Polyspermy‐preventing mechanisms in the golden hamster egg

Abstract The zona pellucida of a fertilized egg recovered from an artificially inseminated female reacts to capacitated spermatozoa differently from the zona pellucida of an unfertilized egg. The zona of a fertilized egg shows a sperm‐refractory response, “zona reaction,” so that capacitated spermatozoa neither firmly attach to nor penetrates it. On the other hand, the zona of an unfertilized egg shows no such sperm‐refractory response and many spermatozoa attach to and penetrate it. The zona reaction was observed not only in recently penetrated eggs but also throughout cleavage. Judging from the earliest fertilization stage at which we observed a complete zona reaction, it is estimated that this reaction is completed in less than 15 minutes after sperm attachment to the vitellus. One hundred per cent of the fertilized eggs recovered from females five hours after artificial insemination were refertilized in vitro after removal of the zona pellucida by trypsin‐treatment. The incidence of refertilization decreased when the eggs were recovered at later times. Only 3% of the eggs recovered at nine hours after artificial insemination were refertilized. Judging from the fertilization stage at which eggs were no longer refertilizable, it has been estimated that the completion of the vitelline surface block to polyspermy is established in not less than two nor more than three to three and one‐half hours. Possible causes of polyspermy encountered with in vitro fertilized eggs are discussed.