Sperm interaction with the zona-free hamster egg.

The interaction of sperm with the zona-free hamster egg was studied. Hamster sperm were capacitated in Tyrodes media, containing 50% heat-inactivated serum and used to inseminate zona-free eggs in BWW containing 10% serum. Capacitated sperm began fusing with eggs within the first hour of insemination and by 3 h penetration had ceased as indicated by the absence of further changes in the mean number of sperm incorporated per egg. Penetration by capacitated hamster sperm was linearly related to the log of the motile sperm concentration at concentrations above 10(4) cells/ml. The viability of sperm and eggs in culture was limited in these studies to approximately 3-5 h. The existence of a block to unlimited sperm penetration of the zona-free egg was sought in reinsemination experiments. A relatively low sperm concentration was used to initiate egg responses, followed, at timed intervals, by reinsemination with a high challenge concentration of sperm. Subsequent polyspermy levels reflected the presence or absence of the egg's block to polyspermy response. In order to circumvent the problems arising from the rapid aging of hamster sperm in culture, mouse sperm were employed, a convenience afforded by the lack of species specificity in this egg. Reinseminated eggs incorporated additional sperm during the challenge; therefore, the hamster egg is not capable of preventing unlimited sperm penetration. The implications of these findings to the use of the zona-free hamster egg test in fertility evaluation is discussed.