**1. Introduction**

342 Endometriosis - Basic Concepts and Current Research Trends

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Endometriosis is defined as the presence of endometrial-like tissue outside the uterus, which induces a chronic, inflammatory reaction (Kennedy *et al*., 2005). Infertility is one of clinical manifestation of endometriosis showed by the difference of fecundity. At our tertiary hosital pelvic endometriosis is frequently found in infertile women: 23,8 % (1987), 37,2 % (1993) and 50 % (2002) cases of diagnostic laparoscopy (Samsulhadi, 2002).

An association between endometriosis and infertility has repeatedly been reported in the literature, but an absolute cause-effect relationship has not yet been confirmed. The controversy regarding whether endometriosis is a cause of infertility or an incidental finding is ongoing (ASRM, 2006; Gupta *et al*., 2008). Many theories of endometriosis which may impair fertility have been suggested during the years, and new hypotheses and approaches to the problem have arisen with the application of assisted reproduction techniques (Garrido *et al*., 2002). Data on the impact of endometriosis on the results of in-vitro fertilization and embryo transfer (IVF-ET) treatment are not consistent. Several theories have been proposed to identify the pathomechanism of infertility in endometriosis. None of these theories can completely explain these association. Based on many reports the possible mechanisms that could cause infertility in endometriosis are pelvic adhesion and endometrioma and also excess production of inflammatory factors in micro environment that both play a role in alteration fertility function. Severe endometriosis is associated with pelvic adhesions leading to a possible mechanic or anatomic disturbance of fertility, in the other hand a mild stage may have a direct and indirect negative effect on folliculogenesis, oocyte development, sperm function, embryogenesis, and endometrium receptivity (Barnhart *et al*., 2002). Our aim in the present review is to describe an update on several approaches of the pathomechanism of infertility in endometriosis, based on its impact on a number of pathologic conditions, such as: pelvic adhesion and endometrioma, abnormal folliculogenesis and impaired oocyte function, altered sperm function, reduced embryo quality, and impaired endometrium receptivity.

#### **2. Pelvic adhesion and endometrioma**

The mechanisms by which endometriosis impairs fertility have not been completely determined but are likely varied. Ovarian involvement and adhesion that block tubal motility and pick-up of the egg could be a main causative of mechanical interference on

Pathomechanism of Infertility in Endometriosis 345

with endometrioma had smaller numbers of follicles developed, oocytes harvested, and

In women with endometriosis, pelvic adhesions contain estrogen and progesterone receptors, and produce basic fibroblastic growth factor and vascular endothelial growth factor, implying a regulation of pelvic adhesion formation by steroid hormone. Zang (2010) found that both the percentage and the density of protein gene product (PGP) 9.5-positive nerve fibres in ovarian endometriotic lesions were significantly higher in women with ovarian endometriosis who had pelvic adhesions than in those women with ovarian endometriosis and no pelvic adhesions (Zang et al., 2010). It is suggested that ovarian endometriotic lesions may be innervated through mediating effects of peritoneal inflammatory cytokines and growth factors including IL-1, IL-6 and TNF-a, in women with pelvic adhesions, thus leading to an increase of nerve fibres in ovarian endometriotic lesions

Infertility associated with the advanced stages of endometriosis may be explained by pelvic adhesion and endometrioma as described above. The mechanism of infertility associated with endometriosis without adhesion and endometrioma, such as minimal or mild endometriosis as well as the negative impact of all stages of the disease on infertility is poorly understood. Many possibilities have been suggested, ranging from abnormal folliculogenesis to impaired endometrium receptivity (Arici *et al*.*,* 1999). Peritoneal fluid, a biologic fluid present in the abdominal cavity, has been a focus of research on endometriosis because of the extent of information it potentially carries about the disease. The proximity of peritoneal fluid to endometriotic lesions shows the milieu in which the immune mediators associated with the local inflammation of endometriosis can be studied. It has been suggested that such alterations in cytokines and growth factors interfere with

