Electron Microscopic Study of Viruses in Mouse Thymuses during Leukemogenesis by Urethan

Abstract

A systematic search for viral particles was carried out in thymuses of C3Hf inbred mice treated with leukemogenic doses of urethan. Thymuses collected during the tumor latent period and thymic lymphosarcomas were examined. A group of 38 mice was treated, starting at 10 days of age, with 5 i.p. doses of urethan (lmg/g body weight) once every two days. This treatment induces approximately 30 per cent thymic lymphosarcomas in C3Hf mice, after an average period of 20 weeks, whereas no spontaneous thymic lymphosarcomas develop in untreated controls. Six animals with thymic lymphosarcoma were killed between 16–32 weeks of age; the others were sacrificed at 3,5,10 and 14 weeks of age when they did not have any recognizable thymic tumor either grossly or microscopically. As control, thymuses of a group of 28 untreated mice were examined. Mature and immature type C particles morphologically indistinguishable from the murine leukemogenic viruses and intracisternal A particles were observed. The immature C particles and intracisternal A particles were found in all the groups of treated and untreated mice. Intracisternal A particles were more numerous in the untreated than in the treated animals. The number of A particles in the neoplastic thymuses was far inferior than that observed in the treated animals killed before development of lymphoma. The amount of immature C particles was always much less than that of A particles. No significant difference in the number of immature C particles was observed in all experimental groups. Mature C particles were only seen in the thymus of a few treated or untreated mice, never in thymic lymphosarcomas. Within the limits of this investigation, the results presented are not consistent with the view that urethan may act by stimulating viral multiplication.