TEMPORAL AND SPATIAL REGULATION OF MICROGLIAL TESSELLATION DURING DEVELOPMENT

During development, microglia, the resident immune cells in the brain, proliferate, and colonize the brain parenchyma until they reach a perfect tessellation covering all the brain parenchyma. This mosaic-like distribution is fundamental for the effective surveillance of the brain parenchyma, supporting precise immune responses of the microglial population. However, how this colonization is orchestrated to reach a tessellated distribution remains to be elucidated. To comprehend how microglia achieve their adult density and 3D tessellation we combined experimental data from postnatal mice and 3D mathematical modelling to define the contribution of potential spatial cues and test hypothesis such as contact-inhibition in the control of their proliferation and distribution. The analysis of the single nearest neighbour distance did not show significant differences across ages for all microglia, but proliferative microglia were more distant from each other compared to mitotically quiescent cells, suggesting a potential spatial cue in the control of microglial proliferation. To understand the colonization dynamics in space and integrate our experimental data, we developed a mathematical approach using a 3D agent-based model to analyze the local density of proliferative and mitotically quiescent microglia over time. We combined the model with experimental analysis of their spreading dynamics using long-term 3-photon in vivo imaging in P2-P4 mice, which allowed us to determine microglial velocity and trajectories during brain colonization. Elucidating the laws of attraction and repulsion controlling microglial tessellation will provide a better understanding of the microglial developmental trajectory towards functional maturity.