Beginnings, Growth and Development - Frequently Asked QuestionsOK, these are questions that have come up time and again. Try the following first...before you leap to your email.
1. I would always suggest that you initially look at the lecture/lab handouts and slides for the answer.
2. Next go to your trusty embryology Textbook, which will definitely have your answer!
3. Still stuck? Search the UNSW Embryology website or the online NCBI Bookshelf or Developmental Biology (Gilbert).
If it is simply a term or definition you need then use the Glossary. The received questions below are in no particular order.
Page Links: Introduction | Cord Histology | Where are the Lecture Slides? | How do the Brain vesicles develop? | How do chorionic villi form? | What is Mesenchyme? | Do I need to know all the exact days? | Gastrulation | How the amnion covers the embryo? | Yolk sac is formed from endodermal tissue? | What happens to the yolk sac? | Amniotic sac is formed from? | Difference in clinical and embryonic weeks? | Abnormal and normal development? | Gamete development? | Specific zones of neuroblast proliferation | Relationship between the chorion and placenta | Which ectoderm cells form the neural plate? | Origin of extraembryonic mesoderm? | When does the differentiation of the hypothalamus occur? | Added Lab Links | Quick Lecture Links | Glossary
Correction An earlier version of the labelled Placental Cord cross-section image (and given in some Practical 8 class tutorial/descriptions) had transposed the labelling of vein and artery. The current online version, shown right, is now correct. (More? Placenta Histology) |
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Links were printed in the student guide and entered in the Map as Key References. I also said in both the lecture and all the associated labs (and the labs have links to the page..."that all my lectures are placed on the UNSW Embryology site" and I gave the link (and we start the labs on the class notes page each time)!
Lecture Development of the embryo/fetus - 1 (12 May 10-11am, Clancy)
Lecture Development of the embryo/fetus - 2 (13 May 10-11am, Central Lecture Block 7)
Mesenchyme is embryonic connective tissue. It is also used to describe the spread, unorganised cellular nature (mesenchymal) of this tissue. Mesenchyme refers only to the cellular organization, it can have different embryonic origins, for example it can be derived from mesoderm or neural crest in origin.
I hate these negative questions, tha always start with "do I need to know?" My answer would always be yes you do need to know, otherwise why would you be here? But in this case, I will break my rule, at this point you do not need to know the exact day by day timings (a general understanding will do).
The point is to identify what is happening when in development, the sequence, and eventually you can build a bigger picture of what is happening in the embryo/fetus at different times.
This is relevant when we start considering "critical periods" of development when maternal effects can have serious effects on specific systems and leave others relatively untouched.
(More? Week by Week)
The lecture notes state that the mesencephalon divides into the secondary vesicles of mesencephalon and metencephalon. However, we were under the impression that mesencephalon remains the same and becomes known as the midbrain.
Early Brain Structure - 3 primary vesicles
Early Brain Structure - 5 secondary vesicles
From the 3 primary vesicles developing to form 5 secondary vesicles
How does the scyncytiorophoblast form the placenta, chorionic villi etc.?
If I did say the 2nd week, it was an error, gastrulation occurs in the 3rd and 4th week. Week 1 and 2 are before and early implantation: this earlier period is about blastocyst, trophoblast and bilaminar embryo development.
(Quicktime movie in a new page Exocoelomic Vesicle) |
Initially it is the folding of the embryonic disc ventrally bringing with it the amnion (attached at the disc edge) from the dorsal side around ventrally. This occurs on the disc right and left, as well as above and below the notochord. Later changes in relative size and relationships can be best answered by looking at the animation of the early developing embryo and its surrounding sacs and membranes. |
Yolk sac wall is formed from endoderm and extraembryonic mesoderm. The embryo is initially smaller than the yolk sac, but as it continues to grow faster than the yolk sac, it then lies bent over (ventrally) the top of the yolk sac and folding "grabs" part of the sac which will now lie inside the embryo (fore-, mid-, hind-gut) and connecting stalk (allantois) and the rest outside the embryo as the visible yolk sac. Continued growth of embryo and connecting stalk isolates the yolk sac ball to the peripheriy connected by a yolk stalk (see the animation below).
