During normal development, cells divide, then differentiate to adopt their individual form and function in an organism. Under most circumstances, mature cells cannot transdifferentiate, changing their fate to adopt a different form and function. Because differentiated cells cannot usually divide, the repair of injuries as well as regeneration largely depends on the activation of stem cell reserves. The mature cochlea is an exception among epithelial cell layers in that it lacks stem cells. Consequently, the sensory hair cells that receive sound information cannot be replaced, and their loss results in permanent hearing impairment. The lack of a spontaneous cell replacement mechanism in the organ of Corti, the mammalian auditory sensory epithelium, has led researchers to investigate circumstances in which transdifferentiation does occur. The hope is that this information can be used to design therapies to replace lost hair cells and restore impaired hearing in humans.