Induction of the acrosome reaction in sperm by exposure to low temperature increases their rate of fusion with zona-free hamster oocytes.

The fusion rate of human sperm with zona-free hamster ova was investigated after induction of the acrosome reaction by exposure to a low temperature (4 degrees C). Sperm were collected from 14 patients, and selected by the 'swim-up' method. The sperm were incubated for 24 h at either room temperature (control group) or at 4 degrees C (low temperature group), followed by additional incubation at 37 degrees C for 3 h. The mean sperm penetration rate, number of swollen sperm heads as well as the number of sperm attached to the oocyte increased significantly after exposing sperm to low temperature. The sperm penetration rate showed a significant correlation (Spearman test, r = 0.572, n = 28, P less than 0.0035) with the acrosome reaction in the low temperature group. These results were associated with an increase in the rate of penetration of hamster ova observed in this study, presumably due to the increase in induction of the acrosome reaction by low temperature. Incubation of sperm at low temperature might be useful in the evaluation of so-called false negative results in the zona-free hamster test.