Numerous past investigations into human implantation have tended to pay attention to the mechanism of embryo adhesion to endometrial cells or embryo invasion into endometrial stromal tissue. For successful pregnancy, however, embryo penetration through the endometrial epithelial cell (EEC) sheet is also absolutely required. To improve the performance of assisted reproductive technology, implantation studies should also focus on EEC dynamics, in particular, the action of EECs during embryo penetration. Although only EEC apoptosis has been proposed as a mechanism for the formation of the embryo penetration route, we have also recently reported that characteristic EEC collective motion, regulated by the epithelial to mesenchymal transition, also plays a key role in permitting embryo penetration. We review here how EECs form embryo penetration routes in light of our findings.
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