PAX-8 expression in cutaneous ciliated cysts: evidence for Müllerian origin.

Cutaneous ciliated cysts (CCC) are exquisitely rare, benign cystic lesions demonstrating simple, ciliated epithelial linings reminiscent of fallopian tube epithelium. Most commonly, CCC show a predilection for the lower extremities of young reproductive age women and demonstrate immunohistochemical positivity for estrogen and progesterone receptors, supporting the theory that they are derived from ectopic Müllerian rests. PAX-8 is a paired box gene, important in the development of Müllerian and thyroid organs and has utility in the identification of tumors of Müllerian, renal, and thyroid origin. Prompted by the precedent studies on PAX-8 immunohistochemical expression in tumors of Müllerian origin, this article aimed to explore the utility of this antibody in defining the histogenesis of 2 bona fide cases of CCC, both occurring in young reproductive age women. Herein, 2 prototypic index cases of CCC with strong nuclear positivity for estrogen and progesterone receptors are shown to also have positive nuclear staining for PAX-8, further supporting their likely Müllerian origin. These data support the designation of these lesions as cutaneous Müllerian cysts, distinct from potential ciliated cysts of eccrine origin.