Occurrence of fasciculated microtubules at nodes of Ranvier in rat spinal roots.

Fascicles of microtubules, previously considered to be unique to the initial segments of myelinated axons, were found at nodes of Ranvier of sensory and motor axons in rat spinal roots, though their occurrence was limited to the proximal portion of the axons. No fasciculated microtubules were noticed in the internodes of myelinated axons or in unmyelinated axons. In transverse sections, microtubules in fascicles were characteristically cross-linked by short filamentous strands at a centre-to-centre distance of 36-40 nm. In sensory axons, the density of fasciculated microtubules at the node of Ranvier was 45-48% at the level of the dorsal root ganglion and decreased progressively as thin sections were made more distal to the ganglion in both directions. In motor axons, microtubule-fascicles involved about 24% of the microtubules in the ventral rootlets, but they were rarely seen in distal portions of the ventral roots. No fasciculation of microtubules was discerned in the sciatic and saphenous nerves. These findings suggest that cross-linking protein(s) for fasciculation, which are synthesized in the perikaryon and primarily used for the initial segment, may be further transported to fasciculate microtubules at some proximal nodes of Ranvier.