Effects of cryopreservation on the human sperm acrosome and its response to A23187

The proportion of human spermatozoa from 28 ejaculates to lose their acrosomes during cryopreservation was measured and correlated with the number that became immotile or lost the integrity of their plasma membrane. The ability of washed spermatozoa to acrosome react in response to A23187 before and after cryopreservation was compared. Motility was assessed by time-lapse photography; intact acrosomes were stained with fluorescein conjugated Pisum sativum agglutinin and dead spermatozoa were stained with bisbenzimide (H33258). Twenty-four per cent of spermatozoa lost their acrosomes during freezing and thawing, but the number that did so was not correlated with the number that became immotile or non-viable. Frozen spermatozoa exhibited fewer spontaneous acrosome reactions than did fresh spermatozoa (5 versus 13% after 4 h), but they responded to A23187 in a similar way. Although frozen spermatozoa were significantly more likely to die during the incubation, the data do not suggest that degenerative acrosome loss had a major influence on the results. In the hamster egg test frozen-thawed spermatozoa achieved more penetrations than did fresh spermatozoa when stimulated with 0 or 1 μmol A23187 l^-1 but considerably fewer when stimulated with 4 μmol A23187 l^-1. The following conclusions were made. First, cryopreservation damage to the acrosome, the plasma membrane and the flagellum can occur independently. Second, acrosome function is maintained after cryopreservation as long as the organelle remains mechanically intact. Third, some spermatozoa that lose their acrosomes during cryopreservation remain viable and can fuse with zona-free hamster eggs.