Cell surface structure of rodent sperm heads.

Replicas of critical point dried rodent sperm were examined by transmission electron microscopy. The surface of rat sperm heads appeared to be coated with regularly spaced 90A lamellar material. The plasma membrane overlying the acrosomal region of guinea pig spermatozoa displayed a regular scalloped array of lamellar structures. In replicas, the surface of Chinese hamster spermatozoa appeared coated by an array of small tubles and vesicles in the region overlaying the acrosome. It was possible to obtain replicas of the outer acrosomal membrane by removing the plasma membrane of mouse spermatozoa by brief treatment with Hank's balanced salt solution containing 0.2-0.5% MgC12. Replicas of the surface of the outer acrosomal membrane reveal evenly spaced, hexagonally-packed 90A particles similar to those which have been observed by other workers in replicas of freeze fractured outer acrosomal membranes. The finding of hexagonally arranged structures on the surface of the outer acrosomal membrane which appear very similar to those which have been observed in the plane of the membrane suggests that protein molecules in the plane of the membrane may protrude through or in some other manner distort the membrane surface. The post-acrosomal region of mouse sperm displays parallel lamellae with 100A spacing. If surface changes occur on sperm heads during maturation or capacitation, it should be possible to detect them with this technique.