AXOPLASMIC ORGANELLES AT NODES OF RANVIER .1. OCCURRENCE AND DISTRIBUTION IN LARGE MYELINATED SPINAL ROOT AXONS OF THE ADULT CAT

Using light microscopy (LM) and electron microscopy (EM) we have exami ned the occurrence and distribution of axoplasmic organelles in large myelinated nerve fibres of the L(7) ventral and dorsal spinal soots of the cat with special reference to the paranode-node-paranode (pnp)-re gions. 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-p aranode-region. Further, a veil of Toluidine blue positive material wa s observed in about 50% of the paranode-node-paranode-regions. In abou t 25% of these paranode-node-paranode-regions the veil lay distal to t he midlevel of the paranode-node-paranode-region and in the remainder it lay proximally. Electron microscopy suggested that the ultrastructu ral equivalents of the dark-blue bodies and of the veil were dense lam ellar bodies and a diffuse granular material, respectively. Our calcul ations indicate that from 70% to more than 90% of some organelles (den se lamellar bodies, multivesicular bodies and vesiculo-tubular membran ous organelles) present in an axon are accumulated in the paranodenode -paranode-regions. The occurrence of these organelles in the individua l paranode-node-paranode-regions varied within wide limits also in adj acent fibres. The dense lamellar and multivesicular bodies dominated t he distal part of the paranode-node-paranode-regions while the vesicul e-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 par anode-node-paranode-regions analyzed ultrastructurally, seven were cla ssified as 'fully segregated', that is 67% or more of the lamellar and multivesicular bodies, present in the whole paranode-node-paranode-re gion, lay distal to the mid-level, and 67% or more of the vesiculo-tub ular membranous organelles lay proximal to it.