Maternal age affects oocyte developmental potential at both ends of the age spectrum

Effect of maternal age on ATP content and distribution of mitochondria in bovine oocytes

Our objective was to understand how maternal age influences the mitochondrial population and ATP content of in vivo matured bovine oocytes. We hypothesized that in vivo matured oocytes from older cows would have altered mitochondrial number and distribution patterns and lower cytoplasmic ATP content compared to the oocytes obtained from younger cows. Follicles ≥5mm were ablated in old cows (13 to 22 yrs, Old Group, n = 7) and their younger daughters (4 to 10 years old, Young Group; n = 7) to induce the emergence of a new follicular wave. Cows were treated twice daily with eight doses of FSH starting 24 hr after ablation (Day 0, day of wave emergence). Prostaglandin F2alpha (PGF) was given on Days 3 and 3.5, LH on Day 4.5, and cumulus-oocyte-complexes were collected 18-20 hours post-LH by ultrasound-guided follicular aspiration. Oocytes were either processed for staining with MitoTracker Deep Red FM or for ATP assay. Stained oocytes were imaged with a Zeiss LSM 710 confocal microscope, and mitochondria were segmented in the oocyte volume sets using Imaris Pro 7.4. In vivo matured oocytes obtained from old cows were similar in morphological grades to those from young cows. However, the oocytes of COC from older cows had 23% less intracellular ATP (27.4±1.9 vs 35.7±2.2 pmol per oocyte, P = 0.01) than those of young cows. Furthermore, the average volume of individual mitochondria, indicated by the number of image voxels, was greater (P<0.05) in oocytes from older cows than in those from younger cows. Oocytes from older cows also tended to have a greater number of mitochondrial clusters (P = 0.06) and an increased number of clusters in the central region of the oocytes (P = 0.04) compared to those from younger cows. In conclusion, our study demonstrated that maternal age was associated with a decrease in the cytoplasmic ATP content of in vivo mature oocytes and an altered distribution of mitochondrial structures. These findings suggest that maternal age may negatively influence the developmental competence of oocytes from older cows.

Human follicular fluid elicits select dose- and age-dependent effects on mouse oocytes and cumulus-oocyte complexes in a heterologous in vitro maturation assay

Follicular fluid (FF) is a primary microenvironment of the oocyte within an antral follicle. Although several studies have defined the composition of human FF in normal physiology and determined how it is altered in disease states, the direct impacts of human FF on the oocyte are not well understood. The difficulty of obtaining suitable numbers of human oocytes for research makes addressing such a question challenging. Therefore, we used a heterologous model in which we cultured mouse oocytes in human FF. To determine whether FF has dose-dependent effects on gamete quality, we performed in vitro maturation of denuded oocytes from reproductively young mice (6-12 weeks) in 10%, 50%, or 100% FF from participants of mid-reproductive age (32-36 years). FF impacted meiotic competence in a dose-dependent manner, with concentrations >10% inhibiting meiotic progression and resulting in spindle and chromosome alignment defects. We previously demonstrated that human FF acquires a fibro-inflammatory cytokine signature with age. Thus, to determine whether exposure to an aging FF microenvironment contributes to the age-dependent decrease in gamete quality, we matured denuded oocytes and cumulus-oocyte complexes (COCs) in FF from reproductively young (28-30 years) and old (40-42 years) participants. FF decreased meiotic progression of COCs, but not oocytes, from reproductively young and old (9-12 months) mice in an age-dependent manner. Moreover, FF had modest age-dependent impacts on mitochondrial aggregation in denuded oocytes and cumulus layer expansion dynamics in COCs, which may influence fertilization or early embryo development. Overall, these findings demonstrate that acute human FF exposure can impact select markers of mouse oocyte quality in both dose- and age-dependent manners.

