Axoplasmic organelles at nodes of Ranvier. I. Occurrence and distribution in large myelinated spinal root axons of the adult cat.

Using light microscopy (LM) and electron microscopy (EM) we have examined the occurrence and distribution of axoplasmic organelles in large myelinated nerve fibres of the L7 ventral and dorsal spinal roots of the cat with special reference to the paranode-node-paranode (pnp)-regions. Ninety-eight percent of the 550 Toluidine Blue-stained paranode-node-paranode-regions examined in the light microscope contained dark-blue bodies accumulated distal to the midlevel of the paranode-node-paranode-region. Further, a veil of Toluidine Blue positive material was observed in about 50% of the paranode-node paranode-regions. In about 25% of these paranode-node-paranode-regions the veil lay distal to the midlevel of the paranode-node-paranode-region and in the remainder it lay proximally. Electron microscopy suggested that the ultrastructural equivalents of the dark-blue bodies and of the veil were dense lamellar bodies and a diffuse granular material, respectively. Our calculations indicate that from 70% to more than 90% of some organelles (dense lamellar bodies, multivesicular bodies and vesiculo-tubular membranous organelles) present in an axon are accumulated in the paranode-node-paranode-regions. The occurrence of these organelles in the individual paranode-node-paranode-regions varied within wide limits also in adjacent fibres. The dense lamellar and multivesicular bodies dominated the distal part of the paranode-node-paranode-regions while the vesiculo-tubular membranous organelles dominated the proximal part, i.e. the organelles showed a mutual proximo-distal segregation with reference to the midlevel of the paranode-node-paranode-region. Of seventeen paranode-node-paranode-regions analyzed ultrastructurally, seven were classified as 'fully segregated', that is 67% or more of the lamellar and multivescular bodies, present in the whole paranode-node-paranode-region, lay distal to the mid-level, and 67% or more of the vesiculo-tubular membranous organelles lay proximal to it.