It possesses an inner cell mass, or embryoblast which subsequently forms the embryo, and an outer cell mass, or trophoblast which forms the placenta. The human blastocyst arises after compaction and comprises 70-100 cells. It is preceded by a zygote, the fertilized egg cell, and succeeded by an embryo.
Blastocyte formation begins at day 5. Differential expression of cells in the morula is thought to be the cause of the lineage divergence of different cell types. For example, the Oct-3/4 transcription factor is restricted to the ICM, whereas Cdx2 is restricted to the trophoblast. This differential transcription factor expression is likely to be the result of positional effect - cells in the middle of the preceding zygote are in a different environment to those on the outside, thus causing differential expression.
The blastocyst consists of two primary cell lines:
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.