The force awakens: insights into the origin and formation of microglia

• Microglia are derived from erythromyeloid precursors (EMPs) from the yolk sac.
• No turnover of microglia with blood cells during adulthood.
• Bone marrow-derived phagocytes can enter the brain during adulthood after priming of the CNS.
• Cytokines and transcription factors ensure microglia generation and homeostasis.

Microglia are tissue resident macrophages of the central nervous system (CNS) that maintain homeostasis and respond to immune challenges. New genetic fate mapping tools have revealed a yolk sac origin of microglia. Once established in the CNS, microglia persist throughout the lifetime of the organism behind the blood–brain barrier and maintain themselves by self-renewal. Recent studies uncovered a broad spectrum of microglial functions that are influenced by the dynamism of brain formation and neuronal wiring. This review focuses on current findings concerning microglia origin and formation during development and discusses the factors important for microglia survival and function.