I've written at some length in the past about "the brain's silent majority": the 90 percent of the cells in our central-nervous system that aren't nerve cells. Collectively called "glia" (a rather demeaning term that comes from the Greek word for glue), this collection of non-nerve brain cells has long gone unsung.
Stanford neurobiologist Ben Barres, MD, PhD, has been in the forefront of research proving that, despite their relatively sparrowlike demeanor in comparison to nerve cells' pyrotechnic bursts of electrochemical activity, the three glial cell types - astrocytes, oligodendrocytes, and microglia - are silent superstars, juggling scores of tasks crucial to brain development and function.
In a just-published review in Science, Barres and two colleagues describe the role of microglia (which - unlike nerve cells or the other glial-cell types, all descended from brain-based precursor cells - trace their developmental origins to the immune system) in brain health. The upshot: Malfunctioning microglia may underlie many neurodegenerative conditions worrisome to an aging population as well as to physicians and researchers.
Barres and his co-authors write:
The meaning of the term "inflammation" has undergone considerable evolution. Originally defined by Celsus' four cardinal signs of "tumor, rubor, calor et dolor" [swelling, redness, heat, and pain], inflammation typically [involves the leakage] of blood cells [from the bloodstream into tissues].... The word "neuroinflammation," however, is increasingly used to identify a radically different set of conditions ... specific to the central nervous system (CNS). Whereas viral, bacterial, and autoimmune diseases of the CNS can resemble their [non-nervous-system] counterparts [in formal appearance], the concept of "neuroinflammation" has gradually expanded to also describe diseases that display none of Celsus' cardinal signs, [that] do not attract conventional inflammatory cells, and that most neuropathologists would classify as degenerative rather than inflammatory. These "inflammatory" changes are restricted to a cell type exclusive to the CNS: the microglia.
Among the many jobs performed by microglia is their surveillance of the entire brain for signs of injury, to which they rapidly respond by migrating to the site, tackling pathogens, and gobbling up damaged nerve-cell components to speed repair. Another item on the microglial resume is what the Science review's authors call the "sculpting of neural circuits": Microglia help prune away excessive or inappropriate brain-cell connections.
But one can readily imagine how that latter process, if it gets out of hand, could cause problems. The authors suggest that the pathological underpinnings of not only neurodegenerative diseases such as Alzheimer's but even so-called neurodevelopmental conditions including autism-spectrum and obsessive-compulsive disorders - not traditionally thought of as inflammatory - may turn out to harbor a microglial component. "As our tools and technology for studying microglia grow," they write, "so too will our understanding of their role in the healthy brain."
Previously: Unsung brain-cell population implicated in variety of autism, Brain police: Stem cells' fecund daughters also boss other cells around, Research shows eating berries may boost brain health
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