We describe a pilomatricomal horn on the right arm of a 39-year-old man. Although initially the tumor was clinically thought to be a verruca vulgaris, the microscopic features were similar to those found in classic pilomatricoma, except for the epidermal location and the presence of a cutaneous horn. Light microscopy showed replacement of the epidermis by basaloid cells, with masses of cornified material containing shadow cells that formed a cutaneous horn. Whereas classic pilomatricoma is confined to the deep reticular dermis or subcutis, the present case represents a unique heretofore unreported epidermal variant of pilomatricoma that pathologists should be aware of to differentiate it from malignant epidermal tumors.

Expression patterns of hair and epithelial keratins and transcription factors HOXC13, LEF1, and beta-catenin in a malignant pilomatricoma: a histological and immunohistochemical study.J Cutan Pathol. 2006 Jan;33(1):1-9.

BACKGROUND: We have previously shown that benign pilomatricomas not only maintain the sequential expression of the hair matrix and precortex keratins hHa5 and hHa1 of normal hair follicles in their transitional cell compartment, but also preserve the association of hHa5 expression with that of its regulatory homeoprotein HOXC13 in the lower transitional cell compartment. In contrast, hHa1 expression in the upper transitional cell compartment is uncoupled from the nuclear co-expression of the LEF1/beta-catenin complex seen in normal hair follicles (Cribier et al., J Invest Dermatol 2004; 122: 1078). METHODS: Formalin-fixed paraffin sections of the tumor were examined using a panel of mono- and polyclonal hair and epithelial keratin antibodies as well as antibodies against HOXC13, LEF1, and beta-catenin. RESULTS: Morphologically, the malignant pilomatricoma investigated here clearly deviated from the described major tumor type by a large number of differently sized parakeratotic squamoid whorls emerging within the mass of basaloid cells and surrounded by cells remembering transitional cells, but only rarely containing shadow cells and signs of calcification. We show that hHa5/HOXC13 co-expression was maintained in transitional cell areas, in which hHa1 expression was much stronger than in benign pilomatricomas, but again uncoupled from concomitant nuclear LEF1/beta-catenin expression. Surprisingly, however, and in clear contrast to benign pilomatricomas, these transitional cells co-expressed the epithelial keratins K5, K14, and K17, with the latter being as strongly expressed as hHa1, both also staining the entire inner mass of the parakeratotic whorls. CONCLUSIONS: Although the malignant pilomatricoma investigated here was distinctive in that it contained a multitude of parakeratinizing whorls and no signs of calcification, it shared both hHa5/HOXC13 co-expression and disrupted hHa1/beta-catenin-LEF1 expression in its transitional cell compartment around the whorls with benign pilomatricomas. However, in clear contrast to the latter, transitional cells of the malignant tumor also strongly expressed the epithelial keratins K5, K14, and K17. We speculate that the observed dominance of the epithelial differentiation pathway over the competing conventional shadow cell differentiation pathway may prevent massive calcification of the tumor.

Aggressive pilomatrixoma of the infra-auricular area: a case report. Auris Nasus Larynx. 2005 Dec;32(4):407-10. Epub 2005 Sep 28.

Although pilomatrixomas are well known among dermatologists and dermatopathologists, head and neck surgeons confronted with these lesions in the infra-auricular region do not consider this benign neoplasm in the differential diagnosis. Aggressive pilomatrixoma is a benign tumor of the hair matrix cells affecting mainly children. Histologically, the border between aggressive pilomatrixoma and pilomatrix carcinoma is still not clear. We report the case of a 15-year-old Turkish boy suffering from an aggressive pilomatrixoma of the infra-auricular region and review the literature about this unclear entity.

The common ultrasonographic features of pilomatricoma.J Ultrasound Med. 2005 Oct;24(10):1397-402.

