Morphological correlates of aging in the chinchilla cochlea.

The inner ears from 80 chinchillas ranging in age from premature to 19.2 years were examined as plastic-embedded flat preparations to determine the morphological changes associated with aging. Three of the four forms of human presbycusis defined by Schuknecht were found in the chinchillas. All animals had losses of sensory cells or sensory presbycusis. Inner (IHCs) and outer hair cells (OHCs) degenerated at a rate of about 0.29% and 1.0% per year, respectively. Age-related degeneration of inner (IPs) and outer pillars (OPs) occurred at a much slower rate. In four animals (5%) the dendritic processes of some of the spiral ganglion cells had degenerated in areas where the loss of sensory cells was minimal. This pathological change is likely equivalent to neural presbycusis. Six animals (7.5%) had regions of degeneration of the stria vascularis or strial presbycusis. The other common finding in the aging cochleas was the presence of lipofuscin or age pigment. Lipofuscin deposits were found to accumulate in the subcuticular region of OHCs, IPs and OPs, near the endolymphatic surfaces of many of the supporting cells and in the epithelial cells of Reissner's membrane. The IHCs accumulated much less lipofuscin. The morphological changes seen in the ears of aging chinchillas were qualitatively similar to those seen in the temporal bones of aging humans although the magnitude of the changes was considerably less. These results suggest that some of the damage found in aging human cochleas may be due to aging plus exposure to one or more ototraumatic agents.