Histogenesis and fine structure of peritoneal tumors produced in animals by injection of asbestos

Primary tumors of the peritoneal cavity were produced in rats and mice by the intraperitoneal injection of crocidolite asbestos. The earliest discernible neoplastic stage consisted of many small, pedunculated nodules scattered over the surfaces of the viscera, diaphragm, and body wall. These nodules contained a central core of reticulin or collagen surrounded by layers of pleomorphic connective-tissue cells. The surfaces of the nodules were covered by a single layer of epithelial cells similar to normal mesothelium. In later stages some nodules remained distinct and became quite large, but often the tumor spread over the peritoneal surfaces as a uniform sheet. The cells found in sheets were mostly of the same type as those in large nodules: pleomorphic connectivetissue cells in the central regions and epithelial cells on the surface. In some advanced tumors, however, the cells had a spindle-cell pattern similar to a fibroma or fibrosarcoma. In this form the neoplasms were locally invasive; however, at earlier stages no signs of invasion could be detected. Electron microscope studies showed only a slight difference in ultrastructure between the different tumor cell types, and it is suggested that the growths arose from undifferentiated mesenchymal cells in the sub mesothelial tissues. These cells retained their normal pleomorphic pattern or gave rise to either epithelial (mesothelial) cells on free surfaces or spindle cells in deeper layers.