Blastocyst

The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and a fluid-filled blastocyst cavity known as the blastocoels or the blastocystic cavity. The human blastocyst comprises 70-100 cells. Blastocyst formation begins at day 5 after fertilization in humans, when the blastocoels opens up in the morula.