Neuroglia

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 in a rat brain stained with an antibody against GFAP

23 week fetal brain culture astrocyte

Neoplastic glial cells stained with an antibody against GFAP (brown), from a brain biopsy

Glial cells, sometimes called neuroglia or simply glia (Greek γλία, γλοία "glue"; pronounced in English as either /ˈɡliːə/ or /ˈɡlaɪə/), are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain and peripheral nervous system.^[1]

As the Greek name implies, glia are commonly known as the glue of the nervous system; however, this is not fully accurate. Neuroscience currently identifies four main functions of glial cells:

1. To surround neurons and hold them in place,
2. To supply nutrients and oxygen to neurons,
3. To insulate one neuron from another,
4. To destroy pathogens and remove dead neurons.

For over a century, it was believed that the neuroglia did not play any role in neurotransmission. It was only in 2010 that it was recognised that the glial cells do have some effects on certain physiological processes like breathing,^[2][3] and in assisting the neurons to form synaptic connections between each other.^[4]

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 embryogenesis 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, and release gliotransmitters such as ATP, which modulate synaptic function.

Glial cells are known to be capable of mitosis. By contrast, scientific understanding of whether neurons are permanently post-mitotic,^[5] or capable of mitosis,^[6] is still developing. In the past, glia had been considered to lack certain features of neurons. For example, glial cells were not believed to have chemical synapses or to release transmitters. They were considered to be the passive bystanders of neural transmission. However, recent studies have shown this to be untrue.^[7]

For example, astrocytes are crucial in clearance of neurotransmitters 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 play a role in many neurological diseases, including Alzheimer's disease. Furthermore, at least in vitro, astrocytes can release gliotransmitter 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.^[8] 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, they are more of a partner to neurons.^[9] They are also crucial in the development of the nervous system and in processes such as synaptic plasticity and synaptogenesis. Glia have a role in the regulation of repair of neurons after injury. In the central nervous system (CNS), glia suppress 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 peripheral nervous system (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 the (CNS) and the (PNS), 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. Schwann cells are also known as neuri-lemmocytes. These cells envelop nerve fibers of the PNS by winding repeatedly around a nerve fiber with the nucleus inside of it. This process creates a myelin sheath which not only aids in conductivity, but it also assists in the regeneration of damaged fibers. Oligodendrocytes are another type of glial cell of the CNS. These dendrocytes resemble an octopus bulbous body and contain up to fifteen “arm like” processes. Each “arm” reaches out to a nerve fiber and spirals around it, creating a myelin sheath. This myelin sheath insulates the nerve fiber from the extracellular fluid as well as speeds up the signal conduction in the nerve fiber.^[10]

Types

Different types of neuroglia

Microglia

Microglia are specialized macrophages capable of phagocytosis that protect neurons of the central nervous system.^[11] 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 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).

Macroglia

Other

Pituicytes from the posterior pituitary are glia cells with characteristics in common to astrocytes.^[22] Tanycytes from the hypothalamus descend from radial glia.^[23]

Capacity to divide

Glia retain the ability to undergo cell division in adulthood, whereas 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.^[24]

Embryonic development

Most glia are derived from ectodermal tissue of the developing embryo, in particular 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.

History

Glia were first described in 1856 by the pathologist Rudolf Virchow in a comment to his 1846 publication on connective tissue. In his 1858 publication `Cellularpathology´, he described glial cells in more detail.^[25]

Numbers

Neuroglial cells are generally smaller than neurons and outnumber them by five to ten times; they comprise about half the total volume of the brain and spinal cord.(Clinical Neuro-Anatomy, Richard S. Snell, 7th edition) ^[26] The ratio differs from one part of the brain to another. 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.^[26]

Most cerebral cortex glia are oligodendrocytes (75.6%); astrocytes account for 17.3% and microglia (6.5%)^[27]

The amount of brain tissue that is made up of glial 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%.^[28] Moreover, humans are known to have the most abundant and largest astrocytes of any animal.^[12]

Additional images

• 

  Oligodendrocyte

• 

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

• 

  Transverse section of a cerebellar folium

See also

• Polydendrocytes

References

Notes

Bibliography

• Brodal, Per (2010). "Glia". The central nervous system: structure and function. Oxford University Press. p. 19. ISBN 978-0-19-538115-3. 
• Kettenmann and Ransom, Neuroglia, Oxford University Press, 2012, ISBN13: 9780199794591 |http://ukcatalogue.oup.com/product/9780199794591.do#.UVcswaD3Ay4|

Further reading

• "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". Current Opinion in Neurobiology 14 (3): 346–52. doi:10.1016/j.conb.2004.05.007. PMID 15194115. 
• Artist ADSkyler(uses concepts of neuroscience and found inspiration from Glia)

External links

Wikimedia Commons has media related to Glia.

Audio

• "The Other Brain"—The Leonard Lopate Show (WNYC) "Neuroscientist Douglas Field, explains how glia, which make up approximately 85 percent of the cells in the brain, work. In The Other Brain: From Dementia to Schizophrenia, How New Discoveries about the Brain Are Revolutionizing Medicine and Science, he explains recent discoveries in glia research and looks at what breakthroughs in brain science and medicine are likely to come."
• "Network Glia" A homepage devoted to glial cells