Blastocyst

Blastocyst
Carnegie stage	3
Days	5
MeSH	Blastocyst

A pig blastocyst. The bar is 0.1 mm.

The blastocyst is the structure formed in early embryogenesis, after the formation of the blastocoel, but before implantation.

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 proper. The human blastocyst arises after compaction and comprises 70-100 cells. It is preceded by a zygote, the fertilized egg cell.

Blastocyst formation begins at day 5 after fertilization in humans.^[1] Differential gene expression 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 expessed at a higher level in the trophoblast than the ICM. 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.

Formation of the blastocyst

The blastocyst consists of two primary cell lines:

• the inner cell mass, also known as the "embryoblast"
• the trophoblast

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.

References

External links

• Blastocyst photos at different stages of development
• Diagram at weber.edu

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This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated.