(A–E). Histopathology of large (A, C) and small (B, D) pulmonary arteries. Both histological, H&E (A, B) and immunofluorescent, PECAM-1/CD31 (C, D) staining’s demonstrate marked expansion of the vasa vasorum capillary network in adventitial, perivascular regions. Quantitative morphometric analyses demonstrated that the volume density (Vv) of vasa vasorum is significantly greater in neonatal calves with severe hypoxia-induced pulmonary hypertension compared with normoxic controls (E). (F, G), Angiogenic responses in the adventitia of a human patient with pulmonary fibrosis and associated pulmonary hypertension. Medium-sized pulmonary artery stained with CD31, demonstrating evidence for capillary network expansion in the perivascular area (medial/adventitial region, arrow) (F). CD31 immunohistochemical evidence of capillary proliferation (arrow) (G). Bars, 500 μm in A and C, 100 μm in B, D, F, and 25 μm in G.