Figure 1.

Etiology of endometriosis: retrograde menstruation and impaired immune environment. Following the retrograde menstruation theory proposed by Sampson, stromal and glandular components of shed endometrial tissue reflux into the pelvic cavity during menstruation. The regurgitated fragments trigger an aberrant infiltration of multiple immune cells into the endometriotic niche, inducing reduced cytotoxicity and aberrant cytokine secretion. The impaired microenvironment, in return, contributes to the angiogenesis, neurogenesis, and endometriotic lesion progression.