Regional localization of RNA and protein metabolism in Schwann cells in vivo.

Schwann cells, which form and maintain extensive myelin sheaths, have the bulk of their lipid and protein synthesis restricted to the compact 'perinuclear' zone at the centre of the internode. Using teased fibre and quantitative electron microscopical autoradiography, we demonstrated that additional protein synthesis takes place in the lengthy processes of Schwann cell cytoplasm. This 'so-called' superficial cytoplasmic channel network forms a branching and anastomozing array that stretches between the perinuclear region and the distant paranodes. Protein synthesis apparently does not extend from this surface network into the Schmidt-Lanterman incisures or paranodal loops that circumscribe compact myelin. To maintain protein synthesis in these lengthy processes, Schwann cells transport a portion of their RNA along the superficial cytoplasmic channels at a rate (0.1 mm per day) that appears to be slightly lower than the transport rate reported for RNA along dendrites of hippocampal neurons in culture (0.5 mm per day). Nearly a week is required for labelled RNA to be transported from the Schwann cell nucleus to the paranodal terminals of the longer channels. The existence of this extended protein synthesis is not limited to myelinating Schwann cells. Schwann cell processes associated with small calibre axons also appear to synthesize some of their own proteins as the RNA needed to catalyze local translational events is transported into these processes.