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Old Tuesday, April 08, 2008
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Default Gastrulation

GASTRULATION
I. DEFINITION

A. Profound, but well-ordered, rearrangements of cells in the embryo.

B. Active movements of cells into the interior of embryo to form germ layers (ectoderm, endoderm, mesoderm).

C. Internal cavity is formed = archenteron (gastrocoel).

D. Rearrangements necessary for establishment of primary organ rudiments (discrete cell masses which give rise to the major organ systems).



II. PROMINENT PHYSIOLOGICAL FEATURES

A. Morphogenetic movements of cells.

B. Cell division slows down.

C. Growth is insignificant.

D. Increase in oxidative metabolism.

E. Nuclei become more active in controlling cells; increase in paternal influence.

F. New kinds of proteins synthesized.



III. MAJOR TYPES OF GASTRULATION

A. Gastrulation within context of a sphere.

1. Amphioxus - invagination of blastula to form double-walled cup.

2. Outer cells = ectoderm.

3. Inner cells = mesoderm and endoderm.

4. Cavity formed = archenteron.

5. Blastopore = narrowed opening of "cup".

B. Development of germ layers as 2-dimensional sheets upon mass of passive yolk.

1. Amphibians - yolk in vegetal hemisphere.

2. Birds and reptiles - telolecithal eggs.

3. Mammals - gastrulation similar to birds and reptiles, even though eggs possess little yolk.

4. Yolk makes simple inpocketing impractical.



IV. AMPHIOXUS GASTRULATION
A. Wall of blastula flattens and invaginates to form "cup".

B. Mesodermal crescents - initially along dorsal and ventral rims of "cup".

C. Notochord - forms from mesoderm along dorsal surface of archenteron.

D. Mesoderm breaks up into mesodermal segments; cavities form within segments and eventually fuse to give rise to coelom.

E. Neural plate = flattened region on dorsal part of gastrula; sinks downward and is covered by overgrowth of epidermal epithelium.

F. After sinking, neural plate rolls into neural tube (primary organ rudiment giving rise to nervous system).

1. Neuropore = anterior opening to outside.

2. Neurenteric canal - temporarily connects archenteron to neural tube.



V. AMPHIBIAN GASTRULATION (Frogs)
A. Blastula cannot inpocket due to yolk.

B. Bottle cells - elongate, begin invagination in marginal zone.

1. Rudiment of blastopore = groove; dorsal lip forms 1st.

2. Blastopore spreads laterally and ventrally until it forms a continuous ring.

C. Yolk plug = yolky, vegetal cells completely enclosed by ring-shaped blastopore.

D. Involution - cells roll around lip of blastopore into interior.

E. Epiboly = expansion of dorsal blastoderm due to thinning; as layer of cells expands, cells are pushed around the lips of the blastopore into the interior.

F. Germ layers:

1. Ectoderm = cells left on the outside.

2. Endoderm = cells of roof, floor, and walls of archenteron.

3. Mesoderm - spreads out between ectoderm and endoderm as a continuous sheet.

a. Chordamesoderm - involutes around dorsal lip of blastopore; gives rise to head mesoderm and notochord.

b. Lateral plate mesoderm - involutes around lateral and ventral lips.

c. Anterior region which mesoderm never reaches gives rise to mouth.

G. Primary organ rudiments:

1. Neurulation: Neural plate --> neural folds --> neural tube.

2. Segmental plate = thick dorso-lateral mesoderm next to notochord which breaks into discrete segments called somites.

3. Thin lateral plate mesoderm will line coelom, and is connected to somites by intermediate mesoderm, which will give rise to the excretory system.

4. Lateral plate mesoderm consists of 2 layers:

a. Somatic mesoderm (parietal) = outer layer lining body wall.

b. Splanchnic mesoderm (visceral) = inner layer covering organs.

c. Coelom = space in between these two layers.





VI. GASTRULATION IN BIRDS
A. Pre-gastrulation stages occur before egg is laid, making study of early gastrulation difficult.

B. Primitive streak: cells of blastoderm condense and thicken at posterior end of area pellucida, grows or extends cranially up to 2/3 across area pellucida. At 18 hr., streak begins to regress.

1. Primitive knot (Henson's node) = local thickening of tissue at cephalic end of primitive streak (roughly equivalent to dorsal lip of amphibian gastrula).

2. Primitive pit = depression in center of primitive knot; continuous posteriorly with primitive groove.

3. Primitive groove - formed by coordinated mass movements of cells down from surface to interior of embryo = immigration.

4. Primitive ridges (folds) = thickened edges of primitive groove.

C. Cells of epiblast move toward primitive knot and groove, and migrate down, spreading out laterally and cranially.

1. Endoderm = 1st cells to immigrate, replace the hypoblast.

2. Mesoderm = cells following endoderm.

a. Extends and spreads laterally and forward.

b. Chordamesoderm - immigrates thru primitive pit and extends cranially to form head process, which gives rise to the notochord. The head process elongates as the streak regresses.

3. Ectoderm = cells of epiblast remaining on the surface.

D. Proamnion = anterior region not reached by mesoderm.

E. Neurulation and primary organ rudiments:

1. Neural plate - forms in ectoderm anterior to primitive knot, developing in cranio-caudal sequence following regressing primitive streak (cells still immigrating in posterior streak). Neural tube rolls up similar to amphibians.

2. Notochord - chordamesoderm condenses to form distinct rod.

3. Mesoderm:

a. Segmental plate mesoderm - along side notochord, becomes divided into somites.

b. Intermediate mesoderm - connects somites with lateral plate mesoderm, does not become segmented.

c. Lateral plate mesoderm - gives rise to somatic and splanchnic layers which enclose coelom.

4. Endoderm - anterior, posterior, and lateral folds ventrally undercut embryo to form gut tube. Anterior and posterior intestinal portals = openings in ends of gut tube into underlying yolk.
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