The local microenvironment of peritoneal fluid surrounding the endometriotic implant is immunologically dynamic and links the reproductive and immune systems. Peritoneal fluid contains a variety of free floating cells, including macrophages, mesothelial cells, lymphocytes, eosinophils and mast cells (Oral *et al.,* 1996). The peritoneal fluid of women with endometriosis have confirmed an increased number, concentration and activation of macrophages which may induce proliferation of cells that are involved in inflammation through secretion of factors such as IL- 1, IL-6, and TNF-α (Oral *et al*., 1996a). Other studies similary found that levels of cytokines, such as, IL-6, IL-8 and TNF-α increased in the peritoneal fluid of women with endometriosis (Arici *et al*., 1996), meanwhile endometriotic implants also secreted various cytokines including IL-1, IL-6, IL-8, TNF-α in the peritoneal cavity in patients with endometriosis (Oral *et al*., 1996b). Cytokines, which are produced by many cell types in peritoneal fluid, play a diverse role as toxic effect in constructing the peritoneal environment that induces the development and progression of endometriosis and

Peritoneal fluid bathed the ovaries, hypothetically the inflammatory components in peritoneal fluid in women with endometriosis might diffuse into the ovarian follicles, or by

mature oocytes (Nakahara *et al*, 1998).

in women with ovarian endometriosis ( Zang *et al.,* 2010).

folliculogenesis, ovulation and fertilization (Arici *et al*.*,* 1999).

endometriosis-associated infertility (Harada *et al*., 2001).

**3. Abnormal folliculogenesis and impaired oocyte function** 

fertility especially in severe endometriosis due to a possible mechanic or anatomic disturbance such as extensive pelvic adhesions. Pelvic endometriosis, the most common form of the disease, could be associated with increased secretion of pro-inflammatory cytokines, impaired cell-mediated immunity and neo-angiogenesis. Barnhart *et al*. (2002) found that compared with women with mild endometriosis, women with severe endometriosis have a statistically significantly lower pregnancy rate and implantation rate, have fewer oocytes obtained at ovarian retrieval, and have a lower peak estradiol concentration (Barnhart *et al.,* 2002).

Adhesion formation involves three important components: 1) acute inflammatory response, 2) fribinolysis, and 3) metalloproteinases and their tissue inhibitors. Celluler mediators within the peritoneal fluid can potentially modulate inflammatory responses over a large surface area due to the liquid nature of the peritoneal fluid. There are three important pro-inflammatory cytokines involved in adhesion formation: interleukine (IL)- 1, IL-6 and Tumor Necrosis Factor (TNF)-α (Cheong *et al.,* 2002). Endometriosis is associated with signs of pelvic peritoneal inflammation including increased volume of peritoneal fluid, increased concentration of peritoneal fluid macrophages, and increased peritoneal fluid concentrations of IL-6, IL-8, TNF-α and other cytokines and growth factors. Indeed, these cytokines have been reported to increase the endometrial–peritoneal adhesion *in vitro* (D'Hooghe and Debrock, 2002). Pelvic adhesion secondary to endometriosis is the most accepted reason for infertility, presumably via dysfunction of the fallopian tube or ovary. Inflammatory cytokines, IL-6, IL- 8 and TNF-α produced by endometrial cells probably contribute to the adhesion process. IL-8 has been shown to stimulate the adhesion of endometrial cells to fibronectin. TNF-α has also been reported to promote endometrial stromal cell proliferation *in vitro* and endometrial stromal cell adhesion to extracellular matrix components (Garcia-Velasco and Arici, 1999). These pelvic adhesion inhibits ovum capture after ovulation.

Cysts of endometriosis (endometriomas) may become adherent to the uterus, bowel or pelvic side wall. Any of these anatomic distortions can result in infertility. The presence of an ovarian endometrioma greater than 1 cm in diameter is classified as stage III (moderate) or more in the revised American Society for Reproductive Medicine (ASRM) classification of endometriosis, but unfortunately, the staging system does not correlate well with a woman's chance of conception following therapy (ASRM, 2006). The impact of an ovarian endometrioma on infertility remains controversial, despite the number of studies that have been performed. Suzuki *et al* (2005) found that endometriosis, even after diagnostic laparoscopy with treatment when necessary, clearly affects the number of oocytes as well as the number of transferred embryos but not embryo quality and the related parameters of pregnancy, as indicated by the fertilization rate, embryo quality, implantation rate, pregnancy rate, and live birth rate, irrespective of the presence of an ovarian endometrioma (Suzuki *et al,* 2005). Nakahara (1998) found that the proportion of apoptotic bodies in the membrana granulosa cells and the cumulus cells from patients with endometrioma is significantly higher than that in patients without endometrioma. Based on these studies endometrioma prove the existence of a more advance stage of endometriosis than the non existence of endometrioma. The existence of endometrioma is considered one of the indicators of endometriosis in the ovary due to the increase of the apoptosis in the follicle and gave, in turn, the follicle an atretic status. Consequently, patients with endometriosis

fertility especially in severe endometriosis due to a possible mechanic or anatomic disturbance such as extensive pelvic adhesions. Pelvic endometriosis, the most common form of the disease, could be associated with increased secretion of pro-inflammatory cytokines, impaired cell-mediated immunity and neo-angiogenesis. Barnhart *et al*. (2002) found that compared with women with mild endometriosis, women with severe endometriosis have a statistically significantly lower pregnancy rate and implantation rate, have fewer oocytes obtained at ovarian retrieval, and have a lower peak estradiol

