Ultrastructure of the early human implantation in vitro.

Four hatched human blastocysts obtained after in-vitro fertilization and development were placed on monolayer cell cultures of human endometrial epithelium, and subsequently examined by transmission electron microscopy. All four blastocysts became adherent to the monolayer and three implanted and exhibited outgrowth of their trophoblastic cells. During implantation the blastocysts differentiated into mural and polar trophoblastic cells, and embryonic cells including endodermal cells. The endometrial cells were displaced and stacked into a multilayer at the periphery of the implantation sites, allowing the trophoblastic cells to come in contact with the culture dish. The endometrial cells displayed local exo- or endo-cytosis where they contacted the trophoblastic cells. The trophoblastic cells were not observed to be phagocytosing endometrial cells. These observations suggest that human blastocysts portray an intrusive type of implantation during the initial stages.