Colocalization of endogenous albumin, cubilin, and megalin in endocytic compartments of normal rat proximal tubules. Section of rat kidney proximal tubule incubated with rabbit anti-rat albumin (1:10,000), sheep anti-rat megalin (1:50,000), and monoclonal mouse anti-rat cubilin (1:2,500) followed by incubation with gold conjugated goat anti-rabbit immunoglobulins (10 nm), donkey anti-sheep immunoglobulins (15 nm), and goat anti-mouse immunoglobulins (5 nm). Megalin (15-nm gold particles; arrows) and cubilin (5-nm gold particles; large arrowheads) colocalize (a) at the brush border (BB) and in endocytic vesicles (E) as well as in the recycling compartment dense apical tubules (D). Albumin (10-nm gold particles; small arrowheads) colocalize with the receptors in endocytic invaginations (I) and vesicles (E) and is concentrated in later endocytic compartments and lysosomes containing only little labeling for the receptors (b). Notice that most tubular structures (D) contain only labeling for the receptors supporting efficient recycling of these in contrast to albumin, which is degraded (b). ×58,000.