Electron microscopic serial section analysis of nodes of Ranvier in lumbosacral spinal roots of the cat: ultrastructural organization of nodal compartments in fibres of different sizes

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Abstract
The general ultrastructural organization of nodes of Ranvier in peripheral nerve fibres from 2 to 20 μm in diameter (D) was investigated in the adult cat using serially sectioned ventral and dorsal spinal roots. The study was performed in order to collect and systematize information considered necessary for a morphometric analysis of the node of Ranvier. In all cases a node of Ranvier could be divided into a central nodal axon segment and a surrounding nodal Schwann cell compartment. The latter included a nodal gap matrix substance, more or less overlapping nodal Schwann cell collars and, as a rule, also a Schwann cell brush-border emanating from the nodal Schwann cell collars and occupying the nodal gap. The relative size and the organization level of the nodal Schwann cell compartment increased with increasing fibre size up to a fibre diameter of 8–10 μm. At this fibre size the nodal gap was of a fairly even height (1 μm) all around the nodal axon and contained a thick brush-border of densely packed, more or less radially arranged Schwann cell microvilli. In very small fibres (D < 3 μm) the nodal gap was low (10 μm in diameter the relative size and the degree of structural order of the nodal Schwann cell compartment decreased with increasing fibre size. Drastic sectorial variations in nodal gap height and local thinning-out of the brush-border became prominent features in the largest fibres. The possiblein vivo organization of the nodal Schwann cell compartment is discussed. Preliminary calculations indicate that the extracellular space directly surrounding the nodal axon might be quite small and that the area open for free communication between this extracellular space and the endoneurial space might be very much restricted, measuring as little as 2% of the area of the nodal axolemma. Algorithms for calculating various nodal structural parameters are discussed.