Simple epithelial cytokeratin‐expression in seborrheic keratosis

The cytokeratin expression of seborrheic keratosis was studied by means of immunohistochemistry and compared with that of normal human skin. The following findings were obtained in seborrheic keratosis: (1) a partial lack of high molecular weight cytokeratin (#1/68 kD, # 10/56.6 kD) in all ten cases examined; (2) the detection of cytokeratin typical for simple epithelia (#8/52.5 kD, # 18/45 kD, # 19/40 kD) in eight often cases; and (3) the detection of cytokeratin #5/58 kD in supra‐basal cells in 5 of 10 cases. An immunoelectron‐microscopic investigation, using an anti‐keratin antibody against cytokeratin #19/40 kD, revealed a whirl‐like arrangement of keratin filaments within immunoreactive cells, in contrast to a linear, parallel arrangement in non‐immunoreactive cells. Cells known to express cytokeratin typical for simple epithelia, such as sweat gland cells or Merkel cells, were not observed. The altered cytokeratin gene‐expression in seborrheic keratosis may be attributable to de‐differentiation of tumor cells or potential re‐differentiation towards embryonic keratinocytes.