Figure 2  Pulmonary arteries of the muscular type displaying obstructive arteriopathy in lungs of patients with PAH. A Medial hypertrophy with smooth muscle cell proliferation and pronounced adventitial fibrosis. Magnification x200, Weigert-hematoxylin-phloxine-saffron staining (WHPS). B Concentric non-laminar intimal fibrosis comprising numerous myofibroblasts (arrows). C Eccentric intimal fibrosis corresponding to organized thrombotic material. Br: bronchus, Ar: pulmonary artery. Magnification x100, HES staining. D Thrombotic lesion, so called "colander-like lesion", with partial recanalization by microvessels. Note the similarity to plexiform lesions (F). Magnification x100, HES. E Concentric laminar intimal fibrosis, so called „onion-skin lesion“. Magnification × 200, HES. F Plexiform lesion with proliferation of small sinusoid-like vessels on a fibrotic matrix. Note surrounding dilated vessels. Magnification x100, HES. G Multiple dilation lesions being the sentinel of the centrally located plexiform lesion. Magnification × 40, Elastica-van-Gieson staining (EvG). H The same plexiform lesion after immunohistochemical staining with anti-CD3, a T-lymphocytic marker. Note the perivascular distribution of the inflammatory infiltrate. Magnification x100.