A. Characteristics of normal microglia
Microglia are mesoderm-derived cells (in contrast to the macroglia -- astrocytes, oligodendrocytes, and ependyma -- which are derived from neuroectoderm). Microglia are the CNS representatives of the reticuloendothelial system. One major function is to serve as a fixed macrophage system.
B. Pathological reactions
Microglial cells become activated in response to injury and may respond by
developing elongated nuclei (rod cells)
forming aggregates around areas of tissue necrosis or dying neurons (neuronophagia, microglial nodule or shrub
In addition to microglia, blood-derived macrophages serve as phagocytic cells in the CNS.