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|Blastocyst just before implantation|
|A human blastocyst, 5 days after fertilisation|
|Gives rise to||Gastrula and inner cell mass|
The blastocyst is a structure formed in the early gestation of vertebrates. It is preceded by the morula. It possesses an inner cell mass (ICM), or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, surrounding the inner cell mass and a fluid-filled cavity known as the blastocoele. The human blastocyst comprises 70-100 cells.
Blastocyst formation begins at day 5 after fertilization in humans and is closely related to blastula.
The blastocyst develops from the morula, a solid ball of about 16 undifferentiated, spherical cells within the zona pellucida. As cell division continues, the blastomeres change their shape and tightly align themselves against each other, forming the fluid-filled cavity called the blastocoele.
The cells now forming differentiate into two types, an inner-cell mass growing on the interior of the blastocele and cells growing on the exterior of it. In 24 to 48 hours, the zona pellucida breaches, referred to as hatching. This removes the constraint on the physical size of the cell mass.
The cells on the exterior of the blastocyst begin excreting an enzyme which erodes epithelial uterine lining and creates a site for implantation. The cells surrounding the blastocyst now destroy cells in the uterine lining, forming small pools of blood, which in turn stimulate the production of capillaries. This is the first stage in the growth of the placenta. The inner cell mass of the blastocyst divides rapidly, forming two layers. The top layer becomes the embryo, and cells from there occupy the amniotic cavity. At the same time, the bottom layer forms a small sac.
The blastocyst consists of two primary cell types:
- the inner cell mass, also known as the "embryoblast"
- 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.