Incorporation of [14C]glycine into protein of the adult rat peripheral nerve: effects of inhibitors

Myelin sheath remodelling in remyelinated rat sciatic nerve

In order to elicit de- and remyelination adult rat sciatic nerves were injected with diphtheria toxin dissolved in phosphate buffered saline (PBS). Control nerves were injected with PBS alone. After survival times of 1-10 weeks, the animals were perfused with glutaraldehyde. Specimens from the injected nerves were processed for light microscopic (LM) examination of teased fibres or for electron microscopic (EM) examination of longitudinal thin sections. LM examination of teased fibres after survival times of 6-10 weeks, showed that most remyelinated internodes are 150-300 microns long. In addition, some exceptionally short Schwann cell sheaths, with lengths of 15-150 microns, occur intercalated between conventional remyelinated internodes. EM analysis of thin sections showed that axonal evaginations penetrate in between the terminating myelin lamellae in fibres with nodal widening and/or paranodal demyelination, at early stages of demyelination. Such alterations are not present in relation to myelin sheaths formed during remyelination, which commences about 3 weeks after injection. In addition, some scattered contracted Schwann cell sheaths, which may be as short as 5-10 microns, occur at all stages. These are more frequent shortly after onset of remyelination than at later stages, and they are either composed of a cytoplasmic investment bordered by heminodes, or a more or less distorted myelin sheath bordered by nodes of Ranvier. This picture is very similar to the myelin sheath remodelling observed in regenerated rat sciatic nerves, and in some developing nerves with a mismatch between nerve growth and internodal elongation. It is concluded that a myelin sheath remodelling occurs in de- and remyelinated rat sciatic nerve, presumably as a result of the lack of longitudinal growth.

Radioautographic studies of choline incorporation into peripheral nerve myelin

This radioautographic study was designed to localize the cytological sites involved in the incorporation of a lipid precursor into the myelin and the myelin-related cell of the peripheral nervous system. Both myelinating and fully myelinated cultures of rat dorsal root ganglia were exposed to a 30-min pulse of tritiated choline and either fixed immediately or allowed 6 or 48 hr of chase incubation before fixation. After Epon embedding, light and electron microscopic radioautograms were prepared with Ilford L-4 emulsion. Analysis of the pattern of choline incorporation into myelinating cultures indicated that radioactivity appeared all along the length of the internode, without there being a preferential site of initial incorporation. Light microscopic radioautograms of cultures at varying states of maturity were compared in order to determine the relative degree of myelin labeling. This analysis indicated that the myelin-Schwann cell unit in the fully myelinated cultures incorporated choline as actively as did this unit in the myelinating cultures. Because of technical difficulties, it was not possible to determine the precise localization of the incorporated radioactivity within the compact myelin. These data are related to recent biochemical studies indicating that the mature myelin of the central nervous system does incorporate a significant amount of lipid precursor under the appropriate experimental conditions. These observations support the concept that a significant amount of myelin-related metabolic activity occurs in mature tissue; this activity is considered part of an essential and continuous process of myelin maintenance and repair.