Scanning electron microscopy of isolated peripheral nerve fibres

Summary The surface morphology of normal myelinated nerve fibres prepared in different ways for scanning electron microscopy has been studied and compared with the surface features of similar fibres undergoing retrograde changes. Nodes of Ranvier, paranodal specializations, artefactual fractures of the myelin, and the endoneurial collagen sheaths are described. A regular pattern of elevations, usually with a pitted or depressed surface seen on normal myelinated fibres after certain preparative procedures are thought to be artefacts produced during preparation and to be related to the neurokeratin network.Alterations in the surface structure of fibres central to long-standing nerve transections include irregular protuberances, serial surface corrugations and large swellings, all associated with demyelination. Fibres that have undergone retrograde degeneration consist of endoneurial tubes with focal swellings occupied by macrophages or myelin debris, together with fine unmyelinated and small myelinated regenerating axons. Strict centrifugal progression of myelination of regenerating axons was not observed.We thank Mr. R. A. Willis for his collaboration and for taking the SEM photographs of normal nerve fibres, and the Cambridge Scientific Instrument Co. Ltd. for permission to reproduce the SEM photographs of experimental nerve fibres. We also thank Dr. A. Boyde for access to his SEM and for helpful comments on interpretation of the scanning electron micrographs, Prof. J. Z. Young, Dr. P. K. Thomas, and Dr. R. H. M. King for discussion, and Messrs. P. Reynolds and D. Gunn for photography.A grant from the Muscular Dystrophy Group of Great Britain is gratefully acknowledged.