Background: Endometrium is derived from intermediate mesoderm via mesenchymal to epithelial transition (MET) during development of the urogenital system. By retaining some imprint of their mesenchymal origin, endometrial epithelial cells may be particularly prone to return to this state, via epithelial to mesenchymal transition (EMT). We hypothesized that pelvic endometriosis originates from retrograde menstruation of endometrial tissue and that EMT-like and MET-like processes might be involved in the pathogenesis of pelvic endometriosis.

Methods: We investigated commonly used molecular markers for EMT, including cytokeratin, E-cadherin, N-cadherin, vimentin, S100A4 and dephosphorylated beta-catenin by immunohistochemistry in different forms of pelvic endometriosis: deep infiltrating endometriosis, ovarian endometriosis and superficial peritoneal endometriosis (red and black lesions), as well as samples of menstrual endometrium, other benign ovarian cysts (mucinous and serous cyst adenoma), and abdominal scar endometriosis for comparison.

Results: Epithelial cells of red peritoneal lesions and ovarian endometriosis showed less epithelial marker (cytokeratin, P < 0.0001) expression and more mesenchymal marker (vimentin and/or S100A4, P < 0.0001) expression than those of menstrual endometrium. In contrast, epithelial cells of black peritoneal lesions and deep infiltrating endometriosis showed more epithelial marker (E-cadherin) expression than those of menstrual endometrium (P < 0.03), red peritoneal lesions (P < 0.0001) and ovarian endometriosis (P< 0.0001), but maintained expression of some mesenchymal markers (vimentin, S100A4). In addition, dephosphorylated beta-catenin protein expression was significantly higher in epithelial cells of deep infiltrating endometriosis (P < 0.0001) than in epithelial cells of red and black peritoneal lesions and ovarian endometriosis.

Conclusions: EMT-like and MET-like processes might be involved in the pathogenesis of pelvic endometriosis.