Intraepidermal cytokeratin 7 expression is not restricted to Paget cells but is also seen in Toker cells and Merkel cells

Histologically, extramammary Paget's disease and mammary Paget's disease (M PD) are characterized by large atypical cells distributed throughout the ep idermis. Although classic examples of these disorders are easily diagnosed on morphologic grounds, some cases may cause differential diagnostic proble ms. Immunohistology with a wide variety of antibodies has been used as an a id for the identification of Paget cells, for their distinction from other entities, and for investigation of the origin or nature of the disorder. Re cently, cytokeratin 7 has been proposed as a specific and 100% sensitive ma rker for Paget's disease. We studied 22 cases of mammary Paget's disease an d 22 cases of extramammary Paget's disease with and without an underlying m alignancy for their reactivity with monoclonal antibodies to cytokeratin 7 (CK7) and cytokeratin 20 (CK20). Our studies show that anti-CK7 is an effec tive but not 100% sensitive marker for Paget cells, staining 21 of 32 cases of mammary Paget's disease and 19 of 22 cases of extramammary Paget's dise ase, whereas CK20 stained 0 of 17 cases of mammary Paget's disease and 6 of 19 cases of extramammary Paget's disease. We also demonstrate that CK7, bu t not CK20, highlights intraepidermal clear cells with bland nuclear featur es (Toker cells) that have been reported in 11% of normal nipples. By using CK7 as a marker, however, we were able to identify Toker cells in most of the nipples we studied: 8 of 15 nipples from mastectomy patients without Pa get's disease, and 15 of Is autopsy cases (both male and female) with norma l breasts and nipples. It also permitted us to perform more extensive pheno typing on them, showing that Toker cells share similar antigens with Paget cells and with cells lining the underlying normal lactiferous ducts. Tn 7 o f 15 cases containing CK20-positive Merkel cells, CK7 was also seen to stai n Merkel cells. In infrequent cases, Toker cells or Merkel cells may be so numerous focally that a CK7 stain may raise the possibility of involvement of the nipple by Paget's disease. An awareness of the CK7 reactivity of Tok er cells and Merkel cells as well as attention to the cytologic features of the case should avoid this problem.