Cytoplasmic maturation of bovine oocytes: Structural and biochemical modifications and acquisition of developmental competence

Chemical reversion of age-related oocyte dysfunction fails to enhance embryo development in a bovine model of postovulatory aging

PURPOSE

There are no clinical treatments to prevent/revert age-related alterations associated with oocyte competence decline in the context of advanced maternal age. Those alterations have been attributed to oxidative stress and mitochondrial dysfunction. Our study aimed to test the hypothesis that in vitro maturation (IVM) medium supplementation with antioxidants (resveratrol or phloretin) may revert age-related oocyte competence decline.

METHODS

Bovine immature oocytes were matured in vitro for 23 h (young) and 30 h (aged). Postovulatory aged oocytes (control group) and embryos obtained after fertilization were examined and compared with oocytes supplemented with either 2 μM of resveratrol or 6 μM phloretin (treatment groups) during IVM.

RESULTS

Aged oocytes had a significantly lower mitochondrial mass and proportion of mitochondrial clustered pattern, lower ooplasmic volume, higher ROS, lower sirtuin-1 protein level, and a lower blastocyst rate in comparison to young oocytes, indicating that postovulatory oocytes have a lower quality and developmental competence, thus validating our experimental model. Supplementation of IVM medium with antioxidants prevented the generation of ROS and restored the active mitochondrial mass and pattern characteristic of younger oocytes. Moreover, sirtuin-1 protein levels were also restored but only following incubation with resveratrol. Despite these findings, the blastocyst rate of treatment groups was not significantly different from the control group, indicating that resveratrol and phloretin could not restore the oocyte competence of postovulatory aged oocytes.

CONCLUSION

Resveratrol and phloretin can both revert the age-related oxidative stress and mitochondrial dysfunction during postovulatory aging but were insufficient to enhance embryo developmental rates under our experimental conditions.

Description of a new quantitative method to assess mitochondrial distribution pattern in mature equine oocytes

The Mitochondrial distribution pattern or MDP in mammalian oocytes serves as an indicator of their cytoplasmic maturity, with a heterogeneous pattern associated with mature cytoplasm. Currently, MDP assessment involves fluorescent labelling of mitochondria followed by visual evaluation, as no quantitative method exists. Our objective was to develop a quantitative approach to assess MDP in mature equine oocytes. Equine oocytes, obtained by ovum pick up (OPU) were matured in vitro, and only metaphase II oocytes were used in the study (n = 56). Following denudation, oocytes were fixed, stained with MitoTracker™ Red CMXRos (50 nM in TCM-199 with Hank´s salts and 10% FBS) for 15 min at 38 °C, and then incubated with 2.5 µg/ml Hoechst 33342 for 10 min at 38 °C. Confocal microscope images were acquired, and the oocyte's MDP was visually classified as either homogeneous (HoD; n = 17) or heterogeneous (HeD; n = 39). For quantitative analysis, Fiji-ImageJ software was employed. Background subtraction was performed, and a 1-pixel line along the diameter was drawn to calculate the intensity profile. Fluorescence intensities were normalized, and ratios of peripheral to central fluorescence intensity were determined. Student´s t-test was used for comparations; MDP ratio was (mean ± standard error of the mean): 0.8 ± 0.02 for HoD and 0.3 ± 0.02 for HeD (p < 0.001). These results demonstrate concordance between quantitative and qualitative MDP assessment in mature equine oocytes. Our study describes a new approach to quantify mitochondrial distribution pattern and cytoplasmic maturation in mature equine oocytes.

Follicular fluid exosome-derived miR-339-5p enhances in vitro maturation of porcine oocytes via targeting SFPQ, a regulator of the ERK1/2 pathway

