Microglia,the resident macrophages of the brain,originate from yolk sac progenitors and invade the brain at embryonic stages to progressively become integrated in the parenchyma.However, the mechanisms driving the acquisition of the mature microglia phenotype during development are yet unclear.Our hypothesis is that the intrinsic genetic programs that establish the microglial identity early on are fine-tuned by the brain environment.To test this hypothesis,we focused on the hippocampus and the cerebellum,two structures with protracted development.We found that microglial development occurs in two stages:an initial colonization followed by a functional maturation.Seeded microglial progenitors were highly proliferative in the first postnatal days and the microglial population reached a maximum density by postnatal day P14.To model the colonization process,we developed a series of mathematical simulations using differential equations and Monte Carlo simulations.The optimal model fitness was obtained using a two-cell type model,in which a small population of proliferative microglia faded off by P5 and gave rise to a large population of quiescent,post mitotic microglia.After the initial seeding and colonization,maturity was reached by acquisition of a branched morphology and optimal phagocytosis efficiency, which occurred between P14 to P21.The model also predicted an elongation of cell cycle duration with age,that was experimentally validated by sequential injections of EdU-BrdU at different time windows:at P2 we found a cell cycle duration of 12 hours,increasing by P7 to 24 hours.Deciphering the microglial maturation program is highly relevant because early alterations could be genetically imprinted and lead to long-term functional deficits,impacting on the onset and progression of neurodevelopmental and neurodegenerative disorders.