Hair cell replacement in the avian inner ear following two exposures to intense sound

The present study is concerned with the degree to which the avian cochlea retains the capacity to regenerate hair cells following repeated exposures to intense sound. Two groups of chicks were exposed once to an intense pure tone for 48 h at either 2 or 16 days of age. In a third group, they were exposed to the same stimulus at both ages. Structural alterations of the auditory epithelium were assessed both qualitatively and quantitatively at 0, 12 or 26 days following the single exposure at 2 days of age, and at 0 or 12 days following the single or second exposure at 16 days of age. The numbers of hair cells lost in the twice-exposed birds and those exposed once at 2 days of age were approximately 24 and 36%, respectively, and were not significantly different. Interestingly, the single exposure at 16 days of age caused greater hair cell loss (61%). Twelve days after overstimulation, the hair cell population in all experimental groups returned to near normal levels due to the emergence of new hair cells. This observation of hair cell replacement extends the early findings that birds are able to repair their acoustically damaged ears after either a single or repeated overexposure.