Abstract. Inverted papillomas of the skin occurred in five dogs. Lesions were 1-2 cm, circumscribed, flasklike structures below the level of the surrounding normal skin. Walls of the structures consisted of hyperplastic epidermis, forming thin papillary projections on thin fibrovascular stalks. Cells in the stratum granulosum had clear cytoplasm, numerous keratohyalin-like granules of various sizes, and poorly defined intranuclear inclusions. These cells stained positively for papillomavirus group-specific antigens by both the peroxidase-antiperoxidase and avidin-biotin methods. Virions with a mean diameter of 35.7 nm were present within nuclei in cells of the stratum granulosum when examined by electron microscopy. In situ DNA hybridization, using a canine oral papillomavirus probe, localized papillomavirus DNA in canine oral papillomas, but not in canine cutaneous squamous or inverted papillomas, suggesting that a different papillomavirus type was present in the latter lesions. Although these lesions resembled intracutaneous cornifying epitheliomas (keratoacanthomas), they appear to be a distinct lesion, probably with a different etiology Dogs develop a variety of cutaneous neoplasms; the etiology of many are unknown. Papillomavirus antigens have been detected in cutaneous squamous papillomas and some squamous cell carcinomas, but not in canine intracutaneous cornifying epitheliomas, trichoepitheliomas, pilomatrixomas, or sebaceous adenomas. 16 We describe here five cases of inverted papillomas in dogs which superficially resemble intracutaneous cornifying epitheliomas (keratoacanthomas), are morphologically distinct from the common cutaneous squamous papilloma, and contain papillomavirus antigens.
Materials and MethodsSurgical biopsies from five dogs were fixed in 10% neutral buffered formalin, processed routinely, embedded in paraffin, sectioned at 6 pm, and stained with hematoxylin and eosin (HE). Serial sections were tested for the presence of papillomavirus group-specific antigens by both the peroxidaseantiperoxidase (PAP)16 and avidin-biotin complex (ABC) techniques. Tissues removed from paraffin blocks of three cases were reembedded in epon-araldite, sectioned, and examined in an electron microscope to confirm the presence of virus.Sections for in situ hybridization were deparaffinized, treated with a prehybridization mixture of 50% deionized formamide in 0.3 M NaCl and 0.03 M Na, citrate, and incubated with the hybridization mixture.6 The hybridization mixture was prepared using cloned canine oral papillomavirus DNA24, nick translated in the presence of 66.8 pmoles ,%-labelled dATP.I3J7 The resulting 35S-labelled DNA had a specific activity of greater than 1 x lo8 cpmhg. Following a 4-hour incubation with the hybridization mixture, the sections were rinsed, dehydrated with ethanol, and applied to NTB-2 nuclear track emulsion (Kodak). The emulsion was exposed for 7 days, slides developed in a 1 : 1 mixture of D-19 (Kodak) and H,O, rinsed in 1 O/o glacial acetic acid, fixed in Kodak Rapid Fix, stained wi...