Fig. 7. Neurogenic divisions have a longer cell cycle and FGF accelerates cell cycle. (A) Cell cycle times for divisions in control (untreated) and FGF-exposed slices. Box plot showing quartile ranges for cell cycle times of all control cells (PP+PN divisions, n=54); PP divisions (n=49); PN divisions (n=5); following exposure to FGF, all divisions (n=65); apically localized divisions (n=57); non-apical divisions (n=8). Statistical comparison was performed using the non-parametric two-tailed Mann-Whitney U-Wilcoxon rank sum test for pairwise comparison, as our data sets are not normally distributed; a P-value of <0.05 provides evidence that there is a 95% chance of a difference between compared data. PP compared with PN divisions, P=0.013; apical FGF compared with PP divisions, P=0.632; apical FGF compared with PN divisions, P=0.01; non-apical FGF compared with PP, P=0.048; with PN, P=0.002; and with apical FGF divisions, P=0.048). (B) Exposure to FGF produces only progenitor-generating divisions. A single cell located basally for 10 hours in the presence of FGF divides apically to produce two daughter cells (red and green dashed outlines), which exhibit attenuated interkinetic nuclear migration. Some nuclei also divide non-apically (blue dashed outline). MIP of 30 images captured at 1.5 µm intervals. Scale bar: 10 µm. (B') Lineage tree for cells represented in B. (See Movie 6 in the supplementary material.) (C) Comparison of cleavage plane orientation of divisions in control and FGF-treated slices, measured live from 1.5 days up to 38 hours. Error bars indicate s.d. (control: 84 divisions, 12 slices, three experiments; FGF-treated: 86 divisions, seven slices, three experiments.) FGF treatment does not result in a significant change in cleavage plane orientation. One-way ANOVA comparison of cleavage plane orientation of cells in live control and FGF-treated tissue (P=0.57), alpha level set at 0.05. (D) Proportion of cells dividing non-apically (>10 µm away from the apical surface) increases when tissue is treated with FGF. In fixed tissue 15±6% of mitotic cells are found away from the apical surface (505 cells, in 12 sections from six embryos); in live control tissue, 20±4% of divisions are non-apical (687 cells in 28 slices from 11 experiments). In FGF-treated tissue the proportion of non-apical divisions increases to 36±9% (300 cells, in 10 slices from six experiments). An ANOVA test between live control and live FGF-treated tissue provides evidence that there is a 95% chance of a statistical difference between these two populations (P=0.00004). Error bars indicate s.d.