Plexiform lesion from a patient with severe pulmonary hypertension demonstrating the exuberant proliferation of cells that comprise the lumen of the small pulmonary artery. Plexiform lesion from a patient with severe pulmonary hypertension demonstrating the exuberant proliferation of cells that comprise the lumen of the small pulmonary artery. Multiple slit-like vascular spaces are all that remain of the original lumen (Haematoxylin-eosin). Reproduced from [19] with permission from the publisher. Andrew Peacock Eur Respir Rev 2013;22:20-25 ©2013 by European Respiratory Society