**1. Introduction**

262 Endometriosis - Basic Concepts and Current Research Trends

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historical account. *Virology*, Vol. 384, No. 2, (February 2009), pp. 260-265, 0042-

Endometriosis is a common gynecological disease. It is unique to have benign histology but with malignant characteristics. Easy recurrence, multiple organ involvement and malignant transformation potential make endometriosis a complex disease. Multi-factors contribute to the pathophysiology. Recently, endometriosis has been regarded as a stem cell disease (Sasson & Taylor, 2008). Some studies have provided evidence of the possible existence of stem cells in endometrial tissue (Gargett et al., 2004, 2007). We successfully isolated eutopic and ectopic endometrial mesenchymal stem cells (EN-MSCs) derived from one donor to examine the genetic difference analysis that provided a powerful tool for investigating the disease origin (Kao et al., 2011). Our results are consistent with the concept that endometriosis is a stem cell disease. We identified and characterized the MSCs from ectopic and eutopic endometrium by *in vitro* cell characteristics, including serpiginous morphology, surface biomarkers, a lack of gap junctional intercellular communication and the ability of differentiation and transdifferentiation into adipocytes, osteocytes, chondrocytes, neural cell and cardiomyocytes. In an *in vivo* animal study, we found the ability of invasion in eutopic and ectopic MSC.

The origins of endometrium stem cells are still under debate. Bone marrow is one of the origins. Bone marrow MSC circulates to the endometrium and reprograms into the endometrial MSC.

Aims of the chapter are to discuss about the endometrial stem cell identification and characterization and the stem cell theory of endometriosis will be discussed by search from the PubMed publications. The results will update our knowledge of the novel theory -- stem cell as the root of endometriosis.
