Microglia

Microglia are the brain's own macrophage population, but they are not blood-derived: they arise from yolk sac progenitors, colonise the central nervous system before the barrier closes, and maintain themselves by local self-renewal. In healthy tissue they are highly ramified and constantly extend and retract processes, sampling their surroundings and contacting synapses. They respond to pathogen-associated patterns through Toll-like receptors and to nucleotides released by injured cells, and they secrete cytokines including tumour necrosis factor alpha and interleukin-1 beta. They also prune synapses during development through complement, a mechanism reactivated in disease.

Genetics made them central to neurodegeneration research. Coding variants in TREM2, a microglial receptor, substantially raise Alzheimer's disease risk, and many common risk loci found by genome-wide association are expressed preferentially in myeloid cells rather than neurons. In humans the usual in-vivo measure is positron emission tomography with a translocator protein ligand.

Activation is not a direction of travel with a good end and a bad end. The same cell clears debris and plaque, supports remyelination, strips synapses and amplifies injury, depending on stimulus and timing, so blanket suppression is a poor design goal and the M1 and M2 labels borrowed from macrophage culture do not describe brain states.

Two errors are common. Claims that a compound reduces microglial activation often rest entirely on Iba1 immunostaining intensity or a change in cell shape, morphological proxies rather than functional measurements. And translocator protein imaging is read as neuroinflammation when the ligand also binds astrocytes and vascular cells, and when a common binding polymorphism sorts people into high, mixed and low affinity groups that must be genotyped before any comparison holds.