Membrane particle changes attending the acrosome reaction in guinea pig spermatozoa

Abstract

To examine the freeze-fracture appearance of membrane alterations accompanying the preparation of sperm membranes for fusions - the first preparatory stage occurring before physiologic release of the acrosomal content, the second afterward - the authors induced the acrosome reaction in capacitated guinea pigs spermatozoa by adding calcium to the mixture. The most common features observed before fusion of the acrosomal and plasma membranes were the deletion of fibrillar intramembranous particles from the E-fracture faces of both membranes, and the clearance of globular particles from the P face of the plasma membrane - events taking place near the terminus of the equatorial segment. Large particles, >12 nm, remained not far from the cleared E-face patches. The P face of the outer acrosomal membrane is virtually clear from the outset. In addition, when fusion was completed, occasional double lines of large particles transiently embossed the P face of the plasma membrane (postacrosomal) side of the fusion zone. Behind the line of fusion, another series of particle-cleared foci emerged. Induction of the acrosome reaction in media containing phosphatidylcholine liposomes resulted in their apparent attachment, incorporation, or exchange in both the originally and secondarily cleared regions.