Chronic demyelination in mouse peripheral nerve produced by lysophosphatidyl choline and X-irradiation: ultrastructural observations.

The effects of X-irradiation on demyelination and remyelination were studied in the peripheral nerve of the mouse. Three days after injection of lysophosphatidyl choline into one sciatic nerve, a 20 Gy dose of X-rays was administered to the hind limb. At survival times ranging from 4 days to 6 months after injection, the nerves were examined by light and electron microscopy. Removal of myelin debris was retarded and remyelination delayed or prevented. The myelin sheaths which did form were thin and the configuration of Schmidt-Lanterman incisures and nodes of Ranvier was abnormal. Some of the chronically demyelinated fibres formed focal node-like complexes: patches of finely granular material coated the inner aspect of the axolemma, the external surface was covered by slender processes of Schwann cell cytoplasm, and an electron-dense lamina was present in the enlarged periaxonal space. Elsewhere demyelinated axons and their ensheathing Schwann cells were separated by gap junctions or transverse bands. The present findings indicate that the morphological differentiation of structures thought to be characteristic of nodes of Ranvier can take place in the absence of remyelination.