Intra-axonal ferric ion-ferrocyanide staining of nodes of Ranvier and initial segments in central myelinated fibers.

Ferric ion and ferrocyanide were used to stain central nervous tissue from the spinal cords of rats following fixation in cacodylate-buffered aldehydes. At the nodes of Ranvier in myelinated fibers, the stain was localized primarily on the inner surface of the unmyelinated nodal axolemma, as had been reported previously for peripheral nodes. Unmyelinated initial segments of myelinated neurons were similarly stained, but the axon hillock, cell body and dendrites were not stained. Synapses also exhibited no staining. Details of stain localization and comparison of these results with other ultrastructural data suggest that the stain is specific for the node of Ranvier and the axon initial segment, and are consistent with the idea that the axolemma at these sites may be structurally different from the cell membrane in other regions of the neuron, including paranodal and internodal regions of the axon. The electron-dense substance underlying the cytoplasmic surface of the membrane at the nodes and initial segments may represent a substrate that serves to bind together membrane structures in specialized regions of the axolemma.