These are the smallest neuroglial cells. The cell body is flattened. The processes are short. These cells are frequently seen in relation to capillaries. As already stated they differ from other neuroglial elements in being mesodermal in origin. They are probably derived from monocytes that invade the brain during fetal life. They are more numerous in grey matter than in white matter. They become active after damage to nervous tissue by trauma or disease and act as phagocytes.