Morphogenesis of rat muscle spindles after nerve lesion during early postnatal development

The influence of innervation on muscle spindle morphogenesis has been investigated in rat hind-limb muscles by sectioning the sciatic nerve, with suture of the stumps, at various postnatal stages. After nerve section at 4 or 7 days of age a proportion of spindles survived during the denervation phase and developed, during the subsequent reinnervation phase, into atypical structures. The reinnervated spindles were recognized by the presence of a limiting capsule but lacked the characteristic distinction of equatorial and polar regions. The intrafusal fibres were fewer than normal and were indistinguishable in size and fine structure from extrafusal fibres; they had a single motor endplate and lacked sensory nerve terminals. In reinnervated muscles of animals operated at 13 and 22 days of age there was a progressive tendency towards a restoration of normal spindle structure and innervation. These findings indicate that muscle spindle morphogenesis is profoundly altered by nerve lesion at early development stages, apparently as a result of inadequate sensory reinnervation. This study also shows that the differentiation of intrafusal fibres is dictated by their specific pattern of innervation and is not intrinsically predetermined.