Death pathways in noise-damaged outer hair cells

Using morphological criteria, death pathways in outer hair cells (OHCs) were determined in chinchilla organs of Corti that had been exposed to a high- or moderate-level octave band of noise (OBN) centered at either 0.5 or 4-kHz. The specimens were part of our large collection of plastic-embedded flat preparations of chinchilla cochleae. Three death pathways were identified: (1) oncotic – swollen, pale-staining cell with a swollen nucleus, (2) apoptotic – shrunken, dark-staining cell with a pyknotic nucleus and (3) a newly defined third pathway – no basolateral plasma membrane but cellular debris arranged in the shape of an intact OHC with a nucleus deficient in nucleoplasm. To minimize the secondary loss of OHCs from the entrance of endolymph into the organ of Corti, the specimens used for quantitative analysis of death pathways had the following characteristics: (1) the level to which they were exposed was less than or equal to 95 dB SPL, (2) the exposure duration was 6–216 h, (3) fixation for microscopic examination took place in vivo 1–2 h post-exposure and (4) there were no focal OHC lesions in the organs of Corti. Fifty-eight noise-exposed cochleae met these criteria. In these specimens, degenerating and missing OHCs were classified as to which death pathway the cells had followed or were following. Nine non-noise-exposed cochleae were also evaluated for OHC death pathways. The number of OHCs following the third death pathway was significantly greater in the noise-exposed cochleae than the non-noise-exposed cochleae for total exposure energies greater than those produced by 75 dB SPL for 216 h to a 0.5-kHz OBN and 57 dB SPL for 48 h to a 4-kHz OBN. In cochleae exposed to either octave band, OHCs dying by oncosis or apoptosis were uncommon.