Ultrastructure and innervation of regenerated intrafusal muscle fibres in heterochronous isografts of the fast rat muscle

Extensor digitorum longus (EDL) muscles from 2- to 28-day-old rats were gra fted into EDL muscles of adult inbred recipients (n = 8). At 1-6 months aft er the operation, experimental muscles were excised and the ultrastructure and innervation of regenerated muscle spindles was examined. Regenerated mu scle spindles (n = 36) in isografted EDL muscles contained 4.3 +/- 0.2 (mea n +/- SEM) encapsulated muscle fibres. These "intrafusal" muscle fibres lac ked nuclear bag and nuclear chain accumulations, which are characteristic o f normal muscle spindles; thus, they rather resembled thin encapsulated ext rafusal muscle fibres. In the same sample, myelinated axons were found in 3 3 (92%) muscle spindles, but no sensory terminals were found. These finding s demonstrate that regenerated spindles in isografted EDL muscles were not reinnervated by spindle-specific sensory axons, but exclusively by motor ax ons. Typical intracapsular motor endplates (MEPs) were found in one third o f regenerated spindles examined. Their motor terminals contained accumulate d mitochondria and synaptic vesicles. As is characteristic for MEPs, axolem ma and sarcolemma were separated by a synaptic cleft about 60 nm wide that contained a basal lamina. The underlying sarcolemma formed either small inf oldings or none at all, and the subsynaptic area contained only small subsa rcolemmal accumulations of mitochondria. It is apparent that the structures described here as "regenerated muscle spindles" do not perform their norma l physiological function as stretch receptors because they lack the sensory innervation. The present results show that regeneration and reinnervation in heterochronous isografts corresponds to that previously described in aut otransplanted free muscle grafts. The results also show that, during muscle spindle regeneration, intrafusal satellite cells develop into extrafusal-l ike muscle fibres, apparently due to their motor innervation.