Presence of uterine pinopodes at the embryo-endometrial interface during human implantation in vitro

In order to study changes occurring on the surfaces of human endometrial epithelial cells in the presence of an implanted blastocyst, we used scanning electron microscopy for investigation of five endometrial biopsies and three human implantation sites obtained in vitro. All specimens showed areas with endometrial pinopodes, separated by cells displaying microvilli or cilia at the apical surface. Pinopode formation was more pronounced in endometrial biopsies than in cell cultures. All blastocysts adhered to pinopode presenting cells. Endometrial surface changes were not seen around the blastocysts. The results of this study demonstrate that cultured endometrial epithelial cells are capable of pinopode formation. Furthermore, endometrial epithelial pinopodes, generally considered as a marker of endometrial receptivity, seem to be directly involved in the adhesion of the blastocyst to the endometrial surface.