The Fetal Allograft Revisited: Does the Study of an Ancient Invertebrate Species Shed Light on the Role of Natural Killer Cells at the Maternal-Fetal Interface?

Figure 3

The human maternal-fetal interface. Fetal blood enters and exits the placenta via the umbilical cord. Fetal vessels lie at the core of each placental villae and all villae are lined externally by trophoblast cells. The cells of the inner layer (green) are called villous cytotrophoblast cells. Those of the outer layer develop from the villous cytotrophoblast through syncytialization and are called syncytiotrophoblast (orange). Floating villae are completely bathed in maternal blood; anchoring villae traverse the intervening blood-filled space to attach to the maternal decidua. At the tips of the anchoring villae, a subpopulation of cytotrophoblast cells leaves the villae to invade the maternal endometrial decidua. Here, these extravillous cytotrophoblast (blue) cells encounter populating maternal immune cells. Some extravillous cytotrophoblast cells will invade the maternal uterine arteries and become endovascular trophoblast cells (blue). Appropriate vascular invasion by endovascular trophoblast cells results in remodeling of the uterine vasculature (inset, right) and adequate placental perfusion. Poor vascular invasion by endovascular trophoblast cells results in limited remodeling of the uterine vasculature (inset, left), a condition associated with adverse pregnancy outcomes.