Hypoblast

The hypoblast is as a structure of early embryonic development of many vertebrates besides the epiblast, the embryo and the trophoblast blasts a part of the developing blastula or blastocyst. He is often considered to be the precursor of the endoderm. In some species, however, could be demonstrated that all germ layers of the epiblast emerge by cells of the epiblast migrate toward the hypoblast and these displace. However, the hypoblast is no less important, he is responsible for example for the formation of the yolk sac and the emergence of the prechordal plate.

The hypoblast of the human embryo differentiates into the second week of development of the cells of the embryo blasts pointing towards the blastocyst cavity back. It consists of a single layer of flat cells. Hypoblast cells migrate to the sides of the blastocyst cavity and dress the inner wall cells. This layer of cells is called Heuser membrane and forms the inner wall of the primary yolk sac. Hypoblast cells also colonize the gap spaces that form between the yolk sac and the faster growing cytotrophoblast. This network of emigrated cells is called the extraembryonic mesoderm and surrounds both the yolk sac and the amniotic cavity.