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Figure 1 Main histopathological features of pulmonary arteriopathy (see text for details). (All histological sections were stained with Verhoeff-van Gieson unless specified.) Medial hypertrophy: (A) preacinar pulmonary artery, x80; (B) intra-acinar artery, x600. Concentric laminar intimal thickening: (C) intra-acinar artery, x500; (D) pre-acinar artery. The vessel was decorated with anti-smooth muscle actin antibodies (SMA), revealing the intimal thickening (white arrow) to be composed of SMA-positive cells, x150. Eccentric (E) x100 and concentric nonlaminar (F) x100 intimal thickening of pre-acinar arteries. Plexiform lesions: (G) intra-acinar artery decorated with SMA showing SMA-negative endothelial cells (arrow) surrounded by a rim of SMA-positive cells (rusty color), x380; (H) pre-acinar artery adjacent to a plexiform lesion (arrow) and dilation lesions (shown by asterisk) x60. (I) Dilation lesions (arrows), x40; (J) colander-like lesion, HE x400; (K) lymphomonocytic arteritis, HE, x300.