Changes in Epithelial Cell Polarity and Adhesion Guide Human Endometrial Receptivity: How In Vitro Systems Help to Untangle Mechanistic Details.

Tissue remodeling of human endometrium occurs during the menstrual cycle to prepare for embryo adhesion and invasion. The ovarian steroid hormones 17β-estradiol and progesterone control the menstrual cycle to achieve the receptive state during the "window of implantation" (WOI). Here, we focus on the human endometrial epithelium and its changes in polarity, adhesion, cytoskeletal organization and the underlying extracellular matrix enabling embryo implantation. The adhesion and invasion of the trophoblast via the apical plasma membrane of epithelial cells is a unique cell biological process, which is coupled to partial epithelial-mesenchymal transition (EMT). Given the fundamental species differences during implantation, we restrict the review mainly to the human situation and focus on cell culture systems to study the interaction between human trophoblast and endometrial cells. We summarize current knowledge based on the relatively scarce in vivo data and the steadily growing in vitro observations using various cell culture systems.