Interactions of spermatozoa with the female reproductive tract: inspiration for assisted reproduction.

Artificial insemination with sexed semen, in vitro fertilisation and intracytoplasmic sperm injection have been used to reproduce animals, but often not as successfully as natural mating. Learning more about how spermatozoa normally interact with the female tract can provide inspiration for developing improvements in assisted reproduction. The present review focuses on Bos taurus, because more is known about this species than others. At coitus, bull spermatozoa are deposited into the anterior vagina, where they rapidly enter the cervix. Cervical mucus quickly filters out seminal plasma from spermatozoa, unlike most assisted reproduction protocols. Spermatozoa that reach the uterus may require certain cell surface proteins to swim through the uterotubal junction. Shortly after passing through the junction, most spermatozoa are trapped in a storage reservoir by binding to oviducal epithelium, in the case of cattle via bovine seminal plasma (BSP) proteins coating the sperm head. As ovulation approaches, spermatozoa capacitate and shed BSP proteins. This reduces sperm binding to the epithelium and releases them from storage. Motility hyperactivation assists spermatozoa in leaving the storage reservoir, swimming through oviducal mucus and the cumulus oophorus, and penetrating the oocyte zona pellucida. Chemotactically regulated switching between asymmetrical (i.e. hyperactivated) and symmetrical flagellar beating may also guide spermatozoa to the oocyte.