Early changes in the pleural mesothelium following intrapleural inoculation of the mineral fibre erionite and the subsequent development of mesotheliomas.

Changes in the pleura of rat lungs after intrapleural inoculation of the fibrous zeolite, erionite, have been examined from the earliest stages of reaction through to the eventual development of mesotheliomas. The initial changes involve haemorrhaging and pleural inflammation and proliferation with localized destruction of the elastic membrane under the visceral pleura. This allows cell proliferation into the lung parenchyma with fibres being able to penetrate into the lung. The chronic stimulation of the pleura by erionite eventually leads to the development of mesotheliomas which are invasive or compressing. The tumours are derived from the epithelial cells of the mesothelium (as shown by cytokeratin staining) or the subserosal cells beneath the mesothelium. Both types of mesothelioma can be invasive and some show an unusual property of 'tracking' along the blood vessels in the parenchyma as they invade. In dose-response terms for mesothelioma formation, erionite is over 200 times more tumourogenic than crocidolite (blue) asbestos.