Histologic and functional aspects of the endometrium in the implantatory phase.

The endometrium is receptive to an intrusive blastocyst during a limited time span, defined as the implantation window. No definite clinically applicable receptivity marker has been discovered yet. Current approaches of the endometrium are focusing on factors emerging in the implantation window or retrieved from animal or in-vitro studies. These data fail to correlate endometrial parameters with ongoing pregnancy. Morphological tools are limited by difficulties in interpretation and inter-observer variability. In recent studies, extremely deviant endometrial morphology on the day of oocyte retrieval in IVF cycles with embryo transfer has been associated with a deleterious effect on clinical pregnancy. Gene expression data have identified clusters of genes involved in the regulation of the implantation window. Ideally, future studies should link endometrial morphology and function in precisely timed biopsies, analyzing functional biological pathways in a selected endometrial target area.