Virus-like Particles in 3-Methylcholanthrene-induced Primary and Transplanted Sarcomas of Germ-free Rodents¹

Primary and serially transplanted 3-methylcholanthrene-induced sarcomas of germ-free mice and rats and the thymuses of tumor-bearing and tumor-free experimental animals were examined by electron microscopy for virus particles. The results can be summarized as follows: (a) Small doughnut-like particles (Type A) about 75 mµ in diameter were found mainly in association with the endoplasmic reticulum of the cell in all primary and serially passaged tumor tissues of germ-free mice, of both the C3H and the AKR strain. (b) In addition, doughnut-shaped particles and particles with a large dense nucleoid (Type C), both about 100 mµ in diameter, were also detected in the intercellular spaces of primary and passaged tumor tissues of both the C3H and the AKR strain of mice. These Type C particles were more numerous in the latter mouse strain. (c) Both types of virus particles observed in the tumor tissues were also present in the thymus tissues of tumor-bearing and tumor-free mice. (d) Virus-like structures have not been noted in similarly induced tumor tissues nor in the thymuses of germ-free Sprague-Dawley, Fischer, and Wistar strain rats. The above results may indicate that the detection of virus-like particles in methylcholanthrene-induced tumors were reflections of the viral status of the experimental animals and that they probably represent “viral contamination of the tumor tissues” in vivo. The fine structures of the basic, methylcholanthrene-induced tumor cells are described.