Oocyte quality depends on the appropriate acquisition of cytoplasmic and nuclear maturation, with the latter depending on the presence of a normal cell spindle [26, 27]. The meiotic spindle of human oocytes in metaphase II (MII) is a temporary dynamic structure consisting of microtubules associated with the oocyte cortex and their network of subcortical microfilaments [28, 29]. This microtubular structure functions primarily by assisting chromatid segregation, concomitant with the extrusion of the second polar body, ensuring the end of the meiotic process. The meiotic spindle of the oocyte is extremely sensitive to factors such as oxidative stress (OS), which promotes meiotic anomalies and chromosome instability and is associated with increased apoptosis and impairment of preimplantation embryo development [29–31].

Noninvasive spindle analysis of in vivo matured oocytes from infertile patients with mild (stages I and II) and advanced (stages III and IV) endometriosis , and patients without endometriosis undergoing stimulated cycles for intracytoplasmic sperm injection (ICSI) did not demonstrate significant differences between groups in terms of the nuclear maturation stage, the percentage of oocytes in metaphase II with visible spindles, and the spindle localization [32]. However, it is important to state that there are no studies evaluating the accuracy of PolScope microscopy for the detection of meiotic anomalies in human oocytes (by comparing noninvasive analyses with invasive analyses after immunostaining for morphological visualization of both microtubules and chromatin by high-performance confocal microscopy). Thus, although no differences were observed in the percentage of oocytes in metaphase II with visible spindles between infertile women with and without endometriosis, we cannot conclude that the percentage of meiotic anomalies in oocytes matured in vivo would be similar for the two groups.

Endometriosis may be associated with OS [33, 34]. The disease is associated with chronic inflammation, and reactive oxygen species (ROS) are inflammatory mediators that modulate cell proliferation [34]. Endometriotic cells may suffer endogenous OS due to increased ROS production, changes in the pathways of ROS detoxification, and a fall in catalase levels, as observed in tumor cells [35]. In the presence of pelvic endometriosis, there may be macrophage activation in the peritoneal cavity, which in turn may promote an increased production of ROS, nitrogen, cytokines, prostaglandins, and growth factors. As a consequence, OS occurs, generating lipid peroxidation as well as peroxidation of their degradation products and of the products formed by its interaction with low-density lipoproteins and other proteins. OS may also damage endometrial mesothelial cells and induce the onset of adhesion sites for endometrial cells, favoring the development and progression of endometriotic foci [36]. The activity of superoxide dismutase (SOD), one of the enzymes responsible for ROS neutralization, appears to be significantly higher in the ectopic endometrium of endometriomas than in eutopic endometrium [36, 37].

The data suggest a tendency to a higher production of free radicals in women with endometriosis, associated with a potential change in antioxidant capacity, which may supposedly contribute to the occurrence of OS. This, in turn, may be related to the pathogenesis of the disease, its progression and its possible consequences regarding fertility. However, this increase in the levels of antioxidant enzymes in the eutopic and ectopic endometrium of women with endometriosis may be both a primary event and an event secondary to the increased production of ROS in an attempt to prevent oxidative damage.

There is a positive association between infertility related to endometriosis, advanced disease stage and increased serum hydroxyperoxide levels, suggesting an increased production of systemic reactive species in women with endometriosis [38]. These data, taken together with the reduction of serum vitamin E and glutathione levels observed by our group, suggest the occurrence of systemic OS in women with infertility associated with endometriosis.

The peritoneal fluid (PF) of women with endometriosis promoted meiotic oocyte anomalies and embryo apoptosis in an experimental murine model, with OS being the potential mediator [39]. However, these authors used mice as the experimental model, limiting the extrapolation to humans of the data obtained. Preliminary results from our laboratory regarding the cell spindle and chromosome distribution of in vitro matured oocytes obtained from stimulated cycles of infertile women with endometriosis have suggested a potential delay or impairment of meiosis I, supporting the correlation between endometriosis and meiotic oocyte anomalies [40].

Since the follicular environment is extremely important for the process of oocyte maturation, changes in the composition of the follicular fluid (FF) may influence the maturation and quality of oocytes, affecting fertilization, early embryo development and subsequent pregnancy [41]. Differences in the constituents of FF have been reported between women with and without endometriosis [20, 34, 42, 43], suggesting that FF may influence the acquisition of oocyte competence in women with this condition. We investigated also if OS in the follicular microenvironment may be involved in female infertility by comparing FF from infertile women undergoing controlled ovarian hyperstimulation (COH) for ICSI between women who achieved pregnancy and those who did not. We compared the levels of five OS markers in the FF and found higher total antioxidant capacity (TAC) in the FF of infertile women who did not achieve clinical pregnancy compared to those who did, suggesting that the occurrence of OS in this microenvironment may be related to compromised ICSI outcome, a fact that needs further confirmation [44].

Due to inconsistent results about the association between minimal and mild endometriosis and infertility [13, 45], and in order to examine the mechanisms of infertility related to mild endometriosis , we evaluated the effect of FF from infertile women with mild endometriosis on nuclear maturation and the genesis of meiotic oocyte anomalies during in vitro maturation (IVM) of bovine oocytes. We found that FF from infertile women with mild endometriosis may compromise nuclear maturation and the meiotic spindles of in vitro matured bovine oocytes [46]. In fact, markers of OS have been observed in the FF of infertile women with endometriosis who underwent ovarian stimulation for assisted reproduction procedures [47]. As previously mentioned, OS may promote meiotic oocyte anomalies and chromosome instability, and is associated with increased apoptosis and impairment of preimplantation embryo development [29–31]. These findings led us to hypothesize that OS in the follicular microenvironment may impair nuclear maturation and may promote the genesis of meiotic oocyte anomalies in infertile women with endometriosis . Our results open new insights into the pathogenic mechanisms of infertility related to mild endometriosis , suggesting that FF from infertile women with mild endometriosis may be involved in the worsening of oocyte quality of these women, a hypothesis that should be evaluated in future studies, including patients with UI.

