High resolution scanning electron-microscopic study on the three-dimensional structure of the sarcoplasmic reticulum in the slow (tonic) muscle fibers of the frog, Rana nigromaculata.

The three-dimensional structure of the sarcoplasmic reticulum (SR) in the slow (tonic) fibers of the rectus abdominis muscle of the Japanese meadow frog (Rana nigromaculata nigromaculata Hallowell) was examined by high resolution scanning electron microscopy, after removal of the cytoplasmic matrices by the osmium-DMSO-osmium procedure. The SR forms a repetitive network throughout these fibers. At the level of the Z-line, a slender transverse tubule (T-tubule) runs transversely to the longitudinal axis of the myofibril. Small, spherical or ovoid terminal cisternae couple laterally with the T-tubule at intervals of 0.4-1.0 microns, and form a "terminal cisterna-T-tubule complex" on whose surface tiny indentations are occasionally seen. Each terminal cisterna gives rise to a few sarcotubules that run in various directions, divide frequently and form circular or oval meshes of diverse sizes in front of the A- and I-bands. The sarcotubules usually form small meshes in the middle of the A-band, but occasionally fuse and form a poorly developed H-band (fenestrated) collar.