What Happens to Endometriosis During the Menstrual Cycle?

INTRODUCTION: The objective of this study is to determine the structural changes in endometriosis throughout the menstrual cycle. MATERIALS AND METHODS: This retrospective comparative study was undertaken in a gynaecological unit of a university teaching hospital and looked at the immunohistochemical appearances of epithelial cells of the endometrium and endometriosis in 17 cases at various stages of the menstrual cycle, particularly during menstruation. RESULTS: The epithelium in endometriosis lesions undergoes the same cyclical morphologic changes that are observed in eutopic endometrium. In particular, each of the six cases of endometriosis observed during the active bleeding phase showed evidence of epithelial shedding of the terminally differentiated secretory-phase epithelial cells and their almost immediate replacement by small undifferentiated cells. CONCLUSION: The cyclical shedding/regeneration of endometriotic epithelium during menstruation has not previously been recognised, and it may have significant implications for the understanding of the aetiology and best management of endometriosis.