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EXPERIMENTAL ASBESTOSIS--GROSS A DE TREV1LLE 641 Fig 3.---Minimal, moderately healed asbestotic lesion in rat exposed to chrysotile dust for one month, then postured and killed 24 months later. Lesion consists of a partially thickened alveolar duct. Mural thickening is caused by hypocelOifer connective tissue that has ob literated alveoli near bottom of field (hematoxylin and eosin, (X 150). stioes of the precollagenous stroma of the inflammatory tissue. When examined one year or longer after the intratracheal dust injection, asbestotic fibrosis was found in all rats so injected. The characteristic lesion was sharply limited to the alveolar duct as well as to the short re spiratory bronchiole and consisted of moder ate to severe collagenous thickening of the wall, often also associated with striking hy perplasia of smooth muscle (in more than one third of the injected animals). The col lagenous mural thickening involved much of the wall and resulted in the obliteration of many of the evaginating alveoli. Generally speaking, the involvement of the alveolar ducts, even in animals injected with but a Fig 4--Field tame at in Fig 3 after tilver Impregnetion chows that scar is composed on nonbrsnching. largely parallel, thick collagen fibers that are fairly densely arranged (Gordon and Sweets. (X 150). single dose of 3.5 mg of chrysotile dust, was greater than the minimal lesion observed in rats that had inhaled high concentrations of the dust for one month. As judged from the examination of a single section of both lungs of the rats, greater dust dosage was often, but not always, associated with an increased incidence of thickened alveolar ducts. The occurrence of larger collagenous scars con taining small remnants of air spaces seemed also to be related to the multiplicity of the asbestos dust injections, as was a metaplasia to the columnar variety of the epithelium lining the surviving evaginating alveoli. The sharply delimited mural thickening of the alveolar ducts (Fig 3) and respiratory bronchioles, with the collagenized, nonarboresoent, thick stromal fibers (Fig 4), and the associated reduced oellularity (Fig 3), Arch Environ Health--Vol IS, Nov 1967 I' > 8005 1461 PRODUCED BY FORD