In this study, we aimed to investigate cytoplasmic maturation and miRNA expression of mature oocytes cultured in porcine follicular fluid exosomes. We also examined the effect of miR-339-5p on oocyte maturation. Twenty eight differentially expressed miRNAs were detected using miRNA-seq. We then transfected cumulus oocyte complexes with miR-339-5p mimics and inhibitor during culture. The results showed that exosomes increased endoplasmic reticulum levels and the amount of lipid droplets, and decreased ROS levels, lipid droplet size, and percentage of oocytes with abnormal cortical granule distribution. Overexpressing miR-339-5p significantly decreased cumulus expansion genes, oocyte maturation-related genes, target gene proline/glutamine-rich splicing factor (SFPQ), ERK1/2 phosphorylation levels, oocyte maturation rate, blastocyst rate, and lipid droplet number, but increased lipid droplet size and the ratio of oocytes with abnormal cortical granule distribution. Inhibiting miR-339-5p reversed the decrease observed during overexpression. Mitochondrial membrane potential and ROS levels did not differ significantly between groups. In summary, exosomes promote oocyte cytoplasmic maturation and miR-339-5p regulating ERK1/2 activity through SFPQ expression, thereby elevating oocyte maturation and blastocyst formation rate in vitro.

Could the presence and proportion of three/multiple pronuclei (3PN/MPN) zygotes indicate the cytoplasmic maturation state of oocyte cohort in conventional IVF patients?

Although abnormally fertilized zygotes with three or multiple pronuclei (3 PN/MPN) are commonly believed to be associated with improper maturation of the oocyte cytoplasm in conventional IVF cycles, no studies investigated the association between the proportion of MPN zygotes and the maturation state of the oocyte cohort. We compared the cytoplasmic maturity of oocytes from conventional IVF cycles with different proportions of 3 PN/MPN zygotes. A total of 1428 conventional IVF patients with ≥6 oocytes retrieved and fresh embryos transferred were divided into 4 groups according to the proportions of 3 PN/MPN zygotes. The pregnancy outcomes and the proportion of nuclear immature oocytes were analyzed to suggest the cytoplasmic maturation state of the oocyte cohort. Our results showed that the group with a low proportion of 3 PN/MPN zygotes had a higher clinical pregnancy rate (CPR) than those without 3 PN/MPN zygotes (P < 0.05). However, the live birth rate (LBR) was not significantly different between the two groups. The implantation rate (IR), CPR, and LBR did not differ between the low-proportion and high-proportion 3 PN/MPN groups. The proportion of nuclear immature oocytes on day 1 was highest in the group without 3 PN/MPN zygotes (23.8 %) and gradually decreased with an increased proportion of 3 PN/MPN zygotes (P < 0.001). Therefore, the presence of 3 PN/MPN zygotes after conventional IVF may indicate a more mature cytoplasmic state of the oocyte cohort, and the increased proportion of 3 PN/MPN zygotes is associated with an increased maturation state of the whole oocyte cohort. The occurrence and proportion of 3 PN/MPN zygotes may serve as an indicator for the cytoplasmic maturity of the oocyte cohort and help clinicians evaluate the efficiency of ovarian stimulation and optimize the stimulation protocols in subsequent cycles.

Fibronectin 1 supports oocyte in vitro maturation in pigs

Oocyte in vitro maturation (IVM) based on the follicular fluid (FF) environment can exploit untapped resources, however, what FF factors regulate oocyte maturation remains unclear. This work demonstrated that serum and FF significantly promoted oocyte polar body extrusion (PBE) and subsequent embryo development, and FF was especially effective. Fibronectin 1 (FN1) was predicted as one potential candidate to regulate oocyte maturation by proteomics. FN1 transcription obviously decreased, and the protein expression significantly increased and migrated to plasma membrane or even outside during oocyte IVM. Treatment with 10 ng/mL FN1 significantly improved oocyte PBE rate. FN1 significantly upregulated the percentage of regular spindle morphology, downregulated the γ-H2AX level, decreased the levels of ROS and apoptosis, and increased GSH and mitochondrion contents by ameliorating the expression of corresponding genes. Moreover, FN1 significantly increased the p-PI3K level to enhance the activation of PI3K signaling pathway. In conclusion, this study discovers and confirms that FN1 is one factor in FF that significantly enhances oocyte maturation, and the underlying mechanism is that FN1 ameliorates oocyte nuclear and cytoplasmic maturation by promoting the activation of PI3K signaling pathway.

