Maternal recognition of pregnancy and immunological implications of trophoblast-endometrium interactions in equids

Fertilized eggs reach the uterus of the mare by the sixth day after ovulation whereas unfertilized eggs remain lodged in the fallopian tubes for many months. However, embryo removal studies indicate that the fetal message for luteal maintenance in the pregnant mare is not transmitted until Days 14-16. The equine endometrial cups comprise a series of small, ulcer-like endometrial outgrowths which are present in the pregnant horn of the uterus between 40 and 150 days of gestation. Each cup consists of a discrete colony of large, gonadotropin-secreting, trophoblast cells derived from a specialized region of the fetal membranes which invade the maternal endometrium around Day 37 after ovulation. Histological examination of endometrial cups from horses and donkeys, carrying intra- and interspecific conceptuses and after reciprocal transfer of hybrid zygotes, indicates maternal recognition of paternal transplantation antigens on the endometrial cup cells. This leads to a successful cell-mediated immune reaction which destroys the cups. The development of the local immune response is paralled by a rapid rise in the titre of paterno-specific cytotoxic antibodies in maternal serum. The precise function of endometrial cups in the maintenance of equine pregnancy remains to be elucidated.