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Neuroglia of the brain shown by Golgi's method.

Astrocytes can be identified in culture because, unlike other mature glia, they express glial fibrillary acidic protein.

Glial cells, commonly called neuroglia or simply glia (Greek for "glue"), are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for the brain's neurons. In the human brain, there is roughly one glia for every neuron with a ratio of about two neurons for every three glia in the cerebral gray matter.^[1]

As the Greek name implies glia are commonly known as the glue of the nervous system, though this is not medically accurate. The four main functions of glial cells are to surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons. They also modulate neurotransmission.^[2]

[edit] Functions

Some glial cells function primarily as the physical support for neurons. Others regulate the internal environment of the brain, especially the fluid surrounding neurons and their synapses, and nutrify neurons. During early embryogeny, glial cells direct the migration of neurons and produce molecules that modify the growth of axons and dendrites. Recent research indicates that glial cells of the hippocampus and cerebellum participate in synaptic transmission, regulate the clearance of neurotransmitters from the synaptic cleft, release factors such as ATP, which modulate presynaptic function, and even release neurotransmitters themselves. Unlike the neuron, which is generally considered permanently post-mitotic^[3], glial cells are capable of mitosis.

Traditionally glia has been considered to lack certain features of neurons. For example, glial cells were not believed to have chemical synapses or to release neurotransmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies have shown this to be untrue. For example, astrocytes are crucial in clearance of neurotransmitter from within the synaptic cleft, which provides distinction between arrival of action potentials and prevents toxic build up of certain neurotransmitters such as glutamate (excitotoxicity). It is also thought that glia plays a role in Alzheimer's disease. Furthermore, at least in vitro, astrocytes can release neurotransmitter glutamate in response to certain stimulation. Another unique type of glial cell, the oligodendrocyte precursor cells or OPCs, have very well defined and functional synapses from at least two major groups of neurons. The only notable differences between neurons and glial cells are neurons' possession of axons and dendrites, and capacity to generate action potentials.

Glia ought not to be regarded as 'glue' in the nervous system as the name implies; rather, it is more of a partner to neurons. They are also crucial in the development of the nervous system and in processes such as synaptic plasticity and synaptogenesis. Glia has a role in the regulation of repair of neurons after injury. In the CNS, glia suppresses repair. Glial cells known as astrocytes enlarge and proliferate to form a scar and produce inhibitory molecules that inhibit regrowth of a damaged or severed axon. In the PNS, glial cells known as Schwann cells promote repair. After axonal injury, Schwann cells regress to an earlier developmental state to encourage regrowth of the axon. This difference between PNS and CNS raises hopes for the regeneration of nervous tissue in the CNS. For example a spinal cord may be able to be repaired following injury or severance.

[edit] Types

[edit] Microglia

Microglia are like specialized macrophages capable of phagocytosis that protect neurons of the central nervous system. They are derived from hematopoietic precursors rather than ectodermal tissue; they are commonly categorized as such because of their supportive role to neurons.

These cells comprise approximately 15% of the total cells of the central nervous system. They are found in all regions of the brain and spinal cord. Microglial cells are small relative to macroglial cells, with changing shapes and oblong nuclei. They are mobile within the brain and multiply when the brain is damaged. In the healthy central nervous system, microglia processes constantly sample all aspects of their environment (neurons, macroglia and blood vessels).

[edit] Macroglia

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[edit] Capacity to divide

Glia retain the ability to undergo cell division in adulthood, while most neurons cannot. The view is based on the general deficiency of the mature nervous system in replacing neurons after an injury, such as a stroke or trauma, while very often there is a profound proliferation of glia, or gliosis near or at the site of damage. However, detailed studies found no evidence that 'mature' glia, such as astrocytes or oligodendrocytes, retain the ability of mitosis. Only the resident oligodendrocyte precursor cells seem to keep this ability after the nervous system matures. On the other hand, there are a few regions in the mature nervous system, such as the dentate gyrus of the hippocampus and the subventricular zone, where generation of new neurons can be observed.

[edit] Embryonic development

Most glia are derived from ectodermal tissue of the developing embryo, particularly the neural tube and crest. The exception is microglia, which are derived from hemopoietic stem cells. In the adult, microglia are largely a self-renewing population and are distinct from macrophages and monocytes which infiltrate the injured and diseased CNS.

In the central nervous system, glia develop from the ventricular zone of the neural tube. These glia include the oligodendrocytes, ependymal cells, and astrocytes. In the peripheral nervous system, glia derive from the neural crest. These PNS glia include Schwann cells in nerves and satellite glial cells in ganglia.

[edit] History

Glia were discovered in 1856 by the pathologist Rudolf Virchow in his search for a 'connective tissue' in the brain.

[edit] Numbers

The human brain contains roughly equal numbers of glial cells and neurons with 84.6 billion glia and 86.1 billion neurons.^[1] The ratio differs between its different parts. The glia/neuron ratio in the cerebral cortex is 3.72(60.84 billion glia; 16.34 billion neurons) while that of the cerebellum is only 0.23 (16.04 billion glia; 69.03 billion neurons). The ratio in the cerebral cortex gray matter is 1.48 (the white matter part has few neurons). The ratio of the basal ganglia, diencephalon and brainstem combined is 11.35.^[1]

Most cerebral cortex glia are oligodendrocytes (75.6%) then astrocytes (17.3%) and least for microglia (6.5%)^[5]

The amount of brain tissue that is made up of glia cells increases with brain size: the nematode brain contains only a few glia, a fruitfly's brain is 25% glia, that of a mouse, 65%, a human, 90%, and an elephant, 97%.^[6]

[edit] Additional images

Oligodendrocyte

Section of central canal of medulla spinalis, showing ependymal and neuroglial cells.

Transverse section of a cerebellar folium.

[edit] References

[edit] External links

• Media related to Glia at Wikimedia Commons
• "The Mystery and Magic of Glia: A Perspective on Their Roles in Health and Disease." Neuron 60, November 6, 2008 by Ben Barres
• Role of glia in synapse development
• Overstreet L (2005). "Quantal transmission: not just for neurons". Trends Neurosci 28 (2): 59–62. doi:10.1016/j.tins.2004.11.010. PMID 15667925.  article
• Peters A (2004). "A fourth type of neuroglial cell in the adult central nervous system". J Neurocytol 33 (3): 345–57. doi:10.1023/B:NEUR.0000044195.64009.27. PMID 15475689. 
• Volterra A, Steinhäuser C (2004). "Glial modulation of synaptic transmission in the hippocampus". Glia 47 (3): 249–57. doi:10.1002/glia.20080. PMID 15252814. 
• Huang Y, Bergles D (2004). "Glutamate transporters bring competition to the synapse". Curr Opin Neurobiol 14 (3): 346–52. doi:10.1016/j.conb.2004.05.007. PMID 15194115. 
• New Source of Replacement Brain Cells Found - glial cells can transform into other cell types and reproduce indefinitely — tricks once thought exclusive to stem cells.
• Artist ADSkyler(uses concepts of neuroscience and found inspiration from Glia)