Contribution of stem cells to epidermal and hair follicle tumors in the dog.

In recent years, cutaneous stem cells have been implicated not only in epidermal and hair follicle renewal and repair but also in epidermal tumourigenesis. Stem cells have been identified in human, rodent, and, recently, canine hair follicles. However, to our knowledge, no studies have been conducted in the dog to elucidate the role of stem cells in the development of hair follicle tumours. In this study, 10 infundibular keratinizing acanthomas, 10 trichoblastomas, 10 trichoepitheliomas,10 pilomatricomas, and four tricholemmomas were retrospectively investigated for the expression of keratin15 (K15) and nestin, two recognized markers of epidermal/follicular and follicular stem cells, respectively. Inaddition, 30 squamous cell carcinomas were evaluatedfor K15 expression. Four samples of normal-haired skinfrom healthy dogs were used as positive controls.Immunohistochemical expression was determined usingcommercially available antibodies (Keratin 15, cloneLHK15, Thermoscientific; Nestin, clone ab7659; Abcam)that had already been tested and described in theveterinary literature. Expression of K15 and nestin inthe cytoplasm of neoplastic cells was semi-quantitativelyscored as negative (0) (< 25% of positive cells), mild (1)(25–50% of positive cells), moderate (2) (50–75% ofpositive cells) and intense (3) (> 75% of positive cells).The intensity of the reaction of single cells was furtherevaluated as faint (+), medium (++) or strong(+++). In normal adult canine skin, K15 was expressedin the outer root sheath cells of the isthmic portion of thehair follicle (bulge region) and multifocally in the basalcell layer of the epidermis. Nestin expression was lesspronounced and restricted to the middle portion of thehair follicle. For K15, expression was negative or mild inthe majority of infundibular keratinizing acanthomas (9/10), pilomatricomas (10/10), and squamous cells carcinomas(27/30). The trichoepitheliomas were the mostheterogeneous group regarding the expression of K15, interms of both percentage of positive cells and intensity ofexpression. Trichoblastomas were moderately to stronglypositive (7/10) and showed the highest intensity ofreaction of all the tumours studied. Tricholemmomaswere either negative (2/4) or moderately to stronglypositive (2/4). In all samples, K15 immunostaining ofneoplastic cells and lobuli was generally more prominentin the external or deep portions of the tumour. Althoughmilder and not equivalent, the trend of nestin expressionin follicular tumours was similar, with the highestpositivity in trichoblastomas and a high variability intrichoepitheliomas. However, the reaction was alwaysvery faint, and nestin was therefore judged a non-optimalmarker for the study of the role of stem cells in hairfollicle tumours of the dog. Our results suggest that hairfollicle stem cells in the bulge may play a role intumourigenesis of tumours originating from this portion,namely trichoepitheliomas and trichoblastomas, wherethe highest K15 immunostaining was observed. The lossof K15 expression in squamous cell carcinomas comparedto normal skin suggests that this event could becritical in the malignant transformation of epidermaltumours, although further studies are needed to confirmthis. [...]