Fig. 4. Ultrastructural localization of filipin sterol complexes on freeze fracture replicas of the head plasma membrane of fixed boar sperm cells. (a) Freeze fracture replica of a large proportion of the plasma membrane of a boar sperm head with filipin labeling type A (Fig. 7A), with a high density of cholesterol-filipin complexes in the apical and equatorial subdomain and lower density in the post-acrosomal subdomain. (b) As in (a), but showing filipin labeling type B (Fig. 7B), with high density of cholesterol-filipin complexes in the pre-equatorial subdomain but no labeling in the equatorial and post-equatorial subdomain. The particles on this replica represent filipin cholesterol complexes. (c) Freeze-fracture replica of a non-labeled sperm cell from a DMSO control experiment showing the absence of the complexes, the small particles represent membrane-associated proteins. Note the pronounced neck region labeling in both labeled replica.