inner cell mass
| ectoderm, mesoderm, endoderm|
In amniote animal embryology, the epiblast is one of two distinct layers arising from the inner cell mass in the mammalian blastocyst or from the blastodisc in reptiles and birds. It derives the embryo proper through its differentiation into the three primary germ layers, ectoderm, mesoderm and endoderm, during gastrulation. The amnionic ectoderm and extraembryonic mesoderm also originate from the epiblast.
In mammalian embryogenesis, differentiation and segregation of cells composing the inner cell mass of the blastocyst yields two distinct layers—the epiblast ("primitive ectoderm") and the hypoblast ("primitive endoderm"). While the cuboidal hypoblast cells delaminate ventrally, away from the embryonic pole, to line the blastocoele, the remaining cells of the inner cell mass, situated between the hypoblast and the polar trophoblast, become the epiblast and comprise columnar cells.
Upon commencement of gastrulation, the primitive streak, a visible, morphological groove, appears on the posterior epiblast and orients along the anterior-posterior embryo axis. Initiated by signals from the underlying hypoblast, formation of the primitive streak is predicated on epiblast cell migration, mediated by Nodal, from the lateral-posterior regions of the epiblast to the center midline. The primitive knot is situated at the anterior end of the primitive streak and serves as the organizer for gastrulation, determining epiblast cell fate by inducing the differentiation of migrating epiblast cells during gastrulation.
During gastrulation, migrating epiblast cells undergo epithelial-mesenchymal transition in order to lose cell-cell adhesion (E-cadherin), delaminate from the epiblast layer and migrate over the dorsal surface of the epiblast then down through the primitive streak. The first wave of epiblast cells to invaginate through the primitive streak invades and displaces the hypoblast to become the embryonic endoderm. The mesoderm layer is established next as migrating epiblast cells move through the primitive streak then spread out within the space between the endoderm and remaining epiblast, which once the mesoderm layer has formed ultimately becomes the definitive ectoderm. The process of gastrulation results in a trilaminar germ disc, consisting of the ectoderm, mesoderm and endoderm layers.
Epiblasts exhibit diverse structure across species as a result of early embryo morphogenesis. The human epiblast assumes a disc shape, conforming to the embryonic disc morphology; whereas, the mouse epiblast develops in a cup shape within the cylindrical embryo.
Gastrulation occurs in the epiblast of avian embryos. A local thickening of the epiblast, known as Koller's sickle, is key in inducing the primitive streak, the structure through which gastrulation occurs.