PERSPECTIVE Changing Face of Microglia

Microglia are resident brain cells that sense pathological tissue alterations. They can develop into brain macrophages and perform immunological functions. However, expression of immune proteins by microglia is not synonymous with inflammation, because these molecules can have central nervous system (CNS)–specific roles. Through their involvement in pain mechanisms, microglia also respond to external threats. Experimental studies support the idea that microglia have a role in the maintenance of synaptic integrity. Analogous to electricians, they are capable of removing defunct axon terminals, thereby helping neuronal connections to stay intact. Microglia in healthy CNS tissue do not qualify as macrophages, and their specific functions are beginning to be explored. Microglia are less numerous than neuro-glia but about as common as nerve cells.However, until 1991, a leading textbook of neuropathology stated (at the beginning of its chapter on microglia) that “... the microglia have become the most controversial element of the central nervous system; indeed, their very ex-istence is in doubt, ” and the chapter concluded, “... the term ‘microglia,’which implies a single dis