Endometriosis is explained by :

Implantation Theory : Sampson's theory of endometriosis is reflux of menstrual endometrium through the fallopian tubes and its subsequent implantation and growth on the pelvic peritoneum and the surrounding structures. Sampson observed that in cases of uncomplicated endometriosis, the fallopian tubes were usually patent. Coelomic Metaplasia Theory : Meyer and Ivanoff (1919) propounded that endometriosis arises as a result of metaplastic changes in embryonic cell rests of embryonic mesothelium, which are capable of responding to hormonal stimulation. Embryologically, Mullerian ducts arise from these same tissues, hence such a transformation in later life seems plausible. Metastatic Theory : While the above theories can explain the occurrence of endometriosis at the usual sites, they found it difficult to explain its occurrence at less accessible sites like the umbilicus, pelvic lymph nodes, ureter, rectovaginal septum, bowel wall, and remote sites like the lung, pleura, endocardiumband the extremities. Hence it was suggested by Halban et al. (1924) that embolization of menstrual fragments through vascular or lymphatic channels occur, and this leads to the launching of endometriosis at distal sites. Endometrial tissue has been retrieved in pelvic lymphatics in 20% women with endometriosis. Hormonal Influence : Whatever the initial genesis of endometriosis, its fuher development depends on the presence of hormones, mainly oestrogen. Pregnancy,oopherectomy,irradiation,before pubey,menopause,anti estrogenic substances,smoking-----negative factors for endometriosis.Cyclical hormones stimulate its growth, but continuous hormone secretion or therapy suppresses. Immunological Factor : The peritoneal fluid in endometriosis contains macrophages,cytokines and natural killer (NK) cells which clear blood spilled into the peritoneal cavity. Impaired T cell and NK cell activity and altered immunology in a woman may increase the susceptibility to proliferation and growth. Other Factors : Other factors implicated in the occurrence of endometriosis are genetic, multifactorial, vaginal or cervical atresia encouraging retrograde spill. The more frequent the cycles, and the more the bleeding, greater is the risk of endometriosis. Prostaglandins secreted by endometriotic tissue may exacerbate chronic pain and dysmenorrhoea. Risk factors are polymenorrhagia, retroveed uterus which increases the risk of retrograde spill. A woman who has undergone tubectomy operation rarely develops endometriosis. History of familial tendency is repoed in 15%. Genetic basis accounts for 10% of endometriosis; incidence in first-degree relative is sevenfold. It may be that several factors are involved in the aetiology of endometriosis at different sites and none of the above theories fits into the development of endometriosis Reference: Shaw's Textbook of gynaecology,16th edition page no 410