Molecular specializations of the axon membrane at nodes of Ranvier are not dependent upon myelination

Abstract

Nodes of Ranvier from normal and 'dystrophic' mice have been examined with quantitative freeze-fracture electron microscopy. Regions of nodal, paranodal and interparanodal axolemma of normal fibres are clearly distinguishable on the basis of particle size distributions in electron micrographs of freeze-fractured replicas. Protoplasmic fracture faces of normal nodes of Ranvier, contain approximately 40% 100 A particles and about 25% elongated particles 150 by 250 A. Paranodal and interparanodal membranes contain a more uniform distribution of smaller diameter particles. 'Dystrophic', mice of the 129/ReJ-Dy strain have a genetic defect of Schwann cell development and myelinogenesis. Axons of the sciatic and deep peroneal nerves in dystrophic mice, which appear to be normally myelinated, possess approximately the same distributions of particles as axons in normal mice. However, in affected regions of the ventral and dorsal roots, Schwann cell wrappings may be missing, creating heminodes of Ranvier where the myelination terminates or begins again. At such heminodes, there is a circular band of axonal membrane which bears particles of sizes and packing densities similar to that found at normal nodes. High voltage electron microscopic examination of 0.25--1 micron thick sections from these hemi-nodal regions reveals the presence of a filamentous layer beneath the particle-rich membrane. In addition, completely amyelinated regions of root axons contain particle patches having size-density distributions similar to that of both normal and hemi-nodal membranes. Thus, the nodal membrane displays a characteristic particle-size distribution profile. The occurrence of this particle profile does not appear to be dependent upon the presence or absence of Schwann cells. These observations suggest that the functions subserved by the numerous particles at the node of Ranvier are not dependent upon myelination for their local differentiation within the axonal membrane.