Galactose-specific adsorptive endocytosis: an ultrastructural qualitative and quantitative study in cultured rat hepatocytes.

Using galactosylated bovine serum albumin coupled to colloidal gold (galBSA-CG) as a probe, the receptor-mediated endocytosis pathway has been analyzed qualitatively and quantitatively at the ultrastructural level in cultured rat hepatocytes. The results showed that galBSA-CG was specifically recognized by the asialoglycoprotein receptor of the hepatocytes, thus confirming biochemical findings. The probe was preferentially bound in coated pits of the cell surface. When bound elsewhere on the plasma membrane, it apparently moved towards coated regions. In both cases, it was then internalized via coated vesicles. The galBSA-CG passed through pleiomorphic tubular structures and in endosomes, a pool of smooth-surfaced vesicles of various size (50-350 nm), before transiently accumulating in multivesicular bodies. The latter then fused with lysosomes where the glycoproteinic moiety of the probe was degraded, as judged by the flocculated aspect of the accumulated gold particles. About 10% of the internalized ligand was recycled back to the cell surface via secreting vesicles containing lipoprotein-like particles without having apparently passed through lysosomes, which suggests the existence of a pre-lysosomal sorting mechanism of the endocytosed material. Functional recovery of the morphologically restored biliary polarity of hepatocytes in culture was indicated by the fact that galBSA-CG finally appeared in the reconstituted bile canaliculi.