Assessment of the morphology and significance of the lymph nodal and hepatic lesions produced in rats by the feeding of certain mineral oils and waxes - Proceedings of a pathology workshop held at the Fraunhofer Institute ofToxicology and Aerosol Research Hannover, Germany, May 7-9, 2001

A Panel of medical and veterinary pathologists reviewed published and unpub lished reports dealing with studies of various white mineral oils and waxes in F344 and Sprague-Dawley rats. They also had available and studied histo logic slides from both subchronic and chronic studies of certain mineral hy drocarbons 90-day oral study of low melting point wax (LMPW) in female Fisc her 344 and Sprague-Dawley rats; 90-day studies of P15H* and P70H white oil and high melting point wax (HMPW) in male and female F344 rats and 24 mont h study of P70H white oil in male and female F344 rats. The Panel also revi ewed mineral oil-induced alterations in tissues of human patients (liver, h epatic lymph node and spleen). The Panel agreed that certain of the mineral hydrocarbons produced lesions in the mesenteric lymph nodes and liver of t he F344 rat and these lesions were best described as microgranulomas/granul omas. The lesions were fundamentally similar in both organs, although varyi ng in severity with dose and type of mineral hydrocarbons. The Panel agreed that hepatic lesions with inflammatory cell infiltration, necrosis, and fi brosis were produced only by feeding of LMPW and the lesions were confined to F344 rats and not found in Sprague-Dawley rats. The most severe granulom atous lesions in the mesenteric lymph nodes were found in high dose LMPWfed F344 rats. The microgranulomas were similar in subchronic and chronic stud ies. Also, little difference existed between controls and treated F344 rats in the incidence and severity of the lesions after 2 years of feeding P70H white oil. The Panel agreed that some slight reversibility existed for the se lesions, but also agreed that complete resolution was unlikely as regres sion of the lesions in the rat would likely be slow. The Panel agreed that a minimal severity infiltrate of mononuclear inflammatory cells occurred in the base of the mitral valve in a slightly increased incidence in F344 rat s fed LMPW. The Panel concluded that these mitral valve alterations had lit tle if any toxicologic significance as the focal infiltrate was minimal in severity, occurred in controls, occurred in association with murine cardiom yopathy, and were unlike the responses in the liver and mesenteric lymph no des. The Panel agreed that the lesions observed in the liver and mesenteric lymph nodes of F344 rats exposed to MHCs, especially the LMPW, were differ ent morphologically from changes observed in lymph node, liver, and spleen of humans that were mineral oil-users, These changes in humans are usually found incidentally in tissues taken at biopsy or autopsy. The MHC-induced l esions can be considered incidental and inconsequential in humans. Granulomatous lesions are produced in rats by MHC-feeding, especially in li vers of the F344 rat with certain of the MHCs, but are not found in human t issues. This suggests a heightened and perhaps different type of toxic resp onse in the rat compared to humans. The MHC-associated alterations in human s are ubiquitous, present after a certain age in most, if not all, humans, and consist of intra-and extra-cellular oil droplets with a minimal macroph age (including giant cells) response.