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.
Role of microglia in disease
Microglia synthesize amyloid precursor protein (APP) in response to excitotoxic injury. Plaques result from abnormal proteolytic cleavage of membrane bound APP. Amyloid plaques can stimulate microglia to produce neurotoxic compounds such as cytokines, excitotoxin, nitric oxide and lipophylic amines, which all cause neural damage. Plaques in Alzheimer's disease contain activated microglia. A study has shown that direct injection of amyloid into brain tissue activates microglia, which reduces the number of neurons. Microglia have also been suggested as a possible source of secreted β amyloid.