OBJECTIVES: The purpose of this series was to describe typical ultrasonographic features of 20 cases of pilomatricoma and to improve its diagnostic rate with the use of an ultrasonographic approach. METHODS: For 20 pilomatricomas in 19 patients with preoperative ultrasonography from 1995 to 2004, we reviewed age, sex, symptoms, duration, referring clinician, and tumor sites. The ultrasonographic findings were retrospectively analyzed for tumor location, shape, size, margin, echo texture, echogenicity, presence, amount, and shape of calcification, presence of a hypoechoic rim, and Doppler flow pattern. RESULTS: The mean age of the 19 patients was 6.9 years (range, 1-21 years), and the female-male ratio was 1.1:1. Patients had a painful palpable mass in 10 cases (50%). Nine lesions occurred in the neck, 5 in the cheek, 2 in the preauricular region, and 4 in the extremity. All tumors were located in the subcutaneous layer. The mean size of the tumors was 13.4 mm. Fourteen pilomatricomas (70%) appeared as well-defined oval masses. Tumors were heterogeneously hyperechoic in 80% of cases. All tumors had internal echogenic foci. A hypoechoic rim was seen in 17 cases (85%). Doppler flow signals were observed in the peripheral region in 14 cases (70%). A correct preoperative diagnosis was made in 33% on the basis of clinical findings and in 76% by ultrasonography. CONCLUSIONS: Diagnosis of pilomatricoma should be considered when a well-defined mass with inner echogenic foci and a peripheral hypoechoic rim or a completely echogenic mass with strong posterior acoustic shadowing in the subcutaneous layer of the head, neck, or extremity is found on ultrasonography.

Aberrantly differentiated cells in benign pilomatrixoma reflect the normal hair follicle: immunohistochemical analysis of Ca-binding S100A2, S100A3 and S100A6 proteins.Br J Dermatol. 2005 Feb;152(2):314-20.

BACKGROUND: Pilomatrixoma is a common benign cutaneous tumour containing differentiated hair matrix cells. This tumour is mainly composed of basophilic, transitional, shadow and squamoid cells. Although some S100 proteins are expressed in a tissue-specific manner in the hair follicle (e.g. S100A2 in the outer root sheath, S100A3 in the cortex and cuticle, and S100A6 in the inner root sheath), little information is available concerning their distribution in the aberrantly differentiated tissues of pilomatrixoma. OBJECTIVES: To characterize the disordered epithelial elements of pilomatrixoma by localizing S100A2, S100A3 and S100A6 proteins. METHODS: Immunohistochemistry and dual-immunofluorescence microscopy were performed on 22 pilomatrixoma specimens using antibodies specific to the three proteins. RESULTS: Tissue-specific distribution of the S100 proteins investigated was preserved in the morphologically disordered tumour tissues. Anti-S100A2 antibody stained squamoid cells and putative outer root sheath cells; basophilic and potential hair matrix cells were occasionally stained. S100A3 staining was found in transitional cells and putative cortical cells, and was strong in both dispersed cells and hair-like structures surrounding cells which were presumably cuticular cells. Anti-S100A6 antibody labelled some S100A3-negative transitional cell strands, potentially inner root sheath cells. CONCLUSIONS: The epithelial elements of pilomatrixoma can be characterized using S100 proteins as biochemical markers. Our results show that pilomatrixomas retain a certain degree of differentiation indicative of distinct hair-forming cells.

Pilomatricoma with a bullous appearance.J Cutan Pathol. 2004 Sep;31(8):558-60.

BACKGROUND: Pilomatricoma is a benign, cutaneous neoplasm with differentiation toward hair matrix. The tumor is usually a deep-seated, solitary, firm nodule with overlying normal epidermis. Pilomatricoma with a bullous appearance is very rare. METHODS: A 16-year-old Chinese girl with a soft, purplish, translucent bulla on the left shoulder for 6 months and a nodule underlying the bulla is described. RESULTS: The histopathologic findings were consistent with pilomatricoma. There was extraordinary dilation of lymphatic vessels in the overlying dermis, which explains the clinical bullous appearance. The surrounding dermis had an edematous appearance. A Verhoeff-van Gieson stain disclosed the marked reduction of the elastic fibers, and an alcian blue stain was negative. CONCLUSIONS: The patient is diagnosed as bullous variant of pilomatricoma.