Dark Microglia Are Abundant in Normal Postnatal Development, where they Remodel Synapses via Phagocytosis and Trogocytosis, and Are Dependent on TREM2

This study examined dark microglia--a state linked to central nervous system pathology and neurodegeneration--during postnatal development in the mouse ventral hippocampus, finding that dark microglia interact with blood vessels and synapses and perform trogocytosis of pre-synaptic axon terminals. Furthermore, we found that dark microglia in development notably expressed C-type lectin domain family 7 member A (CLEC7a), lipoprotein lipase (LPL) and triggering receptor expressed on myeloid cells 2 (TREM2) and required TREM2, differently from other microglia, suggesting a link between their role in remodeling during development and central nervous system pathology. Together, these results point towards a previously under-appreciated role for dark microglia in synaptic pruning and plasticity during normal postnatal development.