New HCs adopt the IHC fate.

<p>(A–A') Confocal image of EGFP and Espin double staining at the top of the HC layer. Cochleae were dissected from <i>PLP/CreER^T+; Atoh1-HA+; CAG-EGFP+</i> mice given tamoxifen at P0 and P1 and analyzed at P9. Arrows show the same new HC with the straight line–shaped stereociliary bundles. (A') The same new HC viewed at the confocal YZ angle. (B–D) Whole-mount Fgf8 <i>in situ</i> hybridization of control cochleae at P0 (B), P8 (C), and P10 (D). (E) Whole-mount Fgf8 <i>in situ</i> hybridization in the cochlea from a <i>PLP/CreER^T+; Atoh1-HA+</i> mouse that was given tamoxifen at P0 and P1 and analyzed at P10. Arrows point to Fgf8+ cells. (F) Comparison of cell numbers from <i>in situ</i> hybridization of Fgf8+ cells and new HCs (Myosin VI+/Atoh1-HA+) at P10 showed no differences. (G–G’’’) Triple staining of Atoh1-HA, parvalbumin, and GFP in the cochlea of <i>PLP/CreER^T+; Atoh1-HA+; Fgf8-GFP+</i> mice at P10. Arrows target the new HC expressing Fgf8. OHCs: outer hair cells; IHCs: inner hair cells. Scale bars: 10 µm (A), 20 µm (B–E, G’’’).