The Prediction of IVF Outcomes with Autologous Oocytes and the Optimal MII Oocyte/Embryo Number for Live Birth at Advanced Maternal Age

Background and Objectives: Delayed childbearing in advanced age might be associated with a low prognosis for achieving pregnancy. Therefore, it is important to establish a predictive tool that will optimize the likelihood of a live birth at advanced age. Material and Methods: The retrospective study was conducted at the Ferona Fertility Clinic in Novi Sad (Republic of Serbia), between January 2020 and May 2021. The survey included 491 women aged ≥35 who met the inclusion criteria and who were subjected to an IVF (in vitro fertilization) treatment cycle. Results: The average number of retrieved oocytes, MII (metaphase II) oocytes, and developed embryos significantly decreased in advanced age. Age was also found to have a significant adverse effect on pregnancy and live birth rates. In women aged ≥35, 10/12 MII oocytes or 10/11 embryos are required for reaching an optimal live birth rate/cumulative live birth rate. Optimal CLBR (cumulative live birth rate) per one oocyte was achieved when 9 MII oocyte were retrieved. Conclusions: The study indicates that the cut-off for increased risk is ≥42 year. However, despite low live birth rates, autologous IVF for these women is not futile. An increase in the number of retrieved mature oocytes and a generation of surplus cryopreserved embryos could reinforce LBR (live birth rate) and CLBR. Clinicians should be very cautious in counseling, as autologous IVF may only be applicable to women with good ovarian reserve.

Equine maternal aging affects the metabolomic profile of oocytes and follicular cells during different maturation time points

Introduction: Oocyte quality and fertility decline with advanced maternal age. During maturation within the ovarian follicle, the oocyte relies on the associated somatic cells, specifically cumulus and granulosa cells, to acquire essential components for developmental capacity. Methods: A nontargeted metabolomics approach was used to investigate the effects of mare age on different cell types within the dominant, follicular-phase follicle at three time points during maturation. Metabolomic analyses from single oocytes and associated cumulus and granulosa cells allowed correlations of metabolite abundance among cell types. Results and Discussion: Overall, many of the age-related changes in metabolite abundance point to Impaired mitochondrial metabolic function and oxidative stress in oocytes and follicular cells. Supporting findings include a higher abundance of glutamic acid and triglycerides and lower abundance of ceramides in oocytes and somatic follicular cells from old than young mares. Lower abundance of alanine in all follicular cell types from old mares, suggests limited anaerobic energy metabolism. The results also indicate impaired transfer of carbohydrate and free fatty acid substrates from cumulus cells to the oocytes of old mares, potentially related to disruption of transzonal projections between the cell types. The identification of age-associated alterations in the abundance of specific metabolites and their correlations among cells contribute to our understanding of follicular dysfunction with maternal aging.

Control of mitochondrial integrity influences oocyte quality during reproductive aging

Reduced quality in oocytes from women of advanced maternal age (AMA) is associated with dysfunctional mitochondria. The objective of this study was to investigate the mechanisms controlling mitochondrial quality during maternal aging in mouse and human oocytes. We first evaluated the expression of proteins involved in the mitochondrial unfolded protein response (UPRmt) and mitophagy in in vivo matured metaphase II (MII) oocytes collected from young and aged mice. Expression of UPRmt proteins, HSPD1 and LONP1, and mitophagy proteins, total-PRKN and phosphorylated-PRKN, was significantly decreased in aged compared to young oocytes. Treatment of aged oocytes during in vitro maturation with the mitochondrially targeted antioxidant mitoquinone (MQ) specifically restored total-PRKN and phosphorylated-PRKN expression to levels seen in young oocytes. We next investigated whether maturing young oocytes under a high-oxygen environment would mimic the effects observed in oocytes from aged females. Phosphorylated-PRKN expression in oxidatively stressed young oocytes was reduced compared to that in oocytes matured under normal oxygen levels, and the mitochondrial DNA (mtDNA) copy number was increased. Treating oxidatively challenged young oocytes with MQ restored the phosphorylated-PRKN expression and mtDNA copy numbers. Treatment of oxidatively challenged oocytes with MQ also increased the co-localization of mitochondria and lysosomes, suggesting increased mitophagy. These data correlated with the developmental potential of the oocytes, as blastocyst development and hatching of oxidatively stressed oocytes were reduced, while treatment with MQ resulted in a significant increase in blastocyst development and hatching, and in the percentage of inner cell mass. Consistent with our results in mice, MII oocytes from women of AMA exhibited a significant decrease in phosphorylated-PKRN and total-PRKN compared to those of young women. Our findings suggest that the protein machinery to control the health of the mitochondria via UPRmt and mitophagy may be compromised in oocytes from aged females, which may result in inefficient clearance of dysfunctional mitochondria and reduced oocyte quality.

