Blastocyst

A blastocyst is the stage of early mammalian development that follows the morula. As cleavage continues, the cells of the solid morula pump fluid inward, opening a cavity called the blastocoel. The resulting blastocyst, which in humans forms about four to five days after fertilization and holds a hundred or more cells, has two distinct parts: an outer layer of trophoblast cells and an inner cell mass clustered at one end. It is the mammalian form of the blastula seen in other animals.

The two parts have different fates. The trophoblast attaches to the lining of the uterus, invades it and forms the embryo's share of the placenta; it also releases human chorionic gonadotropin (hCG), the hormone detected by pregnancy tests. The inner cell mass develops into the embryo itself and some of its membranes, such as the amnion. Before it can implant, around day 6 in humans, the blastocyst must hatch from the zona pellucida, the protective coat it inherited from the egg. Many blastocysts never implant.

Cells of the inner cell mass are pluripotent, able to form any tissue of the body, and are the source of embryonic stem cells. In in vitro fertilization, embryos are often grown to the blastocyst stage before transfer to the uterus.