Lymphatic vessels in peritoneal endometriotic lesions

Purpose: While the exact pathogenesis of endometriosis remains unclear, it has been hypothesized that fragments of viable endometrial tissue shed at menstruation can transit via the lymphatic circulation and establish endometriotic lesions at distant sites. Mounting evidence suggests that related parameters such as immune cell densities are altered in peritoneal endometriotic lesions. However, lymphatic vessels in lesions have not been previously investigated. In this study, the presence and distribution of lymphatic micro-vessels were characterized in peritoneal endometriotic lesions and surrounding tissues. Methods: Immunohistochemical staining was performed with an antibody specific for lymphatic endothelium (podoplanin, D2-40 clone) on 48 peritoneal endometriotic lesion specimens and 15 samples of normal surface peritoneum. Lymphatic micro-vessel density (LVD) was quantified in and around endometriotic lesions, and in normal surface peritoneum using an automated cellular imaging system. Results: LVD in endometriotic stroma (mean +/- SD = 31.1 +/- 28.8 vessels per mm(2)) was significantly increased compared to both adjacent (8.8 +/- 7.6, P<. 001) and distant sub-peritoneum (17.7 +/- 12.0, P=. 002). In addition, lesion-adjacent LVD was significantly lower than distant (P<. 001). No statistically significant differences in LVD were observed between endometriotic stroma and normal surface peritoneum from women without endometriosis (31.1 +/- 26.5) or at different phases of the menstrual cycle. Conclusions: This is the first report of LVD in peritoneal endometriotic lesions. LVD is increased in the stroma of peritoneal lesions compared to the surrounding sub-peritoneal tissue. Lymphatic microvessels play an important role in the trafficking of immune cells within endometriotic lesions and may be involved in lesion establishment.