Synantocytes: the fifth type of glia? In comparison with astrocytes.

To date four types of glial cells have been identified in central nervous system: astrocytes, oligodendrocytes, microglia, ependymocytes. The latest results indicate the existence of the fifth glial type-synantocytes from the Greek word synanto that is for contact. Synantocyte processes reach neurons, astrocytes, oligodendrocytes, microglia, synapses, myelin sheaths and nervous fibres' nodes of Ranvier. Morphologically, synantocytes are similar to astrocytes, but they do not contain, like astrocytes, glial fibrillary acidic protein and S-100 beta protein. Synantocytes show expression of NG2 surface chondroition sulphate proteoglycan. Moreover, these cells in contrast to astrocytes do not have membrane transporters for glutamate (Glu), but have receptors for Glu and gamma-aminobutyric acid, whose activation can contribute to keeping up ion balance in CNS. Synantocytes are components of synapses, participate in neuronal cytoskeleton stabilization and control myelin integrity, mediate oligodendrocytes answer to nervous fibres' damage and form glial scars. Thus, there are evidences that synantocytes and astrocytes make separate glial population, playing important functions in neuroprotection.