Fig. 5

Fetal compact myocardium expands into pre-existing trabecular layer to form hybrid myocardial zone. a Immunostaining for tdTomato and TNNI3 on P7 Hey2-2A-CreER;R26-tdTomato heart sections. The boxed regions in the inner, middle, and outer myocardial wall are magnified. Tamoxifen was induced at E12.5. b Transverse sections of P7 heart from base to apex. c Quantification of the percentage of tdTomato⁺ cardiomyocytes (CM) in the inner, middle and outer myocardial wall. n = 4. d Immunostaining for tdTomato and TNNI3 on transverse sections shows different contribution of tdTomato⁺ CMs in the inner i, middle m, and outer o myocardial wall. e Cartoon figure showing the fate mapping of embryonic trabecular and compact myocardium into the postnatal stage. Yellow lines encircles hybrid myocardium. LV left ventricle, Pa papillary muscle. f, g Immunostaining for tdTomato, TNNI3, and Ki67 (f) or pHH3 (g) on P7 Hey2-2A-Cre;R26-tdTomato heart sections. Quantification of the percentage of proliferating cardiomyocytes (CM) in tdTomato⁺ or tdTomato⁻ populations. n.s. non-significant; n = 4. Scale bars, 500 µm (white) in a, d; 100 µm (yellow) in a, d; 100 µm in f, g