Motor innervation of intrafusal fibers in rat muscle spindles: Incomplete separation of dynamic and static systems

Muscle spindle density is extremely high in the deep muscles of the human neck. However, there is a paucity of information regarding the morphology and immunoreactivity of these muscle spindles. The objective of this study was to investigate the intrafusal fiber content and to assess the myosin heavy chain (MyHC) composition of muscle spindles from human deep neck muscles. In addition to the conventional spindles containing bag1, bag2, and chain fibers (b1b2c spindle), we observed a number of spindles lacking bag1 (b2c spindle) or bag2 (b1c spindle) fibers. Both bag1 and bag2 fibers contained slow tonic MyHCs along their entire fiber length and MyHCI, MyHCIIa, embryonic, and α-cardiac MyHC isoforms along a variable length of the fibers. Fetal MyHC was present in bag2 fibers but not in bag1 fibers. Nuclear chain fibers contained MyHCIIa, embryonic, and fetal isoforms with regional variations. We also compared the present data with our previous results obtained from muscle spindles in human biceps brachii and the first lumbrical muscles. The allotment of numbers of intrafusal fibers and the MyHC composition showed some muscle-related differences, suggesting functional specialization in the control of movement among different human muscles.

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Retention of Pax3 Expression in Satellite Cells of Muscle Spindles

Journal of Histochemistry & Cytochemistry ◽

10.1369/jhc.2009.954792 ◽

2009 ◽

Vol 58 (4) ◽

pp. 317-327 ◽

Cited By ~ 18

Author(s):

Lisa J. Kirkpatrick ◽

Zipora Yablonka-Reuveni ◽

Benjamin W.C. Rosser

Keyword(s):

Skeletal Muscle ◽

Satellite Cells ◽

Latissimus Dorsi ◽

Muscle Spindles ◽

Latissimus Dorsi Muscle ◽

Intrafusal Fibers ◽

Dorsi Muscle ◽

Avian Muscle ◽

Anterior Latissimus Dorsi ◽

Immunohistochemical Techniques

Intrafusal fibers within muscle spindles retain features characteristic of immaturity, unlike the larger and more numerous extrafusal fibers constituting the bulk of skeletal muscle. Satellite cells (SCs), myogenic progenitors, are detected on the surfaces of both intrafusal and extrafusal fibers, but little is known of spindle SCs. We have recently demonstrated that, like their extrafusal counterparts, SCs in muscle spindles of posthatch chickens express paired box transcription factor 7 (Pax7) protein. During vertebrate embryogenesis, myogenic progenitors express both Pax7 and Pax3 proteins. In postnatal mice, Pax3 appears in rare SC subsets, whereas Pax7 is expressed by all SCs within extrafusal fibers. Here we test the hypothesis that Pax3 protein maintains localized expression within SCs of muscle spindles. Immunohistochemical techniques were used to identify SCs by their Pax7 expression within anterior latissimus dorsi muscle excised from posthatch chickens of various ages. A greater percentage of SCs express Pax3 within intrafusal than extrafusal fibers at each age, and the proportion of SCs expressing Pax3 declines with aging. This is the first study to localize Pax3 expression in posthatch avian muscle and within SCs of muscle spindles. We suggest that Pax3-positive SCs are involved in fiber maintenance.

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