Fig. 3. Mitosis and differentiation of vestibular hair cells of mgRb:Rb^-/- embryos. Myosin VI- and phospho-histone H3-immunostaining. (A,A') When compared with controls at birth, utricular sensory epithelia of mutants are hyperplastic because of an excess of myosinVI-positive HCs, some of which have penetrated into the mesenchyme (arrows). (B,B') In contrast to the saccular sensory epithelium of controls in which HCs occupy the lumenal layer, HCs are distributed throughout the epithelium in mutants. (C,C') In contrast to controls, ampullary sensory epithelia of mutants are hyperplastic because of overproduction of myosin VI-positive HCs, some of which invade the mesenchyme (arrow). (D) An oblique section through the utricular sensory epithelium of a mutant shows mitotic figures (arrows) in myosin VI-stained HCs. (E) A cross-section through the utricle shows that mitotic HCs (arrows) have rounded shape and occupy both lumenal and deeper epithelial layers. (F-H') Phospho-histone H3-stained utricular (F,F'), saccular (G,G') and ampullary (H,H') sensory epithelia at birth show higher numbers of mitotic cells in mutants when compared with controls. (I,I') At E14.5, early-differentiating saccular HCs of mutants undergo mitoses, in contrast to controls. Differentiating HCs occupy the lumenal layer and have a large nucleus. Abbreviations: wt, wild type; myo, myosin VI; ph3, phospho-histone H3. Scale bar: 120 µm for A,A',F,F'); 70 µm for B-C',G-I'; 30 µm for D,E.