Some observations on the effects of prolonged asbestos ingestion on cell proliferation in the intestine of aged rats.

The effects of prolonged ingestion of amosite, crocidolite, and chrysotile UICC standard reference asbestos samples were examined in groups of laboratory rats. Animals were given over 250 mg per week for periods up to 25 months and were monitored for the remainder of their life span. Animals were examined for evidence of pathological effects and gastrointestinal mucosal cytokinetic disturbances and for signs of penetration and dissemination of fiber. There was no excess of malignant tumors in the experimental groups when compared with control animals, and no gastrointestinal mucosal abnormalities were found. An examination of the cytokinetic status of a subgroup of animals exposed to asbestos similarly showed no evidence of any adverse effects of prolonged ingestion of amosite asbestos fiber. Detailed electron microscopic examination of various tissue residues for the presence of asbestos fibers was also undertaken. No penetration and/or damage to any of the gut tissues was found. Although occasional fibers were found in a variety of tissue residues, there was no evidence of preferential retention of fibers within any specific tissue and no sign of higher fiber burdens in those animals with tumors. It was concluded that there were no significant adverse effects of prolonged asbestos ingestion in healthy laboratory rats. The implications of these findings are discussed in the light of other published work.