Fig. 1

Phenotypic distinctions between homeostatic and “activated” microglia. Under typical conditions (A), microglia display a ramified morphology and a unique pattern of gene expression, making them identifiable through a number of different techniques. However, a variety of immunogenic stimuli can elicit dramatic morphological and functional changes in microglia. Immunologically activated microglia (B) possess an amoeboid morphology; marked by increased soma size and less ramified processes. Markers for microglia activation vary widely depending on the type and severity of insult, but surface proteins associated with classical immune functions (e.g., antigen presentation, phagocytosis) tend to be increased, while those associated with homeostasis may be reduced. Functions of activated microglia are also highly variable, with the same cells potentially driving tissue damage and repair processes simultaneously. Morphological features may not correspond to assumed functional roles. Nonetheless, classical immune functions of activated microglia include phagocytosing cellular debris, large increases in cytokine signaling, recruitment of peripheral immune cells to the parenchyma, and destroying infected or dying cells