Ultrastructural Modifications of Human Endometrium during the Window of Implantation

The endometrium is a highly dynamic tissue empowered with the capacity to undergo cyclic dramatic changes in response to ovarian steroid hormones, ultimately aiming to create a window of receptivity for blastocyst implantation. Intensive research has been performed to understand and establish morphological and molecular correlates of embryo implantation. However, it still remains a biological mystery particularly in the human, where ethical and moral constraints prohibit in vivo testing and the establishment of an ideal in vitro modeling. Rodent models of embryo implantation are largely irrelevant because the process varies significantly from that in humans. Even among primates, subtle differences exist among species. For maternal preparation of implantation, the endometrial epithelium which is surprisingly hostile towards the embryo implantation, acquires functional status receptive to blastocyst acceptance during a limited period of cycle days, termed as the ‘window of implantation (WOI). This review provides currently available information concerned primarily with the various ultrastructural modifications of endometrium coordinated within the WOI that may signify endometrial receptivity. In the following sections, the dominant features of endometrial differentiation during WOI, including transformations of luminal epithelium, endometrial glands, and stromal decidualization will be discussed from the morphological points of view.