TABLE 1.

Various theories of endometriosis etiology.

Theory	Mechanism
Implantation theory	Retrograde menstruation allowing implantation of endometrial glands in pelvic peritoneum
Coelomic metaplasia	Transformation of peritoneal mesothelium or other cell types into endometrial tissue
Hormonal induction	Oestrogen-driven proliferation of endometrial lesions or metaplasia by hormonal stimulation
Inflammation and oxidative stress	Recruitment of immune cells and their production of cytokines which promotes endometrial growth
Immune dysfunction or deficiency	Prevention of eliminating menstrual debris and promotion of implantation and growth of endometrial lesions
Apoptosis suppression	Survival of endometrial cells by suppression of apoptotic pathways
Genetic	Alteration of cell types to endometriosis lesions
Stem cells	Transformation of haematopoietic, bone marrow or mesenchymal stem cells to endometriosis
Lymphatic, haematogenous metastasis	Spread of endometrial cells by lymphatic or haematogenous vessels
Embryonic cell remnants	Mullerian duct rests, Wolffian duct rests
Multiple factors	Combination of in situ development and endometrial transplantation and implantation
New theory of endometriosis aetiology from primordial germ cells	The eutopic and ectopic endometrium originating from primordial germ cells