Glia
Glia | |
---|---|
Details | |
Precursor | Neuroectoderm for macroglia, and hematopoietic stem cells for microglia |
System | Nervous system |
Identifiers | |
MeSH | D009457 |
TA98 | A14.0.00.005 |
TH | H2.00.06.2.00001 |
FMA | 54536 54541, 54536 |
Anatomical terms of microanatomy |
Glia, also called glial cells (gliocytes) or neuroglia, are non-
Function
They have four main functions:
- to surround neurons and hold them in place
- to supply nutrients and oxygen to neurons
- to insulate one neuron from another
- to destroy pathogens and remove dead neurons.
They also play a role in neurotransmission and synaptic connections,[3] and in physiological processes such as breathing.[4][5][6] While glia were thought to outnumber neurons by a ratio of 10:1, recent studies using newer methods and reappraisal of historical quantitative evidence suggests an overall ratio of less than 1:1, with substantial variation between different brain tissues.[7][8]
Glial cells have far more cellular diversity and functions than neurons, and glial cells can respond to and manipulate neurotransmission in many ways. Additionally, they can affect both the preservation and consolidation of memories.[1]
Glia were discovered in 1856, by the pathologist
Types
Macroglia
Derived from ectodermal tissue.
Location | Name | Description |
---|---|---|
CNS | Astrocytes |
The most abundant type of macroglial cell in the CNS, Astrocytes signal each other using cellular organelles, releasing calcium into the cytoplasm. This calcium may stimulate the production of more IP3 and cause release of ATP through channels in the membrane made of pannexins. The net effect is a calcium wave that propagates from cell to cell. Extracellular release of ATP, and consequent activation of purinergic receptors on other astrocytes, may also mediate calcium waves in some cases.
In general, there are two types of astrocytes, protoplasmic and fibrous, similar in function but distinct in morphology and distribution. Protoplasmic astrocytes have short, thick, highly branched processes and are typically found in gray matter. Fibrous astrocytes have long, thin, less-branched processes and are more commonly found in white matter .
It has recently been shown that astrocyte activity is linked to blood flow in the brain, and that this is what is actually being measured in |
CNS | Oligodendrocytes |
Oligodendrocytes are cells that coat axons in the CNS with their cell membrane, forming a specialized membrane differentiation called insulation to the axon that allows electrical signals to propagate more efficiently.[14]
|
CNS | Ependymal cells |
Ependymal cells, also named ependymocytes, line the spinal cord and the blood-CSF barrier. They are also thought to act as neural stem cells.[15]
|
CNS | Radial glia |
Radial glia cells arise from |
PNS | Schwann cells |
Similar in function to oligodendrocytes, Schwann cells provide myelination to axons in the peripheral nervous system (PNS). They also have phagocytotic activity and clear cellular debris that allows for regrowth of PNS neurons.[18] |
PNS | Satellite cells |
Satellite glial cells are small cells that surround neurons in sensory, parasympathetic ganglia.[19] These cells help regulate the external chemical environment. Like astrocytes, they are interconnected by gap junctions and respond to ATP by elevating the intracellular concentration of calcium ions. They are highly sensitive to injury and inflammation and appear to contribute to pathological states, such as chronic pain.[20]
|
PNS | Enteric glial cells |
Are found in the intrinsic ganglia of the enteric system, some related to homeostasis and muscular digestive processes.[21]
|
Microglia
Microglia are specialized macrophages capable of phagocytosis that protect neurons of the central nervous system.[22] They are derived from the earliest wave of mononuclear cells that originate in yolk sac blood islands early in development, and colonize the brain shortly after the neural precursors begin to differentiate.[23]
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). In a healthy brain, microglia direct the immune response to brain damage and play an important role in the inflammation that accompanies the damage. Many diseases and disorders are associated with deficient microglia, such as Alzheimer's disease, Parkinson's disease and ALS.
Other
Total number
In general, neuroglial cells are smaller than neurons. There are approximately 85 billion glia cells in the human brain,[8] about the same number as neurons.[8] Glial cells make up about half the total volume of the brain and spinal cord.[27] The glia to neuron-ratio varies from one part of the brain to another. The glia to neuron-ratio in the cerebral cortex is 3.72 (60.84 billion glia (72%); 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, with 3.76 for the gray and white matter combined.[27] The ratio of the basal ganglia, diencephalon and brainstem combined is 11.35.[27]
The total number of glia cells in the human brain is distributed into the different types with oligodendrocytes being the most frequent (45–75%), followed by astrocytes (19–40%) and microglia (about 10% or less).[8]
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 hematopoietic stem cells. In the adult, microglia are largely a self-renewing population and are distinct from macrophages and monocytes, which infiltrate an 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.
