embryoblast

Embryoblast, also known as the inner cell mass, is the cluster of cells inside the mammalian blastocyst that will develop into the embryo proper. The blastocyst, formed after fertilization and early cleavage, consists of an outer trophoblast layer and an inner cell mass. The embryoblast is pluripotent, capable of forming most cell types of the body, while the trophoblast gives rise to extraembryonic structures such as the placenta.

Within the embryoblast, cells differentiate into two layers: the epiblast and the hypoblast, forming the bilaminar embryonic disc. The epiblast contributes to the amnion and, through gastrulation, to all three germ layers—ectoderm, mesoderm, and endoderm. The hypoblast forms the primitive endoderm and contributes to extraembryonic tissues such as the yolk sac. The trophoblast surrounds the embryoblast and eventually forms part of the placenta and fetal membranes.

During gastrulation, the epiblast-derived cells migrate to establish the three germ layers, which give rise to virtually all tissues and organs of the embryo. The embryoblast’s primary role is to generate the embryo proper, with supporting extraembryonic structures arising mainly from the trophoblast. In humans and other mammals, the embryoblast stage is a key focus in embryology and assisted reproductive technologies, where the quality of the inner cell mass can be used to assess embryo viability and potential for development.

Terminology-wise, embryoblast is often used interchangeably with inner cell mass, though the term emphasizes its role as the source of the embryo rather than the placenta and related tissues that originate from the trophoblast.