A correlated thin-sectioning and freeze-fracturing study has been made of guinea pig and rat spermatozoa. In sections, the cell membrane over the acrosome has a concanavalin A and ruthenium red reactive glycocalyx which exhibits an ordered pattern related to the lattice of crystalline domains within the plane of the membrane revealed by freeze-fracturing. The cleaved acrosomal membrane also shows a finer linear periodicity in some areas.

The membrane over the equatorial segment of the guinea pig acrosome is marked by a palisade of oblique ridges not observed in the rat. The plasmalemma of the postacrosomal region is rich in membrane intercalated particles, many randomly dispersed, others clustered in rectilinear arrays. A particle-poor zone is found just anterior to the posterior ring. The fold of redundant nuclear envelope posterior to the ring has many nuclear pores in close hexagonal array. The nuclear envelope lining the implantation fossa is devoid of pores. When cleaved it has a particle-free central area surrounded by a broad zone of large, closely packed, hollow particles.

The membrane of the mid-piece in the guinea pig (but not the rat) contains linear strands of 6–8-nm particles oriented circumferentially. The membrane investing the principal piece exhibits the usual randomly distributed particles but in addition, a double row of larger (9 nm) particles runs longitudinally within the membrane over outer dense fiber 1. In the corresponding position in thin sections a local thickening of the membrane is discernible. These observations form a basis for further studies on the functional correlates of these regional specializations of the sperm membrane.

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