Phospho-histone H3 (pH3) immunohistochemistry in tal1, gata3, and gata2a mutants. Lateral views of pH3 (red) and GFP (green) immunohistochemistry in 24 h Tg(-8.1gata1:gata1-EGFP) WT sibling and mutant embryos as indicated (A,B,D,E,G,H). Top panel (1) in each case shows just the red channel and bottom panel (2) shows the merged image of both green and red channels. Tg(-8.1gata1:gata1-EGFP) embryos were used to help orientate to particular rows/cell type regions in the ventral spinal cord. Dorsal, top; anterior, left. Scale bar = 50 microns. Dashed lines indicate ventral limit of spinal cord. Mean number of pH3-positive cells in each D/V row of spinal cord region adjacent to somites 6–10 in WT and mutant embryos (C,F,I). All counts are an average of at least 3 embryos. Cell rows were assigned based on average cell diameters. Error bars indicate SEM. Statistically significant comparisons are indicated with brackets and stars. *P < 0.05, **P < 0.01. For P-values see Supplementary Table 6.