In utero changes of the plasma membrane in the head region of the rat spermatozoa

The plasma membrane of the head of the rat spermatozoa was observed by deep-etch, surface replica, and freeze-fracture methods. The spermatozoa were from three experiments: 1) incubation in Hepes buffer, 2) natural coition, and 3) insemination. The spermatozoa used for experiments 1) and 3) were obtained from the ductus deferens. No change in ultrastructure was observed in the incubation experiment. Randomly oriented rows of particles were observed on the plasma membrane of the ejaculated spermatozoa in the deep-etch replicas. The surface replicas showed a similar structure on the plasma membrane. The surface of the inseminated spermatozoa was smooth. This study shows that the plasma membrane changes in the uterus.