Fig. 2

Alpha-actinin immunofluorescence and ultrastructure of human iPSC-derived skeletal myocytes on day 14 post-differentiation in culture. The left panel shows immunofluorescence labeling of α-actinin photographed at low (A) or high (B) magnification. The right panel depicts the ultrastructure of 14-day-old skeletal myocytes at increasing magnifications from C to E; the boxes delineate regions that are subsequently enlarged. A general view of the sarcomeric banding pattern is presented in C, followed by a detailed image of the different bands, as indicated. Myofibrils full maturity is attested by the presence of H-bands and M-lines. Multiple junctions between SR elements and T-tubules are illustrated in E; note the granular aspect of the SR lumen and the presence of regularly spaced (∼ 25 nm) electron-opaque feet-like structures between SR and T-tubules apposed membranes