A blastocyst is an embryo that has developed to the point of having two different cell sections and a fluid cavity. The blastocyst has an inner cell mass from which the embryo will develop, and an outer layer of cells called the trophoblast, which will eventually form the placenta.

The blastocyst consists of cells forming an outer trophoblast layer, an inner cell mass and a fluid-filled cavity. The blastocyst inner cell mass is the source of true embryonic stem cells that are capable of forming all cell types within the embryo.


Some keypoints:

  • The human blastocyst is composed of 70-100 cells and 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 blastocoele or the blastocystic cavity.
  • The trophoblast combines with the maternal endometrium to form the placenta in eutherian mammals.
  • Before gastrulation, the cells of the trophoblast become differentiated into two strata: the ectoderm of the chorion plays a role in the development of the placenta; the endoderm differentiates and quickly assumes the form of a small sac, called the yolk sac.
  • The embryoblast is the source of embryonic stem cells and gives rise to all later structures of the adult organism.
  • The floor of the amniotic cavity is formed by the embryonic disk, which is composed of a layer of prismatic cells, and the embryonic ectoderm, which is derived from the inner cell mass and lies in opposition to the endoderm.


Watch a Blastocyst hatch below:


A Hatching Blastocyst from Wijnland Fertility Clinic