Aim: To investigate a possible follicular origin of extramammary Paget's disease (EPD). EPD is a predominantly intraepidermal tumour with extensive involvement of adnexal structures and high recurrence rates suggesting a follicular stem cell origin. Cytokeratin (CK) 15 and CK19 are considered markers for follicular stem cells located in the hair follicle bulge region.
Methods and results: Formalin-fixed paraffin-embedded tissues of 12 cases of primary EPD (three anal, nine vulvar) were studied immunohistochemically with antibodies to CK15 and CK19. All cases of EPD showed polygonal Paget cells in the interfollicular epidermis, hair follicles, sebaceous and apocrine glands distributed individually, in nests and in gland-like areas. The polygonal Paget cells were intimately associated with small, flat, mitotically active, 'compressed' keratinocytes. The large Paget cells uniformly expressed CK19 in 12/12 EPD. The small 'compressed' keratinocytes showed strong cytoplasmic CK15 staining in 9/12 EPD with focal accentuation, while the polygonal Paget cells were negative.
Conclusions: These histological and immunohistochemical observations allow the following conclusions: (i) the small, flat, 'compressed' keratinocytes are an integral part of EPD; (ii) the dual cell population is reminiscent of sebaceous glands with mature sebocytes and germinative keratinocytes; (iii) since both cell types express cytokeratins typical for follicular differentiation, EPD may be a proliferation of adnexal stem cells residing in the infundibulo-sebaceous unit of hair follicles and adnexal structures.