Characterization of filaments within the subacrosomal space of rat spermatids during spermiogenesis

Two types of filaments were observed within the subacrosomal space of rat spermatids. The first of these types was characterized as actin by demonstration of actin filament affinity for myosin S-1 subfragments. Actin filaments were noted in the subacrosomal space shortly after the acrosomal sac made contact with the nucleus. As the acrosome increased its surface area contact with the spermatid nucleus, the number of layers of subacrosomal filaments increased. Pre-treatment with detergent, which in addition to permeablizing cells to allow entry of S-1, also caused the acrosome to vesiculate and the subacrosomal space to widen. In such preparations filaments were more easily visualized and appeared to extend between the nuclear and acrosomal membranes, indicating, but not proving, attachment to these membranes. During spermatid clongation, the number of actin filaments in the subacrosomal space increased greatly, especially over the dorsal convex region of the spermatid head. The polarity of the majority of filaments was not ascertainable since filaments were tightly packed within the narrow subacrosomal space. In late spermiogenesis (steps 18 and 19), actin filaments were no longer detected within the subacrosomal space. A second and much thicker type of filamentous structure was observed in the subacrosomal space of spermatids at steps 14-17 of spermiogenesis. About 14 nm in diameter (10-15 nm measurement range depending on fixation protocol utilized), these filaments did not decorate with myosin S-1 subfragments and were found in subacrosomal regions not containing actin. Fourteen nanometer filaments were seen in parallel array along the ventral folded portion of the nuclear membrane and extended partially around the nucleus. Like actin filaments. 14 nm filaments were not seen in the subacrosomal space during late spermiogenesis.