**4. Etiology and pathophysiology of endometriosis**

The source of endometrial implants is not fully understood. However, there are evidences for competence of some women over others (Di and Guo 2007). The reasons for susceptibility of some patients for development of ectopic lesions are not clear. Comparative microarray analysis of gene expression in patients with ectopic endometrial cells and that of eutopic endometrium has demonstrated an alternative pattern of gene expression in two groups (Taylor, Lundeen et al. 2002; Giudice 2003; Giudice 2006). This is also seen between patient and non-affected women (Gogusev, de Joliniere et al. 1999; Taylor, Lundeen et al. 2002; Giudice 2003; Giudice 2006; Wu, Strawn et al. 2006).

Although etiology of disease has not been clearly understood, a number of theories have been proposed in correlation with the pathogenesis of endometriosis which includes: genetic and environmental factors, immune system, retrograde menstruation, coelomic metaplasia, embryonic rest theory, lymphovascular metastasis and stem cell-based theory. Clinical manifestation of endometriosis and ectopic formation of endometriotic lesions usually considered as end point result of several aberrant biological process.
