Ultrastructure of human blastocyst-endometrial interactions in vitro

The interactions of seven human blastocysts with cultured endometrial cells were investigated by light microscopy and transmission electron microscopy . Trophoblastic-endometrial contact was observed at the lateral border of e ndometrial epithelial cells where trophoblast and endometrial epithelial ce lls shared apicaljunctional complexes and desmosomes. The first sign of pen etration was invasion of a trophoblastic cytoplasmic protrusion between end ometrial epithelial cells. Ln broad contact areas, lateral displacement of endometrial epithelial cells and formation of a peripheral pseudostratified epithelium were observed. When trophoblastic cells were interposed fully a mong endometrial epithelial cells, they formed a penetration cone and appea red to dislodge endometrial epithelial cells from the stromal compartment. A single penetration cone only was found in each specimen. Endometrial or t rophoblastic degeneration was not observed. Formation of multinucleate (gre ater than or equal to three nuclei per cell) trophoblast cells was not obse rved, but many cells displayed areas with abrupt disappearance of well-defi ned plasma membranes, which is indicative of syncytium formation. In this s tudy, adhesion and penetration occurred at the same time. The human blastoc ysts penetrated the endometrial surface epithelium by intrusive penetration . Epithelial penetration was achieved primarily by cellular syncytiotrophob last-like cells and the first indications of syncytium formation were obser ved simultaneously with penetration of the epithelium.