On place and time: microglia in embryonic and perinatal brain development. 
Microglia, the brain-resident macrophages, play key roles in regulating synapse density and homeostasis in the postnatal and adult brain.  However, microglia enter the brain during embryogenesis and recent studies have revealed additional early functions of these immune cells in prenatal and perinatal cerebral development.  Such findings are of importance since prenatal inflammation and microglia dysfunction have been associated with several neurodevelopmental disorders.  This review provides a selective overview of the early roles of microglia, their link with a specific spatiotemporal distribution and how they can be modulated by intrinsic factors or environmental signals.