An electron-microscopic study of the endothelium in mammalian bronchial microvasculature.

The endothelial ultrastructure and permeability in sequential segments of the bronchial microvasculature of the rat, guinea pig, and hamster, were examined electron-microscopically before and after perfusion with horseradish peroxidase (HRP) or tannic acid. Transendothelial channels are common in the bronchial venules of all three species. Fenestrated capillaries are present just beneath the bronchial epithelium. The endothelial fenestrae, transendothelial channels, and leaky endothelial junctions in venules are permeable to tracers such as HRP and tannic acid, indicating that the microvasculature of the bronchial circulation may be more permeable than that of the pulmonary circulation.