Origin of the membrane for the acrosomal process: is actin complexed with membrane precursors?

The formation of the acrosomal process requires an increase of 40-150% in the plasma membrane limiting the cell body. In freeze fracture replicas and thin sections this new membrane behaves as a typical unit membrane. Since in sperm there are no vesicles or surface projections which could account for the increase in surface, we suggest that the new membrane is generated from precursors stored with the actin in the periacrosomal region. Consistent with this suggestion is the observation that compounds such as didansyl cysteine and N phenyl-1-napthylamine which fluoresce when they enter a hydrophobic environment stain the periacrosomal material.