Bilaminar Blastocyst - Becoming Bilaminar

Becoming Bilaminar

The zygote first transformed into a morula through cleavage and then more divisions lead to a blastocyst that consisted of just a trophoblast, and a embryoblast. By the second week, the embryoblast has begun separating into two layers: the epiblast and hypoblast also called the primitive endoderm. At the embryonic pole of the blastocyst, the amniotic cavity finds a home between the epiblast and the hypoblast. The epiblast stretches to surround the cavity very quickly and this layer of the epiblast becomes known as the amnion, which is one of the four extraembryonic membranes. The rest of the hypoblast and epiblast, not including the amnion, is what contributes to the bilaminar embryonic disc (bilaminar blastoderm/blastocyst), which sits between the amniotic cavity and the blastocyst cavity. The embryo proper and extramembryonic membranes are later derived from the embryonic disc.