Mammalian sperm chemotaxis and its association with capacitation

Much progress has been made in recent years in establishing mammalian sperm chemotaxis and understanding sperm capacitation. Thus far, chemotaxis to follicular fluid has been established by a variety of means in human and mouse spermatozoa. It was found that only a small fraction of a given sperm population (averaging around 10%) is chemotactically responsive and that this fraction constitutes capacitated (ripe) spermatozoa. Both the chemotactic responsiveness and the capacitated state are transient (with a lifetime of 50 min to 4 h) and they occur only once in the sperm's lifetime. It has been proposed that the role of sperm chemotaxis in mammals (at least in humans) is selective recruitment of capacitated spermatozoa for fertilizing the egg, and that the role of the continuous replacement of chemotactic/capacitated spermatozoa is to prolong the time during which capacitated spermatozoa are available in the female reproductive tract. The sperm chemoattractants have not been identified, but they appear to be heat-stable peptides. Although the molecular mechanism and the in vivo location of sperm chemotaxis are not known, a number of possible mechanisms and locations are discussed.