Localization of actin in human, bull, rabbit, and hamster sperm by immunoelectron microscopy

Mammalian spermatozoa have previously been shown to contain actin, but its subcellular localization and function have not been elucidated. In this study, actin has been localized at the ultrastructural level in human, bull, rabbit, and golden hamster spermatozoa by a monoclonal antiactin antibody and a preembedding immunogold labeling technique. Specific labeling was localized 1) around the connecting piece in the neck region of sperm from all four species, although a species-specific pattern was evident; 2) on the external surface of the fibrous sheath of human sperm; 3) in the perinuclear space underlying the postacrosomal sheath of bull and rabbit sperm; and 4) between the plasma membrane and outer acrosomal membrane along the concave margin of the hamster sperm head. SDS-PAGE and Western blots immunostained with the monoclonal antibody confirmed the presence of actin in SDS extracts of Percoll-purified sperm from each species.