ABSTRACT
Hair cells (HCs) are the mechanoreceptors, i.e., biological devices converting mechanical energy (e.g., sound pressure or acceleration) into electrochemical energy (i.e., neurotransmission), that are involved in the detection of sound, balance, orientation, and movements. HCs are present in the lateral lines and inner ears of fish and amphibians, in the basilar papilla and vestibule of birds, and in the cochlea and vestibule of mammals. Disease, aging, infection, and exposure to noise or ototoxic drugs cause HC loss. Age and trauma-induced susceptibility is a major health problem because a significant proportion of humans suffer from deafness or balance disorders directly resulting from HC loss. In contrast, HC production is an ongoing phenomenon in the sensory maculae of fishes (14), amphibians (5), and in the vestibular epithelium of birds (2, 6-8). In the auditory and vestibular epithelia of the mammalian inner ear and the auditory epithelium of birds, i.e., the basilar papilla, the production of HCs occurs over only a brief period during the early stages of development (9-12). However, in the basilar papilla, lost HCs following drug or noise-induced damage are replaced through a regenerative process (reviewed in Ref. 13).
