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 described. 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 less damage to their stereocilia than the ordinary IHCs after short-term high-level noise exposure. There was a significant negative correlation between the number of extra IHCs and the size of the post exposure permanent threshold shift. There was no evidence of postexposure generation of extra IHCs in or near the region of hair cell damage. It was concluded that extra IHCs normally occur in rabbits and that they are less susceptible to noise trauma than ordinary IHCs. The negative correlation between extra IHC occurrence and noise susceptibility needs further analysis.