Ultra-high-resolution scanning electron microscopic studies on the sarcoplasmic reticulum and mitochondria of the rat intrafusal muscle fibers. Part I. The capsular sleeve region.

The three-dimensional structure of the transverse-axial tubular system, sarcoplasmic reticulum (SR) and mitochondria in the capsular sleeve region of the intrafusal muscle fibers of the rat lumbrical muscle was observed by ultra-high-resolution scanning electron microscopy after removal of the cytoplasmic matrix by the osmium-DMSO-osmium procedure. Nuclear chain fibers were seen to have slender I-band limited mitochondria, and thick column-forming mitochondria which occasionally exhibited I-band branches. The transverse tubules (T-tubules) ran at the level of the A-band side of the A-I junction and were sandwiched between two large terminal cisternae for most of their length, forming triads. The sarcotubules arising from the terminal cisternae formed single-layer networks at the A-band level, and well developed, double-layer, three-dimensional networks at the I-band level. Occasionally, tetrads and pentads were also seen. Nuclear bag2 fibers possessed column-forming mitochondria of medium size, practically devoid of I-band branches. The T-tubules ran at the level of A-band adjacent to the A-I junction and were coupled with terminal cisternae of various lengths at intervals. As a rule, the SR formed single-layered networks at the level of the A-band and double-layered networks at the I-band level, and was less developed than in chain fibers. Nuclear bag1 fibers contained slender column-forming mitochondria devoid of I-band branches. The T-tubules were located at the level of the A-I junction. Both the T- and the axial-tubules were occasionally coupled with small terminal cisternae, forming dyads or triads. At the I-band level, the SR was well developed and formed single layer networks, but at the A-band level only a few longitudinally arranged sarcotubules and axial tubules were recognized.