Epidermal growth factor upregulates production of supernumerary hair cellsin neonatal rat organ of Corti explants

The organ of Corti is highly ordered, with a single ron of inner hair cells and three rows of outer hair cells. The number of hair cells produced was thought to be limited by the time of their terminal mitosis (i.e. E14 in th e mouse). However, exogenous application of retinoic acid has been shown to stimulate the formation of supernumerary hair cells in organ of Corti expl ants from E13 to E16 mouse embryos. Using late embryonic and neonatal rat o rgan of Corti explants, we investigated the potential for production of sup ernumerary hair cells in more mature auditory sensory epithelia. When newbo rn rat organ of Corti explants were cultured under control conditions, an a rea of supernumerary hair cells was observed in a segment of organ of Corti that was at the junction between the basal and middle turns. In these area s of supernumerary hair cells the number of hair cells increased per unit o f length, but remained constant per surface unit. further demonstrating the supernumerary character of this phenomenon. Organ of Corti explants treate d with epidermal growth factor (EGF) showed a 50% increase in the length of the organ of Corti segment containing supernumerary hair cells. Upregulati on of supernumerary hair cell formation by EGF was found to start and be ma ximal at birth (P0) and to disappear by 2 days after birth (P2). Treatment of EGF stimulated P0 explants with an antimitotic drug, cytosine arabinosid e (ARAc), demonstrated that the production of supernumerary hair cells occu rred independently of cell division.