Bone marrow-derived myeloid cells transiently colonize the brain during postnatal development and interact with glutamatergic synapses.

Although the roles of embryonic yolk sac-derived, resident microglia in neurodevelopment were extensively studied, the possible involvement of bone marrow-derived cells remains elusive. In this work, we used a fate-mapping strategy to selectively label bone marrow-derived cells and their progeny in the brain (FLT3IBA1). FLT3IBA1 cells were confirmed to be transiently present in the healthy brain during early postnatal development. FLT3IBA1 cells have a distinct morphology index at postnatal day(P)0, P7, and P14 compared with neighboring microglia. FLT3IBA1 cells also express the microglial markers P2RY12 and TMEM119 and interact with VGLUT1 synapses at P14. Scanning electron microscopy indeed showed that FLT3 cells contact and engulf pre-synaptic elements. Our findings suggest FLT3IBA1 cells might assist microglia in their physiological functions in the developing brain including synaptic pruning which is performed using their purinergic sensors. Our findings stimulate further investigation on the involvement of peripheral macrophages during homeostatic and pathological development.