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Fig. 4 Increased Hh signaling expands ventricular and atrial populations. (A-D) Dorsal views, anterior towards the top. (A,B) vmhc expression at the 18-somite stage and (C,D) amhc expression at the 22-somite stage are expanded in shh-injected embryos. (E,F) At 48 hpf, shh-injected embryos often exhibit enlarged hearts, as indicated by MF20/S46 immunofluorescence. Scale bars: 100 μm. (G) Quantification of cardiomyocyte number in wild-type and shh RNA-injected embryos at 52 hpf. Values represent the mean cell number for each category (±s.d.). Asterisks indicate statistically significant differences from wild type (P≤0.0005).