Functions of hyperactivated motility of sperm in the oviduct*

Hyperactivacion of sperm entails increases in the flagellar wave amplitude and the asymmetry of flagellar movement. In simple media on glass slides, hyperactivated sperm are not highly progressive. Nevertheless, we have evidence that hyperactivated sperm within the oviduct possess greater ability to reach the oocyte plasmalemma than normal activated sperm. First, hyperactivated rabbit sperm, flushed from the ampulla at the time of fertilization, have been found to swim in circles of greater radii in shallow chambers than in deep chambers. This behavior would tend to bring sperm away from the shallow chambers of the mucosal folds and into the deep chamber of the center of the oviductal lumen wherein lies the oocyte. Second, a study of mouse sperm, observed through the walls of the oviduct, has revealed that they bind to the mucosal surface, and we have now observed that only hyperactivated flagellar bending movements precede their release. Third, hyperactivated sperm may be more capable of penetrating proteoglycans present in the oviduct. A viscous mucus-like substance that traps sperm has been seen by us on the surface of oviductal mucosa from cows and pigs. In a recent study, we used Ficoll to increase the viscosity of medium to model the vis­ cous environment of the oviductal mucus and cumulus matrix. Hyperactivated hamster sperm demonstrated greater ability to penetrate the viscous medium than activated sperm. Finally, an advanced state of hyperactivation may confer an advantage on sperm penetrating the zona pellucida. In an earlier study, hyperactivated hamster sperm were found to further increase flagellar bending after the acrosome reaction. This could confer an increased po­ tential for generating thrust against the zona. In conclusion, it appears that hyperactivation enhances the ability of sperm to reach the site of fertilization and to penetrate the vestments of the oocyte.