Embryogenesis - Formation of The Gastrula

Formation of The Gastrula

During gastrulation cells migrate to the interior of the blastula, as a consequence 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. In diploblastic animals only the ectoderm and the endoderm are present .

• Among different animals, different combinations of the following processes occur to place the cells in the interior of the embryo:
  • Epiboly - expansion of one cell sheet over other cells
  • Ingression - cells move with pseudopods
  • Invagination - forming the mouth, anus, and archenteron
  • Delamination - the external cells divide, leaving the daughter cells in the cavity
  • Polar proliferation - Cells at the polar ends of the blastula/gastrula proliferate, mostly at the animal pole.
• Other major changes during gastrulation:
  • Heavy RNA transcription using embryonic genes; up to this point the RNAs used were maternal (stored in the unfertilized egg).
  • Cells start major differentiation processes, losing their totipotentiality.

In most animals, a blastopore is formed at the point where cells are entering the embryo. Two major groups of animals can be distinguished according to the blastopore's fate. In deuterostomes the anus forms from the blastopore, while in protostomes it develops into the mouth. See Embryological origins of the mouth and anus for more information.