Hypoblast

The hypoblast is a tissue type that forms from the inner cell mass. It lies beneath the epiblast and consists of small cuboidal cells.

Extraembryonic endoderm (including Yolk sac) is derived from hypoblast cells. The absence of hypoblast results in multiple primitive streaks in chicken embryos. The formation of the primitive streak, through which gastrulation occurs, is induced by Koller's sickle.

Structure

In mice

In mouse embryo, the visceral endoderm develop from the primitive endoderm of the blastocyst during the implantation stage covering the epiblast cells and elongates to become an egg cylinder. A distinct morphological domain has been identified by Martin and colleagues, at the distal tip of the mouse egg cylinder, thus this domain was called distal visceral domain (DVE). The DVE cells will move unilaterally to the future anterior until reaching the embryonic/ extra embryonic boundary and at this point, the DVE cells are also named as anterior visceral endoderm (AVE). This migration has been proved to be essential for establishing anteroposterior axis. Besides the AVE, another cell population appears to be separated at the posterior edge of the embryonic egg cylinder, referred to as posterior visceral endoderm (PVE). However, the function of this cell population was not as well studied as AVE.