Their synthesis of procollagen diminishes. Thus, the Masson bodies of bronchiolitis obliterans with organizing pneumonia and the scattered fibroblastic foci in UIP have most of the characteristics of contracting wounds, notably the cytoske-letal phenotype and organization of fibronectin. In addition, in situ hybridization suggests that they express elevated levels of the integrin a5- chain. A notable difference is that in pulmonary fibrosis collagen synthesis can remain active while contractile features are present, emphasizing that the stimulus is protracted compared with the circumscribed injury of a surgical wounds.
Some additional evidence supports the idea of cellular traction in pulmonary fibrosis. Basal lamina often has a sinuous or pleated configuration, reminiscent of the wrinkling of a silicone sheet beneath contracting cultured fibroblasts. Some of the same growth factors that are expressed at elevated levels in pulmonary fibrosis, transforming growth factor P and platelet-derived growth factor, are implicated in the contraction of wounds.
The 4 processes described—interstitial fibrosis, air-space fibrosis, collapse, and contraction—will all occur together, the proportions varying in different lung diseases. They readily account for the impaired gas exchange and reduced lung size that are common to many fibrosing lung diseases.