{"paper_id":"d684a75a-c622-4506-95ed-b86786167ad0","body_text":"Submit Manuscript | http://medcraveonline.com\nIntroduction\nInfertility is known as an inability to conceive (regardless of cause) \nafter one year of consistent (at least two times a week) unprotected \nintercourse when a female partner is younger than thirty-five years \nold or within six months if older than thirty-five years of age. The \noccurrence of infertility is around 14% of the general population, \nthus affecting roughly 1-in-7 couples. Among the most known causes \nof infertility are ovulatory disorders, male factors, fallopian tube \npathology, uterine and/or peritoneal conditions.1 \nAs a rule, the diagnosis of unexplained infertility is established via \nruling out the other causes of infertility after using standard fertility \ntests, including spermogram, ovulation tests, and tubal patency \ntesting. It accounts for almost 40% of cases of female infertility, and \ninvolves from 8% to 28% of couples, experiencing fertility problems.2 \nThe establishment of an unexplained infertility diagnosis is based \non the investigations done prior to diagnosis. Some of the affected \ncouples who are investigated for the unexplained infertility may \nsuccessively conceive spontaneously. Their rate of spontaneous \nsuccessful pregnancy reaches 2-4%. The most significant prognostic \nfactor for successful spontaneous conception is woman’s age, which \nhas a higher conception rates below 30 years. After one year of \nunsuccessful attempts, half of females with unexplained infertility may \nconceive during the subsequent year, and 12% more of the females \nmay conceive within the next two years. 3 But women nowadays \ncannot wait too long since their childbirth plans have already been \ndelayed significantly due to cultural and social changes.\nOxidative stress and mitochondrial dysfunction in \nfemale reproductive system \nA disproportion in the amount of naturally-occurring antioxidants \nand reactive oxygen species (ROS), with the accumulation of the \nROS, creates perfect conditions for oxidative stress. Oxidative \nstress causes lipid peroxidation, protein peroxidation, and genomic \ndamages to DNA and RNA.4,5 The oxidation of phospholipids disturbs \nthe integrity of cellular membranes. The peroxidation of RNA and \nDNA causes its degradation, which can be a trigger to programmed \ndeath of the cells. 5–7 Impaired adaptation mechanisms against the \noxidative stress causes mitochondrial dysfunction within the cell \nby inactivating the enzymes of the mitochondrial electron transport \nchain and by promoting the mutations in mitochondrial DNA. Apart \nfrom that oxidative stress has also been linked to telomere shortening \nand cellular senescence.8 Mitochondrial dysfunction diminishes ATP \nproduction and has a negative impact on the antioxidant synthesis. \nSuch situation creates a vicious cycle when mitochondrial dysfunction \ncaused by the free radicals further increased production of ROS and \nworsen mitochondrial damage.9\nOxidative stress and mitochondrial dysfunction are initiated by \nboth endogenous and exogenous factors. The endogenous factors are \nbiological age, endometrial disorders, polycystic ovarian syndrome \n(PCOS), and premature ovarian insufficiency (POI). 10 While \nexogenous factors comprise of environmental exposure to the inducers \nof ROS - diet, occupational hazards, and assisted reproduction \ntreatment techniques.11\nCellular and subcellular aging mechanisms \ncontributing to infertility\nAging process implicates not only the deterioration of the \nphysiological functions of the organism, but also impairs the fertility \nof the aging individual. The advanced maternal age is associated with \nthe increased risk of adverse obstetric outcomes such as miscarriage, \npreeclampsia and eclampsia, increased occurrence of pre-term and/\nor post-term delivery, low birth weight and neonates that are small or \nlarge for their gestational age, and C-section.12,13 Hence, the constantly \nincreasing occurrence of female infertility and consequent need in \nassisted reproductive technologies. \nAccording to the research of the last few decades, the failures to \nconceive and low pregnancy rates performed both naturally or with \nObstet Gynecol Int J. 2024;15(1):6‒12. 