Atrial and ventricular cardiomyocytes differentiated from rat HAP stem cells had structures of mature cardiomyocytes.

(a) Double immunostaining of cTnT (red)- and MLC2v (green)-positive images, cTnT in the I-band and MLC2v in the A-band showed an alternating pattern of sarcomeres. Scale bars; 10 μm. (b) Double immunostaining of cTnT (red)- and β-catenin (green)-positive images of ventricular cardiomyocytes differentiated from HAP stem cells. Nuclear staining with DAPI (blue). Scale bars; 10 μm. (c-h) Transmission electron microscopy images of cardiomyocytes differentiated from HAP stem cells. (c, d) Non-supplemented cardiomyocytes. Microfibrils with scattered Z-bands (Z) and small-sized mitochondria (Mt). (e-h) Atrial cardiomyocyte (e, f) and ventricular cardiomyocyte (g, h) differentiated from HAP stem cells. Microfibrils with regularly Z-bands (Z). A-bands (A), I-bands (I), H-bands (H) and M-bands (M) were also organized. Large mitochondria (Mt) with complex internal structures. Ventricular cardiomyocytes had T-tubule-like structures (TT) and sarcoplasmic reticulum (SR). N: nuclear. (c, e, g) Scale bars; 2 μm. (d, f, h) Scale bars; 1 μm.