When glial cells were first described over a century ago, in the classic studies by Spanish neuroanatomists Cajal and Hortega, they were envisaged as equivalent to connective tissue, providing only support for the 'real' brain cells – the neurons. However, within the last decade it has become increasingly evident that glial cells play an active and crucial role in two fundamental areas: the development of the mammalian nervous system and the maintenance of normal brain function.
The vast majority of glial cells in the nervous system are classed as macroglia, and, based on structural characteristics, can be further subdivided into oligodendrocytes (oligodendroglia), radial glia and astrocytes (or astroglia). A much smaller group, termed microglia, develop from non-neural tissue and act as macrophages (for review see Thomas 1992). Oligodendrocytes are found in white matter and are the source of the myelin of the axon sheath; their origin and functional properties