Figure 3.

Possible mechanisms of human embryo invasion after attachment to endometrial epithelial cells. (A) After the embryo has firmly attached to the endometrium via several adhesion molecules, luminal epithelial cells receive continuous ephrin A signaling from the attached embryo (#1). (B) The Eph–ephrin system induces reduction of the tight junction of epithelial cells (#2), opening the epithelial cell layer (yellow two-way arrow). This system also promotes cell attachment to the extracellular matrix (#3) without destroying the connection between the embryo-attached epithelial cells and the basement membrane (BM). (C) This system may promote migration of the activated trophectoderm (#4) into the intercellular spaces of epithelial cells with reduced tight junctions.