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Histologic Features of Virus-rich and Virus-poor Shope Papillomas of Cottontail Rabbits¹

Department of Microbiology, School of Medicine, University of Washington, Seattle, Washington 98105

Certain histologic features of virus-rich cottontail papillomas were different from those of tumors poor in virus content. The virus-rich papillomas were characterized by a marked fragmentation of the horny layer and a well-developed granular layer which contained many keratohyalin bodies of unusually large size. A series of histochemical tests have confirmed that such large bodies are, in fact, keratohyalin granules. The virus-poor papillomas showed a rather homogenous horny layer and a granular layer that was ill developed or nonexistent in many areas. The horny layer was much thicker in virus-poor than in virusrich papillomas, and in hematoxylin-and-eosin-stained sections, it was mostly pink in the former and blue in the latter.

Two other types of intracellular bodies were also identified in larger numbers in virus-rich papillomas. One was an eosinophilic cytoplasmic structure; the other was a basophilic intranuclear body. The eosinophilic inclusions were Feulgen-negative, but they sometimes contained a few Feulgen-positive granules. They could be identified in all layers of the papilloma and did not contain the Shope virus antigen, as judged by fluorescent antibody tests. These bodies might represent some form of abnormal cell keratinization. However, the possibility that they may be inclusion bodies of a second virus cannot be excluded.

The intranuclear basophilic bodies appear to represent virusspecific structures. These bodies were Feulgen-positive and occurred in the granular and horny layers. The fact that they contained a nucleoprotein resistant to DNase is consistent with the concept that such bodies might contain a mass of Shope papilloma virus particles. The demonstration by us and other workers that virus particles and virus antigen occur within nuclei in the granular and horny layers supports such a conclusion.

¹ This investigation was supported in part by Research Grant CA 02668 from the National Cancer Institute, NIH, USPHS.

Received 1/14/67. Accepted 6/ 5/67.