248px|Myogenesis and associated Molecules]]

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T12:32:05Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|right|middle|958x348px|Structure of striated muscle]]

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T12:28:26Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

[[File: Myogenesis_molecular.jpg|frame|middle|958x348px|Myogenesis and Associated Molecules]]

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

Predating 1970 the adult musculoskeletal system was heavily researched which provided some overlap with prenatal development, though few studies existed which focused on the fetal stages. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life. <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref> The most common approach is Corticosteroid based medication, which significantly improves muscle strength and function over a short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>
Additionally assisted ventilation, supportive equipment, proton-pump inhibitors, increased excercise, beta-blockers and diuretics culminate to reduce symptoms and increase sufferers quality of life. <ref name="PMID 8143083"><pubmed> 8143083</pubmed></ref> <ref name="PMID 12467747"><pubmed> 12467747</pubmed></ref> <ref name="PMID 19945914"><pubmed> 19945914</pubmed></ref> <ref name="PMID 19945913"><pubmed> 19945913</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

File:Myogenesis molecular.jpg

2014-10-23T12:23:22Z

Z3418989: This describes the transformation of myodermal myotome cells to myofibers and associated signalling molecules. Based upon http://joe.endocrinology-journals.org/content/221/2/R13.long <pubmed>24532817</pubmed> Student number: z3418989 Beginning s...

This describes the transformation of myodermal myotome cells to myofibers and associated signalling molecules.

Based upon

http://joe.endocrinology-journals.org/content/221/2/R13.long

<pubmed>24532817</pubmed>

Student number: z3418989

Beginning six months after publication, I (z3418989) grant the public the non-exclusive right to copy, distribute, or display the Work under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported license, as described at http://creativecommons.org/licenses/by-nc-sa/3.0/ and http://creativecommons.org/licenses/by-nc-sa/3.0/legalcode."

References

<pubmed>24532817</pubmed>

2014 Group Project 8

2014-10-23T11:48:24Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="CEDFF2"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="F5FAFF"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="F5FAFF"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

No studies focusing on the fetal development of muscle existed before the 1970's. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life, these include: <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref>

*Corticosteroid based medication significantly improves muscle strength and function over the short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

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2014 Group Project 8

2014-10-23T11:42:27Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="lightsteelblue"
! Cell type/Stage!! Associated Molecules!! Action
|- bgcolor="lightsteelblue"
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|-
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|- bgcolor="lightsteelblue"
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|-
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

No studies focusing on the fetal development of muscle existed before the 1970's. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life, these include: <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref>

*Corticosteroid based medication significantly improves muscle strength and function over the short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

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Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
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Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

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Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

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2014 Group Project 8

2014-10-23T11:37:38Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons. Historical and current research and models in musculoskeletal development will be addressed, with addition of common muscular congenital fetal abnormalities.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="lightsteelblue"
! Cell type/Stage!! Associated Molecules!! Action
|- bgcolor="slategray"
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="lightsteelblue"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|- bgcolor="slategray"
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="lightsteelblue"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

No studies focusing on the fetal development of muscle existed before the 1970's. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life, these include: <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref>

*Corticosteroid based medication significantly improves muscle strength and function over the short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

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Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
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Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

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Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

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2014 Group Project 8

2014-10-23T11:30:04Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="lightsteelblue"
! Cell type/Stage!! Associated Molecules!! Action
|-
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="lightsteelblue"
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|-
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="lightsteelblue"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

No studies focusing on the fetal development of muscle existed before the 1970's. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life, these include: <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref>

*Corticosteroid based medication significantly improves muscle strength and function over the short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

References to format in later

Biggar, W. (2006). Duchenne muscular dystrophy. Pediatrics in Review, 27(3), pp.83--88.

Birk, D. and Trelstad, R. (1986). Extracellular compartments in tendon morphogenesis: collagen fibril, bundle, and macroaggregate formation. The Journal of cell biology, 103(1), pp.231--240.

Deries, M., Gon\ccalves, A., Vaz, R., Martins, G., Rodrigues, G. and Thorsteinsd\'ottir, S. (2012). Extracellular matrix remodeling accompanies axial muscle development and morphogenesis in the mouse.Developmental Dynamics, 241(2), pp.350--364.

Dom\`enech-Mateu, J., Mart\'\inez-Pozo, A. and Arn\'o-Palau, A. (1994). Development of the tendon of todaro during the human embryonic and fetal periods. The Anatomical Record, 238(3), pp.374--382.
Harel, I., Maezawa, Y., Avraham, R., Rinon, A., Ma, H., Cross, J., Leviatan, N., Hegesh, J., Roy, A., Jacob-Hirsch, J. and others, (2012). Pharyngeal mesoderm regulatory network controls cardiac and head muscle morphogenesis. Proceedings of the National Academy of Sciences, 109(46), pp.18839--18844.

