The mechanism by which macromolecular contents are concentrated during lymph transport was investigated in the collecting lymphatics of the mesentery of the rat. To examine changes in the concentration of lymph macromolecular tracer (fluorescein isothiocyanate-tagged dextran, MW 148,900) in the rhythmically contractile lymphatics, we quantified the fluorescence intensity of the lymph tracer using an intravital microscopy system with single slit-laser (20 μm in thickness) epi-illumination, regardless of the vasomotion of the lymph vessel. The results for the fluorescence intensity measurement at different sites at distances of about 300-1000 μm along unbranched collecting lymphatics revealed that the concentration of the lymph macromolecular contents was significantly increased during their passage from upstream to downstream (P < 0.05). The fluorescence intensity of lymph tracer in the occluded segment of collecting lymphatics around 1000 μm in length was enhanced during the contraction phase and reduced during the relaxation phase. These results suggest that the filtration of macromolecule-free fluid from the lymph across the lymphatic wall during its contraction plays an important role in the lymph concentration mechanism in the collecting lymphatics, probably due to the Starling forces acting across the lymphatic wall.
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