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During gastrulation cells migrate to the interior of the blastula, subsequently forming two (in diploblastic animals) or three (triploblastic) germ layers. The embryo during this process is called a gastrula. The germ layers are referred to as the ectoderm, mesoderm and endoderm.
The two layers are also sometimes referred to as the hypoblast and epiblast. [5] Sponges do not go through the gastrula stage. Gastrulation takes place after cleavage and the formation of the blastula, or blastocyst. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. [6]
The filopodia—thin fibers formed by the mesenchyme cells, found in late gastrulation—contract to drag the tip of the archenteron across the blastocoel. The endoderm of the archenteron will fuse with the ectoderm of the blastocoel wall. At this point gastrulation is complete, and the embryo has a functional digestive tube.
Fish embryos go through a process called mid-blastula transition which is observed around the tenth cell division in some fish species. Once zygotic gene transcription starts, slow cell division begins and cell movements are observable. [4] During this time three cell populations become distinguished. The first population is the yolk syncytial ...
Ingression is a very dynamic process however, and the first sign of an ingressing cell is seen when a future PMC loses its adhesion to hyaline, and cadherin, and increases its adhesion to a basal laminal substrate. These processes occur rapidly, over approximately 30 minutes.
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During the third week, a process called gastrulation creates a mesodermal layer between the endoderm and the ectoderm. This process begins with the formation of a primitive streak on the surface of the epiblast. [6] The cells of the layers move between the epiblast and the hypoblast, and begin to spread laterally and cranially.
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