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| '''Coelom''', meaning "cavity", and major fluid-filled cavities can be seen to form both within the embryo (intraembryonic coelom) and outside the embryo (extraembryonic coelom). The '''intraembryonic coelom''' is the single primitive cavity that lies within the mesoderm layer that will eventually form the 3 major anatomical body cavities ('''pericardial, pleural, peritoneal'''). | | '''Coelom''', meaning "cavity", and major fluid-filled cavities can be seen to form both within the embryo (intraembryonic coelom) and outside the embryo (extraembryonic coelom). The '''intraembryonic coelom''' is the single primitive cavity that lies within the mesoderm layer that will eventually form the 3 major anatomical body cavities ('''pericardial, pleural, peritoneal'''). |
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| == Objectives ==
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| * Understanding of events during the third week of development
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| * Understanding the process of notochord formation
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| * Understanding the process of early somite development
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| * Understanding the process of body cavity formation
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| * Understanding the future fate of mesoderm components
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| * Brief understanding of early heart formation
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| ==UNSW Embryology Links==
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| * '''Mesoderm Slides''' [http://embryology.med.unsw.edu.au/pdf/ANAT2341L6Mesoderms1.pdf Mesoderm Lecture 2008 - 1 slide/page ] | [http://embryology.med.unsw.edu.au/pdf/ANAT2341L6Mesoderms4.pdf Lecture 3 2008 Slides - 4 slides/page] | [http://embryology.med.unsw.edu.au/pdf/ANAT2341L6Mesoderms6.pdf Mesoderm Lecture 2008 Slides - 6 slides/page]
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| * '''Mesoderm''' [http://embryology.med.unsw.edu.au/Movies/week4.htm Week 4 Movies] | [http://embryology.med.unsw.edu.au/Movies/mesoderm.htm Mesoderm Movies] |
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| * '''Mesoderm Notes''' [http://embryology.med.unsw.edu.au/Notes/week4.htm Week 4] | [http://embryology.med.unsw.edu.au/Notes/week4_4.htm Week 4 - Somites] | [http://embryology.med.unsw.edu.au/Notes/coelom.htm Coelomic Cavity Development] | [http://embryology.med.unsw.edu.au/Notes/skmus.htm Musculoskeletal Development] |
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| == Lecture Summary ==
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| The following text is extracted and modified from 2008 lecture slides and should be used as a "trigger" to remind you of key concepts.
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| [[Image:Mesoderm cartoon1.gif]] [[Image:Mesoderm cartoon2.gif]]
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| [[Image:Mesoderm cartoon3.gif]] [[Image:Mesoderm cartoon4.gif]]
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| ===Notochord (Axial mesoderm)===
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| <gallery>
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| Image:Stage7_800x700px.jpg|Stage 7 embryonic disc
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| Image:Stage7_primitive-streak-node.jpg|Stage 7 primitive-streak-node
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| Image:Stage7_cloacal-oral-membranes.jpg|Stage 7 cloacal-oral-membranes
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| Image:Stage7 notochord.jpg|Stage 7 notochord
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| </gallery>
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| ===Mesoderm===
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| [[Image:Stage7_mesoderm.jpg|thumb|Stage 7 mesoderm]]
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| ** generated from epiblast cells migrating through the primitive streak
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| ** epiblast cells expressing fibroblast growth factor (FGF2)
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| ** forms a layer between ectoderm and endoderm with notochord down midline
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| ** present before neural tube formation
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| * divides initially into 3 components
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| <gallery>
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| Image:Stage7_paraxial-mesoderm.jpg|Stage 7 paraxial mesoderm
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| Image:Stage7_intermediate-mesoderm.jpg|Stage 7 intermediate mesoderm
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| Image:Stage7_lateral-plate.jpg|Stage 7 lateral plate
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| </gallery>
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| * Paraxial mesoderm - somites - musculoskeletal structures
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| * Intermediate mesoderm - kidney
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| * Lateral plate mesoderm - body wall structures
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| [[Image:Mesoderm cartoon1.gif]]
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| ===Mesenchyme===
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| * Embryonic connective tissue, describes the cell morphology (Histology is not epithelial organization)
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| ** epithelial to mesenchymal transitions
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| ** mesenchymal to epithelial transitions
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| ===Paraxial Mesoderm===
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| [[Image:Mesoderm cartoon2.gif]]
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| ** lies adjacent to notochord
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| ** Forms 2 components
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| *** Head - unsegmented paraxial mesoderm
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| *** Body - segmented paraxial mesoderm
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| ** Generates trunk muscles, skeleton, dermis of skin, blood vessels, connective tissue
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| * Segmented Paraxial Mesoderm
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| ** segments called somites
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| ** first pair of somites (day 20)
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| ** segmentation imposes a pattern on
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| ** nerves, vasculature, vertebra....
