Figure 2 Usual interstitial pneumonia (UIP) lung vessels. This groups shows distortion of the vascular wall architecture and an increase in red–orange birefringence of collagen fibres in the vascular medial and adventitial layers in all four grades of vascular obstruction (grade I, isolated hypertrophy of the arterial media, grade II, proliferative intimal, grade III, total occlusion of the arterial lumen by fibrosis tissue and grade IV, plexiform lesion) and major proliferation of elastic fibres in the internal and external elastic lamina of the vascular wall (H&E, Picrosirius polarisation and Weigert's resorcin–fuchsin ×200).