Stereocilia of sensory cells in normal and hearing impaired ears. A morphological, physiological and behavioural study.

The aim of the present study was to investigate specific structural alterations in the cochlea of ears with sensory neural hearing loss and to determine the relation between structural damage and loss of auditory function. The results show that with the methods used in this study it is possible to obtain well preserved structures with consistent morphological characteristics. Examination and identification of small pathological alterations were thus possible. The stereocilia of the IHCs were found by scanning electron microscopy to be the structure most susceptible to damage by noise in the rabbit. The extent of IHC damage corresponded rather well with the frequency range for loss of auditory function, particularly threshold shift of the MER. The damage of the IHCs consisted of fusion, fracture and inclination of the stereocilia towards the OHCs. The OHC were frequently left unaltered even when the stereocilia of most IHCs exhibited pronounced alterations. In animals with a severe hearing loss and a postexposure time of more than two months, long "giant" cilia and often also a small thin kinocilium were found on the IHCs. IHC ciliary damage was found also in other species including man, but to a smaller extent than in the rabbit. The stereocilia of IHCs were also damaged by noise in ears of rabbits where the OHCs had been experimentally removed prior to the noise exposure. It was concluded that stereocilia damage is an important morphological alteration in ears with sensory neural hearing loss particularly with NIHL, and that cilia damage contributes to the observed loss of auditory function.