ABSTRACT
The human endometrium is a complex tissue, predominantly composed of epithelial and stromal cells, that is cyclically and primarily regulated by steroid hormones (estrogens and progesterone). Endometrial receptivity is a self-limited period during which the endometrium transiently transforms to acquire the ability to receive a blastocyst and support implantation, a process that is mediated by immune cells, cytokines, growth factors, chemokines, adhesion molecules, and many other compounds (1–3). This transient period, called the window of implantation (WOI), has been postulated to open five days after endogenous or exogenous progesterone action and to close two days later (4, 5). A receptive endometrium and a functionally normal blastocyst, as well as an exquisitely coordinated cross-communication between them, are required for implantation, the process by which the embryo attaches and invades the underlying maternal endometrial tissue.
