Astrocytes are one of the most numerous cell types in the vertebrate central nervous system (CNS) and yet their functions are largely unknown. In the rat optic nerve there are two distinct types of astrocyte1: Type-1 astrocytes develop from one type of precursor cell2, and type-2 astrocytes develop from bipotential, oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells3, that initially give rise to oligodendrocytes (which make myelin in the CNS), and then to type-2 astrocytes4. Type-1 astrocytes form the glial limiting membrane at the periphery of the optic nerve5 and are probably responsible for glial scar formation following nerve transection6. The functions of type-2 astrocytes, which, like oligodendrocytes, are found mainly in tracts of myelinated axons throughout the CNS6, are unknown. In this report we provide evidence that processes from type-2 astrocytes contribute to the structure of nodes of Ranvier, suggesting that the O-2 A cell lineage is specialized for constructing myelin sheaths and nodes in the mammalian CNS.