Pulmonary vascular smooth muscle and its interaction with endothelium. Morphologic considerations

In pulmonary arteries the medial smooth muscle cells and endothelial cells, though separated by an internal elastic lamina, come into contact by way of fenestrations in this lamina. Such a continuous elastic lamina is absent in pulmonary veins facilitating this contact. If vasoconstriction is induced in experimental animals, herniations of medial smooth muscle cells protruding toward endothelial cells provide an extensive and close association and thus a potential interaction between both cell types. This is particularly prominent in the veins. In arteries these herniations, which must penetrate the fenestrations in the elastic lamina, are far less conspicuous. Intimal fibrosis, for instance, as an age change, is not necessarily an impediment for such interaction, since the cells within the intimal layer have all the characteristics of smooth muscle cells and thus may provide a smooth muscle-endothelium contact.