Embryonic factors mediate the maternal age-induced programming of offspring postnatal behavior in mice†

Advanced maternal age is associated with adverse pregnancy and offspring outcomes, including neurodevelopmental disorders. While age-related oocyte and embryonic abnormalities may underlie this association, the aged maternal uterine environment also plays an important role in offspring development and survival. The aim of this study was to evaluate the contribution of maternal age-related embryonic and uterine factors on pregnancy and offspring behavior, by using a model of reciprocal embryo transfer between old and young female mice. Pregnancies were obtained by transferring embryos collected from either old (9-14 months) or young (3-4 months) C57BL/6J female mice to either young or old recipients. The results showed that embryos from old and young donors have comparable developmental potential when transferred to young recipients, whereas no pregnancies were obtained by transferring embryos of young females to old recipients. Moreover, the offspring conceived by aged females displayed altered ultrasonic vocalization and learning skills compared to the progeny of young females, even though they were both prenatally and postnatally fostered by young recipients. These results indicate that maternal factors mostly determine the occurrence of age-related pregnancy complications, whereas the long-term effects of maternal aging on the offspring's behavior could be already established at pre-implantation stages and depend on embryonic factors.

Genome-wide assessment of DNA methylation alterations induced by superovulation, sexual immaturity and in vitro follicle growth in mouse blastocysts

BACKGROUND

In their attempt to fulfill the wish of having children, women who suffer from fertility issues often undergo assisted reproductive technologies such as ovarian stimulation, which has been associated with adverse health outcomes and imprinting disorders in children. However, given the crucial role of exogenous hormone stimulation in improving human infertility treatments, a more comprehensive analysis of the potential impacts on DNA methylation in embryos following ovarian stimulation is needed. Here, we provide genome-wide DNA methylation profiles of blastocysts generated after superovulation of prepubertal or adult mice, compared with blastocysts derived from non-stimulated adult mice. Additionally, we assessed the impact of the in vitro growth and maturation of oocytes on methylation in blastocysts.

RESULTS

Neither hormone stimulation nor sexual maturity had an impact on the low global methylation levels characteristic of the blastocyst stage or was associated with extensive DNA methylation alterations. However, we found hormone- and age-associated changes at specific positions but dispersed throughout the genome. In particular, we detected anomalous methylation at a limited number of CpG islands. Additionally, superovulation in adult mice was associated with alterations at the Sgce and Zfp777 imprinted genes. On the other hand, in vitro culture of follicles from the early pre-antral stage was associated with globally reduced methylation and increased variability at imprinted loci in blastocysts.

CONCLUSIONS

Our results indicate a minimal effect of ovarian stimulation of adult and prepubertal mice on the DNA methylation landscape attained at the blastocyst stage, but potentially greater impacts of in vitro growth and maturation of oocytes. These findings have potential significance for the improvement of assisted reproductive techniques, in particular for those related to treatments in prepubertal females, which could be crucial for improving human fertility preservation strategies.

Non-invasive assessment of oocyte developmental competence

Oocyte quality is a key factor influencing IVF success. The oocyte and surrounding cumulus cells, known collectively as the cumulus oocyte complex (COC), communicate bi-directionally and regulate each other's metabolic function to support oocyte growth and maturation. Many studies have attempted to associate metabolic markers with oocyte quality, including metabolites in follicular fluid or 'spent medium' following maturation, gene expression of cumulus cells and measuring oxygen consumption in medium surrounding COCs. However, these methods fail to provide spatial metabolic information on the separate oocyte and cumulus cell compartments. Optical imaging of the autofluorescent cofactors - reduced nicotinamide adenine dinucleotide (phosphate) [NAD(P)H] and flavin adenine dinucleotide (FAD) - has been put forward as an approach to generate spatially resolved measurements of metabolism within individual cells of the COC. The optical redox ratio (FAD/[NAD(P)H+FAD]), calculated from these cofactors, can act as an indicator of overall metabolic activity in the oocyte and cumulus cell compartments. Confocal microscopy, fluorescence lifetime imaging microscopy (FLIM) and hyperspectral microscopy may be used for this purpose. This review provides an overview of current optical imaging techniques that capture the inner biochemistry within cells of the COC and discusses the potential for such imaging to assess oocyte developmental competence.

