Muscle spindle formation and differentiation in regenerating rat muscle grafts

Abstract

Muscle spindle development and function are dependent upon sensory innervation. During muscle regeneration, both neural and muscul.ar components of spindles degenerate and it is not known whether reinnervation of a regenerating muscle results in Ireestablishment of proper neuromuscular relationships within spindles or whether sensory neurons may exert an influence upon differentiation of these spindles. Muscle spindle regeneration was studied in bupivacainetreated grafts of rat extensor digitorum longus (EDL) muscles. Three types of EDL graft were performed in order to manipulate the extent to which regenerating spindles might be reinnervated: (1) grafts reinnervated following severance of their nerve supply (standard grafts); (2) grafts in which intact nerve sheaths appear to facilitate reinnervation (nerveintact grafts); and (3) grafts in which reinnervation was prevented (nonreinnervated grafts). Complete degeneration of muscle fibers occurred in all grafts prior to regeneration. Initial formation of spindles in regenerating EDL grafts is independent of innervation; intrafusal muscle fibers degenerate and regenerate within spindle capsules that remain intact and viable. The extent of spindle differentiation was evaluated in each type of graft using criteria that included nucleation and ATPase activity, both of which have been shown to be regulated by sensory innervation, as well as the number of muscle fibers/spindle and morphology of spindle capsules. While most spindles contained normal numbers of muscle fibers, most of these fibers were morphologically and histochemically abnormal. Alterations of ATPase activity occurred in all spindles, but were least severe in nerve-intact grafts. While fully differentiated nuclear bag and chain fibers were not observed in regenerated spindles, large, vesicular nuclei, similar to those of normal intrafusal fibers, were present in a small number of spindles in nerve-intact grafts. Sensory nerve terminations were observed only in those spindles that also contained the distinctive nuclei. This study suggests that a specific neurotrophic influence is necessary for regeneration of normal intrafusal muscle fibers and that this influence corresponds to the properly timed sensory neuron-muscle interaction which directs muscle spindle embryogenesis. However, the infrequent occurrence of characteristics unique to intrafusal muscle fibers indicates that reinnervation of regenerating muscle grafts by sensory neurons is inadequate and/or faulty.

Critical period in muscle spindle regeneration in grafts of developing rat muscles

“…Regenerated intrafusal fibres, however, had not differentiated into intrafusal fibre types and exhibited a reaction typical of extrafusal fibres. Similarly, as in most spindles in standard free autografts (Rogers and Carlson 1981;Rogers 1982;Soukup 1988;Cui and Walro 1989;Walro et al 1989;Soukup et al 1990a), afferent axons failed to reinnervate the spindles in our experiments. The lack of nuclear accumulations, the reduced size of the periaxial space and the larger diameters of regenerated intrafusal fibres also indicate that regenerated spindles in our material are like most regenerated spindles (except those found in nerve-intact grafts -48% of spindles according to Quick and Rogers 1983), devoid of the sensory innervation that is essential for their recovery (Zelenfi 1994) and become inappropriately reinnervated by extrafusal motor axons.…”

“…Within the preserved spindle capsules and inside the original intrafusal basal laminae which serve as a scaffolding for regeneration, intrafusal fibres regenerate from intrafusal satellite cells (Milburn 1976;Rogers 1982;Diwan and Milburn 1986). The myosatellites activated during the stage of fibre degeneration divide and fill the persisting basal laminae of the intrafusal fibres with myoblasts, which fuse to form myotubes and further mature into myofibres (Diwan and Milburn 1986).…”

“…Ischaemia, however, does not significantly affect spindle capsules in muscles of adult mammals (Barker and Milburn 1984;Diwan and Milburn 1986). Even after grafting, when both the vascular and nerve supply are disrupted, capsular lesions are very rare (Rogers 1982) and most spindle capsules survive in mature muscles of adult rats (Schmalbruch 1977;Rogers and Carlson 1981;Rogers 1982). Low resistance to ischaemia appears to be inherent in muscle spindle capsules up to the 15th day after birth, since massive capsular degeneration obviously follows muscle grafting.…”

“…Twenty-one spindles (75%) were composed of fibres of one type only, 5 spindles (18%) contained intrafusal fibres of fast 2B and 2A fibre types and 2 spindles (7%) of three fibre types (fast, mixed, slow) (cf. Rogers 1982). Fig.…”

Regeneration and recovery of cat muscle spindles after devascularization.

“…Similarly, though there is intramuscular degeneration of sensory and motor axons and their endings, the endoneurial tubes are left intact and serve to direct neural regrowth along the original pathways. Details of the processes undergone by extrafusal muscle fibres in the rat and cat have been described in numerous studies (see reviews by Carlson 1983Carlson , 1986) and similar information has more recently been provided for intrafusal muscle fibres in the rat (Rogers & Carlson, 1981;Rogers, 1982;Diwan & Milburn, 1986; see also review by Barker & Milburn, 1984). Since regenerated muscle spindles vary considerably in structure and innervation, Diwan and Milburn (1986) classified them into four groups according to degree of abnormality.…”