Carbohydrate-mediated formation of the oviductal sperm reservoir in mammals

Mammalian sperm are trapped in a reservoir in the oviduct until ovulation is imminent. Then, they are gradually released, such that a few meet the oocytes as they enter the ampulla of the oviduct. In the three eutherian species studied to date, sperm are trapped in the reservoir by carbohydrate-mediated binding to the oviductal mucosa. Evidence indicates that a molecule on the surface of the plasma membrane overlying the acrosome binds to a carbohydrate moiety on the surface of the oviduct. While sperm remain bound, they appear to be protected from degradation. When sperm become capacitated, they lose binding affinity for the oviductal mucosa. The mechanism initiating capacitation in the reservoir is unknown; however, it must be tied to the hormonal signalling of ovulation. Hyperactivated motility may assist sperm in pulling off from the mucosal surface as binding affinity declines. The function of the reservoir appears to be to prevent polyspermy and ensure fertilization by providing a small number of sperm in the proper physiological condition for fertilization at the time the oocytes enter the oviduct.