Regeneration in the auditory system.

The auditory organs of birds and mammals normally stop producing sensory hair cells during embryonic development, so loss of those cells later in life results in hearing deficits that have been considered irreversible. In contrast to this, the ears of some fish and amphibians produce hair cells continuously throughout life and even increase in sensitivity. The lateral line organs in the skin of fish and aquatic amphibians also contain hair cells and have long been known to be replaceable through regeneration. Recently, it was discovered that after acoustic trauma or antibiotic poisoning, injured hair cells in the mature auditory organs of birds also could be replaced through regeneration. This is especially notable because it occurs in populations of cells that are mitotically quiescent in undamaged ears. More recent investigations have focused on identifying the cells that give rise to new hair cells during regeneration. In the lateral line organs of salamanders, time-lapse video microscopy has revealed that surviving supporting cells divide to give rise to progeny that can differentiate either as hair cells or as supporting cells. Definitive identification of the progenitors of regenerated hair cells in the avian cochlea awaits further investigation, but evidence that points to two possible candidate cell types is discussed.