Microglia
Also known as: microglial cell, Hortega cells, brain macrophages
The resident macrophages of the CNS, making up ≈5–12% of CNS cells. They derive from yolk-sac progenitors, not neuroectoderm. They survey the brain continuously, phagocytose debris and pathogens, and prune synapses during development.
Overview
Microglia are small cells with elongated, rod- or comma-shaped nuclei. In the healthy brain they are ramified: fine motile processes constantly sample their territory, contacting synapses every few minutes. They originate from yolk-sac erythromyeloid progenitors that colonise the brain before the blood–brain barrier forms, and they self-renew locally throughout life. They are therefore developmentally distinct from all other glia and from bone-marrow-derived monocytes.
When activated by injury, infection or protein aggregates, microglia retract their processes and become amoeboid. In this state they migrate, proliferate and phagocytose debris, dying cells and microbes. They release cytokines (IL-1β, TNF-α), chemokines and reactive oxygen species, and upregulate MHC class II to present antigen. During development and in learning they prune synapses, engulfing complement-tagged synapses (C1q, C3). Chronic activation contributes to neurodegeneration, and microglial genes such as TREM2 are risk factors for Alzheimer disease.
Anatomy
- Histology
Small, dense, elongated nuclei on H&E. Iba1 or CD68 immunostains show the ramified processes. In encephalitis they cluster as microglial nodules, and they surround dying neurons (neuronophagia).
- Development
Mesodermal origin (yolk sac), entering the neural tube around embryonic week 4–5 in humans.
Function
- Immune surveillance of the CNS parenchyma.
- Phagocytosis of debris, apoptotic cells, myelin and pathogens.
- Antigen presentation and cytokine signalling.
- Synaptic pruning and refinement of circuits.
- Support of tissue repair, with potential to drive chronic neuroinflammation.
Clinical significance
HIV and the CNS
HIV infects microglia and perivascular macrophages. Fusion of infected cells produces multinucleated giant cells, the hallmark of HIV encephalitis, and contributes to HIV-associated neurocognitive disorder.
Neurodegeneration
Microglia cluster around amyloid plaques in Alzheimer disease. TREM2 loss-of-function variants raise the risk of Alzheimer disease about three-fold, implicating impaired microglial clearance.
Test yourself
Which glial cell is derived from yolk-sac myeloid precursors rather than neuroectoderm, and acts as the resident macrophage of the CNS?
- Astrocyte
- Microglia
- Ependymal cell
- Oligodendrocyte
Answer: Microglia
Microglia colonise the brain early in embryonic life from yolk-sac erythromyeloid progenitors. They survey the brain, phagocytose debris, prune synapses and present antigen. All the other glia come from the neural tube or neural crest.
Compiled from standard anatomy and physiology references — see the Nervous System sources. For education only; not medical advice.