Distribution of vesicles in cells of air-blood barrier in the rabbit.

Vesicular transport has been proposed as a pressure-independent shuttle mechanism for transport of macromolecules across thin cells. The vesicular populations of cells found in the gas-exchanging parenchyma were quantitated by morphometric means. We found that the numerical densities (number of vesicles/unit of volume) were comparable in endothelial (131/micrometer 3) and epithelial (145/microm3) type I cells but much smaller for interstitial cells. The vesicular loads or numbers of vesicles/unit of surface area were computed separately for each cell front (luminal and abluminal) and found to be similar for all surfaces, ranging from 150 to 196/micrometer 2 except for the interstitial front of epithelial cells where the load was higher (230/micrometer 2). Although this represents a very high transport potential, little can be said about vesicular effective performance because of insufficient knowledge of vesicular biology and biophysics. A frequent observation in the capillary network is attenuations of endothelial cells, which are thinner than the vesicular diameter and devoid of vesicles.