Fig. 1.

Origins and distribution of tissue macrophages. During development, erythromyeloid progenitors from yolk sac and foetal liver give rise to tissue-resident macrophages which persist during adult life as long-lived cells of widely varying mophology that turn over locally. Around the time of birth, bone marrow haemopoietic stem cells (HSC) become the source of blood monocytes, replenishing resident populations with high turnover, such as gut, and in response to increased demand. Therefore, different tissues contain varying mixtures of embryo and marrow-derived macrophages. In response to inflammation, immune and pathologic responses, monocytes infiltrate tissues and give rise to activated macrophages with complex phenotypes. Chronic immune cell aggregates can give rise to macrophage-rich granulomas, containing multinucleated giant cells as a result of monocyte/macrophage fusion. Monocytes contribute to osteoclast multinucleation and also generate functional dendritic cells upon culture in GM-CSF, with or without IL-4. Distinct monocyte populations give rise to DC [111], activated [111] and fibrogenic [18] macrophages