A human blastocyst, 5 days after fertilization. A pig blastocyst. The bar is 0.1 mm.
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 blastocoele or the blastocystic cavity. The human blastocyst comprises 70-100 cells.
Blastocyst formation begins at day 5 after fertilization in humans, when the blastocoele opens up in the morula, a process known as hatching.
Parts of the blastocyst
The blastocyst consists of two primary cell types:
- the inner cell mass, also known as the "embryoblast" (this part of the embryo is used in stem cell research)
- the trophoblast, a layer of cells surrounding the inner cell mass and the blastocyst cavity (blastocoele)
The former is the source of embryonic stem cells and gives rise to all later structures of the adult organism. The latter combines with the maternal endometrium to form the placenta in eutherian mammals.
Formation of the blastocyst
The morula is a solid ball of about 16 undifferentiated, spherical cells. As cell division continues in the morula, the blastomeres change their shape and tightly align themselves against each other. This is called compaction and is likely mediated by cell surface adhesion glycoproteins.
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