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Astrogliosis, also called astrocytosis, is an increase in the number of astrocytes in a person's nervous system to above-normal levels. Astrocytes, also called astroglia, are a type of non-neuron cell, or glial cell, in the nervous system that play an important role in the nervous system. Astrogliosis occurs after the death of neurons due to injury or disease and is one of the body's natural mechanisms for protecting the nervous system from damage.
Astrocytes, so called because of their star-like shape, are cells responsible for tasks such as clearing out and recycling excess neurotransmitters, regulating the amount of ions in the fluid outside of cells, and regulating electrical impulses in the synapses between neurons. They are also involved in repairing and protecting the nervous system by forming glial scar tissue over damaged parts of that system after neuronal death, filling in empty areas left by the deaths of neurons and extending their own cellular membranes to form protective barriers. This function is why astrogliosis accompanies damaging brain infections and injuries, as the body tries to maintain the integrity of the nervous system and protect its surviving neurons from further harm. Unlike most neurons, astrocytes can continue to reproduce through cell division even in adulthood, allowing their numbers to spike upward in response to injury.
The phenomenon of strogliosis is most commonly seen in cases of brain infections caused by malformed proteins called prions, which most commonly affect a new host after being ingested. They are the cause of a group of diseases in many species called transmissible spongiform encephalopathy. Well-known examples include kuru and Creutzfeldt–Jakob disease, which both affect humans, and bovine spongiform encephalopathy, also known as mad cow disease. There are also hereditary prion diseases, such as a hereditary form of Creutzfeldt-Jakob disease and an extremely rare condition called fatal familial insomnia, caused by defective copies of genes responsible for protein replication. Prion infections propagate rapidly as the infecting proteins induce normal, healthy proteins in the body to transform into more prions, killing neurons and spurring the nervous system's astrocytes to grow and replicate to form glial scar tissue to mitigate the damage.
Astrogliosis can help to protect the nervous system from further damage, but it also prevents regeneration afterward. The astrocytes and other glial cells create a physical barrier to the regrowth of axons at the site of the damage and release chemicals that inhibit such axonal growth. As a result, controlling astrogliosis to suppress scar formation is being heavily researched by scientists studying ways to repair nervous system damage.