Trophoblast-uterine interactions in the first days of implantation: models for the study of implantation events in the human.

The most profound changes in the relationship of the trophoblast to endometrial tissues occur in the first 5 days after the initiation of implantation. Not only have the earliest stages--adhesion and epithelial penetration--never been seen in the human, but also the trophoblastic plate and lacunar stages that follow are not available for modern investigative methods. Studies of appropriately timed endometrium and of trophoblast- and endometrium-derived cell lines have important implications for aspects of implantation. Use of nonhuman primates and other animal models for appropriate stages in implantation could further our understanding of direct trophoblast-uterine interactions. The mechanisms involved in epithelial penetration by infiltration of the syncytial trophoblast into the uterine luminal epithelium could be studied profitably using the marmoset or the ferret. Drawing of the blastocyst into an interstitial location might be investigated in the guinea pig. Formation of trophoblastic lacunae can be investigated in the cynomolgus monkey. By using such animal models of events of implantation in situ, implications concerning the molecules involved in adhesion, penetration of junctional complexes, and uterine vessel invasion that have been derived from in vitro or murine studies may be placed in context.