Hair follicle is a small tubular cavtity containing the root of a hair. In this tubular cavity, the sebaceous (oil) glands open. Hair follicles are specialized structures in the skin where hair growth occurs. Hair follicle tumors are due to disordered growth of the hair follicles. These tumors are divided into two main groups; pilomatricomas and trichoepitheliomas.
Trichoepitheliomas arise from keratinocytes (epidermal cell that produces keratin) in the outer sheath of hair follicle or both the sheet and hair matrix (area where the formative cells of hair originate, develop, and are contained).
Pilomatricomas arise from hair matrix.
Both trichoepitheliomas and pilomatricomas are almost always benign in nature. Trichoepitheliomas are small and appear as granular, yellow “cheesy” material often along the back, shoulders, flanks, tail or limbs. Many of these superficial tumors ulcerate and ooze fluid and can become secondarily infected. Pilomatricomas are small and found on trunk of middle-aged dogs. A pilomatricoma feels like a small, hard lump under the skin. This type of tumor grows relatively slowly and usually does not cause pain or other symptoms.
Certain breeds are predisposed to hair follicle tumors including Golden Retrievers, Basset Hounds, German Shepherds, Cocker Spaniels, Irish Setters, English Springer Spaniels, Miniature Schnauzers and Standard Poodles.
Firm, round, elevated, well-circumscribed, hairless, or ulcerated dermoepithelial masses are characteristic of hair follicle tumors. In case of trichoepitheliomas, the cut surface of the masses is gray while in case of pilomatricomas, the cut surface is usually multi-lobbed (lobulated) with white chalky areas.