Fig. 5. Ultrastructural localization of filipin sterol complexes on freeze-fracture replicas of the head plasma membrane of fixed boar sperm cells. Freeze-fracture replica of a sperm cell in which the fracture plane is running across through the sperm head. Filipin/sterol complexes are only visible at sites where the plasma membrane is in the fracture plane, whereas acrosomal and nuclear membranes as well as the protamine condensed DNA (organized into multilammelar sheets) (Balhorn et al., 1999) are devoid of these particles.