Developmental derivation of embryonic and adult macrophages

The macrophage cell lineage continually arises from hematopoietic stem cell s during embryonic, fetal, and adult life. Previous theories proposed that macrophages are the recent progeny of bone marrow-derived monocytes and tha t they function primarily in phagocytosis. More recently, however, observat ions have shown that the ontogeny of macrophages in early mouse and human e mbryos is different from that occurring during adult development, and that the embryonic macrophages do not follow the monocyte pathway. Fetal macroph ages are thought to differentiate from yolk sac-derived primitive macrophag es before the development of adult monocytes, Further support for a separat e lineage of fetal macrophages has come from studies of several species, in cluding chicken, zebrafish, Xenopus, Drosophila, and C. elegans. The presen ce of fetal macrophages in PU.1-null mice indicates their independence from monocyte precursors and their existence as an alternative macrophage linea ge. (C) 2000 Lippincott Williams & Wilkins, Inc.