Postnatal maturation of spindles in deafferented rat soleus muscles

Whether the motor innervation can direct the morphological and histochemical differentiation of developing muscle spindles in the absence of sensory innervation was investigated by deafferentation of the soleus muscle in immature rats. Dorsal root ganglia containing the cell bodies of afferents from the soleus muscle were removed surgically at a stage of postnatal development when spindles already contain the full complement of intrafusal fibers innervated by both afferents and efferents, but when the fibers are histochemically and structurally immature. Experimental soleus muscles were excised one year after deafferentation and sectioned frozen at a thickness of 8 micron. Sections were stained for enzymes indicative of types of muscle fibers and sites of neuromuscular junctions, and were examined by light microscopy. Spindles of muscles that matured in the absence of sensory innervation were abnormal. They lacked the periaxial fluid space and contained fewer intrafusal fibers than did normal spindles. The morphological and histochemical profiles of the encapsulated fibers present in the deafferented spindles more closely resembled extrafusal rather than intrafusal muscle fibers. These observations suggest that deafferentation of the immature spindles induces disintegration of some intrafusal fibers and alters maturation of others. Moreover, motor axons terminated less frequently along muscle fibers in deafferented spindles than on intrafusal fibers of normal spindles. Thus, maintenance of a full complement of intrafusal fibers in the developing spindle, emergence of histochemical profiles typical of normal intrafusal fibers, and development of adult pattern of fusimotor innervation require intact sensory innervation.