Gastrulation
 

[SIZE="4"][COLOR="Red"]GASTRULATION[/COLOR][/SIZE]
[COLOR="Blue"]I. DEFINITION[/COLOR]

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).



[COLOR="Blue"]II. PROMINENT PHYSIOLOGICAL FEATURES[/COLOR]

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.



[COLOR="Blue"]III. MAJOR TYPES OF GASTRULATION[/COLOR]

A. Gastrulation within context of a sphere.

1.[B][U] Amphioxus [/U][/B]- invagination of blastula to form double-walled cup.

2. [B][U]Outer cells [/U][/B]= ectoderm.

3. [B][U]Inner cells [/U][/B]= mesoderm and endoderm.

4. [B][U]Cavity formed [/U][/B]= archenteron.

5.[B][U] Blastopore [/U][/B]= narrowed opening of "cup".

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

1. [B][U]Amphibians [/U][/B]- yolk in vegetal hemisphere.

2.[B][U] Birds and reptiles [/U][/B]- telolecithal eggs.

3. [B][U]Mammals[/U][/B] - gastrulation similar to birds and reptiles, even though eggs possess little yolk.

4. Yolk makes simple inpocketing impractical.



[COLOR="blue"]IV. AMPHIOXUS GASTRULATION[/COLOR]
A. Wall of blastula flattens and invaginates to form "cup".

B.[U] Mesodermal crescents [/U]- initially along dorsal and ventral rims of "cup".

C. [U]Notochord[/U] - forms from mesoderm along dorsal surface of archenteron.

D. [U]Mesoderm [/U]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. [U]Neuropore [/U]= anterior opening to outside.

2. [U]Neurenteric canal [/U]- temporarily connects archenteron to neural tube.



[COLOR="blue"]V. AMPHIBIAN GASTRULATION (Frogs)[/COLOR]
[U]A. Blastula [/U]cannot inpocket due to yolk.

[U]B. Bottle cells [/U]- 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.

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

[B]D. Involution [/B]- cells roll around lip of blastopore into interior.

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

[B]F. Germ layers:[/B]

1. [U]Ectoderm[/U] = cells left on the outside.

2. [U]Endoderm [/U]= cells of roof, floor, and walls of archenteron.

3. [U]Mesoderm[/U] - 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.

[B]G. Primary organ rudiments:[/B]

[U]1. Neurulation[/U]: Neural plate --> neural folds --> neural tube.

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

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

[U]4. Lateral plate mesoderm [/U]consists of 2 layers:

[I]a. Somatic mesoderm (parietal)[/I] = outer layer lining body wall.

b[I]. Splanchnic mesoderm (visceral) [/I]= inner layer covering organs.

[I]c. Coelom[/I] = space in between these two layers.





[COLOR="blue"]VI. GASTRULATION IN BIRDS[/COLOR]
[I]A. Pre-gastrulation stages [/I]occur before egg is laid, making study of early gastrulation difficult.

[I]B. Primitive streak:[/I] 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.

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

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

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

4[U]. Primitive ridges (folds)[/U] = thickened edges of primitive groove.

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

[U]1. Endoderm [/U]= 1st cells to immigrate, replace the hypoblast.

[U]2. Mesoderm [/U]= cells following endoderm.

[I]a. Extends and spreads laterally and forward.[/I]

[I]b. Chordamesoderm [/I]- 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:

[U]1. Neural plate [/U]- 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.

[U]2. Notochord [/U]- chordamesoderm condenses to form distinct rod.

[U]3. Mesoderm:[/U]

[I]a. Segmental plate mesoderm [/I]- along side notochord, becomes divided into somites.

[I]b. Intermediate mesoderm [/I]- connects somites with lateral plate mesoderm, does not become segmented.

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

[U]4. Endoderm [/U]- 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.