Transmission electron microscopy. (A) Murine late embryonic (E16.5), (B) neonatal, and (C) induced pluripotent stem cell-derived cardiomyocytes (iPS-CMs; day 18 of differentiation). For comparison, (D) shows cardiomyocytes in an adult mouse heart. Myofibrils with typical cross striations surrounded by mitochondria are present in all cells but are densely packed and organized only in adult cardiomyocytes. The morphology of iPS-CMs is similar to that of late embryonic/early neonatal cardiomyocytes. m, Mitochondria; n, nucleus; z, sarcomeric z-line.