The identity of microglia, compared to other CNS-associated macrophages in the perivascular space, choroid plexus, and leptomeninges, is established early on from yolk-sac-derived progenitors. Once they colonize the brain parenchyma and differentiate, they can adopt multiple states depending on the particular spatiotemporal context, as shown in more detail in Figure 5. Created with BioRender.com.