{"paper_id":"d9484159-ca4b-4fec-bf9b-555bceef4b3c","body_text":"Understanding endometriosis: a threat to fertility\nAbstract\nEndometriosis is a gynaecological disease that affects female reproductive organs by the growth of endo -\nmetrium-like tissues inside and outside the pelvic cavity. This paper explored how endometriosis can lead \nto increased chances of infertility through a dysregulated immune system and impaired oocyte quality.\nRunning title: Understanding endometriosis\nKeywords: endometriosis, inflammatory cascade, interleukin, oocyte, granulosa cells, cytotrophoblasts\nJeya Verschuren1\nVerschuren et al. \nMedical Journal of Cell Biology 2024\nDOI: 10.2478/acb-2024-0004\nReceived: 26.02.2024\nAccepted: 29.03.2024\n1STN (Student Scientific Society) Anatomia-Klinika-Nauka, Division of Anatomy, Department of Human Morphology and Embryology, Wroclaw \nMedical University, Wroclaw, Poland\n*Correspondence: jeya.verschuren@student.umw.edu.pl\nFull list of author information is available at the end of article\n\nVerschuren et al. Medical Journal of Cell Biology (2024)\n31\nIntroduction\nEndometriosis is a common chronic gynecologi -\ncal disease that affects approximately 1 in 10 wo -\nmen [1]. It is characterised by the growth of endo -\nmetrium-like tissues inside and outside the pelvic \ncavity, mainly affecting female reproductive organs \nsuch as the uterus and ovaries. It can cause symp -\ntoms such as pelvic pain and heavy menstrual ble -\neding. Since these symptoms are non-specific and \noften associated with a typical menstrual cycle, this \ncondition is often misdiagnosed or not diagnosed at \nall. In fact, in patients between 18 and 45 years of \nage, the average delay of diagnosis is 6.7 years [2]. \nAccording to the WHO, endometriosis affects aro -\nund 10% (190 million) of reproductive age women \nand girls globally and specifically 14 million women \naffected across the EU [3]. In Asian countries, preva -\nlence is high and affects approximately 6.8% to \n15.7% [4]. If endometriosis is left untreated, symp -\ntoms will worsen and can lead to infertility. Studies \nhave shown that 30 to 50% of women affected with \nthis disorder are infertile and endometriosis acco -\nunts for up to 25% of general infertility cases [5]. \nThis increase chance in infertility can be connected \nto how endometriosis affects the patients’ immune \nsystem through generation of reactive oxygen spe -\ncies and recruitment of immunocompetent cells, as \nwell as how it impairs oocyte quality by inhibiting \nfollicular granulosa cells.\nImmune system dysregulation\nIn patients with endometriosis, increased per -\ncentages of immunocompetent cells such as macro -\nphages and neutrophils are observed. This points \nto the involvement of the immune system in endo -\nmetrial lesions. In the pelvic cavity of those affected \nwith endometriosis, macrophages are accumulated \nand can lead to the dysfunction of the immune re -\nsponse [6]. When macrophages are activated, they \nrecruit neutrophils and stimulate an inflammatory \nresponse through the release of cytokines which \npromote the angiogenesis of endometrial cells. Such \ncytokines include IL-1, IL-6, and IL-8.\nIL-1\nIL-1 is an interleukin typically responsible for \nregulation of the inflammatory cascade. Through \nbinding to receptors on target cells, they can illicit \ninflammatory responses by recruiting immune cells \nsuch as macrophages and neutrophils [7]. However, \nwhen there is over-secretion such as in endometrio -\nsis, IL-1 can cause tissue damage. IL-1β is a particular \nsubtype of this cytokine involved in pathogenesis by \nforming new blood vessels in “tissues surrounding \nendometriotic lesions by producing vascular endo -\nthelial growth factor (VEGF)” [8]. In physiology, IL-1 \nRa (receptor antagonist) binds to IL-1 receptors \nand acts against these cytokines, diminishing their \namounts before inflammation occurs [9]. However, \nin patients with endometriosis, decreased concen -\ntration of IL-1 Ra is apparent and has even been \nshown to cause dysmenorrhea [10].\nIL-6\nSimilar to IL-1, secretion of IL-6 is also intensi -\nfied in the immune dysfunction found in endome -\ntrial pathology. IL-6 helps to develop and sustain \nendometrial foci and implants by up-regulating \nsecretion of a glycoprotein similar to haptoglo -\nbin called endometriosis protein-I (ENDO-1) [11]. \nSimilar to how haptoglobin binds to free haemo -\nglobin in the blood, ENDO-1 adheres itself to ma -\ncrophages and creates a feed-forward loop that \nprotects the endometrial implants by blocking the \nphagocytic function of macrophages [8]. This dimi -\nnished phagocytosis allows for the survival of these \nimplants as well as an increase in secretion of IL-6, \nwhich will thereby increase ENDO-1 production, re -\nsulting in the feed-forward loop. \nIL-8\nIL-8 is a “chemotactic factor” which recruits im -\nmune cells