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Electron Microscopic Study of Neoplasms Induced by Urethan and X-rays in X/Gf Mice¹

Cancer and Radiobiological Research Laboratory, Department of Health and Hospitals, and Department of Biology, New York University, New York, New York 10032

This study involves a search for ultrastructural characteristics and for the presence of oncogenic viruses in neoplasms induced in X/Gf mice by urethan, by X-irradiation, or by a combination of both agents. The ultrastructural features of mammary carcinomas and thymic lymphomas of the lymphoblastic type are described.

Mammary tumors were characterized by closely adjacent cells, frequently connected by desmosomes, by hypertropic surface microvilli that extended into the lumen of acini, and by a great variation in size and shape of nuclei. Mitochondria varied in number, shape, and size from cell to cell in microscopic sections of the same tumor. Nuclei of bizarre shape showed deep invaginations of the nuclear envelope. Characteristic mammary tumor virus type A and B particles were noted in 2 of 18 carcinomas investigated.

Lymphoblastic thymic lymphomas were characterized by vastly distended cytoplasm with numerous mitochondria, by an abundance of ribosomes and polysomes, and by single and multiple projections of the nuclear envelope embracing portions of the cytoplasm. Dense bodies were frequently noted in cytoplasm and in mitochondria. Type A virus particles were observed in 1 of 19 thymic lymphomas. It is inferred that the type A virus particles in one thymic lymphoma and the mammary tumor virus in two mammary tumors were present in a latent state in these X/Gf mice before they were subjected to treatments. Most recent serological tests revealed that a small percentage of X/Gf mice are susceptible to mammary tumor virus, confirming the electron microscopic observations.

The deep and extensive invaginations of the nuclear membranes, hypertrophic microvilli in mammary carcinoma cells and extensive single and multiple projections of the nuclear membranes of thymic lymphoma cells may be indicative of participation of the membrane systems of target cells in neoplastic transformation.

¹ Supported in part by grants from the Health Research Council of the City of New York and from the Mildred Werner League for Cancer Research.

Received 6/ 5/72. Accepted 9/11/72.