Interrelationship between growth factor and adhesion molecule in the formation of neuromuscular junctions and muscle apindles

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The morphodifferentiation of neuromuscular junctions and muscle spindles in the postnatal rats and Chinese hamsters was observed by ordinary electron microscopy and the interrelationship between basic fibroblast growth factor (bFGF) and neural cell adhesion molecule (N-CAM) during their formation and maturation was examined by immuno-electron microscopy.Motor endings at birth were polyneuronal and subneural apparatuses consisted of shallow cup-like depressions with a few incipient junctional folds. Poly-neuronal motor endings gradually eliminated until the 15th postnatal day and only a motor ending retained. Synaptic gutters became complex and contained increased junctional folds. On the other hand, in muscle spindles at birth, intrafusal myotubes established side-to-side contact with each other without the intercalation of a basal lamina. Sensory endings were confined to the outer surface of the muscle bundle. Thereafter, intrafusal myotubes separated into independent muscle fibers which were innervated by sensory endings and ensheathed in attenuated inner capsule cells.In the examination by immunoelectron microscopy, positive staining for bFGF was observed in both motor and sensory endings until adulthood. Moreover, fibroblasts among muscle fibers and outer and inner capsules of the muscle spindles were also bFGF-positive. On the other hand, N-CAM was detected at motor ending-muscle fiber contacts, but was not necessarily observed at sensory ending-intrafusal muscle fiber contacts.