Regulation of sperm storage and movement in the ruminant oviduct.
Three regions of the ruminant oviduct play different roles in the progress of sperm: the uterotubal junction, isthmus, and ampulla. The uterotubal junction acts as a point of selection of sperm, requiring that sperm are progressively motile and express specific proteins in order to enter the oviduct. The isthmus stores sperm, preserving motility and viability until ovulation. Sperm are stored in the isthmus by binding to its mucosal epithelium. In bovine sperm, binding to the oviductal epithelium is promoted by proteins that are secreted by the seminal vesicles and coat the heads of sperm by associating with plasma membrane phospholipids. Putative oviductal receptors for the seminal vesicle proteins are members of the annexin protein family. Release of sperm from the storage site in the isthmus is gradual, which serves to ensure that sperm in the proper physiological state reach the oocytes at the appropriate moment and also to reduce incidence of polyspermic fertilization. The ampulla supports fertilization and may participate in guiding sperm toward the eggs. Further studies are needed to improve our understanding of the interactions between sperm and the female reproductive tract, in order to develop means to improve fertility in ruminants.