The outer mesoderm layer of the yolk sac differentiates to form blood vessels and red blood cells (blood islands) and forms the vitelline circulation.
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First, look at the above question on the amnion and watch the change in tthe yolk sac (yellow). Now watch the movie below showing just the changes in the yolk sac.
(Quicktime movie in a new page Endoderm (270 Kb)) |
Amniotic sac is lined with ectoderm and covered in extraembryonic mesoderm. The ectoderm lining the sac is outside the embryonic disc and is therefore "extramebryonic" (but we don't usulally give it this name), the extraembryonic mesoderm covering it is initially continuous with the connecting stalk.
The sac begins as a small fluid-filled space on the dorsal side of the embryonic disc, but with growth and folding ventrally of the disc is "drawn" over the edge of the folding disc to cover the embyro. The embryo, then fetus continues to grow entirely within the amniotic fluid-filled sac until term, when it ruptures just before birth.
Clinical weeks are calculated from the only easily accessible data the clinician can access, the last menstrual period (LMP), which you can generally assume is 2 weeks before ovulation and therefore at least the same amount of time before fertilization. Embryonic weeks are from the time of fertilization, a time not easily available to the clinician, and reflects the timing of embryonic development that are covered in all my lecture and lab notes and on this website.
(More? Week by Week | Birth Date Calculator | Childbirth)
In the lectures and labs I have covered mainly the events in normal development focussing on embryological period in detail and broad concepts in fetal development. I did also include, to fit in with your scenario, the factors which can impact on normal development as well as the criterial periods for major impact in various systems. In medicine, you will be eventually dealing with both processes and this was intended as a general introduction that you can build upon.
(More? Critical Periods of Human Development | Abnormal Development)
I did spend more time on egg development than sperm, as I think you will be covering testes in detail elsewhere in histology practicals. I had limited time and thought it would be more relevant initially to use the timeline of oogenesis and the ovary, following this through to ovulation, fertilization and development. You will eventually need to know both, in BGD B we cover development of the gonad.
(More? Week 1 - Oogenesis | Blue Histology - Female Reproductive System | Week 1 - Spermatogenesis | Blue Histology - Male Reproductive System )
In this coverage of neural development in embryology I introduced: development of the neural plate, neural groove, neural tube, neural crest and some abnormalities of neural development. There are within the neural tube regions or "plates" (floor, basal, alar and roof) which will form different neural structures in the brain and spinal cord. I think in the lab I gave the example of the basal plates region in the spinal cord giving rise to the ventral motor horns (motorneurons) and the alar plate giving rise to the sensory interneurons in the mature spinal cord. I also mentioned the neural crest giving rise to the dorsal root ganglia and ventral sympathetic/parasympathetic ganglia.
In the brain, regions of the tube have similar names but give rise to many different CNS structures. You should be generally aware of this (and will eventually need to know) but I did not cover this in detail in the current lectures or labs.
(More? Neural Development)
Ectoderm cells in the midline above the notochord form the neural plate. All ectoderm cells are initially equivalent and cell signalling leads to the neural plate cells differentiating from the surrounding ectoderm, the most obvious cellular morphological change is going from a cuboidal to a columnar epithelium.
(More? Neural Development)
By definition extraembryonic mesoderm is mesoderm outside the embryonic disc and there are several suggested origins for this extraembryonic mesoderm: trophoblast layer, cytotrophoblast, primitive streak (gastrulation), endoderm and yolk sac. Genetic based research in primates suggests a primitive streak (gastrulation) origin. Clearly with onging scientific contention this would be a difficult topic to currently assess at an undergraduate level.
