Ultrastructural analysis of ES-derived cardiomyocytes. (a) Transmission electron micrograph of sectioned beating EB 10 days after plating. Relatively unorganized myofibrillar bundles can be seen in some myocytes. (b) A cell 27 days after plating, displaying a more mature sarcomeric organization. (c) A different cell from the same EB as in b, demonstrating more organized sarcomeres and Z-bands (arrow). (d) High-power electron micrograph showing the presence of a gap junction (arrow) from a cell 16 days after plating. (e) High-power electron micrograph showing the presence of desmosomes (arrow) from a cell 16 days after plating.