Current views on the ontogeny of macrophages and the humoral regulation of monocytopoiesis.

Macrophages do not constitute a stable population but are constantly renewed by an influx of monocytes and by local division of immature mononuclear phagocytes that have also recently arrived in the tissues. The calculated mean turn-over time of tissue macrophages lies between four and six days. During an inflammatory response the production of monocytes in the bone marrow is increased. This process is regulated by a humoral factor, FIM, which is formed and secreted by tissue macrophages. The response of monocyte precursors to FIM is under genetic control.