AVIAN COCHLEAR HAIR CELL REGENERATION - STEREOLOGICAL ANALYSES OF DAMAGE AND RECOVERY FROM A SINGLE HIGH-DOSE OF GENTAMICIN

Hair cell regeneration after acoustic trauma has been conclusively doc umented in birds. Previous studies of aminoglycoside ototoxicity have typically used 5-10 day courses of drug to damage the cochlea and trig ger regeneration, This long-term lesion prevented analysis of the earl y events of regeneration. We set out to determine how much damage woul d occur and how recovery would proceed after a single high-dose inject ion of the aminoglycoside gentamicin. White Leghorn chicks were given a single high dose of gentamicin (100 mg/kg). Three post-injection sur vival groups with age-matched controls were studied: short-term (3-5 d ays), intermediate-term (2 weeks) and long-term (5 weeks). After sacri fice, cochleae were dissected and processed for scanning electron micr oscopy. Using stereological techniques, a quantitative analysis of coc hlear hair cell counts along the proximal 50% of the cochlea was perfo rmed from scanning electron micrographs on 4-7 chicks from each group. Variable degrees of damage were seen 3-5 days after the drug injectio n. All hair cells were lost from the proximal 20% of the cochlea in al l chicks. This complete hair cell loss could extend to 50% of the coch lea. Immature appearing hair cells could be first identified by their immature stereocilia at 3 days, Immature appearing hair cells were pre sent in greatest number in regions which had been denuded of native ha ir cells and in regions where partial loss occurred. Interestingly, im mature appearing hair cells also occasionally appeared in adjacent are as in which there was no apparent loss of native hair cells. Two-week survivors showed an elevation in hair cell number compared to controls in regions which had sustained damage and immediately adjacent region s. This elevation implies that an overproduction of hair cells might o ccur as part of the regeneration response. By 5 weeks after damage hai r cell numbers approximated controls.