Ultrastructure of the acrosome in frozen-thawed boar spermatozoa frozen by the pellet freezing method.

Morphological changes of the acrosome in frozen-thawed boar spermatozoa prepared by a pellet freezing method were examined with a transmission electron microscopy immediately after thawing and 5 hr after incubation of the semen at 37°C. The normality of sperm head morphology after thawing was 5.9 %, but it decreased to 1% after 5 hr of incubation (P<0.05). Prominent morphological changes like swelling of plasma membrane (38.6%), break or loss of plasma membrane (13.9%), swelling of plasma membrane overlying swollen and vacuolated acrosome (13.6%) and swelling of acrosome (11.9%) appeared immediately after thawing. In contrast, after 5 hr of incubation decrease in the swelling of plasma membrane (7.1%) and swelling of acrosome (3.4%) were observed (P<0.05) and further increase in the swelling of plasma membrane with a loss of acrosomal contents (13.7%) and a loss of the acrosome (8.5%) were observed (P<0.05). Swelling of plasma membrane overlying swollen and vacuolated acrosome (22.6%) and swelling of acrosome without plasma membrane (16.4%) showed a tendency to increase after 5 hr of incubation. These results indicate that majority of motile boar spermatozoa have some degree of alterations of the plasma membrane and/or acrosome in sperm heads after thawing.