Fluorescein-labelled dextran (FITC-dextran) of molecular weight 145,000 was used to study vascular permeability to macromolecules by intravital and electron microscopy. Anaesthetised hamsters prepared for intravital observation of the cheek pouch microvasculature were given an intravenous injection of FITC-dextran. Leakage of macromolecules was induced by topical application of bradykinin to the cheek pouch microvasculature and observed in fluorescent light. Leakages occurred only from postcapillary venules of a diameter well below 50 micrometer. The cheek pouch preparation was rapidly fixed by immersion and samples of tissue with intravitally identified leakages of FITC-dextran were studied by electron microscopy. FITC-dextran appeared as black precipitates in the vascular lumen and also outside the lumen in bradykinin-treated animals. In most animal; gaps were found between endothelial cells and these gaps contained dextran precipitates. The results support much other evidence that bradykinin induces macromolecular leakage by opening gaps between endothelial cells in postcapillary venules.

A freeze-drying method has been developed by which fluorescein thiocarbamoyl dextrans (FITC-dextrans) can be localized in thin sections from nervous tissue and muscles. Labelled dextrans with molecular weights of 3,000, 20,000, 70,000 and 150,000 were injected intravenously (i.v.) into golden hamsters and samples from brain, trigeminal ganglia and sciatic nerves were examined 30 min or 4 h later. For comparison experiments were also carried out in mice and some other tracers were tested as well. The dextrans did not pass out of blood vessels in cerebral cortex and white matter. The blood vessels in the trigeminus ganglion were permeable to all of the tested compounds, i.e. even the FITC-dextran with mol.wt. 150,000. Little, if any, i.v. injected dextran could be detected in the endoneurium of sciatic nerve fascicles. Even very high concentrations of dextrans (mol.wt. 3,000 and 150,000) injected around the sciatic nerves did not penetrate the perineurium of the sciatic nerve. As compared with other tracers dextrans have the advantage that they can be obtained in a wide range of molecular sizes. With the proposed technique presented at the end of this article they can be used for studies on vascular permeability in deep tissue like brain, ganglia and peripheral nerve. The use of these tracers will probably be particularly advantageous in investigations concerning the etiology of edematous conditions.