Microglia
Research has discovered dystrophic (defective development) human microglia. "These cells are characterized by abnormalities in their cytoplasmic structure, such as deramified, atrophic, fragmented or unusually tortuous processes, frequently bearing spheroidal or bulbous swellings." The incidence of dystrophic microglia increases with aging. Microglial degeneration and death have been reported in research on Prion disease, Schizophrenia and Alzheimer's disease, indicating that microglial deterioration might be involved in neurodegenerative diseases. A complication of this theory is the fact that it is difficult to distinguish between "activated" and "dystrophic" microglia in the human brain.
Microglia
For a long time it was thought that microglial cells differentiate in the bone marrow from hematopoietic stem cells, the progenitors of all blood cells. However, recent studies show that microglia originate in the yolk sac during a remarkably restricted embryonal period and populate the brain mesenchyme. Additionally, they continuously renew themselves and persist throughout life without replenishment from peripheral monocytic precursors.
Monocytes can also differentiate into myeloid dendritic cells and macrophages in the peripheral systems. Like macrophages in the rest of the body, microglia use phagocytic and cytotoxic mechanisms to destroy foreign materials. Microglia and macrophages both contribute to the immune response by acting as antigen presenting cells, as well as promoting inflammation and homeostatic mechanisms within the body by secreting cytokines and other signaling molecules.