Cells in focus
Glial cells

https://doi.org/10.1016/j.biocel.2004.02.023Get rights and content

Abstract

The nervous system is built from two broad categories of cells, neurones and glial cells. The glial cells outnumber the neurones and the two cell types occupy a comparable amount of space in nervous tissue. The main glial cell types are, in the central nervous system, astrocytes and oligodendrocytes and, in the peripheral nervous system, Schwann cells, enteric glial cells and satellite cells. In the embryo, glial cells form a cellular framework that permits the development of the rest of the nervous system, and regulate neuronal survival and differentiation. The best known function of glia in the adult is the formation of myelin sheaths around axons thus allowing the fast conduction of signalling essential for nervous system function. Glia also maintain appropriate concentrations of ions and neurotransmitters in the neuronal environment. Increasing body of evidence indicates that glial cells are essential regulators of the formation, maintenance and function of synapses, the key functional unit of the nervous system.
Cell facts

  • Throughout the brain, spinal cord and peripheral nerves, neurones are never found except in a close association with glial cells.

  • The turnover rate of mature glia is normally close to zero but most of them respond to injury by rapid proliferation.

  • Glial cells come in many types and have multiple functions in the developing and mature nervous system.

  • Following injury, glia are major regulators of neuronal repair and they are largely responsible for the difference in regeneration capacity between the central and peripheral nervous system.

Section snippets

Introduction: the main glial types

Two main cell types build the nervous system. These are neurones, which are directly involved in electrical transmission and information processing, and glial cells. In all parts of the nervous system, glial cells outnumber neurones by some margin, and they make up a large part of nervous tissue. For instance, glial cells occupy about half the volume of the brain. These cells carry out many indispensable functions, both in development and during the normal function of the mature system (Jessen

Development

The glial cells and neurones of the CNS develop from neural precursor cells of a germinal layer called the ventricular zone, that lines the lumen of the developing spinal cord and the ventricles of the brain. Oligodendrocyte development is better understood in the spinal cord than in the brain. In the cord, oligodendrocytes appear to originate from a tightly restricted area of the ventricular zone in a process that depends on the transcription factors Olig 1 and 2 and the signalling molecule

Guidance

In the developing brain, neurones are often formed at what in cellular terms is a very long way from their final site of residence. Development therefore involves a remarkable amount of neuronal migration, a process in which glial cells play a major role. This has probably been studied most thoroughly in the cerebral cortex and the cerebellum. Here, the radial glial cells mentioned before act as indispensable scaffolds for extensive neuronal migration, involving astrotactin and neuregulin-1

Multiple sclerosis (MS)

This is perhaps the most widely recognised disease associated with glial cells (Lucchinetti & Lassmann, 2001). Multiple sclerosis (MS) is a progressive disease with a significant immune involvement that primarily affects oligodendrocytes. It is characterised by the formation of multiple lesions in the CNS in which myelin is destroyed and oligodendrocytes die. Axons are also adversely affected. The causes of MS are not well understood and effective treatment remains to be developed.

Type 1 Charcot–Marie–Tooth disease (CMT)

CMT is a

Acknowledgements

The work described in this article that was done by the author was supported by grants from the Wellcome Trust and the MRC. I thank Rhona Mirsky and Ashwin Woodhoo for commenting on the manuscript.

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