Effect of Cyclic Adenosine Monophosphate on Connexin 37 Expression in Sheep Cumulus-Oocyte Complexes

Gap junctional connection (GJC) in the cumulus-oocyte complex (COC) provides necessary support for message communication and nutrient transmission required for mammalian oocyte maturation. Cyclic adenosine monophosphate (cAMP) is not only a prerequisite for regulating oocyte meiosis, but also the key intercellular factor for affecting GJC function in COCs. However, there are no reports on whether cAMP regulates connexin 37 (Cx37) expression, one of the main connexin proteins, in sheep COCs. In this study, the expression of Cx37 protein and gene in immature sheep COC was detected using immunohistochemistry and PCR. Subsequently, the effect of cAMP on Cx37 expression in sheep COCs cultured in a gonadotropin-free culture system for 10 min or 60 min was evaluated using competitive ELISA, real-time fluorescent quantitative PCR (RT-qPCR), and Western blot. The results showed that the Cx37 protein was present in sheep oocytes and cumulus cells; the same results were found with respect to GJA4 gene expression. In the gonadotropin-free culture system, compared to the control, significantly higher levels of cAMP as well as Cx37 gene and protein expression were found in sheep COCs following treatment in vitro with Forskolin and IBMX (100 μM and 500 μM)) for 10 min (p < 0.05). Compared to the controls (at 10 or 60 min), cAMP levels in sheep COCs were significantly elevated as a result of Forskolin and IBMX treatment (p < 0.05). Following culturing in vitro for 10 min or 60 min, Forskolin and IBMX treatment can significantly promote Cx37 expression in sheep COCs (p < 0.05), a phenomenon which can be counteracted when the culture media is supplemented with RP-cAMP, a cAMP-specific competitive inhibitor operating through suppression of the protein kinase A (PKA). In summary, this study reports the preliminary regulatory mechanism of cAMP involved in Cx37 expression for the first time, and provides a novel explanation for the interaction between cAMP and GJC communication during sheep COC culturing in vitro.

Mitochondrial deoxyguanosine kinase is required for female fertility in mice

Mitochondrial homeostasis plays a pivotal role in oocyte maturation and embryonic development. Deoxyguanosine kinase (DGUOK) is a nucleoside kinase that salvages purine nucleosides in mitochondria and is critical for mitochondrial DNA replication and homeostasis in non-proliferating cells. Dguok loss-of-function mutations and deletions lead to hepatocerebral mitochondrial DNA deletion syndrome. However, its potential role in reproduction remains largely unknown. In this study, we find that Dguok knockout results in female infertility. Mechanistically, DGUOK deficiency hinders ovarian development and oocyte maturation. Moreover, DGUOK deficiency in oocytes causes a significant reduction in mitochondrial DNA copy number and abnormal mitochondrial dynamics and impairs germinal vesicle breakdown. Only few DGUOK-deficient oocytes can extrude their first polar body during in vitro maturation, and these oocytes exhibit irregular chromosome arrangements and different spindle lengths. In addition, DGUOK deficiency elevates reactive oxygen species levels and accelerates oocyte apoptosis. Our findings reveal novel physiological roles for the mitochondrial nucleoside salvage pathway in oocyte maturation and implicate DGUOK as a potential marker for the diagnosis of female infertility.

Three-dimensional glass scaffolds improve the In Vitro maturation of porcine cumulus-oocyte complexes and subsequent embryonic development after parthenogenetic activation

In vitro maturation (IVM) methods for porcine oocytes are still deficient in achieving full developmental capacity, as the currently available oocyte in vitro culture systems still have limitations. In vitro embryo production must also improve the porcine oocyte IVM system to acquire oocytes with good developmental potential. Herein, we tested a three-dimensional (3D) glass scaffold culture system for porcine oocyte maturation. After 42 h, we matured porcine cumulus-oocyte complexes (COCs) on either two-dimensional glass dishes (2D-B), two-dimensional microdrops (2D-W), or 3D glass scaffolds. The 3D glass scaffolds were tested for porcine oocyte maturation and embryonic development. Among these culture methods, the extended morphology of the 3D group maintained a 3D structure better than the 2D-B and 2D-W groups, which had flat COCs that grew close to the bottom of the culture vessel. The COCs of the 3D group had a higher cumulus expansion index and higher first polar body extrusion rate, cleavage rate, and blastocyst rate of parthenogenetic embryos than the 2D-B group. In the 3D group, the cumulus-expansion-related gene HAS2 and anti-apoptotic gene Bcl-2 were significantly upregulated (p < 0.05), while the pro-apoptotic gene Caspase3 was significantly downregulated (p < 0.05). The blastocysts of the 3D group had a higher relative expression of Bcl-2, Oct4, and Nanog than the other two groups (p < 0.05). The 3D group also had a more uniform distribution of mitochondrial membrane potential and mitochondria (p < 0.05), and its cytoplasmic active oxygen species content was much lower than that in the 2D-B group (p < 0.05). These results show that 3D glass scaffolds dramatically increased porcine oocyte maturation and embryonic development after parthenogenetic activation, providing a suitable culture model for porcine oocytes.

