Synchronization of hair cell regeneration in the chick cochlea following noise damage

Pure-tone overstimulation for prolonged time leads to hair cell death in frequency-specific regions of the cochlear epithelium. Unlike mammals, birds replace missing hair cells by stimulating mitosis in an uncharacterized precursor cell. Regenerated hair cells, initially identifiable by their immature stereocilia and small surface areas, differentiate into mature cells in a manner which parallels embryonic development. In the current study, we examined whether hair cell regeneration is initiated during noise exposure or after the end of acoustic trauma. We exposed 7-to 15-day-old chicks to a 1500 Hz pure tone at 120 dB SPL (re 20 muPa) for 4, 12, and 24 hours and examined the recovering cochlear epithelium with scanning electron microscopy to determine when regenerated hair cells were first identifiable. The earliest evidence of new hair cells appeared roughly 96 hours after the onset of 4-, 12-, and 24-hour exposures. Our previous studies initially identified new hair cells 96 hours after the start of a 48-hour exposure. Therefore, hair cell regeneration follows a similar time course relative to the onset of noise exposure, regardless of the ultimate duration of exposure. Since we estimate that hair cells take at least 48 hours after their genesis to form immature stereocilia, the signal which induces hair cell precursors to re-enter the cell cycle and to divide probably has its initial effects very early during the exposure period. (A previous report of these data was given at the 1991 American Society for Cell Biology conference.)