The organ of Corti is postmitotic in Hey2 mutants by E14.5. Adjacent cochlear sections from Hey2+/- (A-B, E-G) and Hes1-/-;Hey2+/- (C-D, H-J) at E14.5 (A-D) and E18.5 (E-J) were stained for antibodies against p27/Kip1 (A,C), BrdU (B, D), or doubly stained for Myosin6 and BrdU (E-J). Brackets in (A-D) indicate the primordial organ of Corti at E14.5 that is marked by the expression of p27/Kip1. Note that BrdU+ proliferating cells are at the region medial to the developing organ of Corti at this stage. Brackets and arrowhead (E-J) indicate the outer and inner hair cells, respectively. Note that no BrdU+ cells were detected in the organ of Corti following BrdU injections and a chasing period. Scale bar: 10 μm.