Mesothelioma - Pathophysiology and Asbestos
The mesothelium is described as consisting of a single layer of flattened to cuboidal cells which form the epithelial lining of the serous cavities of the body including the peritoneal, pericardial and pleural cavities. With the deposits of asbestos fibres in the parenchyma of the lung , this may result in the penetration of the visceral pleura from where the fibre can then be carried to the pleural surface, consequently leading to the development of malignant mesothelial plaques. The processes leading to the development of peritoneal mesothelioma remain unresolved; however, it has been proposed that asbestos fibres from the lung are transported to the abdomen and associated organs via the lymphatic system. Additionally, asbestos fibres may be deposited in the gut after ingestion of sputum contaminated with asbestos fibres.
Pleural contamination with asbestos or other mineral fibres has been shown to cause cancer. Long thin asbestos fibers (blue asbestos, amphibole fibers) are more potent carcinogens than “feathery fibers” (chrysotile or white asbestos fibers).However, current evidence that smaller particles may be more dangerous than the larger fibers. Remaining suspended in the air, they can be inhaled, and can penetrate more easily and deeper into the lungs. “We probably will find out a lot more about the health aspects of asbestos from [the World Trade Center attack], unfortunately,” said Dr. Alan Fein, chief of pulmonary and critical-care medicine at North Shore-Long Island Jewish Health System. Dr. Fein has treated several patients for “World Trade Center syndrome” or respiratory ailments from brief one to two day exposure of asbsetos near the collapsed buildings.
Mesothelioma development in rats has been demonstrated following intra-pleural inoculation of phosphorylated chrysotile fibres. It has been suggested that in humans, fibre transport to the pleura is critical to the pathogenesis of mesothelioma. This is definitely supported by the observed recruitment of significant numbers of macrophages and other cells of the immune system to localised lesions of accumulated asbestos fibres in the pleural and peritoneal cavities of rats. These lesions continued to attract and accumulate macrophages as the disease progressed. Moreover, the cellular changes within the lesion culminated in a morphologically malignant tumour.
Experimental evidence suggests that asbestos acts as a complete carcinogen with the development of mesothelioma occurring in sequential stages of initiation and progression. The molecular mechanisms underlying the malignant transformation of normal mesothelial cells by asbestos fibres is of course unclear despite the demonstration of its oncogenic capabilities. However, complete in vitro transformation of normal human mesothelial cells to malignant phenotype after being exposed to asbestos fibres has not yet been achieved. In general, asbestos fibres do act through direct physical interactions with the cells of the mesothelium in conjunction with indirect effects following interaction with inflammatory cells such as macrophages.
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