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| ** somites appear in ordered sequence cranial to caudal
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| ** appearance so regular used to stage the embryo
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| ** Hamburger & Hamilton 1951- chicken
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| * thought to be generated by a "clock" (1 pair every 90 minutes)
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| * neural tube begins to close at 4th somite level
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| * 44 pairs of somites
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| === Intermediate Mesoderm===
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| [[Image:Mesoderm cartoon2.gif]]
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| * lies between paraxial and lateral mesoderm
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| * generates urogenital system
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| ** Wolffian duct, kidney
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| ** '''MH''' - covered in Kidney Development Lecture/Laboratory
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| ===Lateral Plate Development===
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| [[Image:Mesoderm cartoon3.gif]]
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| The intraembryonic coelom divides the lateral plate into 2 portions
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| * '''Somatic mesoderm'''- closest to ectoderm
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| ** body wall osteogenic, chrondrogenic and fibrogenic
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| ** except ribs and scapula
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| * '''Splanchnic mesoderm''' - closest to endoderm
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| ** heart and smooth muscle of GIT and blood vessels
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| ===Intraembryonic Coelom===
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| [[Image:Mesoderm cartoon4.gif]]
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| * small spaces (vacuoles) begin appearing within the lateral plate mesoderm
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| * small spaces enlarge forming a single cavity within the lateral plate mesoderm
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| ** divides into 2 parts at about day 18-19
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| * this cavity is called the '''Intraembryonic Coelom'''
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| ** coelom is a general term for a "cavity" and can lie within the embryo (intraembryonic) and outside the embryo (extraembryonic)
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| ** later anatomical spaces within the embryo and fetus can also be described as coeloms
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| * later when the embryonic disc folds the intraembryonic coelom in the lateral plate mesoderm will later form all 3 major body cavities
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| ** Pericardial
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| ** Pleural
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| ** Peritoneal
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| ===Somite Formation===
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| [[Image:Somite cartoon1.png]] [[Image:Somite cartoon2.png]] [[Image:Somite cartoon3.png]] [[Image:Somite cartoon4.png]] [[Image:Somite cartoon5.png]]
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| [http://embryology.med.unsw.edu.au/Movies/mesoderm.htm Mesoderm Movies]
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| * ball forms through epithelialization and interactions (cell-cell, cell-extracellular matrix, ECM) fibronectin, laminin
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| * has 2 populations of cells - peripheral columnar and central mesenchymal
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| * early somite has cavity- somitocoel, cavity is lost during growth
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| * somite enclosed by ECM connected to nearby tissues
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| Somite Specification
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| * Different segmental level somites have to generate different segmental body structures?
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| * somite has to form different tissues?
