Interaction between spermatozoa and the mare's reproductive tract is mediated by biochemical, physical, and inflammatory mechanisms

Sperm transport and elimination from the mare's reproductive tract are important components of the events that lead to fertilization and successful development of an embryo. Following deposition of semen into the uterine body, viable spermatozoa are transported to the oviduct by a combination of sperm motility, uterine contractility, and epithelial cilia activity. This process is facilitated by the release of oxytocin from the pituitary, as well as prostaglandins and estrogen in seminal plasma. The myoelectrical activity in the uterus shows a two-wave pattern following AI. The first immediate phase of contractions is believed to be involved in both sperm transport to the oviduct, and elimination of excess spermatozoa through the cervix. The second phase of contractions coincides with an inflammatory response to spermatozoa. The inflammation is an important physiological component of sperm elimination from the mare's reproductive tract. It is regulated by seminal components. Spermatozoa activate complement in uterine secretion, resulting in chemotaxis of PMNs into the uterine lumen. The PMNs bind to spermatozoa that consequently are removed from the uterus by phagocytosis. Inflammatory mediators, such as PGF2α, cause contraction of the myometrium, and residual spermatozoa, accumulated fluid, and harmful inflammatory products are removed by this second wave of uterine contractions. Once these products are removed from the uterine lumen, the inflammation subsides and the uterine environment returns to its normal state. Specific seminal plasma proteins modulate breeding-induced endometritis, and appear to selectively protect viable spermatozoa from binding to PMNs.