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SUMMARY The method of Due de Chaulnes was employed to determine the mechanical and optical thickness, as well as the refractive index, of transparent tissue layers in living specimens. To this end the reproducible accuracy of the method and its dependence on the adjustment in focusing and on the numerical aperture of the objective was evaluated on test specimens using the procedures of transmitted light, phase‐contrast (PC), and differential interference contrast (DIC) microscopy. The best working conditions were then applied to the actual measurements.

The transport of fluorescent tracer molecules of various molecular weights (MW 340-300,000) within the blood vessels and through the vessel walls into the perivascular tissue of the rat mesentery was studied microscopically. Using a highly sensitive TV-tube the fluorescence intensities were transformed into video signals and recorded for subsequent analysis. The results show that small molecules, such as the water-soluble dye fluorescein-sodium ( F1Na ) (MW 340) can pass through the wall of all blood vessels with minimal delay, whereas the passage of large molecules, such as the serum proteins (MW greater than 60,000), depends very strongly on the diameter of the vessels and on the medium in which the mesentery is embedded during the observation. In this respect, no difference was registered between vessels of the arterial and of the venous part of the microcirculation. The large serum protein molecules moved through the blood vessel walls at specific leakage points. The small dye molecules, however, traversed the wall uniformly along the entire length of the blood vessel. The small-molecule behavior could be described by a passive diffusion model with a cylindrical diffusion source.

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