Herchenhan, A., Bayer, M., Svensson, R., Magnusson, S. and Kj\aer, M. (2013). In vitro tendon tissue development from human fibroblasts demonstrates collagen fibril diameter growth associated with a rise in mechanical strength. Developmental Dynamics, 242(1), pp.2--8.

Hoffman, E., Brown Jr, R. and Kunkel, L. (1987). Dystrophin: the protein product of the Duchenne muscular dystrophy locus. Cell, 51(6), pp.919--928.

Katori, Y., Hyun Kim, J., Rodr\'\iguez-V\'azquez, J., Kawase, T., Murakami, G. and Hwan Cho, B. (2011). Early fetal development of the intermediate tendon of the human digastricus and omohyoideus muscles: a critical difference in histogenesis. Clinical Anatomy, 24(7), pp.843--852.

KINMONT, P. (2008). development of the human achilles tendon enthesis organ. Journal of Anatomy.

Nichol, P., Corliss, R., Yamada, S., Shiota, K. and Saijoh, Y. (2012). Muscle Patterning in Mouse and Human Abdominal Wall Development and Omphalocele Specimens of Humans. The Anatomical Record, 295(12), pp.2129--2140.

Rodriguez-Guzman, M., Montero, J., Santesteban, E., Ga\~nan, Y., Macias, D. and Hurle, J. (2007). Tendon-muscle crosstalk controls muscle bellies morphogenesis, which is mediated by cell death and retinoic acid signaling. Developmental biology, 302(1), pp.267--280.

Rodr\'\iguez-V\'azquez, J., M\'erida-Velasco, J. and Verdugo-L\'opez, S. (2010). Development of the Stapedius Muscle and Unilateral Agenesia of the Tendon of the Stapedius Muscle in a Human Fetus. The anatomical record, 293(1), pp.25--31.

Shwartz, Y., Farkas, Z., Stern, T., Asz\'odi, A. and Zelzer, E. (2012). Muscle contraction controls skeletal morphogenesis through regulation of chondrocyte convergent extension. Developmental biology, 370(1), pp.154--163.

Sussman, M. (2002). Duchenne muscular dystrophy. Journal of the American Academy of Orthopaedic Surgeons, 10(2), pp.138--151.

Tich\`y, M. (1989). The morphogenesis of human sphincter urethrae muscle. Anatomy and embryology, 180(6), pp.577--582.

Tich\`y, M. and Grim, M. (1985). Morphogenesis of the human gluteus maximus muscle arising from two muscle primordia. Anatomy and embryology, 173(2), pp.275--277.

Yiu, E., Kornberg, A. and others, (2008). Duchenne muscular dystrophy. Neurology India, 56(3), p.236.

Zagrebin, A. (1971). Morphogenesis of the lamellar receptors of human striated muscle.Bulletin of Experimental Biology and Medicine, 71(2), pp.199--201.

2014 Group Project 8

2014-10-23T11:26:10Z

Z3418989: /* Molecular and Cellular regulation of fetal myogenesis */

{{ANAT2341Project2014header}}
=Musculoskeletal=
This webpage will be focusing on fetal muscular development.

==Introduction==

The musculoskeletal systems main purpose is to provide the body with structure, stability, support, protection, mineral storage, heat production and movement. It is made up of multiple structures; bone, cartilage, skeletal, tendon, ligaments and joints. This project will be focusing on the fetal development of the Muscular and Tendinous tissue. Both tissue have major embryonic contributions from the somites and during the fetal period undergo myogenesis and fibrillogenesis respectively to form mature tissue. Many molecules, particularly growth factors and proteoglycans regulate the growth of the muscle and tendons.

==Muscle development General Timeline==

Primary myofibers develop in first trimester, Secondary myofibers develop during Second and Third Trimester
We should do a brief overview of primary myofiber formation, and extensive overview of secondary myofiber formation)

==Background Early Embryonic development==
Mesenchymal progenitor cells from somites(occiptal, cervical, thoracic, lumbar, sacral), undergo multiple differentiation stages to create muscle fibers.

http://www.mdconsult.com/books/figure.do?figure=true&eid=4-u1.0-B978-1-4377-2002-0..00015-1--f0025&sectionEid=4-u1.0-B978-1-4377-2002-0..00015-1&isbn=978-1-4377-2002-0&uniqId=464007141-2 (demonstration of myotomes in week 6 and 8)

Skeletal Muscle develops from a process known as myogenesis. Mesenchymal cells which are embryonic connective tissue cels differentiates into embryonic muscle cells, myoblasts. Myoblasts which have single nuclei fuse and elongate to form myotubes which are multinucleated and cylindrical.

