Fig. 6. Model for the role of Notch signaling during hair and supporting cell differentiation in the cochlea. The diagram depicts on the left the signaling that takes place between precursor cells (Specification), and shows on the right the resulting cell fates (Differentiation) that are adopted as a result of this signaling in both wild-type (A) and Notch pathway mutant (B) cochleae. Red cells have reduced Notch activation and will ultimately differentiate as hair cells. The blue cells are those receiving sufficient Notch activation to prevent them from adopting the hair cell fate. The Notch-mediated inhibitory signal acts in two ways: (1) to inhibit the surrounding cells from adopting the hair cell fate; and (2) to suppress continued proliferation of the surrounding cells. In the Notch mutant cochleae precursor cells, the inhibitory signal is reduced. This leads to the production of supernumerary hair cells and the continued cell division of some precursor cells that then preferentially differentiate into supporting cells. HC, hair cell; SC, supporting cell.