Transport, Distribution and Elimination of Mammalian Sperm Following Natural Mating and Insemination.

The integrity of transport, distribution and elimination of sperm in the female genital tract plays a pivotal role for successful reproduction in mammals. At coitus, millions or billions of sperm are deposited either into the anterior vagina (human, primates), the cervix (most mammalian species) or the uterus (pig). In most species, the first anatomical barrier is the cervix, where spermatozoa with poor morphology and motility are filtered out by sticking to the cervical mucus. The second anatomical barrier is the uterotubal junction (UTJ) with its tortuous and narrow lumen. Finally, only a few thousand sperm enter the oviduct and less than 100 sperm reach the site of fertilization. As soon as the sperm enter the oviduct, they form a sperm reservoir enabling them to stay vital and maintain fertilizing capacity for 3-4 days (cow, horse) up to several months (bats). After ovulation, mammalian sperm show hyperactivation which allows them to detach from the tubal epithelium and migrate to the site of fertilization. This review will focus on recent insights of sperm transport, sperm storage and sperm-oviduct interaction in mammals which have been gained by live cell imaging in cows and mice under near in vivo conditions. Detailed knowledge of the biology of spermatozoa within the female genital tract creates the basis for new therapeutic concepts for male subfertility and infertility - an essential prerequisite to increase success rates in assisted reproduction.