EXTRA INNER HAIR-CELLS - PREVALENCE AND NOISE SUSCEPTIBILITY

A total of 39 cochleae, 10 non-exposed and 29 noise exposed cochleae, were analyzed in a scanning electronmicroscope. Inner hair cells (IHCs ) localized on the modiolar side of the ordinary row of IHCs were desc ribed. The mean number of such extra IHCs was 11 per cochlea both for non-exposed and exposed ears, ranging from 0-37. They showed a bimodal distribution with a maximum number at apex and 6-9 mm from apex. The dimension of the cuticular plate was slightly but significantly larger than for the ordinary IHCs. The extra IHCs suffered significantly les s damage to their stereocilia than the ordinary IHCs after short-term high-level noise exposure. There was a significant negative correlatio n between the number of extra IHCs and the size of the post exposure p ermanent threshold shift. There was no evidence of postexposure genera tion of extra IHCs in or near the region of hair cell damage. It was c oncluded that extra IHCs normally occur in rabbits and that they are l ess susceptible to noise trauma than ordinary IHCs. The negative corre lation between extra IHC occurrence and noise susceptibility needs fur ther analysis.