Trichogenic trichoblastoma arising on the supraclavicular fossa with an immunohistochemical study of cytokeratin expression.

We report a 52-year-old female with trichogenic trichoblastoma arising on the supraclavicular fossa. Clinical and histological examinations revealed a thumbnail-sized, elastic, hard, subcutaneous nodule which consisted of keratinous cysts and epithelial cords of basaloid cells with focally follicular differentiation. Based on histological observations, a diagnosis of trichogenic trichoblastoma was made. An immunohistochemical study using a panel of monoclonal antibodies against cytokeratins was performed to investigate the nature and differentiation of this tumour. The most characteristic findings of the immunohistochemistry were CK 8 and 19 expression in the epithelial cords and the outer cells of the cystic structures. These immunoreactivities were similar to those of the outermost layer of the outer root sheath between the lower permanent portion and the upper transient portion, and immunostaining with the other antibodies confirmed this similarity. We can speculate that trichogenic trichoblastoma differentiates mainly toward the outermost layer of the outer root sheath between the lower permanent portion and the upper transient portion, and then into follicular structures.