Transport of protein across lymphatic endothelium in the rat kidney

Abstract

The pathways involved in protein transport across the lymphatic endothelium of the rat renal cortex were studied ultrastructurally. Qualitative and quantitative analyses were made on tissue from control animals (saline injected) and from animals injected intravenously with horseradish peroxidase (HRP) or ferritin. Three types of contacts between adjacent lymphatic endothelial cells were seen in control and experimental tissue. These were end to end, overlapping, and interdigitating. Only one instance of an “open” junction was seen in control tissue. No significant difference was noted in the proportions or widths of these three types of cell contact after the administration of HRP or ferritin. Neither was there any increase in the occurrence of open junctions. HRP was frequently seen in the normal intercellular spaces between adjacent endothelial cells. Coated and uncoated vesicles (diameter 80 to 300 nm) were identified in lymphatic endothelium in both control and experimental tissue and were quantified according to their position in the cell and according to their staining intensity. The volume density of small vesicles (diameter 80 to 100 nm) increased significantly from 0.028 in the control group to 0.049 after HRP injection. Likewise the numerical density rose from 16/μm 3 to 22/μm 3. However, the relative positions of these vesicles did not alter significantly following a protein load. There was a significant increase in the staining intensity of vesicles after the administration of HRP. For example, in control tissue none of the small vesicles was intensely stained in contrast to 8% after HRP injection. Ferritin particles were identified in large intracytoplasmic vesicles (diameter 150 to 500 nm) but not traversing intercellular spaces. It was concluded that (1) the tracer molecules used in this study enter renal lymphatics rapidly after intravenous injection; (2) the major visible routes for transport of the tracer are intracytoplasmic vesicles and normal (as opposed to open) intercellular spaces; and (3) the presence of HRP in the interstitium stimulates an increase in the number of intracytoplasmic vesicles in lymphatic endothelium.