embryoblasts

Embryoblasts, also called the inner cell mass, are the cluster of cells inside the mammalian blastocyst that gives rise to the embryo proper and certain extraembryonic tissues. In the blastocyst, the embryoblast sits on the interior side opposite the outer trophoblast and consists of pluripotent cells capable of forming all embryonic lineages.

During early development, the embryoblast differentiates into two layers: the epiblast and the hypoblast (primitive endoderm). The epiblast gives rise to the amniotic membrane and, after gastrulation, to the embryo proper with the three germ layers (ectoderm, mesoderm, endoderm). The hypoblast contributes to the primitive endoderm and the yolk sac, supporting early nutrition before placental function is established.

Following implantation, embryoblast cells proliferate and reorganize to establish the body axes and subsequent germ layers. The trophoblast, by contrast, forms the placenta and the extraembryonic membranes essential for implantation and fetal support.

Embryoblast cells are considered pluripotent, meaning they can differentiate into cell types of all three germ layers. They express pluripotency-associated factors such as Oct4, Nanog, and Sox2 in many contexts. The health and quality of the embryoblast are factors in early embryo viability in assisted reproduction and developmental biology research.