Endometrial receptivity: selective adhesion competence of rabbit uterine epithelium for trophoblast but not for various tumor cells.

A modification of an established in vitro model for embryo implantation was used to probe the receptive uterine epithelium for any specificity of interaction with various invasive cells other than trophoblast. Endometrial explants consisting of stroma and epithelium taken from pseudopregnant rabbits were cultured in the presence of progesterone in order to regenerate a complete epithelial lining while maintaining the receptive state. Such precultured fragments were brought into contact with multicellular spheroids of different invasive tumor cell lines from different species. In contrast to the trophoblast of the rabbit blastocyst (previous publication), none of the tumor cell lines was able to adhere to intact epithelium of endometrial fragments nor to penetrate it. The uterine epithelium was also an insurmountable barrier for tumor cell spheroids confronted with the epithelium of fresh complex explants consisting of endometrium and myometrium or for spheroids introduced into the uterine lumen of pregnant/pseudopregnant rabbits at the periimplantation phase. However, all tumor cells were able to adhere to and mostly also to invade into the endometrial stroma when it was exposed artificially, i.e. when the epithelium was removed. These results suggest that the receptivity of rabbit uterine epithelium shows a remarkable selectivity with respect to cell type (trophoblast) and species (rabbit, not human, mouse, or rat).