Failure of Zona Penetration of Hamster Spermatozoa After Prolonged Preincubation in a Blood Serum Fraction

Summary. Hamster spermatozoa preincubated in a human blood serum fraction for 2 to 3 hr fertilize over 90% of hamster eggs with intact zonae pellucidae. Spermatozoa preincubated for 4 to 6 hr lose the ability to cross the zona pellucida, but can enter the cytoplasm of zona-free eggs. Electron microscopic examination of briefly incubated spermatozoa showed that the large majority had `intact' or slightly vesiculated equatorial segments, while prolonged incubation led to extensive vesiculation of the equatorial segments. It is inferred that the loss of the ability of spermatozoa to penetrate the zona pellucida is associated with excessive vesiculation of the equatorial segments. In the light of this finding, the possible location of the sperm-borne `zona lysin' is discussed.