Chapter 12 : Microglia

Abstract:

Microglia are the resident macrophages of the brain parenchyma ( 1 ). Although it has long been known that microglia are of myeloid lineage, based on immunocytochemical detection of macrophage-restricted antigens ( 2 ), it has only relatively recently been shown, by fate mapping studies, that these cells are of yolk sac origin and enter the developing neuroepithelium of the central nervous system (CNS) in the embryo ( 3 ). They are present throughout the length of the neuraxis, characterized by their fine processes emanating from a small cell body, and each cell appears to occupy its own territory. The morphology of microglia and their territorial behavior is well illustrated in retina whole mounts ( Fig. 1 ). The density and morphology of the microglia vary between distinct functional divisions of the CNS, with the lowest density found in the cerebellum and perhaps the highest density in the substantia nigra ( 4 ). These regional differences have been well studied in rodents, the most common experimental animal models, and although similar regional differences are seen in the human brain, there are some notable differences. In the rodent brain, the microglia are denser in gray matter than in white matter, while in the human brain, the microglia are denser in the large-fiber tracts that dominate the larger brain ( 5 ).