Fig. 4. Six1 controls proliferation of otic epithelial cells during early inner ear development. (A-D) Transverse sections of otic regions from E8.5 (A,B) and E9.5 (C,D) wild-type and Six1^-/- embryos showing BrdU-labeled cells (orange). BrdU-positive cells were slightly reduced in Six1^-/- otic placode (OP) at E8.5 (B) and by E9.5, BrdU-positive cells were largely reduced in the dorsal region of Six1^-/- otic vesicle (OV, above arrowheads in D). (E) The labeling index was determined by counting the number of BrdU-positive cells from each otic placode or vesicle. Ten ears for each genotype were counted and the numbers were averaged. At E8.5, the number of BrdU-labeled cells in Six1^-/- otic placode was 80% of wild-type embryos. By E9.5, the number of BrdU-labeled cells in Six1^-/- otic vesicle was reduced to 50% of that in wild-type embryos. In Six1^-/- embryos, the number of BrdU-positive cells in the lateral otic vesicle was reduced to 60% (E9.5L), while in the medial half the number was reduced to 40% of that in wild-type embryos (E9.5M). Scale bars: 50 µm.