(from Greek, glia: glue) Glial cells are found throughout the nervous system and were initially thought simply to have very limited functions, to be little more than glue, binding neurons in place. Glia cells—or more colloquially, just glia—in fact have a variety of functions that are critical to the health and operational well-being of the nervous system. They appear to take no direct part in the computations made by NEURONS: they do not form parts of operational NEURAL NETWORKS and are not involved in neuronal signalling in any direct manner. As such glia are of limited interest to biological psychologists keen to explain the relationships between brain systems, behaviour and psychological processing. However, they are critically involved in the operations of the brain and as such deserve interest. There are four principal classes of glia cell: ASTROCYTES (or ASTROGLIA); MICROGLIA; oligodendrocytes (or OLIGODENDROGLIA); and SCHWANN CELLS. These have different forms, distributions and functions.
Astroglia: there are three main types of astrocyte: radial astrocytes (all of which are orientated in a similar plane with respect to the NEURAXIS, and which are found in WHITE MATTER); fibrous astrocytes (or non-radial astrocytes, also found in white matter but more diffusely organized); and protoplasmic astrocytes (with shorter processes than the other forms and found in GREY MATTER). All astrocytes have a more-or-less star shaped appearance (astron is Greek for star; cyte is a suffix meaning CELL, taken from the Greek kytos, a vessel). The star shape is produced by the processes that spread from the cell body, though the exact form varies dependent on type and location. All astroglia can be detected using IMMUNOHISTOCHEMISTRY for GLIAL FIBRILLARY ACIDIC PROTEIN (GFAP) which marks these cells uniquely. Astroglia are involved in a variety of functions. They make contact with both BLOOD vessels and neurons and appear to be involved in ‘trading’ between them. It is possible, for example, that astrocytes can form a reservoir for GLUCOSE, accepting it from the blood stream and feeding neurons at the required rate. (This, of course, is consistent with a role for astrocytes in the operation of the BLOOD-BRAIN BARRIER.) Astrocytes also appear to regulate the ionic composition of the central nervous system (CNS). For example, astrocytes form an interconnected network (the so-called ASTROCYTIC SYNCYTIUM) that is able to move potassium ions (K+) away from sites of high concentra tion to sites of low concentration. Astrocytes are also involved in the reuptake of certain neurotransmitters and neurotransmitter metabolites. Astrocytes are critical for the proper chemical operation of the CNS. Pathologically, the most common type of brain TUMOUR forms from astroglia (and is known as an ASTROCYTOMA).
Microglia: these are small glial cells, making up 10–20% of the total population of glia in the nervous system. Three types—or more correctly, states—can be identified: resting or ramified microglia; activated or reactive microglia; and phagocytic microglia (see brainMACROPHAGE). Various other names have been used to characterize microglia, describing their form or function in different states. The transition of microglia from resting to activated to the phagocytic state appears to be regulated by such things as cell death (see APOPTOSIS) and various forms of damage within the brain, which trigger release of substances such as COMPLEMENT and the CYTOKINE group (see IMMUNE SYSTEM; indeed, the microglia can in many ways be considered as part of the immune system). All microglia can be detected using immunohistochemistry for ox-42 (an ANTIBODY for a complement receptor present on microglia).
Oligodendroglia (from Greek, oligos: few; dendron: tree, glia: glue): these are the glial cells that produce MYELIN within the CNS. OLIGODENDROGLIA (or oligodendrocytes as they are also known) come in four basic types, though there appear also to be some transition forms. These are type I (spherical cell bodies, many processes: found at many levels of the CNS, often associated with blood vessels or fibre pathways); type II (more cuboid than spherical and with fewer processes than type I; only found in white matter); type III (three or four processes only: found in the CEREBELLUM, CEREBRAL PEDUNCLES and CEREBELLAR PEDUNCLES, MEDULLA OBLONGATA and SPINAL CORD); and type IV (associated with the points of entry of CRANIAL NERVES into the CNS: these oligodendroglia are the most similar to Schwann cells). The function of oligodendroglia is to provide myelin to central nervous system neurons: myelin functions as an electrical insulator of axons, speeding axonal conduction velocities. A single oligodendrocyte will provide myelin for more than one neuron (and one neuron will be myelinated by more than one oligodendrocyte). The processes of myelination involves the processes of oligodendrocytes making contact with an axon and then wrapping around it in a spiral fashion. The process of myelination in humans can take very many years: it is not generally completed until individuals are in their mid-teenage years. Myelin can be visualized in the CNS by various histological techniques (see HISTOLOGY; CHEMICAL NEUROANATOMY) but the most effective is the GALLYAS SILVER stain for myelin.
Schwann cells: the most commonly recognized function of Schwann cells is to provide myelin to neurons in the PERIPHERAL NERVOUS SYSTEM (they are not normally found in the CNS). However, Schwann cells do more than this. All nerve fibres in the peripheral nervous system are sheathed by Schwann cells, whose most basic functions are to provide structural support and to interact with surrounding tissues. Schwann cells are discriminated from oligodendroglia not only by location but also by possession of a basal lamina which serves as a border between the Schwann cell-axon unit and the surrounding CONNECTIVE TISSUE. Peripheral nerve fibres are delicate, yet must travel long distances: the presence of Schwann cells allows the development of effective connective tissue support. In addition to this physical function, Schwann cells can also provide myelin, laying it in sections along axons. Unlike oligodendroglia, a single Schwann cell will provide myelin only along one axon, and only for a particular portion of that axon. (The junctions between myelinated sections are the NODES OF RANVIER; see ACTION POTENTIAL.) Schwann cells do on occasion penetrate the CNS, but only in unusual conditions. They appear to be present in the walls of blood vessels, and if the blood brain-barrier is compromised, Schwann cells can enter the CNS. There are reports of them being engaged in remyelination of CNS neurons following brain damage.
Kettenmann H. & Ransom B.R. (1995) Neu roglia, Oxford University Press: Oxford. (This is a remarkably thorough and complete account of neuroglia that will answer almost any question about glial cells.)
Kimelberg H.K. & Norenberg M.D. (1989) Astrocytes. Scientific American 260:66–76.
Ransom B.R. & Sontheimer H. (1992) The neurophysiology of glial cells. Journal of Clinical Neurophysiology 9:224–251.
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