Abstract

Pilomatrical differentiation can be seen in a variety of benign and malignant tumors (e.g., trichoepithelioma/trichoblastoma, panfolliculoma, apocrine mixed tumor, basal cell carcinoma, and pilomatrical carcinoma) with the most common prototype being pilomatricoma. Since it originates from the matrical cells of the hair bulb, pilomatricoma often presents in the deep dermis or subcutis. Epidermal involvement is extremely rare and usually associated with a perforating or transepithelial elimination process. To the best of our knowledge, there are only two case reports of superficial pilomatrical lesions, both with cutaneous horns. In our current cases series, specimens from four patients were included with an average age of 69 years old (51 – 94 years old). All lesions were presented as solitary verrucous or keratotic papules on the extremities (forearm and thighs). Microscopic examination of all lesions reveals verruciform epidermal acanthosis. The two more verruciform-like cases have horn induction while the other two cases do not. All have an intraepidermal proliferation of basaloid matrical cells with small, uniform and round vesicular nuclei. The matrical cells express Wnt signaling pathway markers beta-catenin (strong nuclear and cytoplasmic) and LEF-1, and the lesional cells also express follicular stem cell marker PHLDA-1. The keratinized layer consists of a mixture of keratin and annulate cells with conspicuous cell borders but no cell bridges, suggestive of shadow cells. No similar components are identified in the dermis. Given the morphology and immunochemical expression pattern, these epithelial lesions are consistent with intraepidermal variant of pilomatricoma. Analogous to other benign intraepidermal proliferations (hidroacanthoma simplex, epidermolytic acanthoma, etc.), we propose the term “pilomatrical acanthoma” for these rare lesions.

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