Background: Endotracheal bleomycin administration in rats and other animal species causes rapid development of pulmonary fibrosis, characterized by a transiently increased number of contractile, filament-laden parenchymal cells and increased lung collagen synthesis and deposition. However, the identity and source of the cells that actively synthesize collagen and other extracellular matrix and their relationship to the altered lung structure and function remain uncertain.
Experimental design: In this study, the cells expressing alpha 1(I) and alpha 2(I) procollagen genes were identified and their localization analyzed in control and bleomycin-treated rat lungs at different time points, by in situ and Northern hybridization analyses.
Results: In control lungs, only a few scattered fibroblasts with weak expression of the alpha 1(I) procollagen gene were localized exclusively in the adventitia of the primary and tertiary bronchi, as well as major blood vessels. At day 3 after bleomycin treatment, scattered interstitial cells with significantly increased alpha 1(I) and alpha 2(I) procollagen gene expression were observed in the adventitia of bronchioles, terminal bronchioles, and adjacent small blood vessels. At days 7 and 14, there was a dramatic increase in the number of interstitial cells expressing large amounts of alpha 1(I) procollagen messenger RNA in these areas and extending to the lung parenchyma. This was followed on days 21 and 28 by significant decreases in procollagen gene expression and the number of cells with increased collagen gene expression. Most of the cells with enhanced collagen gene expression were arrayed in a disorganized fashion and were localized mainly around bronchioles, terminal bronchioles, and adjacent small blood vessels as well as in the irregularly distributed fibrotic foci, some submesothelial areas, and injured parenchyma. Northern blot analysis was consistent with the above in situ hybridization observation of the kinetics of collagen gene expression.
Conclusions: The results indicate that in this rat fibrosis model, increased numbers of the interstitial cells with high expression of type I procollagen genes are derived primarily from the fibroblasts in the adventitia of bronchioles, terminal bronchioles, and adjacent blood vessels, as well as the submesothelial region. This then can result in further expansion to adjacent parenchyma and alveolar areas.