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Fig. 1 Progressive planar polarization of the zebrafish floor-plate during somitogenesis. (A) Experimental set-up used to study floor-plate planar polarization in fixed or live embryos. Early (4-12s)- or late (after 18s)-stage embryos, which display floor-plate cells with large apical surfaces, were usually imaged from the top (dorsal view, top; see also B), whereas embryos at intermediate stages (with narrower apical surfaces) were imaged from the side (lateral view; bottom). A p.i. (defined as p.i.=1−(a/b) where ‘a’ is the distance between the BB and the posterior membrane and ‘b’ the distance between anterior and posterior membranes) was used to quantify the BB position along the AP axis. (B,C) Time-course of floor-plate polarization between 6s and 26s. (B) Dorsal views of the floor-plate of flat-mounted embryos showing immunostaining against Centrin (green, BB), ZO1 (magenta, apical junctions) and acetylated-Tubulin (white, cilia) at 12s (left) and 26s (right). Cilia are already visible at 12s but are much longer at 26s. The yellow arrowhead indicates an anterior BB bearing a cilium. (C) Quantification of BB position measured from immunostained samples as shown in A. BB position along the anterior-posterior axis was quantified using the polarization index. Cells were allocated to different categories depending on their p.i. for each stage (6s: seven embryos, 108 cells; 8s: 14 embryos, 224 cells; 10s: 14 embryos, 354 cells; 12s: five embryos, 156 cells; 14s: nine embryos, 208 cells; 16s: nine embryos, 220 cells; 18s: five embryos, 143 cells; 26s: four embryos, 119 cells). ns, P>0.05; ***P<0.001; ****P<0.0001 (Wilcoxon rank sum test). Scale bar: 2 µm in B.