Maternal age affects equine day 8 embryo gene expression both in trophoblast and inner cell mass

BACKGROUND

Breeding a mare until she is not fertile or even until her death is common in equine industry but the fertility decreases as the mare age increases. Embryo loss due to reduced embryo quality is partly accountable for this observation. Here, the effect of mare's age on blastocysts' gene expression was explored. Day 8 post-ovulation embryos were collected from multiparous young (YM, 6-year-old, N = 5) and older (OM, > 10-year-old, N = 6) non-nursing Saddlebred mares, inseminated with the semen of one stallion. Pure or inner cell mass (ICM) enriched trophoblast, obtained by embryo bisection, were RNA sequenced. Deconvolution algorithm was used to discriminate gene expression in the ICM from that in the trophoblast. Differential expression was analyzed with embryo sex and diameter as cofactors. Functional annotation and classification of differentially expressed genes and gene set enrichment analysis were also performed.

RESULTS

Maternal aging did not affect embryo recovery rate, embryo diameter nor total RNA quantity. In both compartments, the expression of genes involved in mitochondria and protein metabolism were disturbed by maternal age, although more genes were affected in the ICM. Mitosis, signaling and adhesion pathways and embryo development were decreased in the ICM of embryos from old mares. In trophoblast, ion movement pathways were affected.

CONCLUSIONS

This is the first study showing that maternal age affects gene expression in the equine blastocyst, demonstrating significant effects as early as 10 years of age. These perturbations may affect further embryo development and contribute to decreased fertility due to aging.

Oocyte aging: looking beyond chromosome segregation errors

The age-associated decline in female fertility is largely ascribable to a decrease in oocyte quality. This phenomenon is multifaceted and influenced by numerous interconnected maternal and environmental factors. An increase in the rate of meiotic errors is the major cause of the decline in oocyte developmental competence. However, abnormalities in the ooplasm accumulating with age - including altered metabolism, organelle dysfunction, and aberrant gene regulation - progressively undermine oocyte quality. Stockpiling of maternal macromolecules during folliculogenesis is crucial, as oocyte competence to achieve maturation, fertilization, and the earliest phases of embryo development occur in absence of transcription. At the same time, crucial remodeling of oocyte epigenetics during oogenesis is potentially exposed to interfering factors, such as assisted reproduction technologies (ARTs) or environmental changes, whose impact may be enhanced by reproductive aging. As the effects of maternal aging on molecular mechanisms governing the function of the human oocyte remain poorly understood, studies in animal models are essential to deepen current understanding, with translational implications for human ARTs. The present mini review aims at offering an updated and consistent view of cytoplasmic alterations occurring in oocytes during aging, focusing particularly on gene and epigenetic regulation. Appreciation of these mechanisms could inspire solutions to mitigate/control the phenomenon, and thus benefit modern ARTs.

Ovarian characteristics and in vitro nuclear and cytoplasmic oocyte maturation in Duroc and Landrace pigs

Differences in total number of piglets born per litter are observed between the Norwegian Duroc (ND) sire and Norwegian Landrace (NL) dam line. The aim of this study was to evaluate ovarian characteristics, and in vitro nuclear and cytoplasmic oocyte maturation in both breeds. One day after weaning, follicular phase ovaries were collected. Ovary length and weight were measured and the number of follicles (< 3 mm and 3–8 mm) was counted. Cumulus‐oocyte complexes (COCs) were collected and matured for 48 hr. To assess cumulus expansion, COC area was analysed at 0 and 20 hr. Nuclear maturation and cortical granule (CG) distribution were analysed at 20 and 48 hr, and total glutathione (GSH) was measured at 48 hr to further elucidate cytoplasmic maturation. In first parity sows, a smaller ovary length and fewer 3 to 8 mm follicles were observed in ND compared to NL. For all sows, ND COCs covered a significantly smaller area at 0 hr, but a higher cumulus expansion ratio was observed at 20 hr compared to NL (364 ± 46% versus. 278 ± 27%, p < 0.001). At 20 hr, more ND oocytes exhibited advanced stages of nuclear maturation, while more NL oocytes showed advanced stages of CG distribution. Nuclear maturation to MII stage at 48 hr did not differ between ND and NL oocytes (90.1% and 87.7%, respectively). Moreover, no significant differences were observed for GSH content or CG distribution after maturation. In conclusion, differences with regard to ovarian characteristics as well as to cumulus expansion, and nuclear and cytoplasmic oocyte maturation at 20 hr were observed between the breeds. Further studies are required to determine if this subsequently affects in vitro fertilization and embryo development.

