Vesicle numerical densities and cellular attenuation: comparisons between endothelium and epithelium of the alveolar septa in normal dog lungs.

The alveolar septa are divided into two anatomically distinct portions: The thin sides consist of capillary endothelium, alveolar epithelium, and their closely apposed (often fused) basal laminae; the thick sides are characterized by prominent interstitial spaces, containing fibrils and cells, which separate the respective basal laminae. Vesicle numerical densities are comparable (approximately 400 vesicles/ micron3 cytoplasm) in the endothelial and epithelial cells on both sides of the septa. Mean vesicle diameters, however, are substantially less in the epithelial cells on both the thin and thick sides. The extent of both endothelial and epithelial attenuation is significantly greater on the thin sides of the septa. Further, epithelial attenuation is more marked than endothelial attenuation on both sides of the septa. The attenuated cellular portions, possibly because of their extreme thinness, are void of vesicles but provide relatively short diffusion distances (20-30 nm) from vessel or alveolar lumen to the basal lamina. Whether these structural differences between endothelial and epithelial cells contribute to physiologic evidence that describes the endothelium as more permeable than the epithelium remains to be established.