The ERβ-cAMP signaling pathway regulates estradiol-induced ovine oocyte meiotic arrest

Although 17β-estradiol (E2) and its receptors (ERs) are reported to play important roles in regulating oocyte maturation, the specific mechanism remains unclear. First, we performed immunohistochemistry analyses to determine the expression of the ERα and ERβ proteins in ovine ovarian tissue. Second, E2 (0.5 ng/mL and 1 μg/mL) were added to pre-IVM medium for 0 h, 1 h and 2 h. The effects of E2 (0.5 ng/mL and 1 μg/mL) on cyclic adenosine monophosphate (cAMP) level in cumulus-oocyte complexes (COCs) and on oocyte meiotic progression were evaluated by ELISA and DAPI staining respectively. Third, the effects of E2 on the gene and protein expression of ERα and ERβ in COCs were investigated by Western blotting and real-time PCR. Afterward, ERβ and cAMP regulators were added to the 2-h pretreatment medium with or without E2 (0.5 ng/mL) to explore the possible interactions among E2, cAMP and ERβ. The results showed that both ERα and ERβ proteins were expressed in ovine cumulus layers and oocytes. E2 significantly increased intra-COC cAMP levels, maintained oocyte meiotic arrest, and promoted ERβ transcript and protein expression. E2 treatment increased the cAMP concentration, which was enhanced by ERβ agonist treatment and remarkably attenuated by ERβ inhibitor treatment. Forskolin plus IBMX treatment increased ERβ protein expression in COCs (P < 0.05), and this was attenuated by Rp-cAMP treatment. In conclusion, E2 (0.5 ng/mL) increased intra-COC cAMP levels by promoting ERβ expression, thereby maintaining oocyte meiotic arrest. cAMP in COCs has a positive feedback effect on ERβ expression, which provides a novel explanation for the positive role of E2 in regulating ovine follicle development and oocyte maturation.

Ameliorating Effect of Sodium Selenite on Developmental and Molecular Response of Bovine Cumulus-Oocyte Complexes Matured in Vitro Under Heat Stress Condition

Selenium (Se), an essential trace element, plays an important role in the antioxidative defense mechanism, and it has been proven to improve fertility and reproductive efficiency in dairy cattle. The present study evaluated the potential protective action of Se supplement of in vitro maturation (IVM) media on the maturation and subsequent development of bovine cumulus-oocyte complexes (COCs) exposed to heat stress (HS). The treatment with Se improved the viability of cumulus cells (CCs) and oocytes (P < 0.05). The proportion of oocytes reached metaphase II (MII) and those arrested at metaphase I (MI) was greater and lower in treatment than control respectively (P < 0.05). Supplementation with Se increased the percentage of cleaved embryos, total blastocysts, and blastocyst/cleavage ratio (P < 0.05). Moreover, the upregulation of CCND1, SEPP1, GPX-4, SOD, CAT, and downregulation of GRP78, CHOP, and BAX in both Se-treated CCs and oocytes were recorded. The upregulation of NRF2 was detected in Se-treated CCs other than in oocytes, which showed upregulation of IGF2R and SOX-2 as the markers of quality as well. Se supplement in IVM media improved the viability, maturation, and the level of transcripts related to antioxidant defense and quality of heat-treated oocytes, which coincided with greater subsequent development outcomes. Se ameliorated the viability of CCs along with upregulation of antioxidative candidate gene expression and downregulation of apoptosis-related ones to support their protective role on restoring the quality of oocytes against compromising effects of HS.