6\n©2024 Yemeliyanova et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, \nwhich permits unrestricted use, distribution, and build upon your work non-commercially.\nUnexplained infertility: a fresh look at the old \nproblem and the novel therapeutic options of its \ntreatment\nVolume 15 Issue 1 - 2024\nMargarita Yemeliyanova,1 Mike KS Chan,1,3 \nMichelle BF Wong,1,3 Dmytro Klokol1,2 \n1European Wellness Biomedical group (EU, APEC), Germany\n2European Wellness Academy, Malaysia\n3FCTI Research & Development GmbH, Germany\nCorrespondence: Margarita Yemeliyanova, MD, European \nWellness Biomedical group (EU, APEC), Germany, \nEmail \nReceived: January 11, 2024 | Published: January 22, 2024\nAbstract\nInfertility affects up to 20% of couples worldwide. Among the frequent causes of female \ninfertility are fallopian tubes-related disorders, hormonal and ovulation disorders, \nendometriosis, and unexplained infertility. The modern-day tendency to delay pregnancy \nhas increased the incidence of age-related infertility, as female reproductive competence \ndecreases with aging. Aging is associated with low-grade inflammation, mitochondrial \ndysfunction, reduced capacity of antioxidant protection system, and stem cell exhaustion in \nfemale reproductive system. Hence, the appropriate actions should be made to address the \ninfertility caused by reproductive aging, oxidative stress, and mitochondrial dysfunction. \nIn recent years, a considerable progress in cell therapy as an emerging approach for the \ntreatment infertility has been made. Cell therapy involves utilizing stem cells, precursor \ncells, cellular extracts, exosomes and other cell-derived therapeutic agents. Cell therapy can \nbe an effective strategy as it provides an interactive, dynamic, specific and individualized \ntreatment.\nKeywords: infertility, cell therapy, stem cells, reproductive function, ovaries, hormonal \ndisorders, mitochondrial dysfunction, oxidative stress, regenerative medicine \nObstetrics & Gynecology International Journal\nReview Article\n Open Access\n\n\nUnexplained infertility: a fresh look at the old problem and the novel therapeutic options of its treatment\n7\nCopyright:\n©2024 Yemeliyanova et al.\nCitation: Yemeliyanova M, Chan MKS, Wong MBF , et al. Unexplained infertility: a fresh look at the old problem and the novel therapeutic options of its \ntreatment. Obstet Gynecol Int J. 2024;15(1):6‒12. DOI: 10.15406/ogij.2024.15.00726\nthe help of assisted reproduction technologies can be related to low-\nquality oocytes and sperm cells. It has also been proven that the \nquality of gametes directly depends on the number of mitochondria \nthey contain and the mitochondrial function. Low gametes quality can \nalso be the reason for a decline in response to ovarian stimulation, \nreduced embryo quality and pregnancy rates, as well as an increased \nincidence of miscarriages and fetal aneuploidy. Couples 35 years \nand above with unexplained infertility are the most representative \ncategory of patients who are implicated to have low gamete quality \ndue to aging of their reproductive system. \nThe aging process itself and any age-related pathologies nowadays \nare largely associated with the mitochondrial malfunction, mostly \ndue to the accumulation of multiple deletions and mutations in the \nmitochondrial DNA (mtDNA) stran. 14 Among other genetic defects \nobserved in aging oocytes are reduction of intracellular antioxidants,15 \nand acceleration of apoptosis in oocytes, 16 abnormal calcium \noscillation signals,17 and high incidence of aneuploidy.18 \nMitochondrial dysfunction in female reproductive \ndisorders \nEmbryonic oocytes are usually formed at the fetal development \nstage and are present inside the ovary for nearly 50 years before \nthey grow and develop into matured oocytes. The ovulation process \nleads to a continuation of meiosis in the immature oocyte, making \nit mature and ready for conception. This implies the process of \nchromosome alignment and separation by the nuclear spindle, leading \nto the reduction of chromosomes to the amount of 23, where another \n23 chromosomes appear to be isolated outside of the oolema and \nenclosed subsequently in the first polar body. Once penetrated by a \nhealthy spermatozoa, what happens next is the extrusion by the oocyte \nof 23 sister chromatids enclosed in the second polar body. That is how \na fertilized zygote receives a normal diploid set of 46 chromosomes. \nThe process of chromosome extrusion outside the oocyte and forming \nthe first and second polar bodies is quite energy-consuming, and it \nappears to be supplied by mitochondria. The oocyte cell encompasses \nthe largest number of mitochondria and mitochondrial DNA \n(mtDNA) copies compared to muscle cells and neurons, which have \nhigher energy requirements and contain thousands of mtDNA copies. \nUpon the follicle recruitment process, the number of mitochondria \nin the oocyte cell is markedly increasing, from 6,000 mtDNA copies \nto 200,000, comprising nearly 50% of the total DNA content in the \noocyte.19,20\nThe structure of mtDNA differs from that of nuclear DNA. It is \nalso a double-stranded circular shape, containing 16,569 pairs, though \nit has no histones or introns like nuclear DNA, which makes it more \nsusceptible to mutations and deletions. The mtDNA includes 37 \ngenes encoding proteins taking part in the respiratory chain of ATP \nproduction. The embryo cells always inherit maternal mtDNA, as the \npaternal one undergoes degradation and elimination via ubiquitination \nreactions.21\nThe aging process is well-known to be accompanied by \nmitochondrial dysfunction and decreased energy production, which \nresults in impaired oocyte maturation, so the very important process, \nsuch as nuclear spindle activity or chromosomal segregation, appears \nto be seriously deteriorated. With age, the mitochondrial energy \nproduction in oocytes depletes, and processes like oocyte maturation \nwith its nuclear spindle activity and segregation of the chromosomes \ndeteriorate.22\n As a result, the aneuploidy rate increases, especially the trisomy, \nfrequently observed in older women’s offspring. There is data that the \nnumber of mutations in mitochondria of the follicle cells increases \nwith age, resulting in impaired reactions of ATP production and \noxidative phosphorylation in older women. 22 It was shown that \nchances for successful implantation strongly correlate with the ATP \nproduction and content in embryo cells. The study demonstrated that \na reduced capacity of oocytes to produce ATP molecules leads to an \nabnormal nuclear spindle and random chromosome spreading.22,23 \nAging of human oocytes is closely associated with shifts in \nmitochondrial function, mitochondrial DNA copies numbers, and \nmitochondrial mutations. 24,25 Simultaneously, in aging oocytes the \nsignificant increase in ROS levels and upregulation of mtDNA stress \ngenes is observed . Notably, the hallmark of the aging oocyte are \nlower mtDNA and a number of essential mitochondrial dysfunction.26 \nResearch data on age-related changes in mtDNA in human oocytes \nsuggests both the decrease, 27 as well as increase of mtDNA. 28 In a \nstudy of Fragouli the mtDNA copy number was decreased in cleavage \nstage embryos in females of a reproductively older age (average age \n40 years), and increased in blastocysts in comparison to females in \na younger reproductive age (average age 35 years). 29 Experimental \nstudies reveal that mitochondria from old mice oocytes are different \ncompared to young mice’ oocytes, and matured oocytes from old mice \nhave considerably lower amount of mtDNA than young mice. The size \nand the total area covered by mitochondria in an oocyte vs total area \nof the oocyte’s cytosol was smaller in oocytes of older mice. Oocytes \nof the elder mice also have a less mitochondrial density compared \nto young ones. In general, the differences in mitochondrial status of \nyoung and old oocytes show the reduction in the functional capacity \nof mitochondria associated with ageing.26\nThe aging oocytes have their telomerase activity also decreased, \nwhich, in its turn, could further contribute to the chromosomal \ndamage in oocytes. 30 The relationship between oxidative stress and \novarian aging is closely linked to abnormal mitochondrial functions, \naccumulated mutations and the downregulation of mitochondrial \nantioxidant gene expression in aged oocytes. 