BACKGROUND: The histogenesis of trichilemmoma remains unclear. OBJECTIVES: To clarify the histogenesis of trichilemmoma by evaluating its cytokeratin (CK) expression. METHODS: In three cases of trichilemmoma, CK expression was studied immunohistochemically using seven antikeratin antibodies against CK1, 10, 14-17 and 19, respectively. RESULTS: CK1 and CK10 were present in keratinizing ductal epithelium. CK14 was present in the whole layer. CK15 was present in suprabasal layers in two cases. CK16 was present in the suprabasal layer, but was absent in keratinizing ductal epithelium. CK17 was present in suprabasal layers and the sebaceous duct-like structure. CK19 was totally absent. CONCLUSIONS: These results showed that trichilemmoma may differentiate mainly towards two directions: infundibular keratinization and proliferation of the outer root sheath with undifferentiated and pluripotent characteristics.

Desmoplastic trichilemmoma: a rare tumor of the eyelid. Eur J Ophthalmol. 2004 Nov-Dec;14(6): 562-4.

PURPOSE: To report an upper eyelid mass which proved to be a desmoplastic trichilemmoma. METHODS: A 60-year-old man had a slowly enlarging upper eyelid mass. The tumor was excised. The pathologic evaluation of the tumor was centered on the differential diagnosis. RESULTS: The clinical appearance of this lesion is nonspecific and can simulate a verruca, follicular keratosis, or basal cell carcinoma. Central desmoplasia, outer root sheath differentiation of the tumor cells, and CD34 positivity are the main characteristics that allow differentiation from basal cell carcinoma. CONCLUSIONS: Proper recognition of a benign neoplasm that may be misdiagnosed as basal cell cancer can prevent aggressive surgical treatment.

Desmoplastic trichilemmoma arising within a nevus sebaceus. Am J Dermatopathol. 1998 Oct;20(5):495-7.

A 74-year-old man presented with a tumor on his scalp that he had since birth. The authors observed a verrucous pink crusted plaque. The tumor was clinically diagnosed as nevus sebaceus and was removed. Histopathologic study showed, arising from nevus sebaceus, a lobular neoplasm with the classic features of trichilemmoma. Desmoplasia was present in the center of the trichilemmomatous area. This description is typical of a special, uncommon type of trichilemmoma that histologically mimics invasive carcinoma.