Hair Cell Regeneration: Mechanisms Guiding Cellular Proliferation and Differentiation

Hair cells (HCs) are vertebrate sensory mechanoreceptors that detect signals from the environment associated with hearing, balance, body orientation, and external water movement. They are derived from special cranial placodes (e.g., otic placode, lateral line placodes) that are transformed into the membranous labyrinth of the inner ear or the lateral line neuromasts during development. HCs are located along with nonsensory supporting cells in specific sensory epithelia in the inner ear of terrestrial vertebrates and also in neuromasts of the lateral line in aquatic animals. Loss of HCs or HC function typically leads to severe sensory deficits. In humans, many deafness-causing mutations that affect HC structure and organization have been identified (Cryns and Van Camp 2004). Epigenetic factors, such as intense or prolonged noise, ototoxic drugs, and certain microbial infections, also lead to HC damage and loss. The effects of HC loss are most pronounced in the elderly human population, of whom as many as 65% experience sensorineural hearing loss and 30% experience vestibular problems such as vertigo. Because HCs are not spontaneously replaced in humans once they are lost, hearing and balance deficits caused by HC loss at any stage of life are irreversible.