31 In fact, this shifts not \nonly concern the mitochondria, but it is also well established that \noxidative stress in main biological molecules (proteins and lipids), as \nwell as in DNA of the aged oocytes causes the decline of the quality \nof the oocytes.32\nIn the ovaries, under normal circumstances the ROS generated \nduring an inflammatory reaction from immune cells and cytokines in \nthe follicular fluid induce oocyte maturation and subsequent follicle \nrupture and ovulation. 33 Hence, the ROS produced at physiological \nlevel by the follicles are necessary for ovulation. On the contrary, \ninhibition of ROS would suppress the ovulation. 34 Conversely, the \nexcess of the ROS resulting in oxidative stress triggers abnormalities \nin female reproductive system, representing premature ovarian \nfailure (POF), POI, and PCOS. POF, which usually means early \nmenopause (before the age of 40 yo) is associated with prematurely \nimpaired ovarian function due to abnormal development or depletion \nof follicles due to increased apoptosis. 35 On the other hand, a drop \nin levels of endogenous antioxidants and increased oxidative stress \nin patients with PCOS produces abnormal formation of cysts and \novarian tissue remodelling, which leads to absence of ovulation and \nsubsequent infertility.36\nAlbeit that ROS are generated in the corpus luteum post-ovulation \nplaying a major role in progesterone synthesis, which is essential \nfor the development of the uterine environment and in regulation \nof implantation, survival, and the progression of pregnancy, the \nexcessive production of free radicals and oxidative stress to the corpus \nluteum disturbs the progesterone synthesis, which can be detrimental \nto embryo and development of pregnancy.37\n\nUnexplained infertility: a fresh look at the old problem and the novel therapeutic options of its treatment\n8\nCopyright:\n©2024 Yemeliyanova et al.\nCitation: Yemeliyanova M, Chan MKS, Wong MBF , et al. Unexplained infertility: a fresh look at the old problem and the novel therapeutic options of its \ntreatment. Obstet Gynecol Int J. 2024;15(1):6‒12. DOI: 10.15406/ogij.2024.15.00726\nIn the uterus, the excessive oxidative stress disturbs morphology \nand function of the uterus, including detachment of the endometrial \nepithelium and possible hindrance of implantation.38 During pregnancy, \noxidative stress can lead immune dysfunction in the uterus that may \nlead to an early pregnancy loss. 39 The impaired anti-oxidative stress \nprotection mechanisms interfere with the successful implantation.40,41 \nOxidative stress and energy production-utilization pathways \naffect not only the oocytes, ovaries and uterus, but also the embryo. \nAs a rapidly developing organism with high energy demand, the \nembryo gets the energy supplied by ATP produced via mitochondrial \noxidative phosphorylation and glycolysis. The studies on mice \ndemonstrated that post-compaction embryos consume times more \noxygen than those in earlier stages of development and tend to shift to \nglucose utilization metabolic pathways. 42 Because the mitochondria \nin embryo cells do not replicate until the blastocyst stage, its pool \nmust be divided between all the numbers of increasing cells during \nthe embryo cleavage. So, its metabolic activity must also increase \naccording to the expanded cellular activity. Hence, we can suggest \nthat the correlation between older maternal age and the risk of \nchromosomal abnormalities occurring in offspring is reasonably \nattributed to depleted mitochondrial activity in the oocytes, which \nleads to both non-disjunction of the chromosomes and arrest of \nembryo development.\nTherapeutic modalities in unexplained infertility in \nBioregenerative medicine\nIn this review, we emphasize that mitochondrial dysfunction \nplays a significant role in reproductive failures and propose that \nreproductive function in women can be improved largely with the use \nof mitochondrial peptides and other mitochondrial nutrients extracted \nfrom the xenogeneic stem cells derived from various fetal tissue \n- ovaries, testis, and placenta at a first place, as well as from other \norgans belonging to hypothalamus-pituitary-adrenal-gonadal (HPAG) \naxis. \nDespite significant advancements in assisted reproduction \ntechniques, such as the intrauterine insemination and the in vitro \nfertilization (IVF), these techniques do not recreate the ideal conditions \nof natural impregnation. The