Impact of Maturation and Vitrification Time of Human GV Oocytes on the Metaphase Plate Configuration

The combination of in vitro maturation (IVM) techniques and oocyte vitrification (OV) could increase the number of useful oocytes in different types of patients. IVM and subsequent OV is the most widely used clinical strategy. Would the results improve if we reverse the order of the techniques? Here, we evaluated survival, in vitro maturation, time to extrude the first polar body (PB), and the metaphase plate configuration of human prophase I (GV) oocytes before or after their vitrification. Specific, 195 GV oocytes from 104 patients subjected to controlled ovarian stimulation cycles were included. We stablished three experimental groups: GV oocytes vitrified and IVM (Group GV-Vit), GV oocytes IVM and vitrified at MII stage (Group MII-Vit), and GV oocytes IVM (Group not-Vit). All of them were in vitro matured for a maximum of 48 h and fixed to study the metaphase plate by confocal microscopy. According to our results, the vitrification of immature oocytes and their subsequent maturation presented similar survival, maturation, and metaphase plate conformation rates, but a significantly higher percentage of normal spindle than the standard strategy. Additionally, the extension of IVM time to 48 h did not seem to negatively affect the oocyte metaphase plate configuration.

The Role of Resveratrol in Mammalian Reproduction

Resveratrol is one of the most investigated natural polyphenolic compounds and is contained in more than 70 types of plants and in red wine. The widespread interest in this polyphenol derives from its antioxidant, anti-inflammatory and anti-aging properties. Several studies have established that resveratrol regulates animal reproduction. However, the mechanisms of action and the potential therapeutic effects are still unclear. This review aims to clarify the role of resveratrol in male and female reproductive functions, with a focus on animals of veterinary interest. In females, resveratrol has been considered as a phytoestrogen due to its capacity to modulate ovarian function and steroidogenesis via sirtuins, SIRT1 in particular. Resveratrol has also been used to enhance aged oocyte quality and as a gametes cryo-protectant with mainly antioxidant and anti-apoptotic effects. In males, resveratrol enhances testes function and spermatogenesis through activation of the AMPK pathway. Furthermore, resveratrol has been supplemented to semen extenders, improving the preservation of sperm quality. In conclusion, resveratrol has potentially beneficial effects for ameliorating ovarian and testes function.

Prospects of Rescuing Young Eggs for Oncofertility

Childhood cancer patients undergoing cancer therapy can be rendered infertile during adulthood. With more girls surviving cancer, fertility preservation in young cancer patients is a major clinical challenge. Advances in egg culture may offer benefits for the fertility of these patients in the future.

The CC-chemokine receptor 2 is involved in the control of ovarian folliculogenesis and fertility lifespan in mice

The chemokine receptor 2 (CCR2) was first described as a chemotactic factor involved in immune responses, but it also plays an essential function in several biological processes. The chemokine (C-C motif) ligand 2 (CCL2) binds to CCR2 triggering G protein-coupled receptor (GPCR) signaling in leukocytes, including activation of PI3K/Akt/mTOR, a key pathway that is also related to follicular activation and survival. However, the potential role of CCR2 in ovarian follicular physiology remain unexplored. Thus, we investigated the role of CCR2 on follicular growth during adult life and aging. Ovaries and oocytes were collected from wild type (WT) mice at 1.5 months old (mo), and CCR2 expression was observed predominantly in oocytes included in growing follicles, as well as after ovulation. Follicle populations were assessed in WT and CCR2-/- mice at 1.5 mo, and CCR2-/- mice had more primordial and less primary and secondary follicles, while there were no differences in antral follicle numbers. Pro-apoptotic genes Bax and Casp3 were downregulated, while anti-apoptotic Bcl2 was upregulated in CCR2-/- mice. To further characterize the role of CCR2 in ovarian aging, follicle populations were assessed in WT and CCR2-/- mice at 1.5, 2.5, 6, 10, and 12 mo. A larger ovarian follicular reserve at 1.5-6 mo was observed in CCR2-/- mice. Finally, CCR2-/- aged mice (6-12 mo) ovulated more oocytes than WT mice. Altogether, these data suggest that CCR2 plays an important role in the regulation of murine folliculogenesis, potentially affecting the reproductive lifespan.