The neural tube and notochord release signaling molecules like Shh, Wnts and [BMP]-4. These signaling molecules act on transcription factors of the MyoD family and Pax <ref name="PMID10809386"><pubmed>10809386</pubmed></ref>.The MyoD or MrF family includes MyoD, Myf-5 , myogenin and Myrf4 <ref name="PMID7748174"><pubmed>7748174</pubmed></ref>. The MyoD family of transcription factors, like MyoD are myogenic bHLH (basic helix loop helix) transcription factors. Pax-3 and the MyoD induce myogenesis, formation of myoblasts. Pax-3 also acts on c-met which is a migratory peptide.

Skeletal muscle is derived from the somites. Paraxial mesoderm segments into somite structures on both sides of the notochord and neural tube.

[[Image: Mesoderm-cartoon3.jpg]]

Somites are mesodermal structures where the dorsal most end of the somite, which is known as the dermomyotome, becomes skeletal muscle and dermis <ref name="PMID9094722"><pubmed> 9094722</pubmed></ref>. A small anterior portion of the paraxial mesoderm remains un-segmented and eventually forms some muscles of the head <ref name="PMID12587921"><pubmed> 12587921</pubmed></ref>.

The myotome lies in-between the scleratome which is ventrally located. The scleratome forms the cartilage and bone of the axial skeleton of the embryo. The dermomyotome is located dorsally and forms the first skeletal muscle in the embryo.
The medial part of the dermomyotome forms the dorsal and intecostal muscles whilst the lateral part of the dermomyotome forms the limb and ventral muscles <ref name="PMID9094722"/>.

As nearly all of the muscular system develops from the mesoderm. The Iris muscle comes from neuroectoderm. And Eosophagus skeletal muscle is derived from transdifferentiation of smooth muscle.

[[Image: Somite_cartoon5.png]]

Myogenesis occurs in two phases; primary and secondary which occur in embryonic and fetal periods Primary myotubes express MHC slow myosin heavy chains <ref name="PMID21204650"><pubmed> 21204650</pubmed></ref>. The primary myotubes form the structure and scaffold upon which secondary myotubes form during secondary myogenesis which occurs in the fetal period <ref name="PMID21204650"/>.

==Molecular and Cellular regulation of fetal myogenesis==

Skeletal myofiber number is set at birth <ref name="PMID5804561"><pubmed>5804561</pubmed></ref>. This was found using experiments on mice and pigs. Similar trends have been observed in humans as found by Widdowson et al (1972) where a huge increase and then levelling of gastrocnemius myofibers were found in the gestational period <ref name="PMID5046781"><pubmed>5046781</pubmed></ref>.

Multipotent mesenchymal cells (MSC) form myoblasts as well as adipocytes and fibroblasts <ref name="PMID10102814"><pubmed>10102814</pubmed></ref>. Therefore specific the nutrients and growth are important in directing the growth of these MSC into either adipocytes or myoblasts <ref name="PMID23100595"><pubmed>23100595</pubmed></ref>.
Myoblasts differentiate to form the embryonic, fetal and adult skeletal muscle and in fact myoblasts differentiate to form all three early in development.

As discussed previously the mesoderm forms somites which divide into the dermomyotome, scleratome and myotome. The myotome becomes the skeletal muscle. Myotome cells migrate to different positions, for example limb buds. Once myotome cells travel to a target destination. Mrf like MYOD1 allow for the transdifferentiation of mesodermal myotome cells to the immature muscle cells, myoblasts.

Then the myoblasts proliferate under the influence of growth factors up until the neonatal period. Following this there is a decrease of growth factors which results in the myoblasts stop proliferating and align. The membranes of the myoblasts fuse together and become myoblasts.

Approximately 100 or more myotubes form a muscle fiber. Note that myoblasts still continually are fusing to this growing mass of muscle cells.Primary myotubes form the scaffolding for which the fetal myoblasts will differentiate into secondary myotubes and add too <ref name="PMID640968 "><pubmed>640968 </pubmed></ref>.

When myoblasts fuse and become myotubes there are number of changes that occur. In the cytoplasm of myotubes features of striated skeletal muscle develops like myofilaments and myofibrils. Myofibrils contain thick myosin and thin actin proteins which repeat along the myofibril to form repeating units known as sarcomeres. The sliding of myosin and actin allows for the contraction of muscle. In sarcomeres the nuclei are pushed to the side.

What is known as primary myofibre forms when after growth factors attract the nerve and it attaches to the bed of muscle. The nerve gives of branches to secondary myofibres that form later.

