Endometrial Receptivity: Cell Biological Aspects

The cell biological basis of endometrial receptivity and trophoblast invasiveness, the two preconditions for implantation ("implantation windowtl), still remains enigmatic. Recently, however, the application of modern concepts of cell biology and developmental biology concentrating on celladhesion and cell polarity seems to open promising new views of these phenomena. At implantation initiation, the trophoblast has to attach viaits apical plasma membrane to the apical plasma membrane of the uterine epithelium. This presents a cell biological paradox since apical plasma membranes of epithelia are normally non-adhesive. Solutions for the paradox are found when taking a side view to processes in embryology that involve interaction of two epithelia, typically combined with epithelium-to-mesenchyme (E-M) transformation, a process that is recently also being discussed to be involved in tumor cell invasion. It has been proposed that some of the molecular events involved in E-M transformation can also be found in both, the acquisition of receptivity by the uterine epithelium and the expression of the invasive phenotype by the trophoblast. While the trophoblast aspect has been discussed in .the light of recent literature by Denker (10), we wiU-concentrate here on the uterine epithelium.