The ultrastructure of normal and reactive microglia.

Normal microglia have a distinct morphology. In rapidly, but now in slowly evolving pathological states the features used to identify the resting cell are often lost. When there is invasion by haematogenous monocytes, phagocytes develop whose origin - cerebral or haematogenous- cannot be ascertained on morphological features alone. These observations stress that microglia are part of the reticulo-endothelial system.