In vivo endocytosis by bristle coated pits of protein tracers and their intracellular transport in the endothelial cells lining the sinuses of the liver. I. The endosomal disposition.

The present observations describe the endocystosis and intracellular transport in the endothelial cells lining the sinuses of the rat liver and occurring 3 min after intravenous administration of two protein tracers: horseradish peroxidase and ferritin. Internalization of the proteins is exclusively by bristle coated pits followed by intracellular sequestration in bristle coated vesicles (diameter 90-210 nm). No other form of endocytosis is present in the endothelial cells of the liver sinuses. The bristle coated vesicles fuse with smooth membrane lined tubules (diameter 20-70 nm) termed transfer tubules. In this way the tracers are deposited into the lumens of the transfer tubules. The transfer tubules in turn fuse with preexisting electron-lucent endosomes which then receive the protein markers. In addition, direct fusion of bristle coated vesicles with endosomes occurs in limited measure. Bristle coated vesicles may fuse with other bristle coated vesicles while losing their clathrin coat, resulting in the formation of new endosomes which may enlarge through fusion with additional bristle coated vesicles or perhaps also with transfer tubules. The sinus endothelium also contains lysosomal vesicles which appear as dense bodies or multivesicular bodies. No protein tracers have been observed in these dense bodies at the time interval (i.e., 3 min) examined. Neither has a direct fusion between endosomes and lysosomes been found. The transfer tubules may be continuous with or may be derived from cisternae of the Golgi complex. No luminal continuity between transfer tubules and endoplasmic reticulum has been observed. The sinus endothelium also contains small bristle coated vesicles (30-40 nm) often attached to Golgi cisternae. These do not participate in the transport of endocytosed protein tracer and are in this respect functionally distinct from the large bristle coated vesicles derived from the endothelial plasmalemma. Serial sections show that there are extensive connections between the endocytic transport organelles: bristle coated vesicles, transfer tubules, and endosomes, possibly forming an intracellular luminal continuum.