Basal cell carcinoma with tricholemmal (at the lower portion) differentiation within seborrheic keratosis.

Abstract

Recent genetic investigations support the idea that basal cell carcinoma (BCC) is trichoblastic carcinoma. However, it is generally thought that clear cell basal cell carcinoma is a result of degeneration rather than tricholemmal differentiation. We report a case of BCC, with clear cell components, that developed within seborrheic keratosis, with histopathological and immunohistochemical findings. The clear cell components in the present case showed the following four characteristics: (i) at the periphery of the aggregations, columnar clear cells were aligned in a palisade along a well-defined basement membrane; (ii) the nuclei of the columnar clear cells were at the pole opposite the basement membrane; (iii) the clear cells contained glycogen; (iv) in the aggregations with clear cell components, there was diffuse positive staining for cytokeratin 7 (CK7) (OV/TLR/30), but only the inner region stained positive for CK17. These four characteristics are comparable to those of the lower portion of normal outer root sheath. In addition, the BCC in the present case was partly composed of squamous cells that contained glycogen and were selectively positive for CK17 - features similar to those of squamous cells in normal outer root sheath. Some clear cell BCCs are simply the result of degenerative change, but other clear cell BCCs may be the result of tricholemmal (at the lower portion) differentiation.