The venous and arterial capillaries of the rete mirabile of the eel swim bladder were perfused with tannic acid both during and immediately following perfusion-fixation with glutaraldehyde. Tannic acid was also introduced into the interstitial spaces through cut faces of the capillary bed. Perfused tannic acid did not permeate the arterial capillary wall because it labeled only luminal membranes and attached vesicles. Tight junctions prevented the movement of tannic acid through arterial interendothelial clefts. The venous capillaries were permeable to perfused tannic acid because both luminal and abluminal surfaces exhibited enhanced electron density. Open endothelial junctions and possibly fenestrae provided direct routes of passage across the venous capillary walls. In arterial capillaries surface caveolae of both luminal and abluminal membranes and apparently free vesicles in the peripheral regions of the cytoplasm contained tannic acid. The greater proportion of internalized vesicular profiles, however, was devoid of tannic acid and no thoroughfare channels of vesicles were observed to conduct tannic acid across the capillaries. More vesicles in venous capillaries were labeled with tannic acid but unlabeled vesicles were also evident. This provides evidence that most capillary endothelial vesicles are discrete entities that lie totally enclosed in the cytoplasm. Fixative-induced fusion of vesicles with the plasma membrane may result in branched clusters and chains of vesicles labeled with tannic acid. This effect would be most pronounced in thin endothelial cells where a greater proportion of the total vesicle population lies closely apposed to the plasma membrane. © 1982 Academic Press, Inc.
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