[Cellular contacts in myelinated fibers of the peripheral nervous system]

Myelination and regional domain differentiation of the axon

During evolution, as organisms increased in complexity and function, the need for the ensheathment and insulation of axons by glia became vital for faster conductance of action potentials in nerves. Myelination, as the process is termed, facilitates the formation of discrete domains within the axolemma that are enriched in ion channels, and macromolecular complexes consisting of cell adhesion molecules and cytoskeletal regulators. While it is known that glia play a substantial role in the coordination and organization of these domains, the mechanisms involved and signals transduced between the axon and glia, as well as the proteins regulating axo-glial junction formation remain elusive. Emerging evidence has shed light on the processes regulating myelination and domain differentiation, and key molecules have been identified that are required for their assembly and maintenance. This review highlights these recent findings, and relates their significance to domain disorganization as seen in several demyelinating disorders and other neuropathies.

Syncytial coupling of neurons in tissue culture and early ontogenesis

Computerized time-lapse video recording was used to detect the process of formation of syncytial couplings between the processes of different neurons in dissociated neuron cultures. These studies showed that once the processes of one neuron had formed connections with another neuron, death of the cell body (its trophic center) was not followed by Wallerian degeneration. Translocation of cytoplasmic varicosities along the branches of one neuron to another was observed over periods of several hours. Electron microscopic studies of the nerve processes of cells in the intramural intestinal plexus in the early postnatal period demonstrated all the transitional states from fusion and perforation of the membranes of contacting dendrites to complete fusion of the neuroplasm of processes with formation of residual membranous structures at the location of the former intercellular contact.