Glial cells (Greek γλία, γλοία "glue"), are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the brain, and for neurons in other parts of the nervous system such as in the autonomous nervous system. In the human brain, there is roughly one glia for every neuron with a ratio of about two neurons for every glia in the cerebral gray matter.
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: 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. For over a century, it was believed that they did not play any role in neurotransmission. That idea is now discredited; they do modulate neurotransmission, although the mechanisms are not yet well understood.
During early embryogenesis glial cells direct the migration of neurons and produce molecules that modify the growth of axons and dendrites. 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 CNS (Central Nervous System), 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 PNS (Peripheral Nervous System), 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.
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.
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.
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%.
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