Fig. 7. p21 expression in the developing inner ear and the phenotype of p21 knockout mice. p21 in situ hybridization; myosin VI and phospho-histone H3 immunostaining; Haematoxylin staining. (A,B) As shown in adjacent sections, p21 and myosin VI are expressed in utricular HCs (arrows) at E16.5. p21 is also found in the utricular non-sensory epithelium. (C) In the cochlea at E16.5, p21 is expressed in HCs and in stria vascularis, spiral limbus and cochlear ganglion neurons. (D,E) Histology of the utricular sensory epithelium of adult p21^-/- mice is indistinguishable from controls, as shown by Haematoxylin staining. (F,G) Histology of the organ of Corti of adult p21 knockouts is comparable with controls. (H) Mitoses are not induced in the cochleas of p21-null mutants, as shown at P2 by phospho-histone H3-staining (compare with control in Fig. 2E). Arrows in G,H indicate HCs. Abbreviations: wt, wild type; ut, utricle; co, cochlea; HE, Haematoxylin; SL, spiral limbus; CG, cochlear ganglion; SV, stria vascularis; IHC, inner hair cell; OHCs, outer hair cells; NSE, non-sensory epithelium; myo, myosin VI; ph3, phospho-histone H3. Scale bar: 70 µm for A-C,H; 80 µm for D-G.