From reservoir to rendezvous: the journey of sperm through the oviduct.

Once viewed simply as a pipeline connecting the ovary and uterus where fertilization and early development occur, the oviduct is becoming more appreciated for its complex functions. The utero-tubal junction (UTJ) restricts the entry of sperm to those meeting specific morphological and molecular requirements. Once sperm passes the UTJ, the isthmus functions as a storage reservoir, retaining sperm before fertilization and prolonging their lifespan, allowing for more asynchrony between sperm deposition and ovulation. Some mammals, such as bats, store sperm for months, but a few days of storage is more common among most species. In the isthmus, some sperm adhere to the oviduct epithelium whereas others are free and moving. Some of the adherent sperm detach from oviduct cells and move to the ampulla, relying on a complex interplay that brings a limited number of sperm to rendezvous with oocytes. Thus, the oviduct is responsible for sperm selection, storage, transport, maturation, and fertilization. Focused primarily on mammals, this review discusses the mechanisms underpinning each of these steps.