Some authors have also associated minimal endometriosis with a defect of steroidogenesis in granulosa cells, represented not only by reduced basal aromatase activity, but also by lower progesterone production in nonstimulated and stimulated cycles [48, 49]. A functional oocyte defect due to abnormal follicular function may be caused by endometriosis [50]. Several other factors were related to abnormal folliculogenesis in endometriosis patients, like a longer follicular phase in IVF cycles, slower follicular growth rate, and reduced dominant follicle size [34]. Thus, anomalies of folliculogenesis associated with endometriosis, if confirmed, may be a potential cause of impaired oocyte quality.

Another way to assess oocyte quality indirectly is the analysis of different markers in cumulus cells (CC). During follicular development, the granulosa cells differentiate into two distinct phenotypes, i.e., the mural population lining the follicular antrum and the population of CC surrounding the oocyte. The former is essential for the production of estrogen and for follicular rupture, while the latter is closely associated with oocyte development. CC function is regulated in part by factors derived from oocytes and the CC, in turn, contribute to oocyte maturation and the oocytes potential for development [51, 52]. In addition, it should be emphasized that CC protect the oocytes from entering apoptosis induced by OS. Some studies have suggested that analysis of gene expression of CC may be used as an indirect predictor of oocyte quality and of the results of ART, with several possible clinical applications [53–55]. Thus, we started studies focusing on gene expression in the CC population. There is a lower expression of the CYP19A1 (aromatase) gene in the CC of infertile women with minimal and mild endometriosis compared to controls [56]. This study opens a new perspective for understanding the pathogenesis of endometriosis-related infertility, suggesting that reduced expression of the CYP19A1 gene in CC might be involved in the impairment of oocyte quality associated with endometriosis. This hypothesis should be confirmed in future studies.

Alterations in immune function are also possible in patients with endometriosis [34]. An increased percentage of B lymphocytes, natural killer cells, and monocyte macrophages in the FF have been noted in a case-control study of patients with endometriosis compared to those with other causes of infertility. This suggests the possibility of an altered immunologic function in the FF of patients with endometriosis [57].

Alterations in both humoral and cell-mediated immunity have been found in the peritoneal environment of endometriosis patients [58], although it is not possible to say at the present time if these alterations are a cause or an effect of ectopic endometrium implantation. In addition, immunoglobulins and complement deposits were observed in the eutopic endometrium of patients with endometriosis [59]. The various components of cell-mediated immunity such as activated pelvic macrophages are increased in the PF of infertile patients with endometriosis resulting in a local peritoneal inflammatory cascade [60]. It was proposed that this inflammatory response can lead to increased ectopic implantation of endometrial tissue, as well as its growth and proliferation [34], although a similar picture may be seen in response to the ectopic implantation of endometrial tissue.

Although there is no consensus, women with endometriosis may have a decreased implantation capacity resulting in lower pregnancy rates , even with IVF. Reduced endometrial receptivity may be secondary to delayed histologic maturation, biochemical disturbances [61] or disturbance in cellular adhesion molecules such as avb3 integrin [61, 62]. It was also found that dysregulation of other select genes in the endometrium of patients with endometriosis may lead to impaired embryonic attachment, embryotoxicity , immune dysfunction, and apoptosis during the window of implantation (WOI) [63] .

Related to answer to infertility treatment in women with endometriosis, it was described that they had a worse prognosis than couples with male factor infertility undergoing COH and intrauterine insemination (IUI) [64]. In IVF, there are mixed reports with some studies observing a detrimental effect of endometriosis on success rates and others not finding an adverse effect on success rates [13]. In general, IVF has a proven benefit over all other treatments, although sporadic reports suggested that endometriosis reduced the success rates with IVF. However, we observed that women with endometriosis undergoing ART have the same chance of clinical pregnancy and live birth, as women with other causes of infertility [25]. Other modalities of adjuvant treatment before IVF were proposed for women with endometriosis and infertility, i.e., medical suppression of their disease with GnRH analogs or surgery with subsequent ovulation induction or IVF. Outcomes may differ when surgery is followed by ovulation induction, compared with surgery followed by expectant management . During IVF cycles, outcome might be affected by prolonged use of GnRH agonists or oral contraceptives before the start of the cycle. Suppression of disease before IVF using GnRH analogs might interfere with the negative effect of endometriosis on cycle outcome since long luteal suppression with GnRH analogs was shown to improve outcomes compared with shorter courses of luteal suppression [65]. Medical treatment of endometriosis alone with oral contraceptives or GnRH analogs has not been shown to increase cycle fecundity rates once the treatment is stopped and only serves to delay attempts at conception [65]. Cumulative pregnancy rates in women with mild endometriosis had a 45 % chance for conception after expectant management, whereas only 20 % conceived with stage III and none with stage IV endometriosis [66]. Laparoscopy excision of mild endometriosis had only a modest benefit on fertility [67].

Since the pathophysiology of endometriosis involves an inflammatory response, cell survival, proliferation, migration, adhesion, invasion, and neoangiogenesis, a high number of medications were tested in preclinical models of endometriosis. This was done due to their theoretic capacity of disrupting important pathophysiologic pathways of the disease, such as inflammatory response, angiogenesis and cell survival, proliferation, migration, adhesion, and invasion [68]. Most of these agents have not been tried in the clinical setting, but further study may help to elucidate the physiopathology pertubations of endometriosis -associated infertility.