Microglia out of place—mapping macrophages across the developing human body

Macrophages (MΦs) are versatile cells found in every organ. Throughout the body, they work tirelessly to maintain tissues. To this end, they are instructed by their respective niches to express organ-specific genes and proteins, resulting in a variety of cell morphologies and highly specialized functions across different tissues. A recent study by Wang et al. used single-cell RNA sequencing to comprehensively profile human MΦ phenotypes across 19 organs between postconception weeks (PCW) 4 and 26. This study revealed a surprising diversity of these cells and suggests new avenues of research.