"...amniotic mesenchyme (usually diploid) derives from early embryonic mesoderm of the primitive streak and not from the hypoblast as is commonly cited" Reference 1
"...the origin of the first extraembryonic mesoderm in the rhesus monkey is probably a two-step process, with formation of a reticulum from primitive endoderm followed by differentiation in situ into mesenchymal cells." Reference 2
"The caudal margin of the primitive streak develops precociously in 12- to 14-day human and macaque embryos, and this appears to be the source of all the extraembryonic mesoderm of the chorion, chorionic villi, and body stalk." Reference 3
We will not cover hypothalamus development in this current BGDA cycle. For those that are interested in an answer to the above question, look at Endocrine Development - Hypothalamus.
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Both the embryo and the amniotic sac in which it floats grow and eventually completely fill the chorionic space. The chorion and the placenta are continuous and contain the same cellular layers. Initially the entire surface of the chorion is covered with developing villi. Over time the villi proliferate and are restricted to the side where the placenta will develop (frondosum, towards the maternal blood lacunae) while the opposite side becomes bare of villi (capsularis, towards the uterine cavity). (More? Placenta Notes |
How does the coelom between the intermediate mesoderm and the splanchnic mesoderm develop into the adult body cavities?
My textbook (Moore & Persaud) says that cleavage of the zygote into blastomeres begins about 30 hours after fertilisation. So what is happening in all that time between fertilisation and zygote cleavage?
What is the origin of extraembryonic mesoderm? Does it come from extraembryonic reticulum secreted between Heuser’s membrane? Does it come from the epiblast?
How does the scinciotorophoblast form the placenta the chorionic villi?
Should we know the specific zones that neuroblast proliferation leads to and what week of embryological development should we know up to in detail?
How does the placenta form? i.e. how is it and the embryo separated during the blastocyst stage?
In the expert tutorial could you please go over how the heart develops in an embryo.
How should we remember the stages? Carnegie? Weeks? Anatomical? Do we need to know all?
Which one(s) are lined with epiblast cells and which are lined with extraembryonic membrane?
Chorionic villi is formed by the syncytiotrophoblast (outer layer) and cytotrophoblast (inner later)?
Lecture 7 Development of the embryo/fetus - 1
Lecture 8 Development of the embryo/fetus - 2
Practical 3 Embryology (fertilisation) (G2/G4 Wallace Wurth)
Practical 6 Embryology (fertilisation to 6 weeks) (G2/G4 Wallace Wurth)
Lecture 7 Development of the embryo/fetus - 1 (28 Jul 11-12am, Central Lecture Block 7)
Lecture 8 Development of the embryo/fetus - 2 (31 Jul 8-9am, Clancy)
Practical 3 Embryology (fertilisation) (G2/G4 Wallace Wurth) Practical 3 Labelled Images
Practical 4 Embryology (fertilisation to 6 weeks) (G2/G4 Wallace Wurth) Practical 4 Labelled Stages Images (not all structures labelled)
Practical 8 Placenta-fetal Membranes (TBA)
Practical 11 Embryology (embryo to fetus) (G2/G4 Wallace Wurth) Practical 11 Labelled Stages Images (not all structures labelled)
The additional listed links now appear on some lab pages to help with understanding development of some organs and structures.
Practical 11
Lab 11 - Carnegie stage 16 Face Development
Lab 11 - Carnegie stage 17 auricular hillocks
Lab 11 - Carnegie stage 22 palatal shelves
Lab 11 - Carnegie stage 22 Endocrine Development - Adrenal Glands
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Most of these questions come from previous 2004, 2006 cycles. I will continue to add 2008 answers as I receive questions.
This page is a brief coverage of some FAQs that have come up in Lecture, Practical, Expert Tutorial or by Email.
Note: Inclusion of a FAQ on this page does not relate to whether the material will be examined or assessed! (I am assuming that you really want to know the answer)
For those with slow internet connections, the UNSW Embryology DVD is available for purchase from the Anatomy office (very recent updates will not appear on the DVD).
Please contact me with your question (after you have followed my rules in the introduction) by email to have an answer posted here.
These notes and linked materials have been prepared for Educational purposes only.
Please email Dr Mark Hill if you wish to make a comment about this current project.