A light microscopic and scanning electron microscopic study of intraventricular macrophages in the brains of aged mice.

Intraventricular macrophages in the aged mouse brain (25-31 months) are found mainly on the surface of the choroid plexus. About half the population are flattened cells similar to those described by a number of authors in adult animals. The remainder are partially or completely distended due to the presence of varying amounts of lipid, either in the form of droplets or of foamy masses. These variations in shape are visible in the scanning electron microscope, as well as in semithin sections. The source of the lipid is unclear. It may result from passage of lipid into the cerebrospinal fluid from the ependyma, because many ependymal cells contain large lipid droplets. Alternatively, the lipid may be the result of phagocytosis of degeneration products of epithelial cells of the choroid plexus, either of complete cells, or of parts of cells extruded into the cerebrospinal fluid.