**1. Introduction**

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Stem cells (SCs) are undifferentiated cells which are able to remain at this state for several generations following cell proliferations. SCs are also able to take part in natural *in vivo* phenomena such as wound repair after physical damages or tissue regenerations.

On this basis, endometrial stem cells (EnSCs), characterized by higher abilities for proliferation, differentiation, fast angiogenesis during menstruation and immune tolerance for embryo during pregnancy, have been considered as a valuable source of stem cells (Gargett, Chan et al. 2007; Meng, Ichim et al. 2007). Several researches have demonstrated presence of highly pleuripotent mesenchymal stem cells in endometrium by differentiation potency to various cell types such as, insulin producer cells (Li, Chen et al. 2010), osteoblasts (Ai, Mehrabani. 2010) , odontoblasts (Ai, Tabatabaei et al. 2009), neurons (Wolff, Gao et al. 2011) (Ai, Tabatabaei et al. 2009) and Myoblast (Ai, Tabatabaei et al. 2009). These findings may open up new opportunities to use endometrial stem cells in tissue engineering and cell therapy (Ai , Mehrabani. 2009).

Additionally our previous studies demonstrated that EnSCs are able to participate at a phenomenon like angiogenesis in 3-D cultures which is similar to early stages of endometriosis (Esfandiari, Ai et al. 2007; Esfandiari, Khazaei et al. 2007; Esfandiari, Ai et al. 2008) which may propose a novel mechanism in pathogenesis of endometriosis compared to the traditional theory that emphasis on retrograde menstruation as a possible factor. These studies are supported by others who demonstrated recruitment of bone marrow-derived mesenchymal stem cells to the endometrium (Taylor 2004) and these all may point to the role of stem cells at the pathogenesis of endometriosis (Figueira, Abrão et al.; Sasson and Taylor 2008). Therefore, endometrial stem cells could be considered as binary role-player in natural function of endometrium and endometriosis and potentially could be used as a target in endometriosis therapy.
