Oligodendrocyte

Oligodendrocytes (from Greek literally meaning few tree cells), or oligodendroglia (Greek, few tree glue),^[1] are a variety of neuroglia. Their main function is the insulation of the axons exclusively in the central nervous system of the higher vertebrates, a function performed by Schwann cells in the peripheral nervous system. A single oligodendrocyte can extend to up to 50 axons, wrapping around approximately 1 mm of each and forming the myelin sheath; Schwann cells, on the other hand, can only wrap around 1 axon.

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Origin

Oligodendroglia arise during development from an oligodendrocyte precursor cell, which can be identified by its expression of a number of antigens, including the ganglioside GD3 ^[2], the NG2 chondroitin sulfate proteoglycan ^[3], and the platelet-derived growth factor-alpha receptor subunit PDGF-alphaR ^[4]. In the rat forebrain the majority of oligodendroglial progenitors arise during late embryogenesis and early postnatal development from cells of the subventricular zones (SVZ) of the lateral ventricles. SVZ cells migrate away from these germinal zones to populate both developing white and gray matter, where they differentiate and mature into myelin-forming oligodendroglia ^[5]. However, it is not clear whether all oligodendroglial progenitors undergo this sequence of events. It has been suggested that some undergo apoptosis ^[6] and that some fail to differentiate into oligodendroglia but persist into maturity as adult oligodendroglial progenitors ^[7].

Function

The nervous system of mammals depends crucially on the myelin sheath for insulation as it results in decreased ion leakage and lower capacitance of the cell membrane. There is also an overall increase in impulse speed as saltatory propagation of action potentials occurs at the nodes of Ranvier in between Schwann cells (of the PNS) and oligodendrocytes (of the CNS); furthermore miniaturization occurs, whereby impulse speed of myelinated axons increases linearly with the axon diameter, whereas the impulse speed of unmyelinated cells increases only with the square root of the diameter.

As part of the nervous system they are closely related to nerve cells and like all other glial cells the oligodendrocytes have a supporting role towards neurons. They are intimately involved in signal propagation, providing the same functionality as the insulation on a household electrical wire.

Satellite oligodendrocytes are functionally distinct from most oligodendrocytes. They are not attached to neurons and therefore do not serve an insulating role. They remain close to neurons and regulate the extracellular fluid.^[8]

Pathology

Diseases that result in injury to the oligodendroglial cells include demyelinating diseases such as multiple sclerosis and leukodystrophies. Cerebral palsy (periventricular leukomalacia) is caused by damage to developing oligodendrocytes in the brain areas around the cerebral ventricles. Spinal cord injury also causes damage to oligodendrocytes. In cerebral palsy, spinal cord injury, stroke and possibly multiple sclerosis, oligodendrocytes are thought to be damaged by excessive release of the neurotransmitter glutamate. Oligodendrocyte dysfunction may also be implicated in the pathophysiology of schizophrenia and bipolar disorder ^[9]. Oligodendroglia are also susceptible to infection by the JC virus, which causes progressive multifocal leukoencephalopathy (PML), a condition which specifically affects white matter, typically in immunocompromised patients. Tumors of oligodendroglia are called oligodendrogliomas.

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