Osteopontin in Human Endometrium: A Role in Endometrial Receptivity and Embryo Implantation?

It is generally accepted that extracellular matrix proteins are not only central in providing the substrate for the anchoring of cells, but, most importantly, they play either permissive or inhibitory regulatory roles in the migration of cells. It has become quite clear that these matrix proteins participate in the signaling events regulating these cellular processes by binding to integrin cell adhesion molecules expressed on the surface of cells (1). During the process of embryo implantation, the trophoblast not only is required to interact directly or indirectly with the endometrial epithelium, but in those animals (including the human) in which invasive implantation is observed, trophoblasts adhere and migrate through the stromal extracellular matrix, continuously degrading and remodeling it (2). In animals exhibiting hemochorial placentation this migratory activity of the trophoblast cells continues until invasion into the maternal vessels and replacement of the endothelial cells with trophoblasts is achieved (3).