Microglia are mononuclear phagocytes that reside within the central nervous system (CNS). They differ from macroglia (astrocytes and oligodendrocytes) in terms of their origin, phenotype and functions, but more closely resemble tissue-resident macrophages in all these aspects. The principal role of microglia is to provide a first line of defence against pathological insults at this primary site. Modern consensus holds that microglia are of myeloid origin, much like tissue-resident mononuclear phagocytes within other organs, and arise during fetal development from progenitors in the yolk sac, liver or spleen or from mesenchymal tissues surrounding the nervous system that subsequently seed the CNS during gestation and perinatally, and differentiate morphologically to ramified and immunophenotypically suppressed adult varieties (20, 31). These intriguing and controversial cells have been the focus of intense scientific research for the past two decades, and the subject of many recent reviews to which the reader is referred (8, 11, 12, 14-16, 18, 20-22, 24-27, 31-34, 54-57)
