Four autopsy cases of pneumoconiosis were studied histopathologically. They consisted of one classical silicosis, one classical asbestosis and two non-classical silicoses. In addition, experimental findings induced by a single intrapleural injection of emulsified silica to rats were also examined. In this report the histogenesis of pulmonary fibrosis was specifically examined. The classical silicosis showed interstitial pulmonary fibrosis and remarkable fibrosis of the bronchial lymph nodes with deposition of numerous silica crystals, while the classical asbestosis showed fibrosis of an intraalveolar type, no particular enlargement of regional lymph nodes, and no asbestos deposition. One of the two cases of non-classical silicosis showed massive fibrosis in the upper fields of both lungs and the other displayed massive fibrosis with extensive necrosis and irregular, cleft-like cavitation. The intrapulmonary dusts in the latter two cases as well as those in the classical silicosis mainly consisted of double-refractile and needle-shaped crystals under light microscopy and were characterized by rich Si with atomic absorption spectrometry. However in those two cases a classical silicotic nodule was not present in the lungs or lymph nodes but both showed fibrosing processes of intraalveolar and interstitial types. In the experiment, granulomas associated with the injected material were present and migrated with time from the pleural surface to the lymph nodes through the pulmonary interstitium without fibrosis in the lungs. From the above mentioned findings in the human materials and the experimental data, the fibrosing process in the classical silicosis, suggested that a blockade of the central lymph canals following lymph stagnation in the lung or lymph nodes may play an important role in addition to the physico-chemical action of silica. The case of classical silicosis was associated with nephrotic syndrome which histologically showed chronic proliferative glomerulonephritis of focal pattern. This fact was of great interest in terms of regarding silicosis as an adjuvant disease.