[[File: Skeletal muscle structure.jpg|frame|right|middle|300x250px|Structure of striated muscle]]

{| class="wikitable mw-collapsible mw-collapsed"
! Muscle Myogenesis Movie
|-
| <html5media height="400" width="400">File:MYOGENESIS_video.mp4</html5media>
|}

External laminae develops for the myotubes and separates the myotubes from surrounding mesenchymal tissue. Sheaths containing the myofibers develop; endomysium, epimysium and perimysium. External laminae and reticular fibers form the endomysium. Perimysium and epimysium layers are the formed by fibroblasts.

There are a greater deal of secondary myofibers than primary and are receptive to nutrients and growth factors.
Muscle regulatory factors (MRFs) control the proliferation of secondary myoblasts. MRF’s are helix-loop-helix transcription factors.

{| class="wikitable"
|- bgcolor="lightsteelblue"
! Cell type/Stage!! Associated Molecules!! Action
|- bgcolor="lightblue"
| Mesodermal Myotome cells || MYOD1|| Allow for transdifferentiation into myoblast
|- bgcolor="lightsteelblue"|
| Early Myoblasts || PAX7 , MYF5|| Act on CDK4 and Cyclin D1 to dephosphorylate Rb and induce cell proliferation <ref name="PMID24532817"/><ref name="PMID22445545"/><ref name="PMID12242286"/>
|- bgcolor="lightblue"
| Mature ‘Committed’ Myoblasts || MRF, MROD || MYOD acts on myostatin to take myoblast out of cell cycle and ready for differentiation <ref name="PMID24532817"/>
<ref name="PMID12242286"/>
|- bgcolor="lightsteelblue"
| Myotubes || Myogenin, MRF4|| Inhibit cell cycle and proliferation by acting on regulatory proteins like P21 to stop the cell cycle allowing for the conversion of myotube to myofiber. <ref name="PMID24532817"/><ref name="PMID10733231"/>
|}

Myoblasts early on express PAX7 and MYF5. PAX7 and MYF5 act on regulatory proteins of the cell cycle like CDK4 and Cyclin D1. CDK4 and Cyclin D1 dephosphorylate RB. RB when phosphorylated is active and inhibits the cell-cell progression in the cell cycle and hence inhibits proliferation. Therefore Cyclin and CDK4 in this case induces cell proliferation. <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID22445545 "><pubmed>22445545</pubmed></ref>, <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref> These creates more fetal myoblasts committed which are committed or programmed to form fetal or secondary myofibers later on. These committed myoblasts express MRF, MYOD. MYOD acts on myostatin to take the myoblast out of the cell cycle and stop it from proliferating <ref name="PMID24532817 "><pubmed>24532817</pubmed></ref>; <ref name="PMID12242286 "><pubmed>12242286</pubmed></ref>.Now the myoblast is ready for differentiation. The phosphorylation of RB at this period helps to slow the activity of the cell cycle. The myoblasts differentiate and fuse to become multinucleated myotubes as discussed previously. In the formed myotubes MRFs like Myogenin and MRF4 act to inhibit cell cycle and proliferation by acting regulatory proteins like P21 to stop the cell cycle <ref name="PMID24532817"/>; <ref name="PMID10733231 "><pubmed>10733231</pubmed></ref> This allows the conversion of the myotube into a myofiber. The myofiber can express desmin an intermediate filament that is increasingly seen as gestation progresses. Growth factors, amino acids and stretch/load activity act on myofibers to affect subsequent hypertrophy maturation of myofibers <ref name="PMID 24532817 "/>

[[File:Mouse limb tissue development.jpg|frame|right|450x350px|Mouse limb tissue development]]

Myofiber hypertrophy is occurs when protein synthesis is greater than protein degradation. The resulting accumulation of protein results in hypertrophy. Therefore maintaining protein levels is important in the hypertrophy of myofibers. This is regulated by nutrients and growth factors (GFs).
Growth factors
There are a large number of growth factors that affect fetal myogenesis. Some of the main GFs as Brown (2014) describes in her article “IGF1, insulin, basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF- β)” <ref name="PMID 24532817"/>; <ref name="PMID2190237 "><pubmed> 2190237</pubmed></ref>, <ref name="PMID22682632"><pubmed> 22682632</pubmed></ref>. Experiments have shown that if IGF1 is removed reduced muscle mass and hypoplasia has resulted. And increased IFG1 expression has resulted in hyperplasia and increased skeletal muscle. IFG1 has also been found to enhance protein synthesis. <ref name="PMID 24532817"/>; <ref name="PMID3546571"><pubmed> 3546571</pubmed></ref>, <ref name="PMID 7744859"><pubmed> 7744859</pubmed></ref>. Insulin similarly has been found to promote fetal muscle growth and protein synthesis.
Basic fibroblast growth factor (bFGF) and transforming growth factor-β (TGF- β) induce proliferation and myogenesis by upregulating cyclin D.

