Maternal Recognition of Pregnancy in the Context of Equine Embryo Transfer

• Key aspects of maternal recognition of pregnancy in the mare are reviewed and discussed in the context of equine embryo transfer.
• Early pregnancy factor and pre-implantation factor and their usefulness in detecting pregnancies before Day 8 are discussed.
• Selective oviductal transport of embryos, embryo mobility and the capsule of the equine embryo are discussed.
• Endometrial cup formation is discussed.

Maternal recognition of pregnancy refers to the sequence of events by which embryo-derived signals prolong luteal function, ultimately assuring ongoing progesterone secretion by the corpus luteum beyond its normal lifespan of the estrous cycle. In this overview, key aspects of maternal recognition of pregnancy in the mare are reviewed and discussed in the context of equine embryo transfer. In mares, the uterine tube has a species-specific, well-developed tunica muscularis which necessitates production of prostaglandin E2 by the equine embryo to facilitate transport to and passage through the uterotubal junction. After entry into the uterus, an acellular glycoprotein capsule forms beneath the zona pellucida, eventually replacing the zona pellucida and surrounding the embryo until the third week of pregnancy. Embryonic mobility is a key aspect of early pregnancy in the mare and is driven by embryo-derived prostaglandins that stimulate uterine contractility. Endometrial prostaglandin production is temporarily attenuated during early pregnancy. After formation of the chorionic girdle and invasion of chorionic girdle cells into the endometrium, endometrial cups are formed resulting in the production of equine chorionic gonadotropin. Equine chorionic gonadotropin is instrumental in the formation of accessory corpora lutea, whose progesterone production is essential in maintaining pregnancy until the placenta produces sufficient amounts of progestogens.