In a healthy body, ROS (reactive oxygen species) and antioxidants remain in balance When the balance is disrupted towards an overabundance of ROS, oxidative stress (OS) occurs OS influences the entire reproductive lifespan of a woman and even thereafter (ie menopause) OS results from an imbalance between prooxidants (free radical species) and the body's scavenging ability (antioxidants) ROS are a double-edged sword – they serve as key signal molecules in physiological processes but also have a role in pathological processes involving the female reproductive tract ROS affect multiple physiological processes from oocyte maturation to fertilization, embryo development and pregnancy It has been suggested that OS modulates the age-related decline in fertility It plays a role during pregnancy and normal parturition and in initiation of preterm labor Most ovarian cancers appear in the surface epithelium, and repetitive ovulation has been thought to be a causative factor Ovulation-induced oxidative base damage and damage to DNA of the ovarian epithelium can be prevented by antioxidants There is growing literature on the effects of OS in female reproduction with involvement in the pathophsiology of preeclampsia, hydatidiform mole, free radical-induced birth defects and other situations such as abortions Numerous studies have shown that OS plays a role in the pathoysiology of infertility and assisted fertility There is some evidence of its role in endometriosis, tubal and peritoneal factor infertility and unexplained infertility This article reviews the role OS plays in normal cycling ovaries, follicular development and cyclical endometrial changes It also discusses OS-related female infertility and how it influences the outcomes of assisted reproductive techniques The review comprehensively explores the literature for evidence of the role of oxidative stress in conditions such as abortions, preeclampsia, hydatidiform mole, fetal embryopathies, preterm labour and preeclampsia and gestational diabetes The review also addresses the growing literature on the role of nitric oxide species in female reproduction The involvement of nitric oxide species in regulation of endometrial and ovarian function, etiopathogenesis of endometriosis, and maintenance of uterine quiescence, initiation of labour and ripening of cervix at parturition is discussed Complex interplay between cytokines and oxidative stress in the etiology of female reproductive disorders is discussed Oxidant status of the cell modulates angiogenesis, which is critical for follicular growth, corpus luteum formation endometrial differentiation and embryonic growth is also highlighted in the review Strategies to overcome oxidative stress and enhance fertility, both natural and assisted are delineated Early interventions being investigated for prevention of preeclampsia are enumerated Trials investigating combination intervention strategy of vitamin E and vitamin C supplementation in preventing preeclampsia are highlighted Antioxidants are powerful and there are few trials investigating antioxidant supplementation in female reproduction However, before clinicians recommend antioxidants, randomized controlled trials with sufficient power are necessary to prove the efficacy of antioxidant supplementation in disorders of female reproduction Serial measurement of oxidative stress biomarkers in longitudinal studies may help delineate the etiology of some of the diosorders in female reproduction such as preeclampsia

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Role of oxidative stress in female reproduction

Oxidative stress is involved in the aetiology of defective embryo development. Reactive oxygen species (ROS) may originate from embryo metabolism and/or embryo surroundings. Embryo metabolism generates ROS via several enzymatic mechanisms. The relative contribution of each source seems different depending on the species, the stage of development, and the culture conditions. Several exogenous factors and culture conditions can enhance the production of ROS by embryos. ROS can alter most types of cellular molecules, and also induce development block and retardation. Multiple mechanisms of embryo protection against ROS exist, and these have complementary actions. External protection, present in follicular and tubal fluids, mainly comprises non-enzymatic antioxidants such as hypotaurine, taurine and ascorbic acid. Internal protection mainly comprises antioxidant enzymes: superoxide dismutase, glutathione peroxidase and gamma-glutamylcysteine synthetase. Transcripts encoding for these enzymes are present in the oocyte, embryo and oviduct. It may be important that these transcripts are stored during oocyte maturation in order to allow the embryo to acquire the aptitude to develop. It is now common to add antioxidant compounds to culture media. Nevertheless, maintaining the pro-oxidant-antioxidant equilibrium in embryos through such supplementation is a complex problem. Further studies are necessary to limit oxidative stress during embryo culture.

Oxidative stress and protection against reactive oxygen species in the pre-implantation embryo and its surroundings

Oxidative stress (OS), a state characterized by an imbalance between pro-oxidant molecules including reactive oxygen and nitrogen species, and antioxidant defenses, has been identified to play a key role in the pathogenesis of subfertility in both males and females. The adverse effects of OS on sperm quality and functions have been well documented. In females, on the other hand, the impact of OS on oocytes and reproductive functions remains unclear. This imbalance between pro-oxidants and antioxidants can lead to a number of reproductive diseases such as endometriosis, polycystic ovary syndrome (PCOS), and unexplained infertility. Pregnancy complications such as spontaneous abortion, recurrent pregnancy loss, and preeclampsia, can also develop in response to OS. Studies have shown that extremes of body weight and lifestyle factors such as cigarette smoking, alcohol use, and recreational drug use can promote excess free radical production, which could affect fertility. Exposures to environmental pollutants are of increasing concern, as they too have been found to trigger oxidative states, possibly contributing to female infertility. This article will review the currently available literature on the roles of reactive species and OS in both normal and abnormal reproductive physiological processes. Antioxidant supplementation may be effective in controlling the production of ROS and continues to be explored as a potential strategy to overcome reproductive disorders associated with infertility. However, investigations conducted to date have been through animal or in vitro studies, which have produced largely conflicting results. The impact of OS on assisted reproductive techniques (ART) will be addressed, in addition to the possible benefits of antioxidant supplementation of ART culture media to increase the likelihood for ART success. Future randomized controlled clinical trials on humans are necessary to elucidate the precise mechanisms through which OS affects female reproductive abilities, and will facilitate further explorations of the possible benefits of antioxidants to treat infertility.

