Anatomic Pathology: Skin Pathology

• Trichoepitheliomas are believed to be poorly differentiated hamartomas of the hair germ. They can occur either as a solitary lesion or as multiple lesions.

• A solitary trichoepithelioma manifests as a firm, elevated, flesh-colored papule or nodule, usually less than 2 cm in diameter. Most commonly, the lesions are seen on the nose, upper lip, and cheek.

• Multiple trichoepitheliomas are transmitted as an autosomal dominant trait. The lesions manifest as small papules with a strong predilection for the central part of the face. The onset of lesions is usually in childhood or at the time of puberty.

• On histologic examination, trichoepitheliomas demonstrate tumor islands in the dermis that comprise uniform basophilic cells usually arranged in a lacelike or adenoid network. The cells sometimes demonstrate a peripheral palisade arrangement and are surrounded by a stroma with an increased number of fibroblasts.

• Because of the islands of basaloid cells, a trichoepithelioma may be misdiagnosed as basal cell carcinoma. However, the presence of well-formed horn cysts, papillary-mesenchymal bodies, and lack of high-grade atypia and mitoses favor the diagnosis of trichoepithelioma. Histopathologic features that favor basal cell carcinoma include the presence of a myxoid stroma and stromal retraction (clefting) around the basaloid islands.

• Focal positive CD34 staining of the fibroblastic stroma has been seen in trichoepitheliomas, whereas it is negative in basal cell carcinoma. Staining of the outermost epithelial layer for bcl-2 has been seen in trichoepithelioma. In contrast, the tumor islands of basal cell carcinoma usually stain diffusely. Immunohistochemistry for CD10 stains the stromal cells but not the epithelial cells in trichoepithelioma, whereas it stains the epithelial cells in basal cell carcinoma.