Kinetics of sperm penetration into and the zona reaction of mouse ova inseminated in vitro

The kinetics of capacitated epididymal sperm penetration into cumulus‐intact and completely cumulus‐free mouse ova were studied. Penetration was complete within two hours of insemination in cumulus‐intact ova incubated with 2 × 10 5 sperm/ml, as evidenced by the attainment of constant values for the mean number of sperm/penetrated ovum. A reduction in sperm viability was not a factor in the cessation of zonae penetration, since high penetration levels were obtained when fresh ova were added to 3‐hour inseminations containing “aged” sperm. With completely cumulus‐free ova, capacitated sperm penetration was initiated between one and two hours post‐insemination, and penetration levels increased over the entire 6‐hour incubation period. Final penetration levels for cumulus‐free ova were generally lower than for their intact counterparts. A constant value for the mean number of sperm/penetrated ovum was obtained after three hours of sperm exposure, indicating that the cumulus‐free ovum also underwent a zona reaction. Similar results were observed when sperm were capacitated in the presence or absence of cumulus and/or epididymal components, suggesting that inadequate capacitation was not the causative agent in the altered penetration of cumulus‐free ova. These results indicate that a zona reaction occurs in both ovum types, although in many cases only after multiple sperm penetration has occurred. In addition, evidence was obtained for a cumulus‐dependent enhancement in the rate of sperm penetration which could not be related to a cumulus involvement in sperm capacitation.