Good question. Read this little exert, it should help you:
Glia, or neuroglia, are the specialized interstitial or stromal cells of the CNS. There are four basic types of glial cells:
astrocytes,
oligodendrocytes,
microglia and
ependymal cells.
1. Astrocytes have many cytoplasmic processes, foot-like extensions, which terminate on blood vessels, neuronal cells bodies, and axons, and surround synaptic terminals and clefts. Functions include biochemical and physical support for neurons, insulation, guidance of migrating neurons during development, and repair following injury. Astrocytes are commonly subdivided into two subclasses: protoplasmic and fibrous. Fibrous astrocytes contain many filaments in long thin processes and are found predominantly in white matter. The protoplasmic form has frequently branched processes and occurs mostly in gray matter.
2. Oligodendrocytes in the gray matter are called satellite cells and function in fluid and respiratory exchange. Oligodendrocytes in the white matter form myelin sheaths in the CNS (Schwann cells form myelin sheaths in the PNS). Each oligodendrocyte supplies myelin sheath segments to many axons while each Schwann cell supplies only one myelin segment.
3. Microglia are small elongated cells -- the nervous system representative of the mononuclear phagocytic (reticulo-endothelial) system. Their normal function is not well understood but they participate in response to injury.
4. Ependymal cells are cuboidal ciliated cells lining the ventricles and the central canal of the spinal cord.
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