Electron microscopic findings in presbycusic degeneration of the basal turn of the human cochlea.

Three human temporal bones with presbycusis affecting the basal turn of the cochlea were studied by light and electron microscopy. Conditions in two ears examined by light microscopy were typical of primary neural degeneration, with a descending audiometric pattern, loss of cochlear neurons in the basal turn, and preservation of the organ of Corti. Ultrastructural analysis revealed normal hair cells and marked degenerative changes of the remaining neural fibers, expecially in the basal turn. These changes included a decrease in the number of synapses at the base of hair cells, accumulation of cellular debris in the spiral bundles, abnormalities of the dendritic fibers and their sheaths in the osseous spiral lamina, and degenerative changes in the spiral ganglion cells and axons. These changes were interpreted as an intermediate stage of degeneration prior to total loss of nerve fibers and ganglion cells as visualized by light microscopy. In the third ear the changes observed were typical of primary degeneration of hair and supporting cells in the basal turn with secondary neural degeneration. Additional observations at an ultrastructural level included maintenance of the tight junctions of the scala media despite loss of both hair and supporting cells, suggesting a capacity for cellular "healing" in the inner ear. Degenerative changes were found in the remaining neural fibers in the osseous spiral lamina. In addition, there was marked thickening of the basilar membrane in the basal turn, which consisted of an increased number of fibrils and an accumulation of amorphous osmiophilic material in the basilar membrane. This finding supports the concept that mechanical alterations may occur in presbycusis of the basal turn.