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The effects of oxidative stress on female reproduction: a review

Endogenous melatonin is synthesized from tryptophan via 5-hydroxytryptamine. It is considered an indoleamine from a biochemical point of view because the melatonin molecule contains a substituted indolic ring with an amino group. The circadian production of melatonin by the pineal gland explains its chronobiotic influence on organismal activity, including the endocrine and non-endocrine rhythms. Other functions of melatonin, including its antioxidant and anti-inflammatory properties, its genomic effects, and its capacity to modulate mitochondrial homeostasis, are linked to the redox status of cells and tissues. With the aid of specific melatonin antibodies, the presence of melatonin has been detected in multiple extrapineal tissues including the brain, retina, lens, cochlea, Harderian gland, airway epithelium, skin, gastrointestinal tract, liver, kidney, thyroid, pancreas, thymus, spleen, immune system cells, carotid body, reproductive tract, and endothelial cells. In most of these tissues, the melatonin-synthesizing enzymes have been identified. Melatonin is present in essentially all biological fluids including cerebrospinal fluid, saliva, bile, synovial fluid, amniotic fluid, and breast milk. In several of these fluids, melatonin concentrations exceed those in the blood. The importance of the continual availability of melatonin at the cellular level is important for its physiological regulation of cell homeostasis, and may be relevant to its therapeutic applications. Because of this, it is essential to compile information related to its peripheral production and regulation of this ubiquitously acting indoleamine. Thus, this review emphasizes the presence of melatonin in extrapineal organs, tissues, and fluids of mammals including humans.

Extrapineal melatonin: sources, regulation, and potential functions.

The roles of cellular reactive oxygen species, oxidative stress and antioxidants in pregnancy outcomes

The mRNA expression of five enzymes: catalase, Cu-Zn-superoxide dismutase (Cu-Zn-SOD), Mn-superoxide dismutase (Mn-SOD), glutathione peroxidase (GPX), and gamma-glutamylcysteine synthetase (GCS) each involved in protection against free radicals was studied in human and mouse oocytes. In the mouse, oocytes were collected at different stages of maturation in order to determine the storage of these transcripts. For the human, germinal vesicle (GV) oocytes harvested during intracytoplasmic sperm injection (ICSI) procedures and failed fertilized metaphase II (MII) oocytes were analysed. Human and mouse were compared in order to determine whether the differential developmental capacity of mouse and human preimplantation embryos in culture could be explained by the variations in the patterns of expression for these enzymes. mRNA expression for these enzymes was examined using reverse transcription-polymerase chain reaction (RT-PCR). In the mouse, all transcripts (except for catalase) were detected, whatever the maturation stage. No qualitative differences were detected between GV and MII oocytes. In human, all the enzymes (except for catalase) were expressed in MII oocytes and Cu-Zn-SOD was particularly highly expressed. Transcripts corresponding to GPX and Mn-SOD were not detected at GV stage but only at MII stage, suggesting that storage could occur between GV and MII stages. However, using 3' end-specific primers for GPX and Mn-SOD, instead of the oligo(dT)(12-18) primer, for the reverse transcription reaction, the transcripts for these antioxidants enzymes have been detected in human oocytes at the GV stage. This suggests the presence of maturation-specific polyadenylation of these transcripts. These enzymes can be considered as markers of cytoplasmic maturation.

Expression of genes encoding antioxidant enzymes in human and mouse oocytes during the final stages of maturation

The mammalian oviduct: biochemistry and physiology

Taurine and hypotaurine seem to be important compounds for sperm survival and capacitation, the fertilisation process and embryo development, and are present in both sperm and genital secretions. Hypotaurine has protective effects against peroxidative damage. We have established a simple method for hypotaurine quantification in sperm and genital secretions. The mean concentration of hypotaurine is significantly higher in bovine than in human spermatozoa and in seminal plasma. We observed that both molecules are secreted by cow, sow, goat and rabbit oviduct epithelial cell monolayers. In rabbit the release is ascorbic acid dependent. Goat oviduct epithelial cells are able to use the transsulfuration pathway to form hypotaurine and taurine from methionine. We were able to identify cysteine sulfinate decarboxylase (EC 4.1.1.29) activity in cow and goat tubal monolayers. Our results demonstrate that hypotaurine and taurine are secreted by oviduct epithelium, and synthesised by tubal cells via the cysteine sulfinic acid pathway. The data obtained emphasise the importance of hypotaurine and taurine for gamete maturation, fertilisation and early embryonic development.

P. Guérin

Papers

Oxidative stress and protection against reactive oxygen species in the pre-implantation embryo and its surroundings

Expression of genes encoding antioxidant enzymes in human and mouse oocytes during the final stages of maturation

The mammalian oviduct: biochemistry and physiology

Hypotaurine and taurine in gamete and embryo environments: de novo synthesis via the cysteine sulfinic acid pathway in oviduct cells

Mammalian oviduct and protection against free oxygen radicals: expression of genes encoding antioxidant enzymes in human and mouse