Scanning electron microscopy of age-related changes in the C57BL/6J mouse cochlea.

Hair cells and nerve fibers inside the organ of Corti of the C57BL/6J mouse, which is known as the precocious presbycusis model, were studied using the scanning electron microscope. For this study, we used thick serial sections cut from celloidin blocks. In the 5-week-old mice, hair cell loss was not seen. The upper tunnel radial fibers crossed the upper part of the tunnel of Corti and entered the Nuel's space between the outer pillar cells. The fibers varied in diameter and showed many varicosities. The basilar fibers emerged into the tunnel of Corti beneath the upper tunnel radial fibers and crossed the floor of tunnel slightly curving basalward. The outer spiral fibers ran along the lateral wall of the Nuel's spaces, sometimes buried in the cytoplasm of Deiters' cells. The nerve endings were clearly seen on the modiolar sides of the outer hair cell bases in the basal turn. In the 30-, 42- and 60-week-old mice, hair cell degeneration was seen both at the basal and apical portions, more pronounced in the former. The outer hair cells were affected more than the inner hair cells. In the basal turn where most of outer hair cells had degenerated, the upper tunnel radial fibers disappeared while the basilar fibers remained. These results suggest that degeneration of the efferent fibers occur earlier than those of the afferent to the outer hair cells.