Hypoblast

The hypoblast is one of two distinct layers arising from the inner cell mass (ICM) during early embryonic development in mammals, birds, and reptiles. It is the first epithelial layer to differentiate. The hypoblast is also known as the primitive endoderm.

Formation and Location

The hypoblast forms through a process involving cellular rearrangements and differentiation within the ICM. In mammals, cells delaminate from the ICM to form a layer on the surface of the blastocoel cavity. In avian and reptilian embryos, hypoblast formation involves a process involving ingression and migration of cells.

Function

The primary function of the hypoblast is to contribute to the formation of extraembryonic membranes, specifically the yolk sac in avian and reptilian embryos, and contribute to signalling to determine the embryonic-abembryonic axis. It does not directly contribute to the tissues of the definitive embryo. The hypoblast’s signaling is crucial for guiding the formation of the primitive streak in the epiblast, the other layer derived from the ICM. The primitive streak is essential for gastrulation, a fundamental process where the three primary germ layers (ectoderm, mesoderm, and endoderm) are established. In mammals, it contributes to the parietal endoderm, which lines the inside of the blastocyst cavity and secretes extracellular matrix components.

Fate

In amniotes, the hypoblast cells are eventually displaced and do not contribute to the formation of the embryo proper. They are replaced by endodermal cells arising from the epiblast during gastrulation. The fate of hypoblast cells involves regulated cell death (apoptosis) or incorporation into extraembryonic structures.