Understanding implantation window, a crucial phenomenon

Embryo implantation is a well-defined and precise process, in which various factors come into play one after the other. There is only a specific period of time during which implantation is possible i.e. "implantation window". Selectins were proposed to have an important role in this phase to ensure suitable rolling of the blastocyst. To prevent the blastocyst from adhering to an area with poor chances of implantation, an important role is played by the repellent activity of MUC-1, which is widely expressed throughout the endometrium and, surprisingly, even increases before implantation. In particular endometrial areas, secretion of chemokines and growth factors will attract the blastocyst to landing platforms known as pinopods. These pinopods are fully developed for only 1 or 2 days and extend over the tips of the microvilli expressing the repellent MUC-1. At this stage, adhesion molecules such as integrins and cadherins intervene to ensure adhesiveness between the embryo and the endometrium.