Embryo-maternal communication during the establishment of equine pregnancy

Abstract

Over the last 30 years, improvements in veterinary management have resulted in higher per cycle pregnancy rates in commercial horse breeding programmes. However, the incidence of early pregnancy loss has not diminished and ~ 10% of pregnancies detected on day 15 fail to survive beyond day 42 of pregnancy. To develop strategies to combat pregnancy loss, it is first necessary to understand why these pregnancies fail. In this respect, a sizeable proportion probably results from intrinsic embryonic abnormalities that cannot be remedied; another significant subset is, however, likely to result from an ‘inadequately receptive’ uterus or failure of the uterus to physiologically adapt to the developing conceptus’ changing needs, as a result of inadequate embryo-maternal communication. Communication between embryo and dam starts during the oviductal period, as evidenced by selective transport of viable embryos into the uterus on day 6 postovulation, whereas unfertilized oocytes remain in the oviduct. Conversely, retarded development exhibited by embryos produced in vitro demonstrates the importance of the oviductal environment to the early embryo. Once in the uterus, the embryo must steer maternal physiology to ensure adequate nutrient provision and prepare the endometrium for implantation. This communication includes ‘maternal recognition of pregnancy’ signalling to extend the lifespan of the primary corpus luteum and ensure continued secretion of progesterone. Although the identity of the embryonic pregnancy recognition signal remains unknown, our understanding of how endometrial function is altered to avert luteolytic prostaglandin F2α release and become receptive to implantation has improved.