Ultra-high-resolution scanning electron microscopic studies on the sarcoplasmic reticulum and mitochondria of the rat intrafusal muscle fibers. Part II: The extracapsular region.

The three-dimensional structure of the transverse-axial tubular system, sarcoplasmic reticulum (SR) and mitochondria in the extracapsular 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 possessed thick to moderately sized column-forming mitochondria with occasional I-band branches. The transverse tubules (T-tubules) running at the level of the A-band side of the A-I junction were sandwiched between two large terminal cisternae for most of their length, forming triads. The sarcotubules arising from the terminal cisternae formed single-layered networks at the A-band level, and well developed, double-layered, three-dimensional networks at the I-band level. Nuclear bag1 fibers possessed 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 layered networks, but at the A-band level only a few longitudinally arranged sarcotubules and axial tubules were observed. Nuclear bag2 fibers contained medium to large diameter column-forming mitochondria without I-band branches. The T-tubules ran at the level of the A-band adjacent to the A-I junction, and were coupled at some intervals with terminal cisternae of various lengths. Swollen, large terminal cisternae were frequently seen.(ABSTRACT TRUNCATED AT 250 WORDS)