The ultrastructure of the various forms of pulmonary arterial intimal fibrosis.

Intimal fibrosis of muscular pulmonary arteries may present in various forms and in varying degrees of severity according to the underlying condition. In patients with pulmonary hypertension, the type of initimal fibrosis is often significant with regard to prognosis and reversibility. For these reasons we have studied the ultrastructure of the thickened intimal layer in aged individuals, where intimal fibrosis occurs as a normal age change, and in patients with pulmonary hypertension associated with fibrosis of the lungs, mitral stenosis, chronic pulmonary thromboembolism and plexogenic pulmonary arteriopathy (either primary or secondary to congenital cardiac defects). In all these forms of intimal fibrosis, the cellular component of the subendothelial intimal layer was apparently almost exclusively the smooth muscle cell. The cells usually had a haphazard arrangement. In primary and secondary plexogenic pulmonary arteriopathy, however, there was a more regular circumferential arrangement. The ultrastructural evidence suggested that the intimal cells were derived from medial smooth muscle cells.