Embryonic macrophages in brown adipose tissue are spatially associated with developing nerves

Brown adipose tissue (BAT) is a specialized adipose tissue that produces heat through sympathetic nerve-dependent thermogenesis and influences glucose and triglyceride metabolism, making it a potential target for obesity-related diseases. In mice, interscapular BAT (iBAT) emerges on embryonic days (E)14.5-15.5, with extensive vessel and nerve networks still maturing postnatally. These developmental stages are critical for BAT, impacting long-term BAT function. In adult mice, macrophages regulate sympathetic innervation and thermogenesis. However, research on BAT macrophage characters, ontogeny, and functions in early life is lacking. Using single-cell genomics and proteomics, fate-mapping techniques, and 3D imaging, we analyzed iBAT macrophages during early development. We discovered the presence of fetal liver-derived macrophages as early as E15.5, which are later fully replaced by bone marrow-derived macrophages. Transcriptomics data highlight mononuclear phagocyte subpopulations, from which most express macrophage-associated genes (Adgre1, Fcgr1, Csf1r) and monocytic markers (Ccr2, Ly6c2) together. Embryonic macrophages interact closely with developing neural networks, while postnatal bone marrow-derived macrophages associate with blood vessels. Lack of fetal liver-derived macrophages disrupts nerve and vessel development and causes downregulation of genes related to neuronal diseases and protein digestion pathways. Our data suggest that fetal liver-derived macrophages, interacting with BATs developing sympathetic neural network, guide the development of tissue morphology. Understanding further the mechanism behind these events can unravel novel targets for BAT immunomodulation.