An Electron Microscopic Study of Spontaneous and Experimentally Induced Leukemia in ICRC Mice¹ Free

Spontaneous, serially transplanted, and experimentally induced leukemias of ICRC mice were studied by electron microscopy in an attempt to detect the presence of virus particles, if any, and to observe the influence of chemical and hormonal treatment on the presence of these virus particles.

The first series of experiments included spontaneous, serially transplanted, and radiation-induced leukemia. The paucity of type C virus particles was quite conspicuous in spontaneous leukemia. Serially transplanted and radiation-accelerated leukemic lesions showed the presence of some type C and intracisternal type A particles. Found in two of these leukemic lesions (thymus and lymphosarcoma), in addition to type C virus particles, were budding and some mature type B virus particles, and numerous intracytoplasmic type A particles. “Viropexis” of type B virus particles has been observed in the lymphosarcoma and in a leukemic thymus gland.

The second series of experiments included leukemia induced in ovariectomized ICRC mice with 20-methylcholanthrene (MCA), pituitary transplants, and ovarian hormones (estradiol and estradiol-progesterone). In ovariectomized ICRC mice, leukemic lesions induced by MCA or pituitary transplants, or by MCA and pituitary transplants, showed type C virus particles and, in most cases, intracisternal type A particles. In leukemia induced in ovariectomized ICRC mice by MCA and estradiol, numerous intracytoplasmic type A particles were observed but no type C virus particles. Numerous intracytoplasmic type A particles, as well as budding and some type B virus particles were found in leukemia (thymus and spleen) induced in ovariectomized ICRC mice by MCA and estradiol combined with progesterone. The large dose of estradiol or a balanced dose of estradiol-progesterone may have stimulated the appearance of numerous intracytoplasmic type A particles, known to be prccursors of mammary tumor virus.