presence of underlying mitochondrial \ndysfunction and failure of anti-oxidative stress defence mechanisms \nin reproductive system, require the supplementation of the human \nIVF culture media with the biologically active molecules capable of \nmodulating the mitochondrial function, stimulators of mitochondrial \ngenesis and biologically active substances with antioxidant properties. \nSuch molecules include naturally-occurring peptides of HPAG axis, \ncoenzyme-Q10, folic acid, vitamins A, C, and E, pantothenic acid, \nmelatonin, resveratrol and others.43,44 \nThus, resveratrol exhibits therapeutic effects in treatment of many \ndiseases due to its anti-aging, antioxidant, anti-inflammatory, insulin-\nupregulating effects, cardioprotective, and anti-neoplastic properties.45 \nResveratrol may be beneficial for the women with impaired ovarian \nfunction, PCOS, endometriosis, and uterine fibroids. 46,47 The \nbeneficial effects of resveratrol are exerted through the sirtuin 1 \n(SIRT1) activation. 48 Another pathway through which resveratrol \ninhibits oxidative stress and inflammation in POI model and exerts \nits anti-apoptotic effects, hence improving the ovarian dysfunction \ncaused by POI, is through the inhibition of the PI3K/AKT and the \nNF-kB signalling pathways. 49–51 In addition, resveratrol inhibits \ntheca-interstitial cell androgen production. Therefore, resveratrol is \nfound beneficial in treatment of PCOS - a condition closely associated \nwith insulin resistance and hyperinsulinemia, theca-interstitial cell \nhyperplasia, and hyperandrogenism.52 In the endometrium, resveratrol \nexhibits anti-apoptotic and anti-proliferative effects. Moreover, \nresveratrol reduces expression of the vascular endothelial growth \nfactor (VEGF), thus aiding in management of endometriosis and \novarian hyperstimulation syndrome, as both of these conditions are \nrelated to the excessive VEGF activity.53\nCoenzyme-Q10 (CoQ10) is the carrier-transporter of electrons in \nthe mitochondrial respiratory chain between complexes I, II, and III. \nThus, CoQ10 participates in the synthesis of ATP. 54 Being a source \nfor superoxide anion, CoQ10 acts both as a prooxidant as well as an \nantioxidant. Its reduced form - the ubiquinol, a potent antioxidant, \nprotects biological membranes from lipid peroxidation. 55 Addition of \nCoQ10 to the treatment protocols improves mitochondrial function, \nand through that pathway may improve the outcome in infertile \npatients. CoQ10 treatment prevented mitochondrial ovarian aging, \nand restored the age-related decline of oocyte quality.56\nLastly, CoQ10 administered to the aged mice restored \nmitochondrial respiratory function and increased glucose uptake in \ncumulus cells, hence helping to improve the reproductive function. \nThe human studies shown that higher levels of CoQ10 in follicles \nare linked to a better embryo quality and higher pregnancy rates. 57 \nCoQ10 supplementation (600 mg/day for 2 months before ovarian \nstimulation) increased ovarian response, fertilization rates, and the \nnumber of high-quality embryos in young women with poor ovarian \nreserve.58 The pre-treatment before IVF with CoQ10 increases \nsuccessful pregnancy rates.59\nFolate, vitamin B9, and its synthetic form called folic acid, is \ncommon in dietary supplements due to its high bioavailability and \nmassive health benefits. 60 The main biochemical function of folate \non the cellular level is the donation of the methyl group for the \nhomocysteine to convert into methionine via methylation reaction, 61 \nwhich is a crucial transitional compound in endogenous synthesis of \nglutathione - the potent intracellular antioxidant. Consequently, folic \nacid exerts protection from oxidative stress by increasing endogenous \nexpression of antioxidant in the cell. Hence, folic acid supplementation \nis widely used in the reproductive field, as well as during pregnancy \nand is essential in achieving favourable pregnancy outcomes.60,61\nCell therapy as a novel therapeutic modality in \nunexplained infertility \nIn recent decade we are gathering more and more evidence that \ncell therapies can provide novel therapeutic paradigms in reversal \nof a wide range of degenerative and age-related disorders. 