Radial glia give rise to perinodal processes

Nodes of Ranvier in the central nervous system in mammals are characterized by the presence of perinodal astrocytic processes. This study examines the association between processes of radial glia and the axolemma at nodes of Ranvier in the spinal cord of the mature axolotl, an animal in which radial glia represent a large portion of the total glial population. The radial glial cells have their cell bodies located close to the central canal. Those situated dorsal to the canal send long processes to the dorsal surface of the spinal cord. Along this trajectory these processes coalesce into large fascicles in the midline and form the dorsal median septum. Slender branches rise from the processes in these fascicles and extend into the adjacent white matter to terminate in close apposition to the axolemma at nodes of Ranvier. This arrangement provides an intracellular pathway extending from the perinodal region to the surface of the spinal cord. Radial glia ventral to the central canal give rise to processes that project to the glia limitans adjacent to the ventral spinal artery. These ventrally projecting processes appear to be more irregular in their branching pattern than their dorsal counterparts. Multiple slender processes are seen in close apposition to the nodal axolemma of myelinated axons in the ventral white commissure, again providing an intracellular pathway that runs from the perinodal region to the cord surface. In one instance a radial glial process was observed to occupy a pocket formed by the invagination of the nodal axolemma. The axonal cytoplasm adjacent to the invagination contained a variety of organelles, e.g. multivesicular bodies, vesicles and endoplasmic reticulum, suggesting that this relationship between the radial glial process and the axon is more than a passive interaction. These observations are consistent with the view that processes of radial glial cells may regulate the extracellular environment adjacent to the nodal axolemma, and/or play an active role in the maintenance of the nodal membrane. The existence of perinodally-directed processes of radial glial cells in the salamander indicates that this axo-glial specialization reflects an important functional interaction preserved across a large segment of the phylogenetic scale.