Role of bristle-coated membrane in the uptake of ferritin by rat macrophages

The distribution of horse ferritin in rat peritoneal macrophages was studied by electron microscopy. In the first few minutes after exposure of the cells to ferritin either in vivo or in vitro, the protein is found in invaginations of the surface membrane not yet isolated from the external medium. These caveolae are characterized by the presence of a prominent bristle coat on their cytoplasmic aspect and a less distinct fuzzy coat on the surface exposed to the exterior of the cell. At later stages ferritin is found in bristle-coated vesicles and later in larger smooth-surfaced vacuoles. Counts of bristle-coated membrane sites before and after exposure to ferritin suggest that ferritin is selectively bound to preformed patches of bristle-coated surface membrane which subsequently invaginate to form the coated vesicles. The question of possible induction of such sites by the presence of ferritin is raised but cannot be settled by the available data.