Inverted Follicular Keratosis in a Dog

Abstract

Inverted follicular keratosis is a benign epithelial tumor in man. It occurs most commonly as a single lesion on the head [I, 21 and is a firm, hairless growth that projects above the surface of the skin. All such lesions are asymptomatic without peripheral erythema or inflammation. The center of the lesion is scaly, and the periphery is smooth and shiny. A 1-year-old female miniature Poodle dog had a small, raised, crusted nodule on the dorsum of the nose. The lesion had been present for about 6 weeks. Onset had been acute with periodic serous discharge and scale formation. There was no pruritis nor self trauma associated with the lesion which was 3 millimeters wide and protruded 3 millimeters above the surrounding skin (fig. I). The nodule consisted of a papillomatous epithelial tumor growing above the surrounding skin. The epidermis at the periphery was raised, blended with the edges of the tumor, and appeared to buttress the central tumor mass. The tumor had several elongated, papilliferous projections, some fused at the bases. The cells at the periphery of the tumor were cuboidal basal epithelial cells with prominent vesicular nuclei, separated from the underlying corium by a basement membrane. Numerous large dendritic melanocytes were scattered throughout the peripheral areas of the tumor; these melanocytes contained great amounts of melanin but little was in the epithelial cells. The cells in the centers of the papilliferous projections showed squamous differentiation and contained abundant eosinophilic cytoplasm and keratohyaline granules. Occasionally formations of squamous eddies were noted. There was little evidence of mitotic activity and no nuclear or cellular atypia. The surface was covered by a parakeratotic scale, and within the center of the tumor small aggregates of keratinous material were present. Dilated and congested vessels and a mild mononuclear cellular infiltrate were in the corium. Although numerous hair follicles were adjacent to the base of the tumor, none penetrated the tumor mass to reach the surface (fig. 2). This lesion has many features associated with inverted follicular keratosis in man, namely the papillomatous surface, one or several invaginations that resemble hair follicles and fingerlike tumor masses with peripheral small basal cells and with small squamous eddies in their centers. The feature that distinguishes this tumor from that in man is a lack of palisading of the peripheral basal cells [2]. Clinically the lesion in our dog resembles that in man. Both are on the head and have scales in the center of the tumor. In man, these tumors are thought to arise from hair follicle infundibulum; the infundibulum is that part of the hair follicle above the opening of the sebaceous gland [2,4]. Tumors of hair follicle origin in the dog [5, 61 do not resemble the tumor in our dog. Inverted follicular keratosis has not been described previously in animals [6]. Inverted follicular keratosis must be differentiated from canine cutaneous papillomatosis which is a solitary growth 151. These two tumors are distinguishable histologically. In canine cutaneous papillomatosis the hyperplastic epidermis is thrown into folds by an ingrowth of hyperplastic dermal tissue and lacks the dermal invasion by epidermal tissue which is a feature of inverted follicular keratosis. Papillomas also lack the squamous eddies and are covered on the surface by hyperkeratotic scale. Inverted follicular keratosis must be further distinguished from squamous cell carcinoma,