In vitro cardiomyocyte coupling model. (A) Neonatal cardiomyocytes (CMs) show greater contraction forces (arrows) than SC-CMs. Force generation at the lateral ends of the cells is depicted as a heat map. (B) Coupling of an immature SC-CM with a stronger nCM (bottom) results in extra focal adhesions (inset) compared with the homogeneous coupling of nCMs (top).