Stromal cells primed by estrogen will differentiate to decidual cells under the influence of progesterone. Epithelial cells are involved in the adhesion process of the embryo.
Receptivity of epithelial cells includes structural changes of the plasma membrane (Murphy, 2000) and the cytoskeleton (Thie ., 2000), causing a transformation of microvilli at the implantation site simultaneous to the stromal decidualization process.
Although implantation itself is a dynamic process occurring between blastocyst and endometrial layers, the priming of the endometrium towards the window of implantation (WOI) is of pure maternal origin.
A reduced endometrial receptivity is found in an increasing number of unexplained infertilities (Lessey ., 1995).
One of the key players during the proliferative phase is the rising female hormone estrogen.Also, at the time of decidualization, increased cytokine production leads to the migration of immune cells into the endometrium, of which the most important are monocytes and uterine natural killer (NK) cells (King, 2000).Immune competent cells have their specific role in the preparation of endometrial receptivity.In the last two decades, a large number of proteins and other molecules, which are expressed in the endometrium in a cycle-dependent manner, have been described.Some of them, such as the insulin-like growth factor binding protein-1 (IGFBP-1), are used as a marker for decidualization at least in experiments.