Sperm selection in the female mammalian reproductive tract. Focus on the oviduct: Hypotheses, mechanisms, and new opportunities.

Research over the past 3 decades has caused a major shift in the way that the oviduct, or fallopian tube, is perceived. Previously, it was regarded as little more than the anatomic site for fertilization, where spermatozoa and oocytes meet as they travel in opposite directions. However, this view has been radically altered by the realization that both spermatozoa and oocytes elicit changes in the biochemical composition of oviductal fluid through the induction of novel gene expression. Moreover, it has also been shown that only a privileged sperm population, selected on the basis of multiple criteria, is permitted to enter the oviduct, where they are subjected to even more selection processes that control their motility and capacitation status, thus either inhibiting or facilitating their progress toward the oocyte. Even more recently, it has become apparent that the oviduct has some ability to differentiate the genetic signatures of X- and Y-bearing spermatozoa. Although how exactly this is achieved is unknown, it prompts us to speculate that the oviduct may also be capable of distinguishing other genetically encoded properties of individual spermatozoa and that there must ultimately be a huge payoff in terms of selective animal breeding.