Figure 5. Ultrastructural analyses and drug responses further confirm net-shaped CM maturation. (A) Transmission electron microscopy images of human stem cell-derived CMs 15 days after cardiac differentiation. Scale bar: 1 μm; sarcomere: white triangle. (B) Pharmacological responses of hiPSC-CMs to cardiac-active compounds. Upper panel: nisoldipine and nifedipine; lower panel: verapamil and isoproterenol. All data were normalized to the sheet-day 15 group and are expressed as means±S.E.M. * Statistically significant differences between individual groups (n≥3; *P<0.05, **P<0.01, ***P<0.001).