Microglia

The smallest of the glial cells, microglia are the immune system cells. It is derived from bone marrow monocytes, and they migrate into the nervous system during the development where they secrete growth factors, guide axons, stimulate discrimination of other glial cells and the formation of blood vessels. They can transform into macrophages that phagocytose debris created by the developmental programmed cell death. In adults they cease being motile unless the nervous system damage happens (trauma, infections, and tumors) whenever they proliferate, revert to their macrophage lifestyle, and are responsible for the inflammatory processes.