The role of estrogen in uterine receptivity and blastocyst implantation

The endometrium is a specialized, hormonally regulated organ that is non adhesive for embryos throughout most of the reproductive cycle in mammals. Thus, the window of implantation is a self-limited period in which the endometrial epithelium (EE) acquires a functional and transient ovarian steroid-dependent status. The luminal EE initiates the adhesion of the developing blastocyst during this period, owing mainly to the presence of progesterone (P) after appropriate 17beta-estradiol (E-2) priming in humans or because of the addition of E-2 after appropriate P priming in rodents. Wen-ge et al. have now demonstrated in mice that low levels of exogenous E-2 can maintain the window of receptivity for an extended period of time, whereas high doses of E-2 can rapidly initiate a refractory state. In summary, levels of E-2 within a very narrow range determine the duration of the window of implantation in uterine receptivity in mice. These outstanding results demonstrate the possibility of manipulating the receptivity window with the use of different doses of E-2.