epiblasten

Epiblasten, or epiblasts, are the outer layer of the embryoblast in the early embryo and form the bilaminar embryonic disc together with the hypoblast. In humans and other mammals, the inner cell mass differentiates into a stacked arrangement where the upper layer is the epiblast facing the amniotic cavity and the lower layer is the hypoblast lining the yolk sac.

During gastrulation, epiblast cells migrate through the primitive streak and give rise to the three germ layers: ectoderm, mesoderm, and endoderm. Ingressing cells form endoderm and mesoderm, while cells that remain on the surface contribute to the ectoderm. Epiblast cells also contribute to extraembryonic tissues; notably, they generate the amnion, with the amniotic cavity forming within the epiblast-derived tissue.

In birds and other vertebrates, the epiblast serves as the outer layer of the blastoderm and is the primary source of the embryo proper, while the hypoblast contributes largely to extraembryonic membranes such as the yolk sac. The pluripotent nature of the epiblast allows it to give rise to all fetal tissues and play a central role in early lineage specification.

Clinical relevance is primarily developmental: disturbances in epiblast formation or function can disrupt gastrulation and lead to early embryonic defects. The study of the epiblast is fundamental to understanding vertebrate embryogenesis and germ-layer formation.