In large mammals, millions of sperm are necessary to fertilize only a few oocytes (reviewed in ). Artificial insemination has been used to increase reproductive efficiency, but still, thousands of excess sperm must be used to ensure fertilization. For example, carefully regulated insemination of cows with semen from highly fertile bulls requires a few million sperm to achieve pregnancy or nonreturn rates over 60%. Of the millions of sperm normally ejaculated in natural mating, only thousands reach the isthmus of the oviduct, and there, most are held back in a reservoir. Only a few reach the ampulla at the time of fertilization (reviewed in ). buy ampicillin
The oviductal reservoir for sperm may serve a number of functions. First, it may prevent polyspermic fertilization, by allowing only a few sperm at a time to reach the oocyte in the ampulla. Sperm numbers have been artificially increased at the site of fertilization in the pig by surgical insemination directly into the oviduct, by resecting the oviduct to bypass the reservoir, and by administering progesterone into the muscularis. In all of these cases, the incidence of polyspermy increased. Second, the oviductal reservoir may maintain the fertility of sperm between the onset of estrus and fertilization. Bull sperm fertility and motility are maintained longer in vitro if the sperm are incubated with oviductal epithelium. Third, the processes of capacitation and motility hyperactivation may be regulated within the reservoir.