Reduced period from follicular wave emergence to luteolysis generated greater steroidogenic follicles and estrus intensity in dairy cows

The onset of productive life in dairy cattle, concomitant to parturition, is accompanied by a substantial decrease in fertility in comparison with non-lactating, nulliparous heifers. Follicular growth patterns differ between parous and nulliparous dairy cattle. Nulliparous heifers ovulate follicles with reduced antral age (RAA). This study aimed to exogenously reduce ovulatory follicle age in lactating dairy cows from 7 to 5 days old. Cows (n = 80) had their estrous cycles synchronized with the Double-Ovsynch program. At the final portion of this program, luteolysis was induced at either 5 (RAA) or 7 (Control) days following follicular wave emergence. RAA outcomes were estimated in comparison with Controls. RAA resulted in smaller follicles 2 days post-treatment. Despite lower serum concentrations of 17β-estradiol before treatment compared with Controls, the rate of increase in this hormone was greater for the RAA treatment. There was no difference in luteolysis rates between treatments. Proestrus (luteolysis onset to estrus onset) was prolonged in RAA cows. Cows with RAA had more intense estruses. Collectively, these results indicate that decreasing the age of the ovulatory follicle may improve the steroidogenic capacity of the dominant follicle and estrus expression intensity in lactating dairy cows.

The oocyte: the key player in the success of assisted reproduction technologies

The ovulation of a mature oocyte at metaphase II of meiosis, with optimal potential to undergo fertilisation by a sperm cell, complete meiosis and sustain the switch to mitotic division, and support early embryo development, involves a protracted and disrupted/delayed series of processes. Many of these are targeted for exploitation in vivo , or recapitulation in vitro , by the livestock industry. Reproductive technologies, including AI, multiple ovulation embryo transfer, ovum pick-up, in vitro embryo production, and oestrus and ovulation synchronisation, offer practitioners and producers the opportunity to produce offspring from genetically valuable dams in much greater numbers than they would normally have in their lifetime, while in vitro oocyte and follicle culture are important platforms for researchers to interrogate the physiological mechanisms driving fertility. The majority of these technologies target the ovarian follicle and the oocyte within; thus, the quality and capability of the recovered oocyte determine the success of the reproductive intervention. Molecular and microscopical technologies have grown exponentially, providing powerful platforms to interrogate the molecular mechanisms which are integral to or affected by ART. The development of the bovine oocyte from its differentiation in the ovary to ovulation is described in the light of its relevance to key aspects of individual interventions, while highlighting the historical timeline.

In vitro embryo production in small ruminants: what is still missing?

In vitro embryo production (IVEP) is an extremely important tool for genetic improvement in livestock and it is the biotechnology that has grown the most recently. However, multiple ovulation followed by embryo transfer is still considered the leading biotechnology for embryo production in small ruminants. This review aimed to identify what is still missing for more efficient diffusion of IVEP in small ruminants, going through the IVEP steps and highlighting the main factors affecting the outcomes. Oocyte quality is essential for the success of IVEP and an aspect to be considered in small ruminants is their reproductive seasonality and strategies to mitigate the effect of season. The logistics for oocyte collection from live females is more complex than in cattle, and tools to simplify this collection system and/or to promote an alternative way of recovering oocytes may be an important point in this scenario. The heterogeneity of oocytes collected from growing follicles in live females or from ovaries collected from abattoirs remains a challenge, and there is a demand to standardize/homogenize the hormonal stimulatory protocols and IVM protocols for each source of oocytes. The use of sexed semen is technically possible, however the low market demand associated with the high costs of the sexing process prevents the routine use of this technique, but its higher availability is an important aspect aiming for greater dissemination of IVEP. New noninvasive approaches for embryo selection are key factors since the selection for transfer or cryopreservation is another difficulty faced among laboratories. Embryo selection is based on morphological traits, although these are not necessarily reliable in predicting pregnancy. Several issues described in this review must be considered by researchers in other to promote the diffusion of IVEP in small ruminants.

