Mouse sperm capacitation in vitro involves loss of a surface-associated inhibitory component

The increasing fertility of epididymal mouse sperm suspensions as preincubation time is extended is accompanied by the inactivation or destruction of an inhibitory component. Alternatively, precocious removal of the component, achieved by centrifugation, leads to significant improvement in fertilizing ability. Suspensions were preincubated for a total of 120 min, with aliquants being removed at 5, 30 and 120 min. By gently washing samples and resuspending in fresh medium, the poor fertility of unwashed 5- and 30-min suspensions was increased such that 30-min washed samples did not differ significantly from fully capacitated, highly fertile 120-min unwashed control samples. When the supernatants obtained during washing of uncapacitated suspensions (5 and 30 min preincubation) were added to capacitated (120 min preincubation) populations, fertilization of cumulus-intact eggs was markedly and significantly inhibited, although fertilization of zona-free eggs was unaffected. In contrast, supernatants from capacitated suspensions were not inhibitory. When suspensions were preincubated in Ca2+-free media, both washing and exposure to hyperosmolal conditions improved fertilizing ability after addition of exogenous Ca2+, although not to the extent seen in control samples. Removal of the inhibitory component therefore increased the response of spermatozoa to Ca2+. The component was shown to be cell-associated and to inhibit the acrosome reaction in capacitated suspensions. Finally, the inhibition was shown to be reversible, with further incubation of inhibited suspensions restoring the original fertility.