Turn-specific and pigment-dependent differences in the stria vascularis of normal and gentamicin-treated albino and pigmented guinea pigs.

The aims of the present study were to determine which structures in the stria vascularis (SV) may depend upon the presence of pigmented melanocytes both for normal morphology and for the expression of gentamicin ototoxicity in the inner ear. These pigment-dependent influences were inferred through comparisons of the SV in pigmented guinea pigs and in albinos containing nonpigmented melanocytes. Results were obtained from 6 albino and 8 pigmented guinea pigs given gentamicin, and from 3 albino and 3 pigmented control animals not receiving the drug. One-month old animals received gentamicin daily (100 mg/kg) for 14 days and recovered for an additional 14 days before being prepared for electron microscopy. The SV from each of the 4 cochlear turns was analyzed using stereological point counting procedures. In control animals, differences were found in the higher cochlear turns, where volume density for the marginal cells in albinos was abnormally large (turns 3 and 4), while the volume density for intermediate cells (melanocytes) was abnormally small (turn 3). Cell volume estimates for the intermediate cells were significantly smaller in the albino than pigmented control animals in the higher cochlear turns, indicating that functional abnormalities may be found in the albino cochlea. In animals exposed to gentamicin, marginal cell volume density was reduced significantly in turn 4 of albinos, but not in any region of the pigmented inner ears. Radial area of SV and estimates of the absolute volumes for marginal cells in albinos given gentamicin also were significantly reduced in turn 1 compared to their controls; such differences were not observed in the pigmented animals. The results indicate that marginal cell size is significantly reduced in albino but not pigmented animals 14 days after gentamicin exposure, and further suggest a role of pigmented melanocytes in ameliorating gentamicin-induced cochlear damage.