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The trophoectoderm is the outer cell layer that surrounds the blastocyst, a day 5 embryo. Its function is to provide nutrients to the embryo. This layer is also known as trophoblast, although this denomination is for before gastrulation.

The cells of the trophoectoderm are in charge of contacting the endometrium to later generate the placenta.

Depending on the appearance of the trophoectoderm, blastocysts can be classified into 4 categories:

Category A
Homogeneous, multi-cellular layer.
Category B
homogeneous trophoectoderm, but with fewer cells than in category A..
Category C
few cells.
Category D
the cells are in the degeneration process.