Figure 7. Proliferation of TopCreER; Rosa^ZsGreen labeled otic progenitors.

Transverse sections through the inner ears of TopCreER; Rosa^ZsGreen embryos exposed to tamoxifen at E7.75 and collected at E9.5 (23–25 somites). EdU was administered one hour prior to embryo harvest. (A–C) GFP (D–F) EdU (G–I) merged. (J–K) Bar graphs display mitotic indices of otic progenitors. Proliferation rate of GFP⁺ cells is equivalent in the dorsomedial (Cre+) (above dashed line) and ventromedial domains and does not differ from the rate for all DAPI⁺ cells. However, the mitotic index of otic progenitors is significantly increased on the dorsomedial (Wnt responsive) compared to dorsolateral (Wnt unresponsive) sides of the otic vesicle, particularly in medial (P=0.0045) and anterior regions (brackets in E, F; P=0.0012, unpaired Student’s t-test). The intercalation of proliferating cells (GFP⁻EdU⁺) between GFP⁺ progenitors may also contribute to their ventral displacement (arrows in I). GFP⁺ cells are occasionally observed several cell diameters from their nearest GFP⁺ neighbor (arrow head in G), consistent with migratory behavior. Abbreviations: DM, dorsomedial; VM, ventromedial; and DL, dorsolateral. Error bars represent s.e.m.