Proliferating cell nuclear antigen (PCNA) and SOX9 expression in the basal turn of W10.4 to W14 human fetal cochlea. (A-C) Basal turn of a cochlea immunostained for PCNA (red) and SOX9 (green) at (A) W10.4, (B) W12 and (C) W14. Cell nuclei were visualized with DAPI (blue). Bracket, the prosensory domain/OC; arrowhead, inner hair cell. Abbreviations: KO, Kölliker’s organ. Scale bars = 40 μm.