Nutrients
Being the building block of proteins, amino acids are important in muscle protein synthesis. But they are more important in adult muscle protein synthesis than fetal muscle protein synthesis. Experiments have shown for example an amino acid infusion did not always result in a fetal muscle growth, only when there was a rise in the insulin levels. And little is known of the interaction of amino acids and growth factors in the context of fetal myogenesis.
Research has shown that fetal or secondary myofibers are more prone to suffer from nutrient deficiency than primary myofibers in pigs and sheep. <ref name="PMID 2041547"><pubmed> 2041547</pubmed></ref>, <ref name="PMID 8014156"><pubmed> 8014156</pubmed></ref>, <ref name="PMID 15317692"><pubmed> 15317692</pubmed></ref>.

Fetal myofiber number have been observed to decrease with detal nutrient deficiency. This is concerning as myofiber numbers are set at birth.
Nutrients are also important for fetal myofiber hypertrophy.
Stretch and loading
Stretch and loading also affect hypertrophy. <ref name="PMID23629510"><pubmed>23629510</pubmed></ref>.

==Tendon Development==

Tendons are connective tissue which join muscle and bone allowing the transmission of force. They organised fibrils which form fibers, which along with fibroblasts are surrounded by connective tissue to form fascicles<ref name="PMID 2337297"><pubmed>2337297</pubmed></ref>. Tendons primary embryonic structure originate from mesenchymal progenitor somite cells with further contributions of Neural Crest and Lateral plate mesoderm.

Appearance of tendons begins in the 20th Carnegie stage and marks the beginning of fibrillogenesis. This process is initiated by fibroblasts in series of extracellular compartments; they enlarge the cells domain into extracellular space. Channels deep in the cytoplasm drive the process of elongation, these channels location is associated with Golgi bodies. <ref name="PMID 8115369"><pubmed>8115369</pubmed></ref>

First type of compartments are formed by collagen containing secretory vacuoles which fuse with surrounding cell membranes. Initial fibrillogenesis is mediated my macromolecular interactions based on vacuole content, with a lesser input from receptor membrane interactions. Fibril groups as fibres close to the cell surface and Secondary extracellular compartments form; at this stage fibroblast are arranged adjacently<ref name="PMID 8115369"><pubmed>8115369</pubmed></ref> . Third level of compartmentalization forms later when fibroblast are adjacent with 2 or more other fibroblasts. As the tendon matures fibres coalesce invading each other with interdigitating processes. Secretory vacuole persist, laterally aggregating with further growth preserving sites for fibril deposit. <ref name="PMID 7780173"><pubmed>7780173</pubmed></ref>
Collagen fiber assembly branch creating fibre networks along fascicles. Fibres branch within tendon fascicle

The main regulatory factors of tendon fibrillogenesis are Leucine-rich repeat proteoglycans.

{| class="wikitable"
|-
! Non-fibrillar components!! Molecular Characteristics!! Function
|-
| Fibromodulin || Keratin sulphate, proteoglycan || Fibromodulin modulates the site-specific cross-linking ultrastructure of collagen, ensuring mechanical strength. Control the pattern of lysyl oxidase-mediated collagen cross-linking by reducing access of the enzyme to telopeptides, by binding to the collagen. <ref name="PMID 24849606"><pubmed>24849606</pubmed></ref>
|-
| Decorin || Chondoirin sulphate, proteoglycan || Regulates expression of multiple leucine-rich proteoglycansins(SLRP) during tendon fibrillogenesis, via Class I and II Small(SLRP). Competes with Biglycan for binding sights on collagen types I-VI . Concentration increases as fetal development continues. <ref name="PMID 16518859"><pubmed>16518859</pubmed></ref>
|-
| Biglycan ||Chrondroitin sulphate and Dermatan sulphate, proteoglycan|| Process of regulation closely mimic Decorin though concentration is maximal expression at day 16-18 during embryonic development, reducing after this point. Competes with Decorin for bonding sights on collagen I-VI. <ref name="PMID 16810681"><pubmed>16810681</pubmed></ref>
|}

==Historical findings==

No studies focusing on the fetal development of muscle existed before the 1970's. During the 70's a majority of research in the field focused on the histologist differences and development of differing fiber types. Research on morphological development was very limited with eh exception of two Czechoslovakian studies in the late 1980's which displayed muscle formation for multiple primordia and sexual differentiation.

