The origins and homeostasis of monocytes and tissue-resident macrophages in physiological situation.

Monocytes and macrophages are critical effectors and regulators of innate immune response. They not only play crucial and distinctive roles in homeostasis, but also contribute to some pathologic processes. The heterogeneity of the macrophage lineage has been widely recognized and, in part, is a result of the specialization of resident macrophages in particular tissue microenvironments. Monocytes are usually known to originate in the bone marrow from a common myeloid progenitor that is shared with neutrophils, and they are then released into the peripheral blood. However, the origin of tissue-resident macrophages, crucial for homeostasis and immunity, has remained controversial until recently. During embryonic organogenesis, macrophages derived from yolk sac and fetal liver precursors are seeded throughout tissues, persisting in the adulthood as resident, self-maintaining populations. After birth, bone marrow-derived monocytes can replenish tissue resident macrophages following injury, infection and inflammation. In this review, we will mainly summarize our current understanding on the origin, ontogeny and fates of tissue macrophages and will briefly discuss the molecular regulation of resident macrophage homeostasis in physiological situation.