The myeloid lineage comprises several functionally and developmentally distinct subsets, including circulating monocytes, and tissue-resident macrophages. Several studies have shown that tissue-resident macrophages, but not circulating monocytes, emerge from the yolk sac during embryogenesis independent of the LT-HSC. Brain and skin resident macrophages (i.e., microglia and Langerhans cells, respectively) emerge at around embryonic day 8 (E8) in a region of the yolk sac known as the blood island, even before the development of the first definitive LT-HSC. The yolk sac-derived (HSC-independent) macrophages then migrate and take long-term residence in the brain and skin of the developing embryo.
Recently, we have identified and characterized two functionally distinct subsets of tissue-resident peritoneal macrophages named small (SPM) and large (LPM) peritoneal macrophages. Currently, we are studying their origin and functional relationship with other tissue-resident macrophages, such as brain microglia, liver Kupffer, and skin Langerhans cells.