In-vivo endocytosis by bristle-coated pits and intracellular transport of endogenous albumin in the endothelium of the sinuses of liver and bone marrow.

The localization of endogenous albumin in the endothelia of the sinuses of bone marrow and liver of rats was studied by means of rabbit anti-rat albumin Fab fragment conjugated with horseradish peroxidase (HRP) visualized by 3,3' diaminobenzidine tetrachloride (DAB)-osmic acid. Albumin reaction product was present in the bristle-coated pits, bristle-coated vesicles, transfer tubules, and in larger vesicles of the endosome-lysosome system. The presence of endogenous albumin in the components of the vacuolar apparatus suggests that these endothelia may be active in the catabolism of endogenous albumin. Because of the ability of albumin to bind to a large variety of chemically different substances, the possible role of endogenous albumin in the internalization of certain substances by the bristle coated pits and bristle coated vesicles of these sinus endothelia needs to be considered.