Improved preservation of rat epididymal sperm for high-resolution low-voltage scanning electron microscopy (HR-LVSEM)

Various fixation protocols were used in an attempt to improve preservation of rat epididymal sperm for high-resolution low-voltage scanning electron microscopy (HR-LVSEM). Wash solutions and fixatives of different composition and osmolarity were tested. Paraformaldehyde and glutaraldehyde concentrations were varied between 0.5% and 3%. Ruthenium red was tested as an additive in both primary fixation and postfixation, or in postfixation alone. HR-LVSEM revealed various degrees of ruffing, folding, blebbing, and peeling off of the plasma membrane, as well as holes of different sizes. The plasma membrane overlying the acrosome and the connecting piece proved to be particularly sensitive to varying fixation conditions. Consistent topographical differences were revealed among the different domains over the sperm head. Most of the differences were considered to be artifacts. Their consistency, however, suggests that structural and biochemical differences exist either within the membrane or in the structures subjacent to the membrane. Primary fixation turned out to be less critical than postfixation. Preservation of a smooth plasma membrane without holes could only be achieved when primary fixation in low aldehyde concentrations, with or without ruthenium red, was followed by postfixation with OSO4 and 1,000 ppm ruthenium red. Examination of thin sections of the same material confirmed that even a considerable number of small holes are difficult to detect in transmission electron microscopy. These results show that with the recent increase in resolution of LVSEM there is need for further effort to improve sample processing.