Sperm transport and elimination from the mare's reproductive tract

Transportation of viable spermatozoa to the oviduct is a rapid process that is completed within 4 hours of breeding. Uterine contractions play a major role in this process, but other mechanisms such as sperm motility, epithelial cilia and a possible effect of seminal PGE on the utero-tubal junction may also be involved. Only a small portion of inseminated or ejaculated semen reach the oviduct. The majority of spermatozoa needs to be effectively eliminated from the uterus before the embryo descends down from the oviduct to the uterine lumen approximately 5 days after fertilization. Myometrial contractions and semen-induced inflammation are responsible for this process. Recent data suggest that sperm elimination from the uterus is a selective process, targeting dead spermatozoa, while viable spermatozoa are protected from PMN-binding and phagocytosis by a specific seminal plasma protein.