**1. Introduction**

182 Endometriosis - Basic Concepts and Current Research Trends

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Little is known about the pathogenesis of endometriosis. The prevailing hypothesis is that following retrograde menstruation, uterine endometrial tissue attaches, invades the peritoneal surface, and becomes vascularized.

The development of new blood vessels represents a crucial step during the establishment of endometriosis because endometriotic implants require neovascularization to guarantee oxygen and essential nutrient supply (Groothuis et al., 2005; McLaren, 2000). The interaction between the ectopic endometrium and the peritoneal tissue is a prerequisite for the induction of angiogenesis and the maintenance of endometriosis

At least, three processes appear to be critical to the establishment of endometriosis, according to the implantation theory: invasiveness, tissue remodeling and interactions between the ectopic endometrium and the surrounding peritoneal tissues (Giudice et al., 2008).

The establishment of endometriotic lesions needs a cascade of neoangiogenic factor, like the vasculatr endothelial factor, cytokines and metalloproteinases: this complex interrelation between factors permit sprouting of capillaries from pre-existing vessels and the subsequent supply for the development of ectopic implants (Hyder and Stancel, 1999).