Capacity to divide
Glia retain the ability to undergo cell divisions in adulthood, whereas most neurons cannot. The view is based on the general inability of the mature nervous system to replace neurons after an injury, such as a
Glial cells are known to be capable of mitosis. By contrast, scientific understanding of whether neurons are permanently post-mitotic,[28] or capable of mitosis,[29][30][31] is still developing. In the past, glia had been considered[by whom?] 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 not be entirely true.[32]
Functions
Some glial cells function primarily as the physical support for neurons. Others provide nutrients to neurons and regulate the
Neuron repair and development
Glia are 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 (or also as neuri-lemmocytes) 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.
Myelin sheath creation
Oligodendrocytes are found in the CNS and resemble an octopus: they have bulbous cell bodies with up to fifteen arm-like processes. Each process reaches out to an axon and spirals around it, creating a myelin sheath. The myelin sheath insulates the nerve fiber from the extracellular fluid and speeds up signal conduction along the nerve fiber.[34] In the peripheral nervous system, Schwann cells are responsible for myelin production. These cells envelop nerve fibers of the PNS by winding repeatedly around them. This process creates a myelin sheath, which not only aids in conductivity but also assists in the regeneration of damaged fibers.
Neurotransmission
Clinical significance
While glial cells in the
In addition to neurodegenerative diseases, a wide range of harmful exposure, such as
History
Although glial cells and neurons were probably first observed at the same time in the early 19th century, unlike neurons whose morphological and physiological properties were directly observable for the first investigators of the nervous system, glial cells had been considered to be merely "glue" that held neurons together until the mid-20th century.[43]
Glia were first described in 1856 by the pathologist Rudolf Virchow in a comment to his 1846 publication on connective tissue. A more detailed description of glial cells was provided in the 1858 book 'Cellular Pathology' by the same author.[44]
When markers for different types of cells were analyzed,
Not only does the ratio of glia to neurons increase through evolution, but so does the size of the glia. Astroglial cells in human brains have a volume 27 times greater than in mouse brains.[47]
These important scientific findings may begin to shift the neurocentric perspective into a more holistic view of the brain which encompasses the glial cells as well. For the majority of the twentieth century, scientists had disregarded glial cells as mere physical scaffolds for neurons. Recent publications have proposed that the number of glial cells in the brain is correlated with the intelligence of a species.[48] Moreover, evidences are demonstrating the active role of glia, in particular astroglia, in cognitive processes like learning and memory[49][50] and, for these reasons, it has been proposed the foundation of a specific field to study these functions because investigations in this area are still limited due to the dominance of the neurocentric perspective.[51]
See also
References
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- ^ "Classic Papers". Network Glia. Max Delbrueck Center für Molekulare Medizin (MDC) Berlin-Buch. Retrieved 14 November 2015.
- Perseus Project.
- ^ "The Root of Thought: What do Glial Cells Do?". Scientific American. 2009-10-27. Retrieved 2023-06-12.
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- ^ Hanani, M. Satellite glial cells in sensory ganglia: from form to function. Brain Res. Rev. 48:457–476, 2005
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- ^ Brodal, 2010: p. 19
- ^ Never-resting microglia: physiological roles in the healthy brain and pathological implications A Sierra, ME Tremblay, H Wake – 2015 – books.google.com
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- ^ Aw, B.L. "5 Reasons why Glial Cells Were So Critical to Human Intelligence". Scientific Brains. Retrieved 5 January 2015.
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- ^ Spadaro, Salvatore (2015-09-01). "Towards a Cognitive Gliascience: A Brief Conceptual Framework". journaljsrr.com. Retrieved 2023-06-12.
Bibliography
- ISBN 978-0-19-538115-3.
- Kettenmann and Ransom, Neuroglia, Oxford University Press, 2012,
- Puves, Dale (2012). Neuroscience 5th Ed. Sinauer Associates. pp. 560–580. ISBN 978-0878936465.
Further reading
- PMID 18995817.
- Role of glia in synapse development Archived 2012-02-07 at the Wayback Machine
- Overstreet, LS (February 2005). "Quantal transmission: not just for neurons". Trends in Neurosciences. 28 (2): 59–62. S2CID 40224065.
- Peters A (May 2004). "A fourth type of neuroglial cell in the adult central nervous system". Journal of Neurocytology. 33 (3): 345–57. S2CID 39470375.
- Volterra A, Steinhäuser C (August 2004). "Glial modulation of synaptic transmission in the hippocampus". Glia. 47 (3): 249–57. S2CID 10169165.
- Huang YH, Bergles DE (June 2004). "Glutamate transporters bring competition to the synapse". Current Opinion in Neurobiology. 14 (3): 346–52. S2CID 10725242.
- Artist ADSkyler (uses concepts of neuroscience and found inspiration from Glia)
External links
- "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.