Uptake of marker particles by in vitro ventilated and perfused rat lung

Colloidal gold marker was present in the capillary lumen and at various stages of pinocytosis and passage through the endothelial cells. It was occasionally located in the peri-endothelial space, close to the basement membrane. The uptake of marker by other cells was not observed. These results suggest the existence of two barriers, the biological (pinocytotic and hydrolytic capacities of the endothelial cells) and mechanical (permeability of the basement membrane).