Fusion of stereocilia on inner hair cells in man and in the rabbit, rat and guinea pig.

The complexity of the problem of sensorineural hearing loss is illustrated by the well-known poor correlation between pure tone thresholds and loss of sensory cells in the organ of Corti and between pure tone thresholds and other auditory functions such as discrimination of speech and thresholds of the intra-aural muscle reflex (MER). The present work demonstrates that two types of inner ear pathology are present in several mammalian species, including man. Both types can be induced by noise. In addition to the earlier well-known loss of outer hair cells, it is shown that abnormal sensory hairs on inner hair cells are an important and frequent finding in acoustically damaged ears. Fusion and inclination of inner hair cell cilia were predominant in rabbits with moderate hearing loss and a rather common finding in rats and humans. In guinea pigs, damage to inner hair cell stereocilia was not common and was seen only in conjunction with considerable loss of outer hair cells. In summary, fusion of the stereocilia of inner hair cells was a common finding in several mammalian species.