Something from my biology a few years ago, HOPE IT HELPS Implantation is defined as the process by which the embryo attaches to the endometrial surface of the uterus and invades the epithelium and then the maternal circulation to form the placenta. Before the initiation of implantation, however, both embryo and endometrium should embark on an elaborated process in a time- and location-specific manner. The crosstalk between a receptive uterus and a competent blastocyst can only occur during a limited time span, known as the “window of implantation” (Psychoyos, 1986; Ma et al., 2003). The window of endometrial receptivity is restricted to days 16-22 of a 28-day normal menstrual cycle, 5-10 days after the luteinizing hormone (LH) surge (Navot et al., 1991). The uterus then continues into the non- receptive period for the remaining cycle as the late luteal
phase until menstruation ensues. It is not surprising that early pregnancy loss, which occurs during the peri-implantation period before pregnancy is confirmed clinically, is common in humans (Norwitz et al., 2001). The risk of spontaneous abortion significantly increases with implantation beyond this window (Wilcox et al., 1999). The synchronized molecular dialogue mediated by cytokines, a variety of growth factors, prostaglandins, matrix degrading enzymes and their inhibitors, and adhesion molecules is required for successful implantation. Infertility, the inability to conceive within a year of unprotected intercourse, affects one in seven reproductive-aged couples in Korea (Kim et al., 2014). It is necessary to investigate the hierarchical pathways that orchestrate the implantation process to develop solutions for implantation failure and to raise fertility rates in women.