<b>IMMUNOCYTOCHEMICAL EVIDENCE FOR A VESICULAR TRANSPORT OF ALBUMIN ACROSS THE EPITHELIUM IN THE RAT INTERLOBULAR BILE </b><b>DUCT </b>

Abstract

Mechanism of transport of serum albumin from intercellular fluid into the bile duct was investigated by a pre-embedding immunocytochemical technique. Rat livers were fixed by perfusion, and vibratome sections were incubated with anti-rat serum albumin (Fab fragments) conjugated with peroxidase. After washing in phosphate-buffered saline (PBS), sections were fixed in glutaraldehyde, followed by incubation in diaminobenzidine (DAB)-H2O2 medium for peroxidase, The sections were then embedded in Epon. On light microscopy, the DAB reaction product (antigenic sites for albumin) was seen in the interlobular connective tissue in which bile ducts were embedded. On electron microscopy, electron dense reaction products were located in interdigitating lateral intercellular space between biliary epithelial cells. The staining was confined to the space at the level of desmosome; zonula adherens and zonula occludens were not stained. On the basal and lateral surface of the epithelial cells, numerous albumin-positive endocytotic vesicles were seen. Multivesicular bodies and peripheral dilatations of Golgi lamellae were also frequently stained. The results suggest that serum albumin is transported across the biliary epithelium by vesicular, intracellular traflic and that diffusion of albumin through the intercellular space between the biliary epithelial cells does not occur under normal conditions.