Hypoblast

The hypoblast is as a structure of the early embryonic development of many vertebrates besides the epiblast , the embryoblast and the trophoblast part of the developing blastula or blastocyst . It is often thought to be the preliminary stage of the endoderm . In some animal species, however, it has been shown that all cotyledons emerge from the epiblast as cells of the epiblast migrate in the direction of the hypoblast and displace it. The hypoblast is no less important, however, as it is responsible for the formation of the yolk sac , for example .

The hypoblast of the human embryo differentiates in the second week of development from the cells of the embryoblast, which point towards the blastocyst cavity. It consists of a single layer of flat cells. Hypoblast cells migrate to the sides into the blastocyst cavity and line its inner wall with cells. This cell layer is also called Heuser's membrane and forms the inner wall of the primary yolk sac. Hypoblast cells also colonize the spaces between the yolk sac and the faster growing cytotrophoblast . This network of migrated cells is called the extraembryonic mesoderm and encloses both the yolk sac and the amniotic cavity .

literature

• Pio Fioroni: General and Comparative Embryology of Animals. Springer, Berlin et al. 1992, ISBN 3-540-55495-5 .