Morphology and pathogenesis of tumors of the thymus and stomach in Sprague-Dawley rats following intragastric administration of methyl nitrosourea (MNU).

Intragastric intubation of MNU (20 mg/kg twice weekly for 9 weeks) into young Srague-Dawley rats resulted in a 100% incidence of thymic lymphomas and gastric carcinomas in animals surviving more than 14 weeks. Progressive thymic atrophy followed the successive MNU administration culminating in marked lymphoid depletion by the 5th week of MNU treatment. At 6 weeks, repopulation of the thymus with lymphoblastic cells was observed, which progressed to thymic neoplasia. The first thymic lymphoma appeared at the 6th week. By the 15th week all rats were either moribund or had succumbed to anoxia caused by pleural effusions associated with thymic neoplasia. Twenty-six of 31 thymic tumors were classified as lymphocytic lymphomas, 2 as histiocytic, 1 as mixed lymphocytic-histiocytic, 1 as bimorphic and 1 as an epithelial thymoma. All but 2 lymphomas were restricted to the thymus. Of these 2 (both histiocytic lymphomas), 1 involved the spleen, liver and abdominal lymph nodes and the other infiltrated locally into cervical and thoracic regions. Significant thymic enlargement occurred only after MNU treatment was stopped (MNU acting as an anti-cancer drug). The gastric carcinomas were restricted to the non-glandular portion of the stomach and developed as a final step following ulceration, hyperplasia and neoplastic transformation. The short latency period and the high incidence of thymic neoplasms represent valuable criteria for future use of this model for pathomorphogenetic as well as immuno- and chemotherapeutic investigations.