While neuroglial cells have been regarded as “support cells” for neurons since the beginning of this century, it has been only in recent years that the nature of such “support” has begun to be appreciated. It is now clear that neuroglial cells provide neurons with essential substrates such as glutamine and remove or inactivate such toxic metabolic products as NH4+ (46, 47, 97, 98), regulate activities of potassium and other ions in the extracellular space (56, 57), and permit saltatory conduction of nerve impulses by forming myelin. In addition to these metabolic support functions, Schwann cells in the peripheral nervous system (PNS) synthesize many proteins necessary for neuronal development and survival; these include extracellular matrix constituents such as type IV collagen, fibronectin and laminin (5, 6, 14), cell adhesion proteins such as N-CAM and myelin associated glycoprotein (MAG) (51), and soluble proteins such as nerve growth factor (NGF) (2, 29, 30). Similar protein synthetic trophic functions are performed in central nervous system (CNS) by astroglia; these include the synthesis of extracellular matrix components and growth factors, for example insulin-like growth factor (1, 13, 20, 21, 58, 65). It is now thought likely, as well, that neuroglia regulate the properties of the blood-brain barrier. CNS neuroglia, probably astroglia, induce brain capillaries to express the tight junction phenotype that is required for a competent blood-brain barrier (89) and it is possible that astroglia participate in the regulation of regional cerebral blood flow by modulating perivascular potassium concentrations (62).