Unusual ultrastructural findings in dendrites of pyramidal neurons in the cerebral cortex of rabies-infected mice

Previous studies using the Golgi technique have demonstrated alterations in the dendritic morphology of pyramidal neurons of the cerebral cortex of mice inoculated with the rabies virus. However, knowledge about the fine structure of dendrites in rabies infection is scarce. This work had the aim of studying the ultrastructure of dendrites in cortical pyramidal neurons of rabies-infected mice. Mice were inoculated intramuscularly with a street rabies virus of canine origin. The animals that showed an advanced stage of disease were fixed by perfusion with glutaraldehyde and paraformaldehyde. Brains were removed and cut on a vibratome to obtain coronal slices of 200 micrometers of thickness. Vibratome slices were subjected to the following treatment: postfixation, dehydration, embedding in epoxy resin and polymerization between glass slides. Ultrathin sections of oriented tissue fragments from the cerebral cortex were obtained and observed under electron microscope. The most significant ultrastructural findings were located within distal dendrites of cortical pyramidal neurons: loss of mitochondria, disorganization and loss of microtubules, formation of vacuoles interrupting the continuity of the cytoplasm and formation of myelin-like figures. These strange myelin figures, which apparently had not been reported in previous studies of rabies, were the most noticeable ultrastructural feature. They also differ from the best known myelin figures formed by concentric lamellae. The possible origin of these myelin figures as result of mitochondrial degeneration is discussed.