Concentric laminar intimal fibrosis 
This morphologically conspicuous phenotype of intimal fibrosis is also known as “onion-skin” or “onion-bulb” lesion. Numerous concentrically arranged fibrotic layers occlude the arterial lumen of small (diameter: 100–200 μm) arteries (Figure 2E). The scary, cell-lacking morphology of this lesion is frequently found in patients suffering from idiopathic PAH and PAH associated to CTD [172]. Nevertheless, immunohistochemical analysis reveals fibroblasts, myofibroblasts and smooth muscle cells.