Differences between Monocyte-Derived and Tissue Macrophages

Abstract

The bone marrow origin of monocytes has been proven, and it is widely accepted that, in vivo, monocytes are the antecedents of the macrophages occurring in inflammatory processes (16). Under in vitro conditions too, monocytes can differentiate into macrophages undergoing morphologically, cytochemically, and biochemically detectable changes similar to those found in vivo (9). Although it has been repeatedly stressed that tissue macrophages, both free and fixed, also derive from blood monocytes (17), this assumption has not been definitely proven. In fact, the study of the fate of blood monocytes in the tissues has been seriously hampered by the lack of reliable criteria for the identification of monocyte-derived macrophages and tissue macrophages. Recently (12), characteristic differences were found in the distribution and nature of perioxidatic (PO) activity between monocytes on the one hand and tissue macrophages, such as those in the peritoneal cavity and liver, on the other. Evidence was also obtained that monocyte-derived macrophages can retain the PO-activity of the monocytes and thus could be distinguished from the resident macrophages (10). The conclusion was drawn that two types of macrophages exist: exudate and resident macrophages, the former being directly derived from monocytes. To verify this conclusion, monocytes isolated from the peripheral blood of normal mice were cultivated in intraperitoneally installed diffusion chambers. In addition, macrophages from the unstimulated peritoneal cavities of mice as well as monocytes in saline-induced peritoneal exudates were studied with respect to the distribution of PO-activity.