5-350 Lecture Notes - Polar Body, Mothers Against Decapentaplegic Homolog 2, Splanchnic

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Outline of Lecture 08
Gastrulation, Organogenesis, and Embryonic Folds
Great diagram on p. 4 of notes!
I. Gastrulation
- Gastrulation is movement of cells from epiblast to inside (begins at 2 weeks)
- Embryonic axes become apparent, germ layers formed, cell fates are restricted
- Primitive Streak
- Found on dorsal midline, posterior
- Epiblast (dorsal side) cells move to streak, through, then spread away
- Primitive node moves anteriorly during this process
- FGF8 signaling may be important for this process
II. Ectoderm
- Arises from epiblast, hence is most dorsal
- Tissues formed:
1) Neural plate (anterior) folds up into a tube and gives rise to CNS
2) Surface ectoderm that surrounds neural plate becomes epidermis
3) Neural crest cells (freed from the two structures above as neural tube pinches
off) migrates throughout embryo and gives rise to lots of cell types
III. Mesoderm (see pictures, p. 5-6)
- Notochord (axial component): regulates development, but disappears later
- Paraxial: pinches off into somites, and somites divide into:
- dermatome (most dorsal), gives rise to dermis
- myotome, gives rise to skeletal muscle of trunk
- sclerotome, gives rise to axial skeleton
- Intermediate mesoderm: gives rise to kidney
- Lateral plate mesoderm: two layers due to coelom
- dorsal somatic layer, combines with ectoderm to make somatopleure, gives rise
to body wall
- ventral splanchnic layer, combines with endoderm to make splanchnopleure,
gives rise to gut, mesenteries, heart, most of circulatory system
- Some endoderm becomes extraembryonic
IV. (Definitive) Endoderm
- Also formed from cells that moved through primitive streak; displaces visceral
- Endoderm gives rise to lining of gut, digestive organs, thyroid
- Endoderm touches ectoderm directly at prochordal plate (near mouth) and
cloacal plate (near anus)
- Visceral endoderm (VE) represses primitive streak on anterior side
- Txn factor vHNF-1 expressed in VE is necessary for gastrulation
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