Structural heterogeneity within the pulmonary microcirculation of the normal rat.

Extra-alveolar microvessels of the rat were described previously as nonmuscular, partially muscular, and muscular (Hislop and Reid, 1978). Based on these structural criteria, the present morphometric results present evidence that mean thickness of the endothelium was significantly greater in the muscular microvessels than in each of the other microvessel categories. The percentage of endothelial surface occupied by attenuated segments (20-30 nm maximal thickness) was greatest in the nonmuscular and partially muscular microvessels. Mean plasmalemmal vesicle diameters were consistent throughout the microvascular units. Vesicle numerical densities, however, were significantly greater in the alveolar capillary endothelium than in endothelia of the nonmuscular, partially muscular, and muscular microvessels. Endothelial vacuoles (diameters greater than 150 nm) were not observed in either the capillaries or the nonmuscular microvessels. Mean diameters and numerical densities of vacuoles observed in endothelia of the partially muscular and muscular microvessels were, however, statistically similar. Subclassification of the partially muscular and muscular microvessels by ultrastructural characteristics as arterioles or venules (Rhodin, 1978; Michel, 1982) served to identify greater numerical densities of vesicles in the endothelium of venules than that of the arterioles. As expected, mean thicknesses of the periendothelial connective tissue spaces varied directly with the relative muscularity of the respective microvessels. Based upon the morphometric data presented in this study, future investigations of pulmonary microvascular functions should include considerations of the observed regional variations of endothelial cell ultrastructure.