The neuroglia (glia, glue) is the supportingand covering tissue of the CNS and has all thefunctions of connective tissue: support, metabolite exchange, and, in pathological processes, digestion of degenerating cells (phagocytosis) and scar formation. It is of ectodermal origin. After Nissl staining, onlycell nuclei and cytoplasm are visible; visual-ization of cellular processes is achieved only by special impregnation methods and by immunocytochemistry. We distinguish three different types of neuroglia: astroglia (macroglia), oligodendroglia, and microglia (A).
Astrocytes have a large, clear cell nucleusand numerous processes which give the cell a starlike appearance (A, C). There are proto-plasmic astrocytes with few processes (usu-ally present in gray matter) and fibrous astro-cytes with numerous long processes (pre-dominantly present in white matter). The latter produce fibers and contain glial fila-ments in cell body and processes. They form glial scars after damage to the brain tissue. Astrocytes are regarded as supporting el-ements, since they form a three-dimen-sional scaffold. On the outer surface of the brain, the scaffold thickens to form a dense fiber felt, the glial limiting membrane, which forms the outer limit of the ectodermal tissue against the mesenchymal meninges. Astrocytes extend processes to blood ves-sels and play a role in metabolite exchange.
In addition, astrocytes play a decisive part in maintaining the interior environment of theCNS, particularly the ion balance. Potassium ions released upon excitation of groups of neurons are removed from the extracellular space via the network of astrocyte processes. Astrocytes probably also take up CO2 released by nerve cells and thus keep the interstitial pH at a constant value of 7.3. Astrocyte processes enclose the synapses and seal off the synaptic cleft. They also take up neurotransmitters (uptake and release of GABA via astrocytes have been demon-strated).
Oligodendrocytes have a smaller, darkercell nucleus and only a few, sparsely branched processes. In the gray matter, they accompany neurons (satellite cells) (B). In the white matter, they lie in rows between the nerve fibers (intrafascicular glia). They pro-duce and maintain the myelin sheath. In the peripheral nervous system the myelin sheath is formed by Schwann cells.
Microglial cells have an oval or rodlike cellnucleus and short, branched processes. They exhibit ameboid mobility and can mi-grate within the
brain tissue. In response to tissue destruction, they phagocytose mate-rial (scavenger cells) and round up into spheres (gitter cell).
The frequently expressed view that micro-glia were not derived from the ectoderm but from the mesoderm (mesoglia) is not sup-ported by evidence.