Misconceptions about the inability of higher vertebrate hair ceils to regenerate have been dispelled, but the signals and factors that control hair-cell regeneration have still to be unravelled.

Our sense of balance and ability to hear depends upon mechanosensitive cells known as hair cells. The hair cells in the mammalian inner ear are distributed among six sensory epithelia - five vestibular and one audi- tory. These sensory epithelia are the three cristae of the semicircular canals, which1 detect angular acceleration, the maculae of the saccule and utricule, which act as gravity detectors, and1 the organ of Corti, which provides information abom complex sounds. Hair cells have a mechanosensitive hair bundle on their apical surface, and displacement of the bundle tip by just a few nanometers in one direction opens non- selective cation channels, depolarizes the cell, and causes the release of neurotransmitter onto the contact- ing afferent nerve fibres. Information about the mechanical stimulus is thereby relayed to the central nervous system. The hair cells in a typical mechanosen- sory epithelium are surrounded by, and separated from each other by the processes of supporting cells - non- sensory cells that sit on the underlying basal lamina (Fig. 1). The nuclei of the supporting cells lie basally whereas those of the hair cells sit closer to the lumenal surface of the epithelium. The apical ends sup- porting-cell processes are coupled to the hair cells by both tight and adherens-type junctions, the hair and supporting cells are arranged as a mosaic within the epithelium. This pattern of cellular organization is found in all vertebrate vestibular organs and also the auditory organs of lower vertebrates. Although the mammalian auditory epithelium, the organ of Corti, conforms to the same basic plan, the supporting cells exhibit considerable structural specialization. Hair cells are susceptible to damage from noise, drugs and infections, they also tend to be lost with age. A significant proportion of the human population suffers from deafness and balance disorders resulting directly from hair-cell loss. Up until only a few years ago, it was generally accepted that only the hair cells of lower ver- tebrates could be generated postembryonically, and that the prenatally produced hair cells of higher vertebrates,