The Effects of Low Temperature on Myelin Formation in
Optic Nerves of Xenopus Tadpoles
The differentiation of myelin by glial cells is under the cell-cell influence of neurons. One proposal to explain this interaction was that neurons synthesized either a myelin component(s) or a regulatory factor that was transported out of the neuronal cell body, moved down the axon using a microtubule-dependent mechanism and released to influence the surrounding glial cells.
Using cold block as a means of inhibiting axonal transport, the myelinating capability of amphibian oligodendrocytes was assessed using ultrastructural methods. The low temperature conditions greatly decreased the number of axonal microtubules (the substrate for the transport mechanism). The compact myelin sheath was unaffected by the cold block, even over an extended period of cold block during the period of normally rapid myelination. However, during cold block the cytoplasmic regions associated with myelin assembly and maintenance (e.g., the paranodal loop regions and inner and outer tongue processes) filled with vesicular membrane profiles. These vesicles were not produced by chemical agents that inhibit axonal transport or by nerve transection. Thus, there was no evidence that axonal transport contributed to the maintenance of the myelin sheath during development.
The alteration in the cytoplasmic regions of the myelinating oligodendrocytes appeared to be due to a local effect on glial cells due to the cold treatment rather than an effect on reduced axonal transport or axonal degeneration. One such local effect could be inhibition of glial protein synthesis. See: Cullen, M.J. and H.deF. Webster. 1989. Inhibition of protein synthesis during CNS myelination produces focal accumulations of membrane vesicles in oligodendrocytes. J. Neurocytology 18:763-774.