Is silica involved in neuritic (senile) plaque formation?

The agent responsible for inducing neuritic (senile) plaque formation in senile dementia of the Alzheimer's type and in the ageing non-demented brain is unknown. Other workers have detected a high concentration of silicon in the rims and cores of senile neuritic plaques. We have therefore looked at whether the reaction of brain tissue to silica particles resembles a neuritic plaque. In this study both fine (10 nm) and coarse (less than 5 microns) particles of silica have been introduced into the brains of rats and mice using a wide range of doses and several methods of administration. The reaction of the brain to the presence of the silica was examined by light and electronmicroscopy up to one year after the injection. The presence of silica particles in the brain resulted in the proliferation of fibrous astrocytes and macrophages and strongly stimulated the production of collagen fibres. Degeneration of some adjacent axons and axon terminals occurred, but there was no detectable deposition of amyloid which is characteristic of senile plaques. Coarse particles of silica invariably produced a more intense reaction than fine particles. The reaction of the brain did not diminish with time within one year of injection. The possible significance of the presence of silica in the plaque as a secondary phenomenon is discussed.