Fig. 1. COUP-TFI^-/- cochlea exhibit supernumerary hair cells and Deiter's supporting cells. (A) Flat-mounts (a-f) and cross-sections (g-j) of wild-type (a,c,e,g,i) and COUP-TFI^-/- (b,d,f,h,j) cochlear duct at P10, stained with Alexa 488-conjugated phalloidin (a-f), anti-myosin VIIa (g,h), and Hematoxylin and Eosin (H&E, i,j). Extra inner hair cells (IHC) were seen in the basal (compare b with a) and apical (compare f with e) regions, and extra outer hair cells (OHC) were seen in the middle (compare numbered rows in d with c) and apical (compare rows in f with e) regions of COUP-TFI^-/- cochlea. OHC stereociliary bundles were often misoriented in COUP-TFI^-/- cochlea (direction of arrows is perpendicular to the kinocilium in f). (g,h) Cross-sections through the apical region of the cochlear duct show the typical pattern of one row of IHC and three rows of OHC expressing myosin VIIa (red) in the wild type, but the COUP-TFI^-/- mutant has supernumerary hair and support cells (arrowheads). Nuclei were counterstained with DAPI (blue). (i,j) H&E-stained sections of apical regions show individual supporting cells underlying every hair cell (arrowheads, compare j with i). Scale bar: 15 µm. (B) Paint-filling of wild-type (a,c,e) and COUP-TFI^-/- (b,d,f) cochlear ducts at E13, E15 and E17 show that COUP-TFI^-/- mutants have a shorter cochlear duct. Scale bar: 100 µm. (C) Graph of hair cell counts (IHC and OHC) from different regions of the cochlear duct at P10, showing more hair cells in the apical region of the COUP-TFI^-/- duct than in the wild type (^*P<0.05).