The changes reported here in the distribution of the Ca2+-dependent actin filament-severing protein scinder-in in the spermatozoa during and following epididymal transit may represent a different response of the gametes to local conditions that altered the [Ca2+]i. Scinderin is a calcium-dependent actin filament-severing protein that has been shown to have a major role in the regulation of exo-cytosis of secretory vesicles during neurotransmitter release by controlling cortical actin disassembly. Perhaps during their transit through the excretory ducts of the testis, the spermatozoa need to modify the state/form of their actin so as to allow emergence of the acrosome reaction, which has been called ‘‘a sperm exocytosis’’.
The finding of scinderin in the neck and in the tail of the epididymal and ejaculated spermatozoa is somewhat more difficult to justify, perhaps because the participation of actin in the movements of the flagellum is ambiguous. Nevertheless, scinderin immunoreactivity in the epididymal and ejaculated spermatozoa (Fig. 4, b-d) was substantially stronger than the immunofluorescence staining detected in the control (Fig. 4a), suggesting that the protein does exist also in the tail. The presence of actin has been reported in the tail of the spermatozoa in virtually all species studied thus far.