The fibrosing process in so-called organized diffuse alveolar damage. An immunohistochemical study of the change from hyaline membrane to membranous fibrosis.

On the assumption that some cases of organized diffuse alveolar damage (DAD) result from organization of hyaline membrane, we collected nine autopsy cases of DAD in various stages of the fibrosing process from hyaline membrane to membranous fibrosis and studied changes in the basement membrane and epithelial cells immunohistochemically. In the majority of cases, the following sequence of events was assumed: the hyaline membrane is first formed at the tip of the alveolar septum, a part of the alveolar duct wall where epithelial cells have disappeared. With time it elongates and completely covers alveolar mouths. In the organizing stage, fibroblasts start to permeate through the alveolar duct walls to replace the hyaline membrane completely and to form membranous fibrous tissue. In a few cases, however, fibrous tissue will fill alveolar spaces to form intraluminal diffuse fibrosis. Alveolar epithelial cells and the basement membrane of the alveolar walls are well preserved until the end of the organizing stage when the basement membrane becomes distorted. We believe that membranous fibrosis represents a form of "alveolar duct damage" and that it differs from diffuse fibrosis, which is indicative of diffuse alveolar damage in the true sense.