Tbx20 overexpression significantly increases cardiomyocyte number. (A–H) Dorsal views of nkx2.5, mef2ca, scl and myl7 expression in non-transgenic (A, C, E and G) and Tg(hsp70l:Tbx20) embryos (B, D, F and H). The expression of nkx2.5 and mef2ca is expanded in Tbx20-overexpressing embryos at the 10-somite stage (A–D), whereas the expression of scl is similar to that of control embryos (E and F). Similarly, myl7 expression is enhanced in Tg(hsp70l:Tbx20) embryos at the 16-somite stage (H). (I and J) Representative confocal projections of myl7:nucGFP hearts from heat-treated non-transgenic (I) and Tg(hsp70l:Tbx20) embryos (J) at 40hpf. (K) Graph of the average number of nucGFP-positive cardiomyocytes in heat-treated non-transgenic (n=8) and Tg(hsp70l:Tbx20) embryos (n=10). Tbx20 overexpression enhanced cardiomyocyte production. (L) Graph of the ratio of BrdU-positive cardiomyocytes to the total number of cardiomyocytes in heat-treated non-transgenic and Tg(hsp70l:Tbx20) embryos (n=7). Overexpression of Tbx20 in early embryos promoted cardiomyocyte proliferation. Error bars indicate standard deviations. Asterisks indicate a significant difference (**p<0.01, ***p<0.001).