Concentric and eccentric non-laminar intimal fibrosis 
Fibrotic lesions of the intimal layer are frequent in PAH-diseased lungs. The intima may be thickened by proliferation and recruitment of fibroblasts, myofibroblasts and other connective tissue cells, and consequently by the interstitial deposition of collagen (Figure 2B, C). In a purely descriptive approach, this thickening may be uniform and concentric, or focally predominating and eccentric. However, the eccentric intimal thickening is frequently observed in cases with thrombotic events and could represent residues of wall-adherent, organized thrombi. Thrombotic lesions, or so called in situ thrombosis, are a frequent pattern in different PAH-subgroups: organization and recanalization of totally occluding thrombotic material may lead to bizarre, fibrotic multi-channel lesions (so called “colander-like” lesions) which can be easily confounded with proliferative complex lesions (see below) (Figure 2D). Frequently, adventitial fibrosis is associated to intimal modifications (Figure 2A, B).