Adhesion formation involves three important components: 1) acute inflammatory response, 2) fribinolysis, and 3) metalloproteinases and their tissue inhibitors. Celluler mediators within the peritoneal fluid can potentially modulate inflammatory responses over a large surface area due to the liquid nature of the peritoneal fluid. There are three important pro-inflammatory cytokines involved in adhesion formation: interleukine (IL)- 1, IL-6 and Tumor Necrosis Factor (TNF)-α (Cheong *et al.,* 2002). Endometriosis is associated with signs of pelvic peritoneal inflammation including increased volume of peritoneal fluid, increased concentration of peritoneal fluid macrophages, and increased peritoneal fluid concentrations of IL-6, IL-8, TNF-α and other cytokines and growth factors. Indeed, these cytokines have been reported to increase the endometrial–peritoneal adhesion *in vitro* (D'Hooghe and Debrock, 2002). Pelvic adhesion secondary to endometriosis is the most accepted reason for infertility, presumably via dysfunction of the fallopian tube or ovary. Inflammatory cytokines, IL-6, IL- 8 and TNF-α produced by endometrial cells probably contribute to the adhesion process. IL-8 has been shown to stimulate the adhesion of endometrial cells to fibronectin. TNF-α has also been reported to promote endometrial stromal cell proliferation *in vitro* and endometrial stromal cell adhesion to extracellular matrix components (Garcia-Velasco and Arici, 1999). These

Cysts of endometriosis (endometriomas) may become adherent to the uterus, bowel or pelvic side wall. Any of these anatomic distortions can result in infertility. The presence of an ovarian endometrioma greater than 1 cm in diameter is classified as stage III (moderate) or more in the revised American Society for Reproductive Medicine (ASRM) classification of endometriosis, but unfortunately, the staging system does not correlate well with a woman's chance of conception following therapy (ASRM, 2006). The impact of an ovarian endometrioma on infertility remains controversial, despite the number of studies that have been performed. Suzuki *et al* (2005) found that endometriosis, even after diagnostic laparoscopy with treatment when necessary, clearly affects the number of oocytes as well as the number of transferred embryos but not embryo quality and the related parameters of pregnancy, as indicated by the fertilization rate, embryo quality, implantation rate, pregnancy rate, and live birth rate, irrespective of the presence of an ovarian endometrioma (Suzuki *et al,* 2005). Nakahara (1998) found that the proportion of apoptotic bodies in the membrana granulosa cells and the cumulus cells from patients with endometrioma is significantly higher than that in patients without endometrioma. Based on these studies endometrioma prove the existence of a more advance stage of endometriosis than the non existence of endometrioma. The existence of endometrioma is considered one of the indicators of endometriosis in the ovary due to the increase of the apoptosis in the follicle and gave, in turn, the follicle an atretic status. Consequently, patients with endometriosis

concentration (Barnhart *et al.,* 2002).

pelvic adhesion inhibits ovum capture after ovulation.

with endometrioma had smaller numbers of follicles developed, oocytes harvested, and mature oocytes (Nakahara *et al*, 1998).

In women with endometriosis, pelvic adhesions contain estrogen and progesterone receptors, and produce basic fibroblastic growth factor and vascular endothelial growth factor, implying a regulation of pelvic adhesion formation by steroid hormone. Zang (2010) found that both the percentage and the density of protein gene product (PGP) 9.5-positive nerve fibres in ovarian endometriotic lesions were significantly higher in women with ovarian endometriosis who had pelvic adhesions than in those women with ovarian endometriosis and no pelvic adhesions (Zang et al., 2010). It is suggested that ovarian endometriotic lesions may be innervated through mediating effects of peritoneal inflammatory cytokines and growth factors including IL-1, IL-6 and TNF-a, in women with pelvic adhesions, thus leading to an increase of nerve fibres in ovarian endometriotic lesions in women with ovarian endometriosis ( Zang *et al.,* 2010).
