Does an actomyosin system supplement the axoneme in mammalian sperm.

Boar sperm fixed in paraformaldehyde, pemeabilized in cold acetone and incubated in the presence of DAPI and Rhodamine-conjugated phalloidin or with fluorescein-conjugated cytochalasin D show brilliant blue staining of the nucleus and brilliant red or green fluorescence respectively in the middle piece and tail regions of the sperm. The results suggest the presence of F-actin in the boar sperm middle piece. Immunocytochemical studies also show the presence of tropomyosin in the middle piece region of boar sperm. Western Blot studies, using lysates of isolated boar sperm middle pieces and tails, indicate prominent sperm-actin bands with similar electrophoretic patterns of rat cardiac and striated muscle. The results strongly suggest that a 41 kDa actin, similar to cardiac or striated muscle actin, is present in sperm middle pieces and corroborates the phalloidin and cytochalasin D localization by fluorescence microscopy. We suggest that the simple sliding filament mechanism of the axoneme may not, by itself, impose the propulsive forces needed to propel large spermatozoa through the viscous prefertilization milieu. We further propose that an actomyosin system associated with the microtubule/dynein complex might play a role in sustained motility of mammalian spermatozoa.