Dual functions of microglia in the formation and refinement of neural circuits during development

•Microglia control the production and survival of neuronal cells.•Microglia regulate neuronal migration and axonal growth.•Microglia control synaptogenesis and synapse elimination.•Microglia exert dual effects by phagocytosis and secretion of molecules.•Dysfunction of microglia leads to developmental brain disorders. Microglia colonize the central nervous system (CNS) parenchyma during embryogenesis and contribute to various developmental processes leading to the formation of refined neural circuits. In this review, we focus on the bidirectional function of microglia during normal CNS development and discuss recent perspectives on the functions of microglia in neural circuit formation. Microglia participate in neurogenesis, migration, axonal growth, and synapse formation and remodeling, all of which are fundamental for the establishment of neural networks, by secreting a variety of molecules toward neurons and phagocytosing both live and dying neurons or their debris. Intriguingly, microglia play dual roles in each of the neurodevelopmental processes that they affect. For instance, microglia modulate synapse numbers by both promoting the formation of new synapses and eliminating unnecessary synapses. The study of the developmental roles of microglia is essential not only for understanding normal CNS development but also for preventing developmental brain disorders caused by microglial dysfunction.