The differing developments of alpha and beta fibers was revealed in a study by University of California published in 1972 using lamb fetuses as an experimental model. Beta muscle fibers are formed during the first stages of fusion, the individual Beta fibers create a network for the alpha fibers to develop on. Red muscle fasciculi are formed by merging of small fiber bundles. White muscles are formed by ongoing addition of alpha fibers. Additionally it was concluded that fetal muscle contraction didn't significantly effect number of fibers present. Most fibers had already been formed by the 20th week, during this period the limit muscle contractions result in little mechanical tension.

===Gluteus Maximus muscle Morphogenesis===

[[File:Gluteus Maximus, Representative Primordia.png|frame|right|450x550px|Labelled drawing of Gluteus Maximus divided to display developmental origin. ]]

A 1985 Czechoslovakian study investigated the development of Gluteus Maximus during the embryonic and fetal periods. Pelvic micro-dissection of human embryos and foetuses with crown length varying from 22-215mm were compared with newborns and adults. The presence of a muscle not present in post-natal adults<ref name="PMID 5043313"><pubmed>5043313</pubmed></ref> was discovered called the coccygeofemoralis or pars coccygea; this muscle originates from sides of coccyx and inserts onto the gluteal tuberosity. In the adult human this muscle fuses with the larger pars sacroiliac or fetal gluteus maximus to create the adult gluteus maximus. Pars sacroiliac originates from ilium and sacrum and inserts onto the gluteal tuberosity<ref name="PMID 15788867"><pubmed>15788867</pubmed></ref>.

In fetus with crown length of less than 45 mm the two muscle primordia are separated by small amount of loose connective tissue. This point onwards the muscles become fused by a small furrow which persists until 215mm crown length. By the time of birth the furrow is absence and the muscles are entirely fused <ref name="PMID 4083527"><pubmed>4083527</pubmed></ref>. The coccygeofemoralis in long tailed mammals remains separate from the gluteus maximus, known as the caudofemeralis muscle <ref name="PMID 1255730"><pubmed>1255730</pubmed></ref>. In these animals its function is lateral flexion of the tail <ref name="PMID 8843689"><pubmed>8843689</pubmed></ref>. This is the sole morphogenesis study on any of the large muscles; it is likely other muscles with multiple origins and insertion have separate fetal muscle primordia.

===Morphogenesis of human sphincter urethrae muscle===

This study was completed in 1989 at the same Czechoslovakian institute of that of Gluteus maximus morphogenesis. . Study displayed three developmental phasess differentiating them by morphogenesis, histology and sexual dimorphism. External urethral sphincter in embryos and fetuses with crown length varying 18-320mm, neonates, children and adults was fixed in formaldehyde and embedded in paraplast. Then cut in series and stained with hematoxylin and eosin for histological analysis. Morphogenesis and Sexual dimorphism specimens were micro-dissected using a stereomicroscope. <ref name="PMID 2610390"><pubmed>2610390</pubmed></ref>
{| class="wikitable"
|-
! Phase!! Time Period!! Characteristics
|-
| Indifferent Phase || Before 10th week || Muscle primordia discernible from neighbouring muscle by week 8. Grow to forms a shallow arch; connecting the urethra and urogenital diaphragm. Consists of condensation of myoblasts up to 9.5 weeks, past this point myotubes and muscle fibres appear.
|-
| Sexual dimorphic Phase || 10th week to Birth || Associated with the development of prostate and vagina. Primordia spread along urethra wall posteriorly.
In Males: Spreads to create the infraprostatic part of external urethral sphincter. Arches anteriorly to join prostate and urethra
In Female: Spreads to create upper part of external urethral sphincter. Lower sixth of sphincter connects anterior and lateral urethral walls, additionally projects to lateral vaginal walls.
|-
| Definite Structuring Phase ||After birth|| Position of urethral sphincter does not alter in relation with prostate and inferior part of vagina. Infraprostatic region in males and upper part in females grow to form a complete ring.
|}

