Many theories have been proffered to explain the histogenesis of endometriosis (Robboy et al., 2009). Generally, they divide into those that favour transplantation of endometrial fragments to ectopic sites, metaplasia of the multipotential celomic peritoneum and induction of undifferentiated mesenchyme in ectopic sites to form endometriotic tissues after exposure to substances released from shed endometrium.