Embryonic macrophages chronologically change their brain-entry routes and competence for neural progenitor-induced microglial differentiation

Microglia arise from macrophage precursors that enter the developing brain and acquire microglial identity, but how these processes are coordinated remains unclear. We show that despite prominent entry of macrophages from the ventricles into the mouse telencephalon at embryonic day (E) 12.5, they enter from the meninges at E11.5, preferentially in the rostral pallium. Consistent with this regional pattern, whole-mount en face imaging revealed that focal holes in the pial basement membrane were most frequently observed in the rostral pallium. We found that neural progenitors, but not endothelial cells, promote acquisition of microglial properties through a process that requires close cellular proximity and partly depends on TGF-{beta} receptor signaling. This responsiveness is substantially stronger in E13.5 than in E17.5 meningeal macrophages, indicating stage-dependent competence to acquire microglial properties. These findings suggest that microglial establishment is developmentally regulated by macrophage access to the telencephalon and responsiveness to local neural cues.