Transmission electron micrographs showing white skeletal muscle (A–C) and myocardial muscle (D and E) from adult zebrafish. Zebrafish striated muscle was prepared under relaxing (A and C) or rigor (B, D, and E) conditions before fixation. Known sarcomeric structures (thin and thick filaments; Z-disks; A-, I-, and H-bands; and M-line), SR, T-tubules only at Z-disks (arrows in A–C), and ghosts of permeabilized mitochondria (D and E) are clearly observed. The inset in A shows grossly distended profiles of SR surrounding the A-band of a myofibril in a rosetta-like arrangement in cross section. In D, an intercalated disk, specific to cardiac muscle tissue, is indicated by an asterisk, but there are no T-tubules in the myocardium. For comparison with cardiac tissue from adult zebrafish, the ultra-structure of a myocardial section from murine embryo (embryonic day 11.5) is shown in F (unpublished data). SL of the skeletal muscle from adult zebrafish is 1.8 µm in relaxing buffer (A), 1.5 µm in rigor buffer (B), and 1.6 µm in the cardiac muscle under rigor conditions (D and E). Bars: A, B, and D–F, 1.0 µm; C, 0.1 µm.