Abstract
The purpose of the work was to develop an in vitro model for the study of lymphatic endothelium and to determine, using this model, whether or not a cytoplasmic process may be involved in transendothelial transport. Segments of canine renal hilar lymphatics were dissected clean, cannulated at both ends, and transferred to a perfusion chamber for measurement of transendothelial protein transport and for ultrastructural tracer studies. The segments were subsequently processed for light and electron microscopy. By both structural and functional criteria the lymphatics were judged to have retained their integrity. At 37 degrees C, 36 lymphatics showed a mean rate of protein transport of 3.51 +/- 0.45 (SEM) micrograms/min per cm2 of lymphatic endothelium. The rate was influenced by the temperature of the system, being significantly reduced by 49% +/- 4.8, 31% +/- 5.3, and 29% +/- 3.9 when the temperature was lowered to 4 degrees, 24 degrees, and 30 degrees C, respectively. When the temperature was raised to 40 degrees C, the rate was significantly increased by 48% +/- 12.2. The vesicular system and the intercellular regions in vessels with increased or reduced rates of transport were analyzed quantitatively to ascertain whether the rate changes could be correlated with ultrastructurally demonstrable changes in either of these postulated pathways. No significant changes in junctional or vesicular parameters were found between the control lymphatics and those perfused at 24 degrees, 30 degrees, and 40 degrees C. At 4 degrees C, the temperature at which the rate of protein transport was maximally reduced, vesicular size decreased, and the number of free cytoplasmic vesicles increased, whereas the number associated with the abluminal and luminal surfaces decreased. We concluded that isolated perfused lymphatic segments transport protein at a relatively constant rate under control conditions, and that this transendothelial transport comprises both temperature-dependent and temperature-independent mechanisms. The findings were considered in terms of the different theories of lymph formation and were interpreted as providing support for the vesicular theory.