The end result of gastrulation is the transformation of the blastula, which consists of a ball or disc of relatively undifferentiated cells into an embryo that contains three germ layers.
The mode of gastrulation depends on the distribution of yolk, but all different types of gastrulation are related to each other in that they are different ways for cells on the outside to get inside (via different types of rearrangement and movement) to form the three germ layers. The cytoskeleton (microfilaments and microtubules) and extracellular matrix play a critical role in providing the motive force and the cues for gastrulation. The position at which gastrulation occurs is determined by local production of a signal that activates small GTPases like Rho, which reorganize the cytoskeleton locally.
During gastrulation, the blastula transforms and the cells begin to manifest their different fates. By the end of gastrulation, three different germ layers have formed: ectoderm (outer); mesoderm (middle); and endoderm (inside). Different cell types arise from these germ layers.
At the end of cleavage, the insect embryo consists of a single epithelial layer (the blastoderm) surrounded a central yolk (as opposed to a blastocoel cavity). As opposed to gastrulating at a single blastopore point, the insect embryo gastrulates along the ventral midline.
The mesoderm arises from invaginating cells of the ventral furrow, and the endoderm arises from cells at each end of the invaginating mesoderm. Fog, concertina and DRhoGEF are the three genes with identified roles in the cell constriction movements of gastrulation.
Sea urchin gastrulation
Sea urchin gastrulation begins with flattening of the vegetal plate on the side of the blastula opposite the apical tuft. Thus, the embryo already has a polarity prior to onset of the morphogenetic changes of gastrulation. A group of blastula vegetal pole cells ingress into the blastocoel cavity; these ingressing cells are called the primary mesenchyme, forming the mesoderm and eventually the skeleton.
After the sea urchin vegatal plate flattens, it invaginates to form an inner pouch called the archenteron. The archenteron gives rise to the endoderm and eventually forms the digestive tract. The opening of the archenteron to the outside is the blastopore, a term reused in amphibian gastrulation.
Next, the archenteron elongates via convergent extension and then ingresses. The number of cells in the archenteron does not increase much during convergent extension, but instead become stretched out to transform a short and fat invagination into a longer and narrower one. Following convergent extension, secondary mesenchyme cells at the tip of the archenteron extend filipodia between the ectoderm and endoderm (primary mesenchyme cells do the same) to form the mesoderm, which forms muscle.
The fish embryo is telolecithal and contains lots of yolk. Cleavage results in a flat mass of cells on one side of the embryo. Gastrulation follows, consisting of two major processes: epiboly and involution. In epiboly, cells at the animal pole (termed the blastoderm or blastodisc in fish) spread out and expand to cover the yolk at the vegetal pole (in amphibians, yolk-filled cells are covered instead).
During epiboly, cells at the edge of the blastoderm began to involute (fold in) and later form the endoderm and mesoderm, with non-involuted cells forming the ectoderm. During epiboly and involution, blastodermal cells converge on one side of the embryo (the future dorsal region) and intercalate via convergent extension to form the axis.
Like fish eggs, amphibian eggs are telolecithal and have bulky yolk at the vegetal pole which prohibit the simple invagination seen in sea urchins. In fish and amphibians, the blastopore is located just above the largest of the yolk. As animal pole cells increase in area and spread over the embryo (via epiboly), the lip of the blastopore widens. Beginning at the dorsal side, cells at the edge of the blastopore lip begin moving inward via involution.
The archenteron -- which gives rise to the endoderm -- is now formed from the vegetal yolky cells that were surrounded via epiboly, and by the cells involuted from the dorsal lip. Other cells that involute over the dorsal lip form the intervening mesoderm. Convergent extension is responsible for the capacity for a small ring of cells just above the dorsal lip to lengthen along the anterior-posterior axis and form the internal endodermal and mesodermal sheets.
Gastrulation in amphibians requires fibronectin, a glycoprotein in the extracellular matrix that provides a substrate for cell migration. Fibronectin binds to integrins (receptors for extracellular matrix proteins). By binding extracellular fibronectin and intracellular actin, integrins link the external environment to the cytoskeleton. In amphibians, the surfaces of cells in the blastocoel roof become covered with oriented fibronectin fibrils. Blastocoels injected with fibronectin antibodies do not undergo gastrulation; the cell layer near the dorsal lip folds repeatedly without going inward.
Bird eggs are extremely telolecithal. After cleavage, the blastodisc is a small group of cells atop the yolk. The blastodisc consists of two layers: the epiblast (surface layer, from which the developing embryo derives) and hypoblast (below the epiblast). Cells converge at one edge of the epiblast, forming a line (the primitive streak) that defines the anterior (aka Hensen's node) and posterior of the embryo.
A slight depression forms in the midline of the primitive streak, and cells ingress through the depression into the space between the epiblast and hypoblast. Cells which ingress toward the anterior (near the node) form the notochord (a mesoderm derivative) and some endoderm; cells which ingress toward the posterior form more mesoderm and endoderm. The head end develops first. The node and the primitive groove, as the passages for cells on there warm to form internal layers, are homologous to the blastopore.
Mammalian gastrulation begins with formation of a primitive streak in the epiblast and continues much like birds. Mammalian eggs have little or no yolk, but mammalian gastrulation is nonetheless similar to bird gastrulation due to evolutionary remnants from ancestral reptiles that laid very yolky eggs. Identical (monozygotic) twins can arise in mammals by formation of two primitive streaks in the epiblast; Siamese twins occur when two primitive streaks do not completely separate.