Structure of cell coats during initial stages of synapse formation on isolated cultured sympathetic neurons

The mordant tannic acid was applied after routine aldehyde and osmium fixation in order to study the ultrastructure of membrane coat material during synaptogenesis in combination cultures of superior cervical ganglion neurons and spinal cord explants. All contact between cultured neurons was mediated by coat material; apposition of outer membrane leaflets never occurred. In most areas, the contact found between membrane coat material lacked special arrangement, but in areas of contact between spinal cord growth cones and sympathetic ganglion neurons, distinct patches of bilateral membrane specialization were seen. Here apposed coat material interdigitated across the intracellular gap, outer membrane leaflets were parallel and dense material was evident in the adjacent cytoplasm. These 'differentiated' contacts were seen both prior to and simultaneously with the appearance in the same area of other synaptic organelles; they did not appear to be precursors of synaptic active zones. Findings suggest that a mosaic of specificities in neuronal coat material may determine the site of 'differentiated' contacts and that these contacts may be relevant to the subsequent formation of a synaptic active zone.