62–64 New \ndevelopments in molecular biology and cellular research have greatly \nexpanded the clinical indications of these therapeutic modalities to \nunexplained infertility as well. 64 The rationale behind this is that \nmajority of disorders leading to the infertility and other reproductive \ndisorders do not appear due to the deficiency in one protein or \nenzyme, but the changes in the complicated signalling on cellular \nand subcellular level. Cell-based therapy is an efficient modality that \nprovides an interactive, dynamic, and individualized treatment, which \naddresses to patient’s pathophysiological conditions. \nUterine tissue immune microenvironment has a crucial role in \nmaintenance of pregnancy. The immunogenic cells, such as Placenta-\nderived stem cells, Thymic progenitor cells and bone marrow-derived \ncells and their cytokines act as the key regulators. These cells express \nnumerous cytokines, such as interleukins IL- 1α and IL-1β, TNF-α \nand exert positive affects on the endometrium. It also promotes the \ninvasion and hemochorial placentation and regulates immune status \nof the embryo during implantation. 65 Thus, Yoshioka in 2006 has \n\nUnexplained infertility: a fresh look at the old problem and the novel therapeutic options of its treatment\n9\nCopyright:\n©2024 Yemeliyanova et al.\nCitation: Yemeliyanova M, Chan MKS, Wong MBF , et al. Unexplained infertility: a fresh look at the old problem and the novel therapeutic options of its \ntreatment. Obstet Gynecol Int J. 2024;15(1):6‒12. DOI: 10.15406/ogij.2024.15.00726\nreported the direct effects of cell therapy on the human endometrium. \nThe study concluded that the intrauterine administration of the stem \ncells, co-cultured in media with added HCG, has notably enhanced \nimplantation and increased the live birth rates in cases of recurrent \nIVF failure.65\nAccording to Hashii et al. stem cells co-culture with luteal cells \nderived from pregnant cases enhanced the Th2 cells cytokines \nproduction (IL-4 and IL-10), induced endometrial differentiation \nand promoted embryo implantation. 66 Th1 cells enhance cytotoxicity \nfunction of NK cells via secretion of cytokines with pro-inflammatory \nproperties (IL-2, INF-γ), thus inhibiting embryo implantation. Th2 \ncells, on the contrary, produce anti-inflammatory cytokines (IL-\n4 and IL-10), which protect the embryo from the immune system \nassaults via suppression of Th1 cells.67 Multiple studies found that the \nequilibrium between Th1/Th2 cytokines profile supported the feto-\nmaternal immune tolerance during pregnancy. 68 Therefore, Th1 cells \nare predominantly produce negative effects on pregnancy, while, Th2 \ncell cytokines play an important part in induction and maintenance of \npregnancy.69\nBased on the numerous reports, the stem cell therapy is an efficient \ntherapeutic modality in the treatment of unexplained infertility. \nMany clinical trials have evaluated the efficiency of stem cell \ntherapies in humans. Use of the xenogeneic precursor stem cells as \ntherapeutic agents has a number of advantages, such as relatively easy \npreparation, abundant sources, and preventable ethical issues. 62 After \nthe implantation procedure, PSC are capable to survive, proliferate \nand differentiate into the finally differentiated cell, except placenta, \nranging from hepatocyte, neural cells, muscles, liver, skin, and \nendocrine cells to oocytes and even sperm. Cultured in vitro, after \nimplantation PSC proliferate in the recipient’s body hence promoting \ntissue remodelling.62\n In the meantime, administering organ-specific xenogeneic fetal \nprecursor stem cells derived from the placenta and organs of the \nhypothalamus-pituitary-adrenals (HPA) and hypothalamus-pituitary-\ngonads (HPG) axis would help to restore and support normal production \nof the hormones regulating menstrual cycle, which facilitate timely \novulation and oocyte maturation for further successful conception and \nembryo development.64,62 Stem cell transplantation as well as clinical \nuse of decellularized organ-specific cell therapy products and cell \nderivatives becomes one of the most promising therapeutic solutions \nfor incurable and untreatable diseases.62\nSome studies reported benefits from the endometrial precursor stem \ncells, which are the inherent endometrial stem cells.70 The engraftment \nof endometrial precursor stem cells has a great potential in the treatment \nof endometriosis. Thus, Tersoglio et al. found that the endometrial \nprecursor stem cells implantation into the thinned endometrium \nallowed to achieve the higher rates of In Vitro Fertilization (IVF) and \nhigher rates of successful pregnancy, especially in cases of repeated \nimplantation failure and/or downregulation of estrogen receptors.71\nMore recently, the female Germ Precursor Stem cells and Ovarian \nPrecursor Stem cells were discovered to be able to promote the ovarian \nregeneration and modulate the ovarian function. For instance, female \ngermline precursor stem cells increased the amount of functional \noocytes.72 \nCellular extracts, procured from the cultured Precursor Stem Cells \n(PSC) are a well-established safe alternative. Clinical applications of \ncell extracts started in the beginning, mid- XX Century in Switzerland \nand Germany. 64,73,74 Stem cells are secreting a broad spectrum of \nparacrine factors, representing the components of the extracellular \nmatrix, adhesion- and binding proteins, enzymes, growth factors, \ncytokines, and chemokines.62,73,74\nPromising potential has been shown by organ-specific cell \nextracts, the cell-free therapeutic, which can be derived and procured \nfrom xenogeneic tissue, possess a great reliability and reproducibility, \neasily manufactured, packaged and transported, can be lyophilized \nor shock-frozen, and do not need to match the donor-recipient \ncompatibility to avoid immune reactions. 73,74 Moreover, compared to \nstem cells, cell extracts have advantages of a lower production time \nand cost, a higher shelf-life, and relatively easy storage method. 73,74 \nOrgan-specific cell extracts can be harvested from various types of \ncells and different culture conditions, to ensure the organ-specificity \nmaintained. The evaluation of biologically active ingredients from \nvarious tissue sources has been done, showing the differences in the \ncomposition, peptide characterization and clinical effects produced.75 \nDifferent types of PSC secret a great variety of cytokines, \nchemokines, and growth factors that produce powerful paracrine \neffects on patient’s endogenous pool of stem cells, as well as \nrejuvenating and regenerative effects on the tissue and organs. \nThey also stimulate cell migration, proliferation, and tissue \nrevascularization, thus promoting the organ’s regeneration. One of \nthe studies has demonstrated that mRNA expressions of interleukins \nIL-1β, IL-6, and IL-8 were significantly downregulated compared to \nthe controls.76 Few studies have demonstrated the more satisfactory \noutcomes in terms of endometrial growth and gestation in patients \nwith thin endometrium following the use of decellularized cell-based \ntherapy protocols.77,78\nMitochondrial replacement therapy in infertility\nMitochondrial substitution therapies target enhancement and/or \nreplacement of the mitochondria inside the oocytes of the patient. 79 \nCurrently there are two different techniques developed: either a \noocyte cytoplasmic transfer from donor oocytes to patients’ oocytes80 \nor a transfer of oocyte chromosomes attached to the meiotic spindle \nfrom the donor’s oocytes to the recipient’s oocytes. 81 Both of these \nmitochondria transfer technologies have comparable clinical \noutcomes, and results in healthy mitochondria identified in the \noffspring population, though with a slightly less ratio of healthy \nmitochondria.82,83\nIn cases when there are no mitochondrial dysfunction diagnosed \nin females with idiopathic infertility, the clinical success of \nmitochondrial transfer is not just realized due to mitochondrial \nreplacement per se , but rather due to numerous other biologically-\nactive factors present in the maternal mtRNA. The success of these \ntreatments combined with absence of adverse reactions and safety \nfor the embryo, should encourage the use of these techniques in \nthe clinical practice of infertility treatment. A more recent clinical \nstudy confirmed that mitochondria replacement and substitution \ntherapy using mitochondria obtained from the patient’s own stem \ncells instead of the allogeneic oocytes is another promising treatment \nmodality to improve embryo quality in patients with