Subcortical maternal complex (SCMC) expression during folliculogenesis is affected by oocyte donor age in sheep

PURPOSE

The age-associated decline in female fertility is largely ascribable to the decrease in oocyte quality. The subcortical maternal complex (SCMC) is a multiprotein complex essential for early embryogenesis and female fertility and functionally conserved across mammals. The present work evaluated expression dynamics of its components during folliculogenesis in relation to maternal age in sheep.

METHODS

The expression of the SCMC components (KHDC3/FILIA, NLRP2, NLRP5/MATER, OOEP/FLOPED, PADI6, TLE6 and ZBED3) was analyzed by real-time PCR in pools of growing oocytes (GO) of different diameters (70-90 μm (S), 90-110 μm (M), or 110-130 μm (L)) derived from non-hormonally treated adult (Ad; age < 4 years), prepubertal (Pr; age 40 days), or aged ewes (age > 6 years).

RESULTS

Specific expression patterns associated with donor age were observed during folliculogenesis for all genes, except ZBED3. In oocytes of adult donors, the synthesis of NLRP2, NLRP5, PADI6, and ZBED3 mRNAs was complete in S GO, while FILIA, TLE6, and OOEP were actively transcribed at this stage. Conversely, Pr GO showed active transcription of all mRNAs, except for ZBED3, during the entire window of oocyte growth. Notably, aged GO showed a completely inverse pattern, with a decrease of NLRP2, TLE6, FILIA, and PADI6 mRNA abundance during the latest stage of oocyte growth (L GO). Interestingly, MATER showed high expression variability, suggesting large inter-oocyte differences.

CONCLUSION

Our study describes the SCMC expression dynamics during sheep oogenesis and reports age-specific patterns that are likely involved in the age-related decline of oocyte quality.

Equine Aging and the Oocyte: A Potential Model for Reproductive Aging in Women

Numerous similarities in reproductive aging have been documented between the mare and woman. Aging is associated with a decline in fertility. In mares and women, oocyte transfer procedures were initially used to establish that oocyte donor age is associated with oocyte quality. Age-associated differences in oocytes include altered morphology, gene expression, and developmental potential. Reactive oxygen species and mitochondrial dysfunction are thought to be important contributors to loss of oocyte quality. In the woman, aneuploidy is a primary consideration with maternal aging. Although misalignment of chromosomes during meiosis has been observed in the mare, less is known in this area. Reproductive aging will be reviewed in the mare and compared with the woman with emphasis on factors that affect oocyte quality and developmental potential. Areas in which the mare could be used as a research model to study reproductive aging in women will be highlighted.

Female ponderal index at birth and idiopathic infertility

Epidemiological studies have demonstrated an increased risk of developing non-transmittable diseases in adults subjected to adverse early developmental conditions. Metabolic and cardiovascular diseases have been the focus of most studies. Nevertheless, data from animal models also suggest early programming of fertility. In humans, it is difficult to assess the impact of the in utero environment retrospectively. Birthweight is commonly used as an indirect indicator of intrauterine development. This research is part of the ALIFERT study. We investigated a potential link between ponderal index at birth and female fertility in adulthood. Data from 51 infertile and 74 fertile women were analysed. BW was on average higher in infertile women, whereas birth length did not differ between the two groups; thus, resulting in a significantly higher ponderal index at birth in infertile women. Ponderal index at birth has been identified as a risk factor for infertility. These results suggest the importance of the intra-uterine environment, not only for long-term metabolic health but also for fertility.