Different functional states of microglia can be defined morphologically. A, ramified microglia exhibit highly branched processes with which they explore their surrounding microenvironment. B, activated microglia retract processes and become enlarged due to organelle build-up and increased metabolic activity. C, phagocytic microglia often appear as rounded brain macrophages. D, dystrophic microglia most characteristically display beaded, twisted or fragmented processes. Human cerebral cortex stained with Iba1 antibody. Bar = 20 μm.