Immunohistochemical observation of cytokeratins in keratinous cysts including plantar epidermoid cyst.

Sixteen cases of epidermal cyst (EC) (6 cases of conventional EC and 10 cases of plantar epidermoid cyst (PEC)) and 9 cases of trichilemmal cyst (TC) were examined by an immunohistochemical technique using various antibodies against cytokeratins and human papilloma virus (HPV) in order to clarify their histogenesis. There was no difference in immunoreactivities between EC and PEC with or without HPV infection. In TC, the inner layers of the cyst wall were stained with the antibody E3 (CK17), and the outermost layer was stained with the antibodies 4.1.18 (CK8) and 170.2.14 (CK19). In PEC and EC, however, the cyst wall didn't react with these antibodies, and differentiation-specific cytokeratins were expressed in the inner layer. These results confirmed that the immunoreactivities of PEC and EC were identical to those of normal epidermis or infundibulum, and that those of TC were similar to the outer root sheath between the lower infundibulum and isthmus. Although PEC has been recently reported to originate from eccrine ducts, there was a significant difference in immunoreactivities between PEC and eccrine ducts. Furthermore, on the basis of the fact that sole skin has no hair follicles, PEC was speculated to originate from epidermal implantation.