Transport and fate of spermatozoa after insemination of cattle.

Sperm capable of fertilizing ova reach the isthmus of cows about 8 h after mating and remain in the caudal 2 cm of the isthmus until ovulation. Then small numbers of sperm move to the site of fertilization at the junction of the isthmus and ampulla. Within a few hours after deposition of semen in the uterine body, most sperm have drained to the exterior in cervical mucus. By 12 to 24 h after insemination, only a few percent of the sperm remain in the reproductive tract, and most of these are in the vagina. Contractions of the reproductive tract appear to be the primary mechanism of sperm transport. Flagellation of sperm is probably required for sperm to enter the folds of the cervix, and flagellation may be helpful or essential for sperm to pass through the uterotubal junction, move from the isthmus to the ampulla, and penetrate ova. High proportions of sperm undergo the acrosome reaction only in the ampulla on the side of ovulation and only after ovulation. The fertilization rate in cattle can be improved by use of semen from high fertility bulls and perhaps by timing insemination with semen from lower fertility bulls after the end of estrus.