Figure 2.

Cell-type-specific and overlapping precursors. Analysis of several null mutations suggest that there is an initial formation of two, partially overlapping, precursor populations, a neuronal precursor characterized by Neurog1 expression and a neurosensory precursor, characterized by Sox2 expression. The 40–80% reduction of hair cell and supporting cell formation in Neurog1 null mice suggests that the size of the common neuronal/neurosensory precursor population varies in different sensory epithelia. The later-expressed bHLH gene Neurod1 does not show this massive effect on hair cells and appears to be exclusively expressed in differentiating neurons. Absence of hair cell differentiation in Sox2 and Atoh1 null mice suggests that these genes are essential for hair cell formation, no matter what origin. Supporting cells depend on the hair-cell-mediated upregulation of Notch (and Hes) for their differentiation and will turn into hair cells in the absence of proper Notch/Hes signaling. Modified after Refs 39,49,61,64,120.