==Current Research and Findings==

===Intermediate tendon of Human Digastricus and Omohyoideus===

Early Fetal Development of the Intermediate Tendon of the Human Digastricus and Omohyoideus Muscles: A Critical Difference in Histogenesis
Katori et al (2011) did a study to understand the development of the intermediate tendon within digastricus and omohyoideus muscles. Both digastricus and omohyoideus have two bellies of muscle with an intermediate tendon. Digastricus has an anterior and posterior belly whilst omohyoideus has a superior and inferior belly. Omohyoideus is supplied by ansa cervicalis and digastricus is supplied by cranial nerves; trigeminal and facial. The difference in innervation gives reason to believe that these muscles are different developmentally. Katori et al (2011) investigate the differences in the formation of the intermediate tendon between these two muscles.
Katori et al (2011) observed in week 7-9 the posterior belly of digastricus was developing the intramuscular tendon, intermediate tendon, with a bulb like terminal tendon. The anterior belly however did not have any intramuscular tendon present within it. The anterior bellies of 50% of specimens moved towards and attached on the bulb like terminal part of tendon. Instead of finding its insertion on reichart’s cartilage or hyoid bone it attaches to terminal pat of posterior tendon. Katori et al hypothesize that this is due impediment such as stylohyoideus, vascular arteries and hypoglossal nerve. How these structures may affect digastricus is for example the thicker hypoglossal nerve separating the posterior tendon from attaching the hyoid bone. Also the stylohoideus’ caudal end may form from mesenchymal condensation near the caudel end of Reichart’s cartilage inhibiting attachment by digastricus. This shows how timing of muscle development affects topography. But eventually muscle fibers in posterior belly intramuscular tendon get converted. The caudal and bulb end of the posterior belly becomes into a tight intermediate tendon at 15 weeks. In half of the specimens in week 15 and week 18 it was observed by desmin immunohistochemistry that anterior belly muscle fibers were integrated into bulb-like and caudal end of posterior tendon.
The omohyoid at 7-9 weeks had a nearly straight and superior-inferior path. Omohyoid becomes angulated by the lateral expansion of the clavicle and shoulder joint (Katori et al, 2011). Omohyoid is tightly fitted in-between think sternocleidomastoid, scalene muscles. The fascia and lymphatic tissue also press the omohyoid towards the sternocleidomastoid. At this time during 7-9 weeks unlike digastricus, omohyoid is a single muscle belly. It reaches its greatness thickness at week 15 at 0.5mm then at the point where tendon develops the thickness decreases. In omohyoideus the intermediate tendon forms secondarily. From week 18 to week 20 the intermediate tendon muscle fibers are replaced by collagen fibers most likely after the tendon has fully shaped itself. Cells along the medial margin of omohyoid were found to be vimentin positive. As vimentin is a intermediate filament its presence is an indication mechanical or osmotic stress (Pekny and Lane, 2007).

===Stapedius===

Rodri´guez-va´ zquez et al (2010) in order to understand the isolated case of a unilateral agenesia of the stapedius tendonduring week 14 of post-conception development (PCd), have tried to understand the way the stapedius muscle develops. The stapedius is essentially formed by two anlagen; one anlagen forms the tendon of the stapedius and the other forms the muscle belly of stapedius. The anlagen forming the tendon are derived from the internal segment of interhyale. The anlagen forming the muscle belly are derived from the 2nd pharyngeal arch near the interhyale, medial to the facial nerve. Interhyale is the internal part of the second branchial arch and it develops into stapedius’ tendon (http://www.drugs.com/dict/interhyale.html ). The observed unilateral agenesia of the tendon of stapedius was found by Rodriguez-va et al (2010) to be due to the internal segment of the interhyale regression. Instead the belly of stapedius was accompanied with a pseudo tendon formed by the external segment of the interhyale. To come to this conclusion the formation of the stapedius and pyramidal eminence was tracked.
Interhyale was observed as a mesenchymal condensation formed at O’Rahilly stage 16 at cranial component of the second branchial arch. The stapes and reichert cartilage are eventually differentiated from the interhyale. At Stage 18 and 19 a mesenchymal bridge forms by the interhyale which bridges stapes and cranial part laterohyale (Reichert’s cartilage). And by stage O’Rahilly stages 20 and 21 the interhyale fully develops.
Rodriguez-va et al (2010) found the belly of stapedius to derive from a blastema which develops adjacent the interhyale to form its own anlage. In O’Rahilly stage 22 Rodriguez-va et al (2010) found that the interhyale began to take on an angular shape and the anlage of the stapedius belly connected with the vertex of the now angular interhyale.
Interhyale developed two segments; internal thick segment and an external thin segment. The thick internal segment as discussed contributes to the formation of the tendon of the stapedius muscle. The external segment begins to regress in the embryonic period and by the fetal period, week 9, the external segment was observed to be fully regressed.
In weeks 10-11 a conical belly continuous with the stapedius tendon was observed and the stapedius tendon attached at the back of the stapes head.The anlage of the pyramidal eminence formed around week 12-14 and around the stapedius muscle belly. The mesenchymal condensation forming the pyramidal eminence grew until it was inhibited around week 15-17.
After week 9 there was not much morphological change of the stapedius. From this study we can see how the stapedius muscle develops its shape and attachments in the embryonic and fetal periods.

