An ultrastructural study of intraventricular macrophages in the brains of aged mice.

Intraventricular macrophages in brains of mice aged 25 to 31 months were examined in the transmission electron microscope. A wide spectrum of phagocytic activity was found ranging from resting, inactive macrophages to extremely large active macrophages. Most of the phagocytosed debris appeared to be of lipid origin and a possible sequence of lipid breakdown was postulated. There was evidence of phagocytosis of degenerating choroid plexus epithelial cells and also of mitosis of intraventricular macrophages. The most striking feature of intraventricular macrophages in the ageing brain was the presence of some extremely large cells in which most of the cytoplasm was filled with phagocytosed debris.