Fig. 6. Mitosis, apoptosis and multinucleation of hair cells of mgRb:Rb^-/- mutants at E18.5, as shown in semi-thin sections. (A) Normal utricular sensory epithelium shows HCs at the lumenal surface and underlying supporting cells. (B) A mutant utricle shows disorganized cell layering and HCs with an atypical nucleus or with two or more nuclei. Some of these HCs show near-normal stereociliary bundles. Some HCs seem to extend from the surface deep into the epithelium. (C) Mitotic cells with rounded morphology and nascent stereocilia (arrow) at the lumenal surface are intermixed with HCs with aberrant nuclear morphologies. A putative supporting cell that has translocated to the surface for mitosis is marked by an arrowhead. (D) Mitotic HCs (short arrows) are found mostly at the lumenal surface, but also in deeper layers, such as in this view where a cell undergoing mitosis is penetrating through the basal lamina. Scattered apoptotic profiles are also found (long arrows). (E,F) A mitotic cell (short arrow) showing signs of degeneration in the cytoplasm. Mitotic cells occasionally showed apoptotic figures (long arrow). (G,H) In contrast to controls, the ampullary sensory epithelia of mutants contain mitotic cells (arrows). In addition, several HCs with abnormal nuclei are found and many of them have stereociliary bundles with a morphology comparable with controls. (I,J) A transverse view through the organ of Corti of a normal cochlea shows four HCs and the underlying supporting cells. In the region of organ of Corti of mutants, high numbers of nuclei are seen, some of them having mitotic figures (short arrows) and a few apoptotic profiles (long arrow). Abbreviations: wt, wild type; ut, utricle; amp, ampulla; co, cochlea; LU, lumen, BL, basal lamina; ME, mesenchyme; HCs, hair cells; SCs, supporting cells; IHC, inner hair cell; OHCs, outer hair cells; P, pillar cell; Ds, Deiters' cell, H, Hensen's cell. Scale bar: 15 µm for A-J.