Microglia are the resident immune cells of the central nervous system and are essential for brain development, homeostasis and response to injury or disease. Originally thought of primarily as inflammatory cells, they are now recognized as highly dynamic regulators of neural circuits.

During development, microglia survey the brain environment with motile processes, pruning excess synapses and shaping neural connectivity. They phagocytose apoptotic cells and debris and secrete signaling molecules that influence neuronal survival and differentiation. Their interactions with synapses are activity dependent, linking microglial function to experience driven circuit refinement.

In the healthy adult brain, microglia remain constantly active, monitoring tissue for signs of damage or infection. They help maintain homeostasis by clearing misfolded proteins, remodeling synapses and regulating myelin turnover. Their responses are finely tuned through receptors for neurotransmitters, cytokines and damage associated molecules.

In disease, microglia adopt diverse reactive states that can be protective or harmful. In neurodegenerative disorders such as Alzheimer’s disease, they recognize aggregated proteins, cluster around plaques, and attempt clearance through phagocytosis. At the same time, chronic activation and release of inflammatory mediators can exacerbate synaptic loss and neuronal dysfunction. Genetic studies have identified microglial genes and pathways that modify risk and progression in these disorders.

Microglial heterogeneity is now a major research focus. Distinct transcriptional and functional states have been identified across brain regions, developmental stages and pathological conditions. Understanding how microglia transition between beneficial and detrimental states, and how they communicate with neurons, astrocytes and the vasculature, is guiding efforts to develop targeted therapies that modulate microglial function in brain diseases.