Human TIMP1 Is a Growth Factor That Improves Oocyte Developmental Competence

Oocyte developmental competence is the ability of a mature oocyte to be fertilized and subsequently support embryonic development. Such competence is gained during folliculogenesis and is facilitated by the bidirectional communication into a compacted cumulus-oocyte complex (COC). Human tissue inhibitor of metalloproteinases-1 (TIMP1) participates in biological processes, including cell growth, differentiation, and apoptosis. This study aimed to evaluate the influence of TIMP1 as a growth factor on the in vitro maturation (IVM) culture of bovine COCs to improve oocyte developmental competence. All TIMP1 treatments (50, 100, and 150 ng/mL) favored the COCs' compaction structure (p < 0.05). TIMP1 at 150 ng/mL produced more oocytes in metaphase II compared to the other treatments (p < 0.05). The 150 ng/mL TIMP1 generated oocytes with the most (p < 0.05) cortical granules below the plasma membrane (pattern I). In a parthenogenesis assay, oocyte IVM in 50 ng/mL of TIMP1 produced the most blastocyst compared to the other treatments (p < 0.05). The Principal Component Analysis (PCA) showed that 50 ng/mL of TIMP1 was the best condition to develop oocyte competence because it was associated with the COC compact and cortical granule pattern I. TIMP1 influences the development of oocyte competence when added to the IVM culture medium of COCs.

Effect of a low ethanol concentration during in vitro maturation of bovine oocytes and subsequent embryo development

The present study investigated the effects of low ethanol exposure on bovine oocytes. Cumulus-oocyte complexes (COCs) were aspirated for the antral follicles of slaughterhouse-derived ovaries. These COCs were incubated in maturation medium containing 0, 0.1, and 0.2% ethanol for 21 h and subjected to fertilization and in vitro development, and then the rates of nuclear maturation, mitochondrial DNA copy number (Mt-cn) and protein (TOMM40), ATP content and lipid content in oocyte, fertilization, and blastulation were examined. Furthermore, COCs were incubated with 0 or 0.1% ethanol and then mitochondrial membrane potential (MMP) and the glucose consumption of COCs was determined. In addition, gene expression in oocytes was examined by RNA sequencing. Ethanol (0.1 and 0.2%) increased Mt-cn and Mt-protein levels whereas 0.2% ethanol increased the blastulation rate and ATP content in oocytes and decreased lipid content in oocytes. Ethanol (0.1%) increased MMP in oocytes and decreased glucose consumption of COCs. Eight stage embryos derived from 0.1% ethanol treated oocytes had higher levels of trimethyl-H3K9 compared with that of nontreated counterpart. RNA sequencing revealed that differentially expressed genes were associated with glycolysis/gluconeogenesis, carbon metabolism, sphingolipid metabolism, amino acid metabolism, and fatty acid degradation pathways. In conclusion, even 0.1% concentrations of ethanol during in vitro maturation considerably affects oocyte metabolism and histone configuration of embryos.

C-X-C motif chemokine ligand 12 improves the developmental potential of bovine oocytes by activating SH2 domain-containing tyrosine phosphatase 2 during maturation†

In vitro maturation of mammalian oocytes is an important means in assisted reproductive technology. Most bovine immature oocytes complete nuclear maturation, but less than half develop to the blastocyst stage after fertilization. Thus, inefficient in vitro production is mainly caused by a suboptimal in vitro culture process, in which oocyte quality appears to be the limiting factor. In our study, a potential maternal regulator, C-X-C motif chemokine ligand 12, was identified by analyzing transcriptome data. C-X-C motif chemokine ligand 12 supplementation promoted the developmental potential of oocytes by improving protein synthesis and reorganizing cortical granules and mitochondria during in vitro maturation, which eventually increased blastocyst formation efficiency and cell number after parthenogenesis, fertilization, and cloning. All these promoting effects by C-X-C motif chemokine ligand 12 were achieved by activating SH2 domain-containing tyrosine phosphatase 2, thereby promoting the mitogen-activated protein kinase signaling pathway. These findings provide an in vitro maturation system that closely resembles the maternal environment to provide high-quality oocytes for in vitro production.

