ABSTRACT
Neuroglia are nonexcitable supporting cells that outnumber neurons at a ratio of 2:1, forming the skeleton of the central nervous system (CNS). They are of both ectodermal and mesodermal origin and are commonly associated with tumors of the CNS. They have only one type of cell process, do not form synapses, and retain the ability to undergo mitosis. Glial cells provide the optimal milieu for neuronal function by balancing the ionic concentration within the extracellular space. They provide nutrients, discard metabolites and cellular debris, and sharpen neuronal signals, preventing ephapsis or cross talk by forming a protective myelin sheath. The glial cells mediate the extent of impulse ow, activity of neurons, and frequency of excitation. Thus, changes in the glial cell membrane potentials may occur as a result of the uctuation in the potassium ion concentration, which, in turn, is affected by level of the generated impulses. They also secrete growth-promoting molecules such as the nerve growth factor (NGF); glial-derived neurite-promoting factor (GNPF), which is a protease inhibitor (trypsin, urokinase, and thrombin); and tenascin. Neuronal sprouting may be facilitated by
glial-derived nexin (GDN), which prevents the digestion of the extracellular matrix molecules by inhibiting proteases secreted by growth cones. Neuroglia also contributes to the formation of the blood-brain barrier, which selectively permits substances and molecules to enter the CNS. They also allow developing neuroblasts to move to their nal destinations. Presence of molecules such as bronectin, laminin, and cellular adhesion molecules may account for the mechanism by which neurons migrate along processes of certain glial cells and not others. Neuroglial cells are classied as macroglial and microglial cells. Macroglial cells are further classied into astrocytes, oligodendrocytes of the CNS, and Schwann cells of the peripheral nervous system (PNS).
