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Implantation is defined as the process whereby the blastocyst attaches to and erodes through the epithelial lining of the uterus (endometrial epithelium). The erosion of the epithelium is accomplished by the trophoblast, which allows the blastocyst to subsequently invade and embed in the underlying tissue (endometrial stroma).

Attachment occurs at the embryonic pole, probably around 6 days. Attachment usually occurs between the surface openings of the uterine glands.

The trophoblastic cells in contact with the endometrial stroma lose their cell boundaries and form a syncytium. This layer of trophoblastic cells is called the syncytial trophoblast (syncytiotrophoblast). Trophoblastic cells adjacent to the blastocoele retain distinct boundaries and form a thin, single layer of cells called the cellular trophoblast (cytotrophoblast). Small cells known as extraembryonic mesoblasts begin to differentiate in situ on the inner surface of the cellular trophoblast. The amniotic cavity begins as a cleft between the embryoblast and cellular trophoblast.

With the appearance of the amniotic cavity the embryoblast becomes the embryonic disc. The embryonic disc consists of two layers, the epiblast and endoderm. The epiblast is a thick layer of potential ectoderm and mesoderm. The endodermal cells border the blastocoele. Because of the two-layered embryonic disc the entire mass of cells is called a bilaminar blastocyst.

Source: Atlas of Human Embryos.