unexplained \ninfertility.84 According to the study, 52 women (age from 27 to 49 \nyears) were treated, that resulted in 61.5% fertilization rate and 23.8% \npregnancy success rate: 11 live births, 1 intrauterine fetal death, and 4 \nmiscarriages. The average implantation rate and birth rate were 18.6% \nand 17.5%, correspondingly. The physical and cognitive development \nof all the babies born was normal, and no mtDNA mutations were \ndetected.84\nApart from the mitochondrial transfer, there are other additional \nnatural treatments, which combine the activities of a direct anti-\n\nUnexplained infertility: a fresh look at the old problem and the novel therapeutic options of its treatment\n10\nCopyright:\n©2024 Yemeliyanova et al.\nCitation: Yemeliyanova M, Chan MKS, Wong MBF , et al. Unexplained infertility: a fresh look at the old problem and the novel therapeutic options of its \ntreatment. Obstet Gynecol Int J. 2024;15(1):6‒12. DOI: 10.15406/ogij.2024.15.00726\noxidant agents, such as melatonin, glutathione, SOD2, catalase that \nprotect cells against oxidative stress. 85 Other antioxidants, such \nas vitamins C and E, as well as coenzyme Q10, provide additional \nadvantages, and can be utilized before more complicated and invasive \ntreatment modalities used.\nConclusion\nUnexplained infertility is associated with various pathological \nconditions with no clear pathogenesis identified, hence with \nno straightforward guidelines regarding the suitable treatment \noptions. However, the research on aging of the reproductive system \nprovides emerging evidences that novel anti-aging and regenerative \nmedicine and translational medicine modalities can give solutions \nto the problems related to infertility. Among such novel methods to \npreserve and restore fertility in women with unexplained infertility \nare various forms of cell-therapy. Cell therapy is an expanding field \nof Bio-regenerative medicine attempting to alleviate numerous \ndiseases involving chronic systemic low-grade inflammation, insulin \nresistance, fibrosis, and diminishing pool of endogenous stem cells. \nFurther characterization of the existing cell products, research of their \nmechanisms of action, and large-scale clinical trials are necessary \nto establishing cell therapy as an effective therapeutic option for the \ntreatment of unexplained infertility.\nThe decreased capacity of defence mechanisms against oxidative \nstress, which is one of the hallmarks of aging, results in a loss of normal \nfunctions of the female reproductive system. Hence, the assisted \nreproductive techniques should be developed in a way to address \ninfertility caused by both reproductive aging in general and oxidative \nstress in particular. The application of stem cells, precursor stem \ncells, cellular extracts, organ-specific peptides, exosomes et cetera in \nregenerative medicine has been well-known and accepted for their \nstrong anti-inflammatory, immunomodulatory, antioxidant capacities \nand stimulatory reparative effects. In addition to conventional cell \ntherapy, the therapeutic use of targeted organ-specific cell therapy \nmodalities has been researched and developed as an effective method \nof addressing to a particular disorder depending on the conditions and \nindividual status of the patient. In conclusion, further development \nof cell therapy protocols as a promising treatment modality against \nunexplained infertility with or without the assisted reproduction \ntechnologies, and further research and development of therapeutic \nmethods of attenuating the mitochondrial dysfunction in aging female \nreproductive system is required.\nAcknowledgments\nNone.\nAuthor contributions\nConceptualization: D.K., M.Y .; methodology: D.K.; writing—\noriginal draft preparation: M.Y ., D.K.; writing—review and editing: \nD.K., M.Y .; supervision: D.K., M.W.; project administration: D.K., \nM.W.; funding acquisition: D.K., M.C. All authors have read and \nagreed to the published version of the manuscript. \nFunding \nNone.\nConflicts of interest \nThe funders had no role in the design of the study; in the collection, \nanalyses, or interpretation of data; in the writing of the manuscript, or \nin the decision to publish the results.\nReferences\n1. 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