HISTOPATHOLOGIC OBSERVATIONS OF THE AGING GERBIL COCHLEA

Age-related histopathologic changes were examined in cochleas from 17 gerbils born and kept in a quiet environment until near the end of the ir life expectancy. Hearing loss varied greatly as did the loss of out er hair cells (OHC). Inner hair cells (IHC) were seldom missing even i n cochleas with severe hearing losses. Flask- and spherical-shaped OHC s were frequently seen in the apical turn. Stereocilia were usually pr esent and orderly on OHCs, but the tallest row of stereocilia on IHCs was often disarrayed and sometimes missing. Alterations in supporting cells were sometimes present in regions of extensive OHC loss. Althoug h pillar cells were seldom missing, the nuclei of outer pillar cells w ere commonly displaced from their normal basal position. The density o f radial fibers appeared similar to that in young gerbils except in th e apical turn of one old ear where a marked loss of radial fibers occu rred without an attendant loss of IHCs. All of the quiet-aged cochleas showed a characteristic clustering of epithelial cells lining the sca la media surface of Reissner's membrane. This structural rearrangement was not accompanied by a significant decrease in the total number of cells forming Reissner's membrane and did not appear to be associated with hearing loss. The findings confirm and extend earlier work showin g that several different types of cells are susceptible to histopathol ogic changes in old ears. The extent of histopathologic changes varied widely as did the degree of hearing loss in animals with a restricted genetic background and maintained under carefully controlled environm ental conditions. It was not possible, based on these initial findings , to relate specific structural to specific functional changes in the aging cochlea. Further light and electron microscopic analysis of othe r regions from these aged cochleas may provide more conclusive data.