Parenchymal Remodeling in UIP

As shown previously, the alveolitis here shows early fibroblast proliferation into an exudate in the alveolus (side of alveolar wall opposite the arrow). With time, this exudate organizes, reepithelializes, and becomes incorporated into the interstitium.

Mechanisms of alveolar remodeling: In an immunohistochemical study of architectural remodeling in the proliferative phase of diffuse alveolar damage, organizing pneumonia, and UIP, it was found that collagen synthesizing cells and fibronectin were present in the fibroblast foci. The fibroblast foci in all of these conditions were almost always outside remnants of basement membrane--an appositional fibrosis--indicating that injury causes alveolar epithelial erosions, and then exudation, organization, and reepithelialization within the air space [1].

In addition to alveolar wall thickening by apposition, another type known as collapse induration also contributes to thickening. As a result of the focal inflammation, some involved alveolar walls collapse and become permanently fused, thereby decreasing alveolar surface area and causing thickening of airspace walls with little or no fibrosis [2]. Collapse induration is an explanation for the traction bronchiolectasis and honeycombing, which occur in the disease.

References

1. Kuhn III C, Boldt J, King Jr T, Crouch E, Vartio T, McDonald J. An immunohistochemical study of architectural remodeling and connective tissue synthesis in pulmonary fibrosis. Am Rev Respir Dis 1989; 140:1693-1703.

2. Burkhardt A. Alveolitis and collapse in the pathogenesis of pulmonary fibrosis. Am Rev Respir Dis 1989; 140:513-524.

Clinical summary Discussion

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