epiblast (generic)

In amniote embryology the epiblast is a tissue derived from the inner cell mass in mammals and from the blastodisc in birds and reptiles, and it lies above the hypoblast. In mammalian embryogenesis its columnar cells sit adjacent to the trophoblast, whereas the cuboidal cells of the hypoblast lie closer to the blastocoele. Although the epiblast is often called the primary ectoderm, its developmental output is far broader: during gastrulation it differentiates to form all three layers of the trilaminar germ disc. In descriptions of the blastoderm it is the outer of the two layers formed during gastrulation, corresponding to primitive ectoderm during that process and to definitive ectoderm afterwards. This dual naming reflects the same tissue observed before and after the gastrulation movements that reorganise it.