Sensory regeneration in the vertebrate inner ear: differences at the levels of cells and species.

The ears of nonmammalian vertebrates are capable of regenerating sensory hair cells after acoustic trauma or ototoxic injury. In contrast, the mammalian inner ear lacks regenerative ability and the loss of hair cells results in permanent deficits in hearing and balance. Comparative observations across all vertebrate classes suggest that regenerative ability was a stem trait and was lost during the course of mammalian evolution. This review provides an overview of regeneration and post-embryonic growth in the vertebrate ear. It is suggested that the lack of regeneration in the mammalian ear was the result of a trade-off between phenotypic plasticity of supporting cells and sensitive high frequency hearing.