An ultrastructural study of the transport pathways across arcuate,interlobar, hilar,and capsular lymphatics in the dog kidney

The ultrastructurally visible transport pathways across arcuate, interlobar, hilar, and capsular lymphatics were studied in canine kidneys with unimpeded lymph and urine flow. The four types of vessels were lined by a nonfenestrated endothelium resting on a discontinuous basal lamina and contained valves. Stereological techniques were applied to determine the volume and numerical densities for cytoplasmic vesicles (80–90 nm in diameter). Values for those parameters increased sequentially in the direction of lymph flow from the kidney. Thus, vesicular volume density rose from 6% of endothelial cytoplasm in arcuate lymphatics to 14% in hilar vessels, and the numerical density increased from 11 vesicles/μm³ to 33 vesicles/μm³. Most of the vesicles lay apparently free within the cytoplasm, the remainder were evenly dispersed along the luminal and abluminal cell borders. Three varieties of interendothelial cell contacts were present: end-to-end, overlap, and interdigitation. Specialized junctional complexes associated with these contacts were either fasciae occludentes or fasciae adhaerentes, although in many instances neither were present. In general, arcuate and interlobar lymphatics had more cell contacts which lacked junctional specializations than did hilar and capsular lymphatic vessels. Only one example out of 676 cell contacts could be classified as “open” (greater than 30-nm gap along the entire contact length). It was concluded that (1) the four types of lymphatic vessels in the present study have ultrastructural features of lymph-forming (as opposed to simple collecting) vessels, (2) the ultrastructurally visible transport pathways in the four types of vessels studied are through “normal” (5- 30-nm wide) intercellular channels and via cytoplasmic vesicles, (3) arcuate and interlobar lymphatics have a structure which is consistent with the formation of lymph from medullary interstitial fluid, (4) hilar and capsular lymphatics may have a role in extrarenal lymph formation, and (5) “open” contacts are not involved normally in translymphatic transport in the kidney.