Mesotheliomas of tunica vaginalis testis of Fischer 344 (F344) rats treated with acrylamide: a light and electron microscopy study

Abstract

The incidence of spontaneous mesotheliomas of the tunica vaginalis testis, documented to be 1.3% in Fischer 344 rats can be increased by long term exposure to acrylamide. The light microscopic and electron microscopic features of tumors developing spontaneously in control animals were compared with those that developed in acrylamide treated animals to determine whether the tumors in the experimental group have any distinctive characteristics. The experimental group comprised three subgroups, i.e. animals exposed to acrylamide in three doses 0.1, 0.5 or 2.0 mg/kg/day over a period of 104 weeks. A total of 35 tumors were examined--7 in control group, 9 in low-dose group, 8 in mid-dose group and 11 in the high-dose group. Most tumors measured less than 1 cm in diameter in aggregate and approximately one-third of all tumors were of microscopic size, less than 5 mm in diameter. All tumors were classified as epithelial mesotheliomas. Three distinct microscopic patterns were recognized (papillary, tubular or solid). All tumors had papillary features, and focally showed some tubular or solid components. No ultrastructural differences were found between tumors of the four groups of animals. It was concluded that the tumors that developed in the tunica vaginalis testis of acrylamide treated animals do not differ morphologically from those that developed spontaneously in control animals. Cellular uniformity, small size of lesions and the absence of perotoneal seeding and metastasis suggests that the mesotheliomas of tunica vaginalis testis of Fischer 344 rats in this study were benign and equivalent to human benign mesothelioma of tunica vaginalis testis or adenomatoid tumors.