The perinodal astrocyte

Several studies have demonstrated the presence of perinodal astrocyte processes at nodes of Ranvier in the central nervous system, suggesting that, in addition to the axon and oligodendrocyte, astrocytes participate in the formation of mature central nodes. The specific association between perinodal astrocyte processes and nodal membrane develops at the time of, or soon after, the appearance of relatively differentiated nodes of Ranvier. This interaction is likely to be mediated by cell adhesion molecules. J1 is a member of a family of glycoproteins that share a common carbohydrate epitope, designated L2/HNK-1, and that have been implicated in cell-cell interactions. This glycoprotein is concentrated at the interface between perinodal astrocyte processes and the nodal region of the axon. Moreover, N-CAM, which is a member of the same family as J1, and cytotactin, an extracellular matrix component produced by glia, are localized at the interface between the axon and perinodal astrocyte processes at nodes of Ranvier. The association of perinodal astrocyte processes with nodal membrane in the central nervous system is similar to that exhibited by perinodal Schwann cell processes at peripheral nodes, and similar functional properties have been suggested for these two glial cell processes, including production of nodal gap substance, buffering of perinodal extracellular ion concentration, and development and/or maintenance of nodal specializations in the axon membrane. Perinodal astrocyte and Schwann cell processes may also function as extraneuronal sites for the synthesis of voltage-sensitive sodium channels, to complement neuronal perikaryal synthesis and axonal transport. Ultrastructural studies on specialized patches of axon membrane within some unmyelinated, demyelinated, and dysmyelinated axons support the hypothesis of a specific role for perinodal astrocyte processes in the assembly, stabilization, and/or maintenance of axolemma with nodal characteristics. These observations suggest a multiplicity of functions for perinodal astrocyte processes at central nodes and implicate the astrocyte as an important component of the node of Ranvier.