Expression of type-specific MHC isoforms in rat intrafusal muscle fibers.

Myosin heavy chain (MHC) expression by intrafusal fibers was studied by immunocytochemistry to determine how closely it parallels MHC expression by extrafusal fibers in the soleus and tibialis anterior muscles of the rat. Among the MHC isoforms expressed in extrafusal fibers, only the slow-twitch MHC of Type 1 extrafusal fibers was expressed along much of the fibers. Monoclonal antibodies (MAb) specific for this MHC bound to the entire length of bag2 fibers and the extracapsular region of bag1 fibers. The fast-twitch MHC isoform strongly expressed by bag2 and chain fibers had an epitope not recognized by MAb to the MHC isoforms characteristic of developing muscle fibers or the three subtypes (2A, 2B, 2X) of Type 2 extrafusal fibers. Therefore, intrafusal fibers may express a fast-twitch MHC that is not expressed by extrafusal fibers. Unlike extrafusal fibers, all three intrafusal fiber types bound MAb generated against mammalian heart and chicken limb muscles. The similarity of the fast-twitch MHC of bag2 and chain fibers and the slow-tonic MHC of bag1 and bag2 fibers to the MHC isoforms expressed in avian extrafusal fibers suggests that phylogenetically primitive MHCs might persist in intrafusal fibers. Data are discussed relative to the origin and regional regulation of MHC isoforms in intrafusal and extrafusal fibers of rat hindlimb muscles.

High incidence of long-chain fibers in spindles of cat superficial lumbrical muscles

Forty-two complete spindle poles of cat superficial lumbrical muscles were analyzed with particular regard to the length and the diameter of intrafusal fiber types. Poles were reconstructed from serial transverse sections of fresh-frozen muscles. The staining module, which was repeated throughout the whole muscle, comprised sections treated for glycogen detection and sections treated for detecting myofibrillar ATPase activity after preincubation at three different pH's (see METHODS). The identification of intrafusal fiber types was essentially based on the ATPase activity of the B region of the intrafusal fibers. Long-chain fibers, i.e., chain fibers that have at least one pole that extends by more than one millimeter beyond the end of the spindle capsule (6), were very commonly observed. Of 42 spindle poles analyzed, 30 (71%) contained at least one long-chain fiber (one in 17 spindle poles, 2 in 11 poles, and 3 in 2 poles). Of 246 poles of chain fibers, 45 (18%) were "long". In four spindles, in which both poles could be completely examined, 10 long-chain fibers were observed. In eight of these, only one pole was long; the opposite pole ended either intracapsularly or at a short distance outside the capsule. Since long-chain fiber poles, presently considered to be among the effectors of static skeletofusimotor (beta) axons, are present in a large proportion of muscle spindles of lumbrical muscles, it would be of particular interest to reevaluate the beta-supply of these muscles by physiological methods.

FINE STRUCTURE OF RAT INTRAFUSAL MUSCLE FIBERS

An ultrastructural study has been undertaken on the equatorial (sensory) region of the rat muscle spindle. Two kinds of intrafusal muscle fibers, a nuclear bag fiber and a nuclear chain fiber, have been identified in this region on the basis of fiber diameter, nuclear disposition, and M-band appearance. The large-diameter nuclear bag fiber contains an aggregation of tightly packed vesicular nuclei, while the small-diameter nuclear chain fiber contains a single row of elongated, well-separated nuclei. Both muscle fibers contain an attenuated peripheral cylinder of myofilaments surrounding a central core of sarcoplasm. Elements of the sarcotubular system, dilatations of the sarcoplasmic reticulum, and the presence of other sarcoplasmic organelles and inclusions are considerably more abundant in the nuclear chain fiber than in the nuclear bag fiber. Leptomeric organelles and membrane-bounded sarcoplasmic granules are present in both intrafusal fiber types and may be situated between the myofibrils or in intimate association with the sarcolemma. The functional significance of some of these structural findings is discussed.