During gastrulation, embryonic cells migrate through an opening within the embryo known as the blastocoele. As the gastrula forms, the remnants of the blastocoele shrink to eventually disappear completely.
The opening into the gastrula is known as the blastopore. The inner cavity created by the infolding is known as the archenteron.
There are five main types of cell movements in gastrulation:
Once gastrulation is complete, organogenesis begins.
During gastrulation, extraembryonic mesoderm forms within the hypoblast or embryonic mesoderm and migrates out to form the blood vessels of the chorion and connect the chorion to the embryo through the umbilical cord.
Sea urchins deviate from simple cleavage at the fourth cleavage. The four vegetal blastomeres divide unequally to produce four micromeres at the vegetal pole and four macromeres in the middle of the embryo. The animal cells divide meridionally and produce mesomeres.
At the beginning of vertebrate gastrulation, the embryo is a hollow ball of cells known as the blastula, with an animal pole and a vegetal pole. The vegetal pole begins to flatten to form the vegetal plate. Some of the cells of the vegetal pole detach and through ingression become primary mesenchyme cells. The mesenchyme cells divide rapidly and migrate along the extracellular matrix (basal lamina) to different parts of the blastocoel. The migration is believed to be dependent upon sulfated proteoglycans on the surface of the cells and molecules on the basal lamina such as fibronectin. The cells move by forming filopodia that identify the specific target location. These filopodia then organize into syncytial cables that deposit the calcium carbonate that makes up the spicules (the skeleton of the pluteus larva).
During the second phase of gastrulation, the vegetal plate invaginates into the interior, replacing the blastocoelic cavity and thereby forming a new cavity, the archenteron (literally: primitive gut), the opening into which is the blastopore. The arechenteron is elongated by three mechanisms.
Second, the archenteron is formed through convergent extension. Convergent extension results when cells intercalate to narrow the tissue and move it forward.
Third, secondary mesenchyme pull the tip of the archenteron towards the animal pole. Secondary mesenchyme are formed from cells that ingress from, but remain attached to, the roof of the archenteron. These cells extend filopodia that use guidance cues to find the future mouth region. Upon reaching the target site, the cells contract to pull the archenteron to fuse with the ectoderm. Once the archenteron reaches the animal pole, a perforation forms, and the archenteron becomes a digestive tract passing all the way through the embryo.
The three embryonic germ layers have now formed. The endoderm, consisting of the archenteron, will develop into the digestive tract. The ectoderm, consisting of the cells on the outside of the gastrula that played little part in gastrulation, will develop into the skin and the central nervous system. The mesoderm, consisting of the mesenchyme cells that have proliferated in the blastocoel, will become all the other internal organs.
There are four kinds of cell movements that drive gastrulation in Xenopus: invagination, involution, convergent extension and epiboly. At the dorsal marginal zone, cells change from a columnar shape to become a bottle cell and form an invagination. At this invagination, cells begin to involute into the embryo. This site of involution is called the dorsal lip. The involuting cells migrate along the inside of the blastocoel toward the animal cap. This migration is mediated by fibronectin of the extracellular matrix (ECM) secreted by the blastocoel roof. Eventually, cells from the lateral and ventral sides begin to involute to form a ring of involuting cells surrounding the yolk plug. These involuting cells will eventually form the archenteron which displaces and eventually replaces the blastocoel. Cells from the lateral marginal zone migrate toward the dorsal midline and intercalate with the cells there. This causes the dorsal involuting cells to undergo convergent extension. The dorsal cells become the first to migrate along the roof of the blastocoel cavity and form the anterior/posterior axis of the embryo. During the involution of cells, the cells of the animal cap undergo epiboly and spread toward the vegetal pole.
The first stage of gastrulation begins with the epiboly of the EVL and the deep cells over the YSL. This epiboly is driven by the migration of nuclei and cytoplasm in the YSL and attachments between the YSL and the EVL. Intercalation of the deep cells with the EVL help drive this movement. At about 50% of epiboly, a fate map similar to that of the Xenopus can be derived. The EVL develops into an extraembryonic membrane and does not contribute to the embryo.
The second stage of gastrulation occurs when the leading edge of the epibolizing blastoderm thickens. The dorsal side forms a larger thickening and is known as the embryonic shield. The deep cells in the embryonic shield form two layers. The epiblast forms near the surface and will give rise to the ectoderm. The hypoblast forms next to the YSL and will form a mixture of endoderm and mesoderm. The hypoblast is formed through involution and/or ingression. The movement of cells in the hypoblast are similar to the involuting mesoderm of amphibians. The end result of gastrulation is an asymmetric involution of cells that form the dorsal structures of the embryo.
As gastrulation proceeds, the primitive streak regresses posteriorly with pharyngeal endoderm, the head process, and the notochord being laid down as it recedes. This results in a temporal gradient of development with the anterior forming organs while the posterior is still going through gastrulation.
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