Background - Although cutaneous stem cells have been implicated in skin tumourigenesis in humans, no studies have been conducted to elucidate the presence and the possible role of stem cells in hair follicle tumours in the dog.Hypothesis - Stem cell markers are expressed in canine epidermal and follicular tumours and can be used to better understand the biology and origin of these tumours.Animals and Methods - In the present study, normal skin sections and 44 follicular tumours were retrospectively investigated for the immunohistochemical expression of keratin 15 (K15) and nestin. In addition, 30 squamous cell carcinomas were evaluated for K15 expression.Results - In normal skin, K15 and nestin were expressed in the outer root sheath cells of the isthmic portion of the hair follicle (bulge region), and K15 expression was also scattered in the basal cell layer of the epidermis. Infundibular keratinizing acanthomas, pilomatricomas and squamous cell carcinomas were mostly negative for K15, trichoblastomas were moderately to strongly positive, tricholemmomas were either negative or strongly positive, and trichoepitheliomas had heterogeneous staining. Nestinexpression was generally faint in all follicular tumours.Conclusions and clinical importance - Our results show that K15 can be a reliable marker for investigating the role of stem cells in hair follicle tumours of the dog, while nestin was judged to be a nonoptimal marker. Furthermore, our study suggests thathair follicle stem cells are present in the bulge region of hair follicles and could possibly play a role in tumourigenesis of canine tumours originating from this portion of the follicle, namely trichoblastomas, tricholemmomas and trichoepitheliomas. The loss of K15 expression in squamous cell carcinomas compared with normal skin suggests that this event could be important in the malignant transformation.
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