Specialized systems and protocols have been implemented in an effort to assist subfertile couples and this has led to the meticulous investigation of the factors affecting fertility at a macroscopical, microscopical and molecular level in order to establish the synergistic conditions that lead to successful reproduction. On this basis, the endometrium plays a crucial role in reproduction, as the tissue lining of the uterus is responsible for nurturing the developing embryo, but importantly and prior to implantation, it is the endometrial state, which is decisive of the fate of the early embryo. This state has been for a long time the subject of diagnostic and therapeutic interventions in the hope that through these, assisted reproduction specialists could improve the pregnancy rates. The current notion suggests that the inability of an embryo to implant is related to the asynchronous changes among itself and the endometrium.
The success of embryo implantation depends on the quality of the embryo, the endometrial receptivity and the synchronization between endometrial changes and embryonic development . The endometrium is in a receptive state when it is capable to accept and support the embryo. Ovarian hormones control its preparation, and various protocols have been suggested . Successful implantation occurs in a definite setting; a functional blastocyst “meets” a receptive endometrium and a synchronized “dialogue” between maternal and embryonic tissues begins [3,4]. The timing of this dialogue, referred as the “window of implantation”, is characterized by significant changes at circulatory and tissue level, as well as in cellular and molecular level, with notable changes in steroid receptors and integrin expression [5-7].