Fig. 1

Extracellular matrix (ECM) composition in healthy (a) and fibrosis affected lungs (b). In healthy lungs, the epithelial cells create a tight barrier that blocks the entry of foreign particles from the inhaled air to the tissue. This is further enforced by the underlying basement membrane (BM) which mainly consists of type IV collagen. The interstitial matrix (IM) is placed below the BM and consists mainly of fibrillar collagens and elastin. In fibrosis affected lung, the continuous epithelial layer is disrupted, and the underlying BM is exposed. The inflammatory response to repeated tissue injury results in the up regulation of proteases and disruption of the BM, exposing the underlying IM to injury. In response to this, fibroblasts are activated and converted to pro-fibrotic myofibroblasts that secrete collagens which accumulate in the IM of the airway wall. Both collagens and elastin undergo proteolytic degradation in the airway and alveolar walls. The processes of synthesis and degradation release ECM fragments which may enter the bloodstream where they can be detected as biomarkers of ECM remodelling. Reproduced with slight modification from [30]