such as neutrophils and macrophages to \nthe site of inflammation [8]. It is found that when \nIL-8 concentrations are increased, there is a high \nproliferation of “ovarian endometrioma-derived \nstromal cells” which can be linked to the growth \nof these cells in the peritoneum [12]. It has been \nspeculated that increased concentrations of these \nimmune cells in the peritoneal fluid are “toxic to \nembryo survival and sperm function” due to the \n“alterations of decidual microenvironment” [12]. \nSpecifically, through the interaction between ma -\ncrophages and cytotrophoblasts. Cytotrophoblasts \nare integral cells making up the outer layer of a bla -\nstocyst (the early stage of an embryo), providing \nnutrients to the growing embryo and later develo -\nping into the placenta [13]. Activated macrophages \nin patients with endometriosis disrupt these impor -\ntant cells by inhibiting their invasiveness, thereby \nbeing detrimental to embryo growth [14].\nROS and OS\nAnother effect of an unregulated immune sys -\ntem is the overproduction of reactive oxygen spe -\ncies (ROS) through stimulation by cytokines and \nmacrophages during inflammation. Although these \nspecies are present in regular physiological states \nand are even associated with supporting female fer -\ntility processes such as “folliculogenesis (the deve -\nlopment of a follicle needed for release of a mature \noocyte), oocyte maturation, and hormone signal -\nling” [12], they must be held in a careful balance \nwith antioxidants or else oxidative stress will occur.\nWhen this balance is lost, oxidative stress (OS) \ncan lead to infertility through damaging oocyte \nDNA and driving post-ovulatory oocytes to apop -\ntosis [12]. This pathomechanism is driven by the \n\nVerschuren et al. Medical Journal of Cell Biology (2024)\n32\nreduction of glutathione levels by OS. Glutathione \nis a key antioxidant regulating apoptosis, therefore \nits decrease leads to morphological changes in the \ncell and eventually cell death [15]. Oxidative spe -\ncies also cause oocyte ageing and cell cycle arrest \nin follicular oocytes, thus ultimately hindering pre -\ngnancy. It is clear that in patients with endometrio -\nsis, the dysregulation of the immune system and the \nconsequent ROS production is not only involved in \nthe pathogenesis of the condition, but can lead to \ninfertility as well.\nOocyte impairments\nOocyte quality is one of the most important fac -\ntors in fertility and can be characterised by an oocy -\nte’s ability to mature and be fertilised [16]. Oocytes \nare immature eggs or ovum found in the ovaries \nthat mature within a follicle and have not been \nfertilised. If there is damage to an oocyte’s quality, \nembryo development may be impaired. In order to \ndescribe how endometriosis affects the abilities of \nan oocyte, we must consider a main factor affecting \noocyte growth: granulosa cells. When primordial \ngerm cells become oogonia and later proliferate \ninto oocytes, the oocytes are encapsulated by a lay -\ner of granulosa cells.\nGranulosa cells\nGranulosa cells are somatic cells in the ovaries \nthat surround an oocyte in a follicle [17]. These gra -\nnulosa cells communicate directly with oocytes in \nprimordial follicles but when the oocytes start to \ngrow, they are separated from the granulosa cells \nby the zona pellucida. The zona pellucida is a coat of \nglycoproteinaceous matrix that surrounds the oocyte \nand is responsible for the “binding of sperm to un -\nfertilised eggs” [18]. Since it completely encapsulates \nthe oocyte, this disrupts the communication between \nthe granulosa cells and the oocyte. Therefore, hete -\nrologous gap junctions begin to form on cytoplasmic \nprojections called transzonal projections to restore \ncommunication [19,20]. Transzonal projections are \nfollicular cell processes branching from granulosa \ncells that penetrate the zona pellucida, reaching the \noocyte. The connection between the oocyte mem -\nbrane and these projections is called a gap junction \n[21]. Through these junctions, granulosa cells pro -\nvide growing oocytes with nutrients like amino \nacids and glucose substrates, as well as paracrine \nsignals such as cAMP and cGMP to regulate oocyte \nproliferation by maintaining oocyte meiotic arrest. \nGiven this, it is clear that functional granulosa cells \nare crucial in healthy oocyte quality. Endometriosis \nimpairs this quality not by attacking the oocytes \ndirectly, but instead by modifying and destroying \nthe granulosa cells.