The Reaction of the Zona Pellucida to Sperm Penetration

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Abstract
A count of the number of sperms seen within the eggs of the rat, mouse, rabbit, sheep and dog. A frequency distribution is given for the eggs of 251 rats and 20 mice, according to the number of sperms they contained. Considering the high proportion of penetrated eggs in both rats and mice, the number of eggs containing more than one sperm (about 20%) was much smaller than chance expectation. This may mean that zona pellucida penetrability decreases after the entry of the first sperm, a reaction not previously demonstrated in the zona pellucida. In eggs containing 2 sperms, the points of entry through the zona were much more frequently in opposite hemispheres than in the same one, suggesting that the zona reaction is a propagated change. The mean time for completion of the reaction is between 10 minutes and 1.5-2 hours. In the rabbit the penetrability of the zona pellucida is not influenced by sperm entry. The mean number of sperms in the perivitelline space of the rabbit egg increased from 0.2 10 hours after coitus to 61.8 16 hours after coitus; thereafter there was no significant change. No sperms were detected in the perivitelline space of 12 fertilized dogs eggs, although numerous sperms adhered to the surface of the zona pellucida. Zona pellucida of both sheep and dog eggs apparently reacts to the entry of the fertilizing sperm to prevent further penetration. Published reports on the number of sperms within the eggs of various mammals are reviewed. Except in rabbit and, perhaps, the mole, the zona pellucida of mammals reacts to sperm penetration in such a way that few, if any, extra sperms can enter the eggs. The importance of this reaction in relation to fertility is discussed.