Fig. 8.

Morphological heterogeneity of F4/80+ microglia in the adult mouse brain. F4/80+ microglia are present in large numbers in all major divisions of the brain, but are not uniformly distributed. There is a more than five-fold variation in the density of immunostained microglial processes between different regions. More microglia are found in gray than in white matter. Microglia vary in morphology depending on their location. Compact cells are rounded, sometimes with one or two short thick limbs, bearing short processes. They resemble Kupffer cells of the liver and are found exclusively in sites lacking a blood–brain barrier. Longitudinally branched cells are found in fibre tracts and possess several long processes which are usually aligned parallel to the longitudinal axis of the nerve fibres. Radially branched cells are found throughout the neuropil. They can be extremely elaborate and there is wide variation in the length and complexity of branching of the processes. The systematic variation in microglial morphology provides evidence that these cells are exquisitely sensitive to their microenvironment. See [38] for further details. Camera lucida drawing courtesy of L.J. Lawson and V.H. Perry. The different panels show: a microglia in the cortex; b macrophages of the subfornical organ, one of the circumventricular organs lacking a blood brain barrier; c microglia of the white matter; d microglia in the ventral pallidum, one of the most densely populated regions of the central nervous system (note the smaller territories of the microglia); e macrophages of the meninges; f macrophages of the choroid plexus. In addition, the central nervous system contains perivascular macrophages which express F4/80 as well as the clearance receptors SR-A and CD206, which are downregulated in resident microglia in normal brain (not shown)