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Gastrulation Gastrulation is a phase early in the development of most animal embryos, during which the morphology of the embryo is dramatically restructured by cell migration. Gastrulation varies in different phyla. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. The purpose of gastrulation is to position the three embryonic germ layers, the endoderm, ectoderm and mesoderm. These layers later develop into certain bodily systems. The ectoderm develops into the skin, nails, the epithelium of the nose, mouth and anal canal; the lens of the eye, the retina and the nervous system. The endoderm develops into the inner linings of the digestive tract, as well as the linings of the respiratory passages. It also forms many glands, such as the liver and pancreas. The mesoderm forms the somites, the notochord, and the mesenchyme, which give rise to the muscles, circulatory and excretory systems of the body. Mammals In humans, gastrulation occurs after implantation, around days 14-16 after fertilization in human embryogenesis. Note, in some livestock species which have a non-invasive implant gastrulation actually occurs before implantation. As the outer cell mass invades the endometrium, the inner cell mass divides into two layers: the epiblast and hypoblast. The hypoblast spreads out and covers the blastocoel to form the yolk sac. The yolk sac is an extraembryonic tissue that produces blood cells similar to the structure that surrounds the yolk in birds. The epiblast further divides into two more layers. The amnion layer forms the fluid filled cavity to surround and protect the embryo during pregnancy. The embryonic epiblast undergoes gastrulation. Gastrulation in mammals is similar to that in birds with the formation of the primitive streak and Hensen's node and the ingression of cells through the primitive groove to form the endoderm and the mesoderm. Thus, gastrulation creates all three germ layers of the embryo: ectoderm, mesoderm, and endoderm. 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 The following description concerns gastrulation in echinoderms, representative of the triploblasts, or animals with three embryonic germ layers. 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. First, the initial invagination is caused by a differential expansion of the inner layer made of fibropellins and outer layer made of hyalin to cause the layers to bend inward. 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. Amphibians During cleavage in amphibians, a higher density of yolk in the vegetal half of the embryo results in the blastocoel cavity being placed asymmetrically in the animal half of the embryo. Unlike in sea urchins, the cells surrounding the blastocoel are thicker than a monolayer. The blastocoel cavity prevents signaling between the animal cap and provides a space for involuting cells during gastrulation. There are four kinds of tissue movements that drive gastrulation in Xenopus: invagination, involution, convergent extension and epiboly. At the vegetal edge of the dorsal marginal zone, cells change from a columnar shape to become a bottle cell and drive invagination. At this invagination, cells begin to involute into the embryo. This initial 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) assembled 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 intercalate with cells closer to the dorsal midline. Directed cell intercalation within the dorsal mesoderm drives 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. Both prior to and during the involution, the animal cap undergoes epiboly and spread toward the vegetal pole.