F.: Electron microscopic identification of viral particles in calcifying epithelioma induced by polyoma virus

SUMMARY-Tumors similar to human calciFying epithelioma of Malherbe were induced in C3HjBi mice by inoculation of polyoma virus. Histological and ultrastructural resemblance of these tumors to the human counterpart, including calcification of cortical cell type of tumor cells, was striking. Electron microscopically the so-called basophilic cells were premature corti-cal, cuticular, and inner root-sheath cells. Predominant, however, was the inner root-sheath cell (Huxley and Henle cells). Hyalinized and calcified cortical cells, imbricate cuticular cells, and typical outer root-sheath cells were identified. Virus particles were found only in tumor cells with the differentiation toward inner root-sheath cells. Visible viral particles first appeared in the nucleus where they were closely related to the nuclear chromatin, and, after proliferation in the nucleus, spread into the cytoplasm and eventually, with the destruction of the host cells, disappeared. Few extracellular viral particles were present. In the nucleus as well as in the cytoplasm they often formed crystalline arrays. Cylindrical viruses were als