An electron microscope study of the response of mesothelial cells to the intrapleural injection of asbestos dust

Studies of the pleural mesothelium in rats, mice, and guinea-pigs following the intrapleural injection of asbestos dust, showed that for 6 months at least this dust did not induce mesothelial hyperplasia. During the first few days after injection some areas of mesothelial cells became rounded and less clearly attached to one another, and a few were found to contain small numbers of asbestos fibres. During this period there was evidence of the penetration of asbestos fibres between the mesothelial cells, into the submesothelial connective tissues. Later in the studies the mesothelium covering most of the pleural cavity returned to normal, but where mesothelium covered asbestos granulomata, the cells were found to be extremely flattened, and without surface microvilli. Usually the mesothelial covering was complete, but in some areas pores were found penetrating the mesothelial cell cytoplasm and leaving areas of connective tissue in direct contact with the pleural cavity. In a few cases mesothelial cells were found lining clefts within the connective tissue of asbestos granulomata.