Aging and oocyte competence: A molecular cell perspective

Follicular microenvironment is paramount in the acquisition of oocyte competence, which is dependent on two interconnected and interdependent processes: nuclear and cytoplasmic maturation. Extensive research conducted in human and model systems has provided evidence that those processes are disturbed with female aging. In fact, advanced maternal age (AMA) is associated with a lower chance of pregnancy and live birth, explained by the age-related decline in oocyte quality/competence. This decline has largely been attributed to mitochondria, essential for oocyte maturation, fertilization, and embryo development; with mitochondrial dysfunction leading to oxidative stress, responsible for nuclear and mitochondrial damage, suboptimal intracellular energy levels, calcium disturbance, and meiotic spindle alterations, that may result in oocyte aneuploidy. Nuclear-related mechanisms that justify increased oocyte aneuploidy include deoxyribonucleic acid (DNA) damage, loss of chromosomal cohesion, spindle assembly checkpoint dysfunction, meiotic recombination errors, and telomere attrition. On the other hand, age-dependent cytoplasmic maturation failure is related to mitochondrial dysfunction, altered mitochondrial biogenesis, altered mitochondrial morphology, distribution, activity, and dynamics, dysmorphic smooth endoplasmic reticulum and calcium disturbance, and alterations in the cytoskeleton. Furthermore, reproductive somatic cells also experience the effects of aging, including mitochondrial dysfunction and DNA damage, compromising the crosstalk between granulosa/cumulus cells and oocytes, also affected by a loss of gap junctions. Old oocytes seem therefore to mature in an altered microenvironment, with changes in metabolites, ribonucleic acid (RNA), proteins, and lipids. Overall, understanding the mechanisms implicated in the loss of oocyte quality will allow the establishment of emerging biomarkers and potential therapeutic anti-aging strategies. This article is categorized under: Reproductive System Diseases > Molecular and Cellular Physiology.

Cadmium exposure impairs oocyte meiotic maturation by inducing endoplasmic reticulum stress in vitro maturation of porcine oocytes

Cadmium (Cd) is toxic metal that can induce various diseases, such as cardiovascular, nervous, and reproductive systems. This study investigated the effect of Cd exposure on porcine oocyte maturation and the underlying mechanism. Porcine cumulus-oocyte complexes were exposed various Cd concentration and tauroursodeoxycholic acid (TUDCA), an inhibitor of endoplasmic reticulum (ER) stress during in vitro maturation (IVM). After IVM, we evaluated meiotic maturation, ER stress, and oocyte quality by Cd exposure. Cd exposure inhibited cumulus cell expansion and meiotic maturation, increased oocyte degeneration, and induced ER stress. The levels of spliced XBP1 and ER stress-associated transcripts, markers of ER stress, were elevated in Cd-treated cumulus-oocyte complexes and denuded oocytes during IVM. Moreover, Cd-induced ER stress impaired oocyte quality by disrupting mitochondrial function and elevating intracellular reactive oxygen species levels while decreasing ER function. Interestingly, TUDCA supplementation significantly decreased the expression of ER stress-related genes and increased the quantity of ER compared with the Cd treatment. Additionally, TUDCA was also able to rescue excessive levels of ROS and restore normal mitochondrial function. Moreover, the addition of TUDCA under Cd exposure greatly ameliorated Cd-mediated detrimental effects on meiotic maturation and oocyte quality, including cumulus cell expansion and MII rate. These findings suggest that Cd exposure during IVM impairs the meiotic maturation of oocytes by inducing of ER stress.

Beneficial effects of fibroblast growth factor 10 supplementation during in vitro maturation of buffalo cumulus-oocyte complexes

Fibroblast growth factor 10 (FGF10) is an important regulator of the mammalian cumulus-oocyte complex that plays a crucial role in oocyte maturation. In this study, we investigated the effects of FGF10 supplementation on the in vitro maturation (IVM) of buffalo oocytes and its related mechanisms. During IVM, the maturation medium was supplemented with a range of concentrations of FGF10 (0, 0.5, 5, and 50 ng/mL) and the resulting effects were corroborated using aceto-orcein staining, TUNEL apoptosis assay, detection of Cdc2/Cdk1 kinase in oocytes, and real-time quantitative PCR. In matured oocytes, the 5 ng/mL-FGF10 treatment resulted in a significantly increased nuclear maturation rate, which increased the activity of maturation-promoting factor (MPF) and enhanced buffalo oocyte maturation. Furthermore, it treatment significantly inhibited the apoptosis of cumulus cells, while simultaneously promoting its proliferation and expansion. This treatment also increased the absorption of glucose in cumulus cells. Thus, our results indicate that adding an appropriate concentration of FGF10 to a maturation medium during IVM can be beneficial to the maturation of buffalo oocytes and improve the potential of embryo development.

