Stereocilium injury mediates hair bundle stiffness loss and recovery following intense water-jet stimulation.

Inner ear hair cells exhibit many pathologies following exposure to intense sound, and the hair bundle is a major site of damage. This paper measures in vitro hair bundle motion on chick cochlear hair cells after intense in vitro and in vivo stimulation to explore the nature of hair bundle injury. Hair bundle stiffness, as well as relative and asymmetric motion of individual stereocilia, is controlled largely by the extracellular tip links, and a change in hair bundle motion was used to assess tip-link destruction following overstimulation. Intense in vitro stimulation caused a loss in stiffness that fully recovered within 10 min post-exposure. Relative and asymmetric stereocilia motion, however, were unchanged following the exposure, implying that tip links remained intact while the core or rootlet of the stereocilia were damaged and subsequently repaired. Intense and prolonged in vivo sound exposures produced stereocilia movements, measured in vitro, that were indicative of damage to stereocilia and tip links. Finally, the relative susceptibility of hair bundles to overstimulation was addressed by comparing stiffness loss with morphological features in the hair bundles. The loss of stiffness significantly increased as the amount of curvature in the hair bundle contour increased.