Macrophage progenitor cells in mouse bone marrow

Macrophages were described by Elie Metchnikoff in 1884 as a population of phagocytic cells present in loose connective tissue'. Initially macrophages were classified as members of the reticuloendothelial system , which included reticular cells, endothelial cells, fibroblasts, histiocytes and monocytes. However, after recognition of the bone marrow origin of macrophages the concept of the mononuclear phagocyte system has been developed which comprises mature macrophages as well as their precursors in peripheral blood and bone marrow. The cells of the mononuclear phagocyte system form a heterogeneous popUlation of cells which are widely distributed throughout the body (Table I) and which have been shown to play essential roles in a broad variety of biological functions (Table 2) (reviewed in 6-9). To date, there is no single characteristic which marks all members of the mononuclear phagocyte system. Cells are instead assigned to the system on the basis of a combination of several characteristics. The lack of a definite mononuclear phagocyte marker and the extensive heterogeneity among members of the system has sometimes made it difficult to detennine whether a particular cell type belongs to the mononuclear phagocyte system or not. For example, in the past there has been much debate whether dendritic cells (Langerhans cells, interdigitating cells, veiled cells, and ill vitro dendritic cells 10) should be included in the mononuclear phagocyte system (reviewed in 11). Recently, an increasing amount of data has accumulated supporting the view that at least part of the dendritic cells do indeed belong to the mononuclear phagocyte system (reviewed in 12). This will be discussed in more detail later in this chapter.

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