The origin of brain macrophages or ldquo;reactive microgliardquo; has been the subject of considerable controversy. The fundamental question is whether or not there is a morphologically and functionally distinct population of cells, called microglia, which are resident in normal brain and differentiate into macrophages in response to inflammatory stimuli. The present study was performed to determine if any cells in the normal brain have the common markers of mononuclear phagocytes; phagocytosis, IgGFc receptors or macrophage specific antigens. In studies of the newborn and the adult murine brain and adult human brain no cells were detected which had any of those markers, although the highly sensitive marker methods were capable of detecting mononuclear phagocytes in all other tissues where they are known to occur. The results suggest that microglia, if they exist as a distinct cell type, are unrelated to mononuclear phagocytes. Furthermore, they suggest, but do not prove, that all inflammatory macrophages are derived form hematogenous precursors.
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