Structural reorganization of hair cells and supporting cells during noise damage, recovery and regeneration in the chick cochlea.

Exposure of the chick cochlea to intense acoustic overstimulation leads to rapid changes in the structural organization of hair cells and supporting cells. During the noise exposure the surface areas of the hair cells are reduced and, concomitantly, the adjacent supporting cells expand. If the exposure continues long enough hair cells bleb out apically and are ejected from the sensory epithelium. Surviving hair cells begin to expand their surface areas again within 24 hours after the termination of noise exposure. By 48 hours of recovery most of the hair cells that have died are ejected from the epithelium and new, regenerating hair cells begin to sprout immature stereociliary bundles. The regenerating stereociliary bundles undergo steps in their maturation that are equivalent to those seen during the normal embryonic development of hair cells. The extent and timing of recovery in the chick cochlea correlates with the length of the noise exposure. Shorter exposures (approximately 12 hours) result in only a few regenerating hair cells but the basilar papilla recovers almost completely within 14 days. Longer exposures (approximately 24 hours) lead to the production of more new hair cells but the recovery of the basilar papilla is incomplete, even after 28 days of recovery.