Intercellular contacts between chick stereocilia after acoustic overstimulation

Replacement of mammalian auditory hair cells

We investigated the potential for hair cell regeneration in neonatal rat organs of Corti grown in culture following destruction of hair cells by neomycin toxicity. Replacement hair cells were observed by light and scanning electron microscopy in lesion sites in the cultures treated with transforming growth factor-alpha, epidermal growth factor or a combination of both factors for 5-7 days post-injury. These new cells had morphological characteristics of immature hair cells. Autoradiographic localization of [3H]thymidine-labelled cells on semi-thin sections indicated that these replacement hair cells did not arise through renewed mitotic division, although an important mitotic proliferation was observed outside the area of supporting and sensory cells.

Re-innervation patterns of chick auditory sensory epithelium after acoustic overstimulation

There is evidence from several studies showing that sensory cells which are destroyed by trauma in the chick auditory epithelium are replaced by new cells. The fate of neurons that innervate the injured and degenerating sensory cells in the lesion, and the temporal sequence of re-innervation of regenerated hair cells are not well understood. This study examined efferent terminals in the chick auditory sensory epithelium following acoustic overstimulation using synapsin-specific immunocytochemistry. Chicks were exposed to an octave band noise (1.5 kHz center frequency, 116 dB SPL, 16 h) and killed on each day from 0 to 9 days postexposure. In the proximal half of control whole mounts of the basilar papillae, synapsin-specific immunoreactivity stained efferent terminals throughout the abneural portion of the sensory epithelium (the short hair cell region). In this area, the labeling appeared as 2-3 bouton-shaped clusters along the abneural edge of each hair cell. After acoustic overstimulation, a lesion was observed at the abneural edge of the papilla where many short hair cells were lost. The center of the lesion was located at 40% distance from the proximal end of each traumatized papilla. Synapsin-specific labeling was not found in sites where expanded supporting cells had replaced missing hair cells. Hair cells which survived the trauma exhibited a shrunken apical area, and synapsin-labeled boutons were observed near their basal domains. New hair cells, which first appeared in the papilla 4 days after trauma, were not initially associated with synapsin-labeled boutons. Regenerated hair cells first displayed contacts with synapsin-labeled boutons 7 days after trauma. Nine days after acoustic overstimulation, most new hair cells appeared to be associated with synapsin-labeled boutons which resembled the normal horseshoe configuration of efferent terminals. The data suggest that direct contact with functional efferent synapses is not necessary for the generation and differentiation of new hair cells.

Return of auditory function following structural regeneration after acoustic trauma: behavioral measures from quail

After measuring baseline behavioral audiograms, three of four behaviorally trained quail and fifteen untrained cohorts were exposed to a 1.5-kHz octave-band noise at 116-dB SPL for 4 h. The trained birds were tested daily following the exposure and showed a steady recovery of absolute sensitivity with a return to normal absolute thresholds by post-exposure days 8-10. Thirteen untrained cohorts were sacrificed after various survival times to evaluate the structural condition of the ear. The cohorts all showed regeneration of sensory cells similar to that seen in chicks. The effects of repeated acoustic trauma on recovery of sensitivity were evaluated by re-exposing the three trained birds and two untrained cohorts 106 days after the first exposure. One of the trained birds was exposed a third time, 113 days following the second exposure. The findings demonstrate that, following acoustic trauma, normal sensitivity returns prior to complete structural regeneration of the sensory epithelium and that repeated acoustic trauma may increase the time course of recovery of normal hearing sensitivity.

Return of auditory function following structural regeneration after acoustic trauma: behavioral measures from quail

After measuring baseline behavioral audiograms, three of four behaviorally trained quail and fifteen untrained cohorts were exposed to a 1.5-kHz octave-band noise at 116-dB SPL for 4 h. The trained birds were tested daily following the exposure and showed a steady recovery of absolute sensitivity with a return to normal absolute thresholds by post-exposure days 8-10. Thirteen untrained cohorts were sacrificed after various survival times to evaluate the structural condition of the ear. The cohorts all showed regeneration of sensory cells similar to that seen in chicks. The effects of repeated acoustic trauma on recovery of sensitivity were evaluated by re-exposing the three trained birds and two untrained cohorts 106 days after the first exposure. One of the trained birds was exposed a third time, 113 days following the second exposure. The findings demonstrate that, following acoustic trauma, normal sensitivity returns prior to complete structural regeneration of the sensory epithelium and that repeated acoustic trauma may increase the time course of recovery of normal hearing sensitivity.