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

Biological, Histological and Ultrastructural Aspects of Hodgkin's-Like Lymphomas in SJL/J Mice

Spontaneous lymphoma developed in 57 % female SJL/J mice, with a mean latent period of 58 weeks and in 33 % male SJL/J mice with a mean latent period of 72 weeks. In (CBA x SJL) F1 and F2, the incidence in females was 79 % and 67 % at a mean age of 107 and 100 weeks respectively while in males it was 53 % and 41 % at 111 and 106 weeks. In reciprocal F1 and F2 hybrids, an incidence of 62 % and 37 % was observed in females at 118 and 109 weeks of age, while in males it was 33 % and 43 % at the mean ages of 72 and 90 weeks. Gross lesions were usually restricted to mesenteric, cervical, axillary and inguinal lymph nodes, Peyer's patches and spleen. Less frequently, lungs, liver, kidney and thymus were also involved. Histological examination of nodes in the pre-neoplastic stage revealed follicular hyperplasia with prominent germinal centers, sinus hystiocytosis, precence of pyroninophilic cells within the paracortical area and medulla, medullary plasmocytosis and epithelioid sarcoid-like noduli. Nodes, spleen and other affected organs examined during overt disease showed a pleomorphic pattern characteristic of type B reticulum cell neoplasm according to Dunn and contained predominantly plasma cells, giant cells or reticulum cells. At the electron microscope, these different cell types were detected within the neoplastic tissue. In addition, cells resembling immunoblasts were frequently observed, which suggests that they might represent proliferating tumor cells capable of differentiating into plasma cells. Type C-immature and A intracisternal particles were repeatedly observed, but no type C-mature particles were seen. Attempts to transplant neoplastic tissue in syngeneic hosts were successful. Cell-free extracts from primary SJL/J lymphomas were injected in newborn RFM and CBA mice. 4 out of 9 RFM mice developed SJL/J-like lymphoma at 64–70 weeks of age, while no lymphomas were found in 8 CBA mice over 70 weeks of age. Freund adjuvant injected intraperitoneally in (CBA x SJL/J) F1 mice did not modify the incidence or the latency of the lymphomas when compared with control animals.

Electron Microscopic Study of Viral Particles in the Pancreas of Normal and Leukemic Mice

An electron microscopic study of viral particles was conducted on pancreas, thymus, and bone marrow of normal and leukemic (C57BL x C3Hf) F1 (BC3) and SWR mice. The leukemic mice had developed thymic lymphosarcoma after neonatal injection of N-nitrosomethylurea or urethan. An intense proliferation of type C particles morphologically identical to the murine leukemia virus was found in the acinar cells of the exocrine pancreas of BC3 mice. The particles were predominantly located in intracytoplasmic vacuoles but they were also numerous in dilated cisternae of the rough endoplasmic reticulum and in extracellular spaces. A substantial quantitative difference of type C particles between the pancreases of leukemic and healthy mice was not observed. No viral particles were found in the pancreas of SWR mice. In the thymus and bone marrow of BC3 and SWR mice a small number of type C particles and intracisternal type A particles was found in all the experimental and control groups. The observation of a high number of type C particles in the exocrine pancreas of BC3 mice is discussed in relation to a viral etiology of chemically-induced lymphomas.