Figure 1.

Sources of macrophages. Macrophages are distributed in tissues throughout the body and contribute to both homeostasis and disease. Erythro-Myeloid Progenitors (EMPs) is the source of pre-macrophages. Adult resident tissue macrophages originate during embryonic development rather than from circulating monocytes. During early gestation, macrophages are first observed and expand in the extraembryonic yolk sac during what is termed primitive hematopoiesis. At this stage in development, macrophages are the only ‘‘white blood cell’’ produced, because restricted progenitors in the yolk sac give rise only to macrophages and red blood cells. Later on, bone marrow derived cells will generate circulating monocytes and be independently recruited to the various organs and tissues.