Sperm dynamics in the female genital tract: Interactions with Fallopian tube microenvironments

After discussing the rate of functional sperm transport to the Fallopian tubes of sheep, cows and pigs, this review focuses on the relatively protracted preovulatory storage of spermatozoa in the caudal portion of the isthmus, with suppressed motility and sperm head adhesion to the epithelium. As well as facilitating the phase of sperm storage, the viscous secretions in this region of the duct may serve to pre vent access of uterine and ampullary fluids into the isthmus sperm reservoir, and similarly entry of a uterine population of polymorphonuclear leucocytes. Ovarian endocrine control of peri-ovulatory sperm activation and release is analysed, with emphasis on a local counter-current vascular exchange of relatively high concentrations of follicular hormones to programme the isthmus and its content of male gametes. The site and time of completion of capacitation are reviewed critically, as are these aspects of hyperactivation and the acrosome reaction. The specific fluid milieu of the ampullary-isthmic junction in the presence of egg investments may be closely involved in triggering these final events for a fertilising spermatozoon, but it is strongly argued also that pre-ovulatory suppression of completion of capacitation is an essential storage strategy. The extremely low initial sperm:egg ratios are considered in terms of (1) molecular modulation of sperm numbers activated from the isthmus sperm reservoir and (2) molecular signals within the cumulus oophorus aggregates of polytocous species. A potential influence of temperature gradients within the Fallopian tubes in guiding viable spermatozoa towards the site of fertilisation is examined, and the post-ovulatory increase in sperm numbers reaching the ampulla after establishment of an il) eversible block to polyspermy is emphasised.