Implantation: Recent Approaches to Understand a Cell Biological Paradox

Concepts of implantation physiology developed on the basis of previous work postulate that embryo implantation is initiated when trophoblast invasiveness coincides with astate of 'receptivity' of the endometrium, the latter being controlled by steroid hormones. However, neither trophoblast invasiveness nor endometrial receptivity are well understood in terms of their cell biological basis: a role of trophoblast-dependent proteinases in implantation has been well documented, but it remains unclear whether they are directly involved in invasion in the strict sense (4). The concept of endometrial 'receptivity' (15,16) has been developed on the one hand on the basis of heterochronous embryo transfer experiments, although these could not equivocally discriminate between generally hostile (embryo-toxic) effects of the uterine milieu and specific inaccessibility of the endometrium for trophoblast attachment and invasion. On the other hand, work on the hormonal control has largely concentrated on decidualization, a process whose significance for trophoblast invasion is far from clear (9,15). We are discussing here new data on cell-cell interactions in embryo implantation which may shed some new light on involved mechanisms. These data have in part been derived from experiments with a novel endometrial organ culture system allowing the study of trophoblast attachment and invasion, in vitro (6,8).