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.
Gliosis
Microglial activation has been shown to be a reactive process in which microglia respond to signals given off by injured neurons. Because various characteristics of microgliosis occur in different time frames after the initial triggering insult, microgliosis must depend on mechanisms which fluctuate temporally based on injured neuronal signals. Studies have shown that in cases of reversible neuronal injury, such as axotomy, neuron signals cause microglia to produce trophic factors, which promote neuron survival. In cases of irreversible injury, however, microglia are induced to release neurotoxic factors that promote increased degeneration of the neuron and more rapid phagocytosis by the microglia. Such specificity of the process of microgliosis indicates that it primarily serves a beneficial purpose, selectively conserving some neural tissue while eliminating others, based on the specific initial CNS insult.