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| * Somite Differentiation
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| * Compartmentalization accompanied by altered patterns of expression of Pax genes within the somite
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| Somite initially forms 2 main components
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| ** ventromedial- '''sclerotome''' forms vertebral body and intervertebral disc
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| ** dorsolateral - '''dermomyotome''' forms dermis and skeletal muscle
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| Somite Axial Specification
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| * rostro-caudal axis appears regulated by Pax/Hox expression, family of DNA binding transcription factors
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| * Movie: Somite Development
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| Sclerotome
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| * sclerotome later becomes subdivided
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| * rostral and caudal halves separated laterally by von Ebner's fissure
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| ** half somites contribute to a single vertebral level body
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| ** other half intervertebral disc
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| * therefore final vertebral segmentation ‚"shifts"
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| Muscle
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| * Myoblast determining transcription factor MyoD is first expressed in the dorsomedial quadrant of the still epithelial somite whose cells are not yet definitely committed
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| ** basic Helix Loop Helix
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| ** form myotome
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| * Myotome component of Somite
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| ** epaxial myotome (dorsomedial quarter) forms the dorsal epimere (erector spinae)
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| ** hypaxial myotome (dorsolateral quarter) forms the ventral hypomere, 3 primary muscle layers which are different at neck, thorax and abdomen
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| [[Image:Stage14 somites limbbuds.png|thumb|Stage 14 Embryo showing somites and limb buds (Week 5)]]
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| * Chick Embryo Mesoderm
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| * Body Musculature - Myotome derivatives-mouse embryo
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| * Lateral Plate Mesoderm
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| * Limb Musculature
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| * Dermomyotome- Muscle (MyoD)
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| * MyoD Pax 3
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| * Somite Differentiation
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| ** migrating neural crest cells enter cranial half, will form dorsal root ganglia (DRG)
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| ** sclerotome bulges ventro-medially towards notochord, then surround and engulf notochord
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| *** not movement of sclerotome, growth of surrounding tissues
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| *** notochord forming nucleus pulposus of IVD
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| Dermomyotome
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| * lateral myotome edge migrates at level of limbs
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| * upper limb first then lower
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| * mixes with somatic mesoderm
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| * dermotome continues to contribute cells to myotome
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| Muscle Development Abnormalities
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| * Duchenne Muscular Dystrophy
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| ** Embryonic muscle development normal and changes occur postnatally
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| ** X-linked dystrophy, large gene encoding cytoskeletal protein - Dystrophin
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| ** progressive wasting of muscle, die late teens
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| * Becker Muscular Dystrophy, milder form, adult onset
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| Axial Segmentation - Somite Specification Signals
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| Chicken Model - Somite formation
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| * Somitomere to Somite
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| * Chicken Stages -regular appearance of somites allowed early experimenters to accurately stage the embryo
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| * Chicken
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| ** Advantages - accessible, easy to manipulate, limb grafts/removal, chimeras, developmental processes
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| ** taxon-Gallus gallus
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| ** develops and hatches in 20-21 days
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| ** Fertilized eggs easily maintained in humidified incubators
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| ** Embryo Staged growth
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| ** Series of Embryonic Chicken Growth
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| ** Hamburger & Hamilton J. Morphology, 88 49 - 92, 1951 [http://embryology.med.unsw.edu.au/OtherEmb/chick1.htm]
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| * Zebrafish, Xenopus
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| * Paraxial Segmentation
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| * Stage 13/14 Embryo
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| ==Take the Quiz==
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| <quiz display=simple>
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|
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| {Mesenchyme refers to the middle layer of the trilaminar embryo
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| |type="()"}
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| - true
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| + false
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| || The the middle layer of the trilaminar embryo is the '''mesoderm''' (meaning middle layer), while most of these cells are mesemchymal in appearance, this term is used to describe the cell histological appearance/organization.
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| {The intraembryonic coelom forms within :
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| |type="()"}
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| - somites
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| + lateral plate
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| - neural tube
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| - intermediate mesoderm
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| ||The intraembryonic coelom forms initially small spaces in the mesoderm layer and coalesce to form a single large "horseshoe-shaped" space within the '''lateral plate mesoderm''' around the embryonic disc. Both young somites (somitocoels) and the neural tube (neural tube lumen) do have cavities, but neither is called the intraembryonic coelom.
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| {All paraxial mesoderm segments into somites.
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| |type="()"}
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| - true
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| + false
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| || While somites do form within paraxial mesoderm, this region remains unsegmented at the level of the head and therefore does not incorporate into somites.
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| {Somites are developmental structures that contribute the following adult structures :
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| |type="()"}
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| - vertebra, notochord, dermis, skeletal muscle
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| + vertebra, intervertebral discs, dermis, skeletal muscle
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| - kidney, body wall connective tissue, sensory ganglia
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| - kidney, gastrointestinal tract smooth muscle, mesentry
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| ||Each somite has specific regions that contribute different components of the embryo. Sclerotome contributes the vertebral column ('''vertebra, intervertebral discs'''). Dermotome contributes the connective tissue layers of the skin ('''dermis''', hypodermis). Myotome ontributes the '''skeletal muscle''' of the body and limbs.
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| </quiz>
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| == References == | | == References == |