Endometrial cups form during pregnancy in mares and are the source of equine chorionic gonadotropin (eCG) and a placenta-associated structure, which is derived from the fetus.
Their invasion of the uterine stroma begins the cells' maturation process, which takes 2–3 days.
[2] Endometrial cups behave somewhat like cells from metastatic tumors, in that they leave the placenta and migrate into the uterus.
Their purpose appears to be to work with other placental cells to control the expression of histocompatibility genes so that the developing fetus is not destroyed by the mare's immune system.
The purpose of these cells In both humans and horses is believed to be to interact with the mother's immune system and increase maternal immunological tolerance of the developing fetus.