(A) Schematic of pig heart stages utilized for this study. (B) Heart weight/body weight (HW/BW in g/kg) ratios were calculated for pigs at P0–6mo. (C, D) Expression of cardiac contractile protein fetal and mature isoforms at P7–6mo in pig left ventricles, with fold change relative to P0. (E) Representative images of Connexin-43 (Cx43) staining of gap junction proteins. Yellow arrows show change in Cx43 expression from lateral punctate staining at P0-P15 to co-localization at CM termini at P30–6mo. Data are mean ± SEM, with *p<0.05, **p<0.01, ***p<0.001, ****p<0.0001 determined by Dunn’s Kruskal-Wallis Multiple Comparisons Tests, in n=4–8 pigs per stage. Asterisk(s) with underline indicate significance compared to P0.