Fine structure of the synaptic endings between sympathetic axons and skeletal muscle cells and of the varicosities in the bundles of neurites in tissue culture.

In cocultures of skeletal muscle and sympathetic ganglia from chick embryos, synaptic boutons on skeletal muscle cells and varicosities in the neuritic bundles were observed electron microscopically. Synaptic endings on skeletal muscles were bulbous. Most synaptic boutons simply made contact with muscle fibers, but some boutons appeared as concave invaginations into the sarcolemma. In the neuritic bundles, numerous varicosities were observed. Close approximations were found between the synaptic boutons and skeletal muscles and between the varicosities and neurites (dendrites or axons), but no membrane thickening nor subsynaptic infolding was observed in either synapse; in our cultures the contacts were characteristic of the autonomic nervous system. In both synapses, three variations were demonstrated by KMnO4 fixation after 5-hydroxydopamine incubation: (i) containing predominantly small dense-core vesicles (noradrenergic type), (ii) predominantly small clear vesicles (cholinergic types), and (iii) a mixture of both small dense-core and small clear vesicles (mixed type). Cytochemically, the varicosities in the neuritic bundles were predominantly noradrenergic at 1 week in culture, and both the synaptic boutons on skeletal muscles and the varicosities in the neuritic bundles contained a mixed population of small dense-core and small clear vesicles at intermediate times, with a gradual shift to cholinergic characteristics. These findings strongly suggest that in this culture system some sympathetic neurons or fibers become cholinergic (neuronal plasticity).