The block to sperm penetration in zonal-free mouse eggs

The rate of sperm penetration and the number of sperm penetrating zona-free mouse eggs were found to be dependent on sperm concentration. At the lowest sperm concentrations examined (10² cells/ml, sperm-egg ratios of approximately 1:1), most eggs were penetrated (75%), and polyspermy was low (19%) following 3 hr of incubation. The number of sperm penetrating the egg was logarithmically related to sperm concentration. All eggs showed a delay of at least 20 min between insemination and penetration, and penetration was complete in approximately 2 hr at 10⁴ sperm/ml; this penetration block was attributed to egg-related changes. The existence and timing of the egg plasma membrane block to polyspermy were evaluated by reinsemination experiments. In this approach, the block was triggered in zona-free eggs with a low concentration of capacitated epididymal sperm at time 0, and the eggs were subsequently challenged with high sperm concentrations. The presence or absence of a block was inferred from the degree of polyspermy observed in these eggs after 3 hr of incubation. Adjusting for sperm concentration-dependent delays between insemination and sperm penetration, a blocking time of approximately 40 min was obtained.