a, Whole-mounted examples of varied location/pattern of spared epicardial cells in ventricles from tcf21:NTR; tcf21:nucEGFP adult clutchmates 3 days after incubation with 10 mM Mtz. White dashed lines, ventricle. b, Differential expression of the NTR transgene among cardiac chambers. In adult tcf21:NTR; tcf21:nucEGFP hearts, EGFP expression is comparable in epicardial tissue covering the atrium, OFT, and ventricle. By contrast, NTR (red, indicated by mCherry) expression is patchy and/or weak in atrium and OFT compared with ventricular expression. c, Section images of ventricles from tcf21:nucEGFP (left) or tcf21:NTR; tcf21:nucEGFP zebrafish (right) treated with 1 mM Mtz (right) for 3 days, and collected 2 days later. Ventricular epicardium was ablated effectively in these experiments. Scale bars, 50 μm.