==Abnormalities==

===Duchenne Muscular Dystrophy(DMD)===

[[File:Progressive myofiber replacement by fibrotic and fat tissue in Dmdmdx rats..jpg|frame|right|600x400px|Progressive myofiber replacement by fibrotic and fat tissue in Duchenne Muscular Dystrophy rats <ref name="PMID 25310701"><pubmed> 25310701</pubmed></ref>]]

Caused by a mutation of the dystrophin protein on locus Xp21; this protein complex connects the cytoskeleton of muscle fibres to the extracellular matrix. Abnormal dystrophin results in a degradation of cellular integrity, excessive penetration of sarcolemma by calcium and water entering mitochondria increasing pressure and bursting. The lack of significant load during fetal development results in minimal wasting, it is detected in postnatal babies at 3-5 years old with muscles resisting gravity being the first to waste. Patients are restricted to wheelchairs by their early teens and have a life expectancy of 25 years. The incidence in male infants is 1 in 36,000.
Typically males are affected while females are carriers. In the offspring of a carrier mother and unaffected father; sons have 50% chance of affected and daughter’s 50% chance of becoming carriers. Since the disease is a terminal illness killing in mid-twenties; affected fathers are not considered in the situation. Absence of affected fathers means it is very unlikely for daughters to be affected.

Duchenne Muscular Dystrophy has no present cure, treatment is aimed at altering onset of symptoms and maximising quality of life, these include: <ref name="PMID 25187493"><pubmed> 25187493</pubmed></ref>

*Corticosteroid based medication significantly improves muscle strength and function over the short term. Patients using the drugs long term gradually have reduced doses to avoid severe side effects which can include; weight gain, behavioural disorders and osteoporosis. Most effective corticosteroids are prednisolone, and deflazacort. Theories on how the steroids work include; include activation of T-Cell pathways, directly reducing muscle regeneration, modulating cell inflammation and enhancement of myogenic precursors. <ref name="PMID 17541998"><pubmed> 17541998</pubmed></ref>

===Amyoplasia===

[[File:Amyoplasia hand contraction.png|frame|right|500x450px|Partial Contracture of Hands, typical manifestation of Amyoplasia]]

Amyoplasia is characterised by replacement of newborn muscle tissue with fat and dense fibrous tissue. Is the most common of Arthrogryposis multiplex congetia disorders; these result in multiple joint contractures. Affected limbs have significantly altered positioning, typically clubfoot is present and elbows are extended<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>. Primary cause is limited fetal movement, muscle is replaced by dense fibrous tissue. Conditions which limit fetal movement include abnormal uterus morphology and reduced amniotic fluid. Presently no genes have been linked to the deformity. <ref name="PMID 9260643"><pubmed>9260643</pubmed></ref>
85-90% of newborns undergo surgery within days of birth primarily on the legs and hips, releasing tendons from contractures. Motion can be improved by casting and splinting, lower limbs are typically cast and upper limbs are typically splinted.
<ref name="PMID 24459070"><pubmed>24459070</pubmed></ref>

{| class="wikitable"
|-
! Involvement subtype!!Percentage of Case
|-
| Four-limb symmetrical || 55%
|-
| Three limb || 5%
|-
| Upper limb only || 17%
|-
| Lower limb only ||16%
|}

===Nemaline Rod Myopathy===

Nemaline rod myopathy is congenital non-dystrophic muscle disease. Sufferers experience general muscle weakness, weakness is most severe in face, neck and limbs. Further clinical manifestations include feeding problems, scoliosis of spine, foot deformities and respiration difficulties. The disease is an inherited both autosomal dominant and recessive, 30% autosomal dominant, 20% autosomal recessive and 50% simplex. In total 6 genes have been associated with the formation of nemaline myopathy with the NEB and ACTA1 genes being the most prominent. Over 60 NEB mutations have been discovered which result in nemaline myopathy, it results in the decreased production and decreased length of the nebulin protein, half of all NM cases are associated with NEB mutations. About 140 mutations to ACTA1 can lead to formation of nemaline myopathy, this results in either aggregation of α-actin fibres preventing functional muscle contraction or total absence of α-actin production.

==References==
Anatomy and variations of palmaris longus in fetuses.<ref><pubmed> 23529313</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23529313]</ref>

Development of the rectus abdominis and its sheath in the human fetus.<ref><pubmed> 22869489</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22869489]</ref>

Sonic hedgehog acts cell-autonomously on muscle precursor cells to generate limb muscle diversity.<ref><pubmed> 22987640</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/22987640]</ref>

The normal growth of the biceps brachii muscle in human fetuses.<ref><pubmed>23468258</pubmed>| [http://www.ncbi.nlm.nih.gov/pubmed/23468258]</ref>

<references/>

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