Trichilemmal differentiation in seborrheic keratosis.

An 88-year-old woman who developed a nodule in a lesion of seborrheic keratosis on the frontal scalp was observed. Histopathologically, the nodule was characterized by lobular acanthosis consisting of glycogen rich clear cells and squamoid cells with squamous eddies and melanin blockade melanocytes. Each lobule was covered with an orthokeratotic horny layer which formed wedge-shaped masses in its central area without formation of an intervening granular layer. The clinical course and histopathological findings suggest that the tumor cells in seborrheic keratosis may tend to a new development of trichilemmal differentiation.