Two phenotypically and functionally distinct microglial populations in adult zebrafish.

Microglia are the tissue-resident macrophages in the central nervous system and are critically involved in immune defense, neural development and function, and neuroinflammation. The versatility of microglia has long been attributed to heterogeneity. Recent studies have revealed possible heterogeneity in human but not in murine microglia, yet a firm demonstration linking microglial heterogeneity to functional phenotypes remains scarce. Here, we identified two distinct microglial populations in adult zebrafish that differ in morphology, distribution, development, and function. The predominant population, phagocytotic microglia, which expresses , is broadly distributed, amoeboid in shape, highly mobile, and phagocytotic. The other white matter-enriched population, regulatory microglia, has ramified protrusions but has limited mobility and phagocytosis capability. These functional differences are further supported by distinct transcriptomes and responses to bacterial infection, where microglia function in tissue clearance and microglia release immune regulators. Our study sheds light on the heterogeneity and functional diversification of microglia.