The re-expression of two myosin heavy chains in regenerated rat muscle spindles

The vascular supply and tendons of the extensor digitorum longus (EDL) muscles of two adult rats were unilaterally severed and the muscles were allowed to regenerate for 40 days. Serial frozen sections of muscle grafts were cut and stained for enzymes that delineated fiber type, sensory endings and motor endings. MF30 and ALD58, two antibodies which react only to intrafusal fibers in normal rat muscle, were reacted against sections of nerve-intact muscle grafts. Data were compared to that from muscles of normal rats. Encapsulated fibers devoid of sensory innervation and some extrafusal fibers in muscle grafts had a weak to moderate reaction to MF30, but no reaction to ALD58. Regenerated, encapsulated fibers with sensory innervation bound both MF30 and ALD58. These data indicate that afferents which reinnervate regenerated spindles retain the capacity to induce expression of spindle-specific myosin isoforms in rats.