Actin, particularly F-actin, has been identified within the subacrosomal space of developing spermatids. Collectively, these authors emphasize that F-actin decreases with the completion of the spermatid’s development until it becomes virtually undetectable by the time mature spermatids are released from the seminiferous tubules. This suggests that the older the spermatid, the more abundant the G-actin and the less abundant the F-actin. Assuming that the principal role of cortical F-actin is to act as a physical barrier that could rigidify the structure that it accompanies, namely, cell membranes, the predominance of one form of actin over the other within the subacrosomal space could influence the shaping of the acrosome to meet specific physiological and structural requirements.
Not surprisingly, scinderin is found in germ cells in which actin has been reported, that is, within a region that corresponds to the subacrosomal space of developing spermatids. In vitro, scinderin has been reported to bind to both G-actin and F-actin. Under basal conditions, when intracellular Ca2+ ([Ca2+]i) is low, 90-95% of scinderin is cytoplasmic, while 5-10% is associated with membrane phospholipids, but when [Ca2+]i is high, most of the scinderin becomes associated with actin and the phospholipids. Therefore, one could anticipate that the changes in scinderin distribution reported here in the spermatids and in the spermatozoa during and following their epididymal transit might have been dictated by [Ca2+]i-in-duced changes in the form of actin.