Two varieties are distinguished on the basis of the morphology of their processes. The fibrous variety, found mostly in white matter, have more slender but well-defined and fewer cell processes. Both varieties of astrocyte play a role in metabolite transfer within the central nervous system. The fibrous astrocytes, in addition, play a role in healing and scar formation in the nervous system. Special histochemical stains for GFAP help identify astrocytes in tissue sections.
Oligodendroglia are smaller than astrocytes and have a denser nucleus and cytoplasm.
These glial cells are seen adjacent to myelinated nerve fibers in the white matter or forming satellite cells to the neurons in the gray matter. Oligodendroglia elaborate central nervous system myelin. The perikaryon of a microglial cell is irregular in shape, and, if elongated, the few processes emanate from both of its poles. Microglia are believed to be the scavenger cells of the central nervous system. The middle layer is the arachnoid, a non-vascular delicate connective tissue coat.