Apoptotic cell removal in development and tissue homeostasis

Cell deletion is a physiological process for the development and maintenance of tissue homeostasis in metazoa. This is mainly achieved by the induction of various forms of programmed cell death followed by the recognition and removal of the targeted cells by phagocytes. In this review, we will discuss cell deletion in relation to the development and function of the innate immune system, particularly of the mononuclear phagocyte system (MPS), its ontogeny and potential role in tissue remodeling in the embryo and adult. Ongoing studies are addressing the roles of professional phagocytes of the MPS and neighboring tissue cells in dying cell removal, and candidate molecules that might attract mononuclear phagocytes to the dying cells. The potential phagocyte must discriminate between living and dying cells; current concepts for this discrimination derive from the observation of newly exposed ligands on the dying cells and new evidence for direct inhibition of uptake by viable cells.