\nAs mentioned earlier, there are high levels of \nROS in the cellular environment of patients with \nendometriosis. This stress affects the endoplasmic \nreticulum (ER), an organelle within the cell that is \nresponsible for synthesising and folding proteins. \nThe oxidative stress stimulates ER stress due to \nthe increase demand for protein folding. When this \noccurs and there is an accumulation of unfolded \nor misfolded proteins in the ER, signal transduc -\ntion cascades called the unfolded protein response \n(UPR) are activated to cope with this increase [22]. \nIn physiological conditions, the UPR is a mechanism \nused to restore homeostasis by “eliminating slow -\n-folding proteins” , allowing the ER to maintain its \nfolding capacity [23]. However, if ER stress persists \nand is severe, the UPR ’s adaptive measures will no \nlonger be sufficient and thus terminal UPR is activa -\nted [24]. Terminal UPR is designed to remove stres -\nsed cells through signalling cell suicide and apopto -\nsis. Specifically, by activating protein kinase R-like \nendoplasmic reticulum kinase, also known as PERK. \nPERK is a vital protein in terminal UPR because it \ncan induce the “expression of proapoptotic tran -\nscription factors” such as activating transcription \nfactor 4 (ATF4) and C/EBP homologous protein \n(CHOP) [25].\nATF4 and CHOP\nThese factors will drive the apoptosis of cells, \nincluding the granulosa cells needed for oocyte de -\nvelopment. With granulosa cells being destroyed, \noocytes are no longer getting the adequate amounts \nof nutrients needed for healthy growth. It has also \nbeen found that oxidative stress can cause damage \nto mitochondria in granulosa cells which can lead \nto the dyssynchronisation of nuclear and cytopla -\nsmic maturation [26]. This impacts fertility because \nit may result in embryo development failure. Given \nthe importance of oocyte quality in fertility and gra -\nnulosa cells vital role in maintaining it, it is evident \nthat the ER stress caused by endometriosis negati -\nvely impacts fertility.\nDiscussion\nIt is clear that there is a multitude of factors \ncaused by endometriosis that can lead to infertili -\nty. The primary focus of treatment research should \nbe on eliminating these consequences before it can \npermanently negatively impact fertility. Drugs that \nhave anti-inflammatory effects or specific inhibitors \nof driving interleukins should be studied as these \nmethods can eliminate the root pathomechanisms \nof endometriosis. However, these solutions may \nonly be helpful if patients are diagnosed before the \ndisease causes irreparable damage to the reproduc -\ntive system. To this day, far too many women are li -\nving with endometriosis and do not realise. Proper \nawareness and education of reproductive health \nwill allow women and girls to recognise irregula -\nrities in their own menstrual cycles and detect this \n\nVerschuren et al. Medical Journal of Cell Biology (2024)\n33\ndisease. Healthcare professionals must also not dis -\nmiss symptoms as typical menstrual manifestations \nas endometriosis can worsen with time. Regarding \nareas of research, future studies should investigate \nif there are any external factors that can lead to the \npathogenesis of endometriosis so that the public \ncan be aware and avoid harmful agents. For a dise -\nase that impacts so many lives, little is known about \nthe exact causes for its pathogenesis. Areas for rese -\narch regarding endometriosis is wide and it is with \nhope that the further we study this disease, the so -\noner we can develop more effective treatments and \npossibly a cure.\nConclusions\nIn conclusion, endometriosis can induce a pro -\n-inflammatory state in which the immune system is \nunregulated and leads to the proliferation of immu -\nne cells and reactive oxygen species, creating a toxic \nenvironment for embryo development. Endometrio -\nsis also affects the quality of oocytes by stimulating \nterminal UPR and destroying the vital granulosa cel -\nls needed for oocyte maturation. Through these me -\nchanisms, it can be concluded that untreated endo -\nmetriosis creates conditions unsuitable for embryo \ngrowth and can therefore be linked to infertility.\nEthical approval\nThe study was a descriptive one. No humans or animals were a sub-\nject of examinations.\nAcknowledgements\nNot applicable.\nCorresponding author\nJeya Verschuren , STN (Student Scientfic Society), Division of \nAnatomy, Department of Human Morphology and Embryology, \nWroclaw Medical University, Chalubinskiego 6a, 50-368 Wro -\nclaw, Poland, e-mail: jeya.verschuren@student.umw.edu.pl .\nConflict of interest statement\nThe authors declare no conflict of interest.\nReferences\n1. Sims OT , Gupta J, Missmer SA, Aninye IO. Stigma and endometrio -\nsis: a brief overview and recommendations to improve psychoso -\ncial well-being and diagnostic delay. Int J Environ Res Public Health. \n2021;18(15):8210; DOI:10.3390/ijerph18158210.\n2. Parasar P , Ozcan P , Terry KL. Endometriosis: epidemiology, diagnosis \nand clinical management. Curr Obstet Gynecol Rep. 2017;6(1):34-41; \nDOI:10.1007/s13669-017-0187-1.\n3. University of Mu� nster. Translational research on endometriosis. TREN-\nDO Project [Internet]. 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