Isolated primary and stem cell–derived cardiac myocytes. (A) Representative images of mononucleated (DAPI, blue) mouse neonate myocytes or mES- or miPS-CMs stained for actin (i) and α-actinin (ii). Bar, 10 µm. (B) Actin OOP as a function of cell type (n = 25, 20, and 9 for neonate, mES, and miPS, respectively). (C) Representative heat maps indicating peak systolic displacement (i) and traction stress (ii). (D) Peak systolic force as a function of cell type (n = 8, 7, and 7 for neonate, mES, and miPS, respectively). Results presented as mean ± SEM. *, P < 0.05.