The fine structure of cutaneous papilloma in the dog was studied in two biopsy specimens fixed in formalin, postfixed in osmium tetroxide, and processed through the Epon embedding technic. The most prominent changes were found in the nuclei of epidermal cells, especially in the granular layer of neoplastic fronds. The affected nuclei contained inclusion bodies in various stages of morphologic differentiation. The inclusions consisted essentially of aggregates of virus particles embedded in the nucleoplasm. The aggregates were often in the form of close-packed crystalline arrays. The individual virus particles in such arrays had a hexogonal profile and measured 450 Å to 490 Å in diameter. In some aggregates, approximately 10 percent of the particles were devoid of the core. These observations present the first morphologic evidence of association of a viral agent with canine cutaneous papilloma. The virus particles found in neoplastic cells had morphologic characteristics indistinguishable from those shown by papilloma viruses.

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This study was supported in part by Research Grant No. CAO 8756 from the National Cancer Institute, NIH, USPHS.

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