Substructure of the postacrosomal sheath of bovine spermatozoa

The substructure of the postacrosomal sheath and its relationship to the plasma membrane and nuclear membrane complex were examined in thin-section, negative-stain, surface-replica, and freeze-fracture preparations. The matrix of the postacrosomal sheath contains a single layer of closely associated 10- to 12-nm filamentous elements aligned parallel to the long axis of the sperm. A precise lateral interaction of the filaments is suggested from negative-stain images which reveal a second set of parallel striations extending over the surface of the sheath at 60 degrees relative to the filament long axis. Several structural differences between the posterior and anterior segments and the outer and inner surface of the postacrosomal sheath were identified. Data on structural specializations of the plasma membrane and nuclear membrane complex which relate to the asymmetric structure are presented and their potential significance in fertilization events discussed.