Glial cells (also called glia or neuroglia), are non-neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses; supportive tissue of the brain, includes astrocytes, oligodendrocytes, ependymal cells and microglia. Unlike neurons, glial cells do not conduct electrical impulses; and they can reproduce. The largest percentage of brain tumors arise from glia. The following functions are recognized:
- support of pyrenophores and their arborizations;
- participation in the myelination of nerve fibers;
- function of “insulator” towards neurons and nerve fibers;
- intervention in the repair of lesions (proliferating and replacing destroyed neurons);
- trophic functions;
- indirect participation in the transmission of the nerve impulse (storing and releasing chemical mediators).
The name glia comes from the Greek word that means “glue,” and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: “This connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.”
Neuroglia can be distinguished into microglia and macroglia; macroglia includes astrocytes, oligodendrocytes, ependymal cells, and Schwann cells. Microglia cells belong to the monocyte-macrophage system, are small and low in number under normal conditions, increasing in number at sites of nerve tissue injury. They present small nuclei, scarce cytoplasm and numerous fine extensions very branched; they are equipped with phagocytosis.
Astrocytes are the most abundant category of neuroglia cells and almost half of the cells contained in the brain. They exhibit a starry shape, with long, branching extensions (gliofilaments). They are connected with other astrocytes, with other glia cells and with neurons. They have a trophic function towards the neurons, allowing the exchange of nutrients between the neurons and the blood, as some of the gliofilaments, through small terminal pedicel-shaped expansions, connect with the capillaries. Astrocytes, forming a continuous cell layer, would help the endothelial cells of the blood vessels of the central nervous system to form the blood-brain barrier (functioning to protect brain tissue from harmful elements in the blood, while allowing the passage of metabolites).
Oligodendrocytes may vary in size; generally they have numerous branching extensions, rather short; in the central nervous system each oligodendrocyte with its extensions can wrap dozens of axons, forming the myelin sheath. Some oligodendrocytes (satellite oligodendrocytes) are closely associated with the cell bodies of neurons.
Ependymal cells are cubic or cylindrical in shape and are densely packed together to form a monolayered covering that plugs the cavities of the brain (brain ventricles) and spinal cord (central canal), termed the ependyma, which retains an epithelioid appearance throughout the life of the individual.
Schwann cells are present only in the peripheral nervous system, where they line the axons, forming the myelin sheath. Each Schwann cell sheaths an internode of an axon, wrapping itself around an axon tract numerous times; the innermost layers are composed primarily of myelin, which forms the cell membrane, while the cytoplasm and nucleus remain in the outermost layers. Myelin functions as an electrical insulator, to increase the speed of nerve impulse conduction.