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: first, an initial colonization driven by highly proliferative progenitors that faded off by P5 and gave rise to a 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. Hence, we have identified the time-window in which there is a switch in the developmental program of microglia from colonization to functional maturation.

To identify the specific signals that drive microglia developmental milestones, we explored the influence of neuronal-related cues such as neuronal scaffolding, secreted factors, and neuronal activity. First, we focused on reelin, a protein involved in the scaffolding used for migration of neuronal progenitors. Then, we focused on neuronal secreted factors such as IL-34, key regulator of microglial proliferation. Finally, to explore the influence of neuronal activity on microglial maturation we inhibited neuronal activity by iDREADDS. Elucidating the regulators of microglial developmental milestones will provide a better understanding of neurodevelopmental disorders and the impact of early alterations on microglial maturation.