CELLULAR-FORMS AND FUNCTIONS OF BRAIN MICROGLIA

Consistent with the recent characterization of microglial cells as mac rophages, an overall picture for the unique function of these cells in CNS tissue has developed. The microglia are derived from blood monocy tes that migrate into the tissue during fetal development and subseque ntly remain after complete formation of the blood-brain barrier. These monocytes give rise to the ramified microglia of adult tissue through the developmental intermediate of amoeboid microglia. Ramified microg lia appear uniquely adapted in contrast to other tissue macrophages ba sed on their stability or lack of turnover and mitotic capability. The ramified cells, while usually downregulated, can convert into active macrophages termed reactive microglia; this conversion appears to occu r nonspecifically in response to any injury. Further, reactive microgl ial cells can fuse to form giant multinucleated cells during viral inf ections. Each microglial cell form possesses a characteristic morpholo gy and differing functional state with regard to macrophage activity. In their role as tissue macrophages, microglia are involved in immune responses, tissue transplantation, and AIDS dementia complex, as well as many other neurological mechanisms and diseases.