Asbestos-induced changes in rat lung parenchyma.

Fischer 344 rats have been exposed to UICC crocidolite by whole-body inhalation procedures for periods of 1 d to 12 mo. Material was obtained from the same location in the left lung, and the numbers of cells in the parenchyma were identified and determined by transmission electron microscopy. An immediate increase (1 d of exposure) was evident in the number of type II cells, suggesting a direct action of the dust on these cells. The number of interstitial and alveolar macrophages showed a significant increase after 3 mo of exposure. The number of alveolar macrophages containing dust particles after a 1-d exposure was 49%, and the corresponding value after 12 mo of exposure was 92%. The longer periods of exposure were associated with an increase in the number of particles per macrophage. Polymorphs appeared in the interstitium at airway bifurcations, prior to their appearance in the alveolar space. These bifurcations were also the initial sites where evidence of cell damage and collagen deposition was seen. In this experiment crocidolite appears to be weakly fibrogenic, and other factors may be needed to produce the marked lesions seen in human asbestosis.