The coasting time affects the quality of cumulus-oocyte complexes in superstimulated ewes

We hypothesized that the coasting time may be beneficial to the quality of cumulus-oocyte complexes recovered from live ewes, as reported in cattle. The present study assessed the effect of coasting times on the quantity and quality of cumulus-oocyte complexes (COCs) in sheep. All ewes were subjected to the "Day 0 protocol", followed by an ovarian stimulation (80 mg of pFSH in three decreasing doses), varying only the coasting time [12 (G12), 36 (G36), or 60 h (G60]. In Experiment 1, data regarding follicular population was assessed. In Experiment 2, the COC quality was checked by their morphology, brilliant cresyl blue (BCB) test, evaluation of chromatin condensation pattern, and oocyte diameter. In Experiment 3, genes related to oocyte developmental competence were evaluated in BCB + COCs. The oocytes in the G60 group had more (P < 0.05) large follicles than the other groups and oocytes with a greater diameter than the G12. Oocyte morphology was similar (P > 0.05) among groups, as well as the BCB + COCs quantity. The G60-oocytes presented a better (P < 0.05) configuration of chromatin condensation compared with the other groups and a greater (P < 0.05) gene expression of BMP15, MATER, ZAR1, and PTGS2 compared with G12, and PTGS2 and HAS2 compared with G36 group. In conclusion, 60 h of coasting time positively affects the quality of COCs recovered after subjecting ewes to the "Day 0 protocol" and ovarian superstimulation. Implementing the appropriate coasting time to a given protocol can positively impact the in vitro embryo production outcomes in sheep.

Cleavage kinetics is a better indicator of embryonic developmental competency than brilliant cresyl blue staining of oocytes

In vitro production of embryos (IVP) is a valuable technology to produce embryos of high genetic value. Despite advances in IVP, the efficiency of culture systems remains low. One method to increase IVP success is the early selection of oocytes or embryos that may have greater developmental potential. Here, we investigated two methods of selection, namely BCB staining and cleavage kinetics, both individually and in conjunction, for improved developmental outcomes in vitro. We hypothesized that a synergistic use of both BCB staining and cleavage kinetics would result in identification of embryos of greater developmental potential. The selection of oocytes by BCB staining does select for those oocytes with higher developmental potential, as noted by a greater blastocyst development between BCB positive (32.6%) and BCB negative (22.0%) on day 8 post-fertilization. However, the utilization of BCB staining and cleavage kinetics in tandem resulted in a complete masking of the effect observed when using BCB alone. We obtained the highest proportion of blastocyst development per selection group using cleavage kinetics alone, in which 53.1% of embryos grouped as Fast produced a blastocyst, which was significantly different from the three other groups (Fast+, Slow, not cleaved). We observed, however, that the separation of embryos by cleavage kinetics did not predict their survival to cryopreservation. In conclusion, in standard culture systems, cleavage kinetics is an effective method for the selection of embryos with increased developmental potential to develop blastocysts, however, it may not be effective to select healthy embryos for transfer following cryopreservation.

Rotenone causes mitochondrial dysfunction and prevents maturation in porcine oocytes

Rotenone is a commonly used insecticidal chemical in agriculture and it is an inhibitor of mitochondrial complex Ⅰ. Previous studies have found that rotenone induces the production of reactive oxygen species (ROS) by inhibiting electron transport in the mitochondria of somatic and germ cells. However, there is little precise information on the effects of rotenone exposure in porcine oocytes during in vitro maturation, and the mechanisms underlying these effects have not been determined. The Cumulus-oocyte complexes were supplemented with different concentrations of rotenone to elucidate the effects of rotenone exposure on the meiotic maturation of porcine oocytes during in vitro maturation for about 48 hours. First, we found that the maturation rate and expansion of cumulus cells were significantly reduced in the 3 and 5 μM rotenone-treated groups. Subsequently, the concentration of rotenone was determined to be 3 μM. Also, immunofluorescence, western blotting, and image quantification analyses were performed to test the rotenone exposure on the meiotic maturation, total and mitochondrial ROS, mitochondrial function and biogenesis, mitophagy and apoptosis in porcine oocytes. Further experiments showed that rotenone treatment induced mitochondrial dysfunction and failure of mitochondrial biogenesis by repressing the level of SIRT1 during in vitro maturation of porcine oocytes. In addition, rotenone treatment reduced the ratio of active mitochondria to total mitochondria, increased ROS production, and decreased ATP production. The levels of LC3 and active-caspase 3 were significantly increased by rotenone treatment, indicating that mitochondrial dysfunction induced by rotenone increased mitophagy but eventually led to apoptosis. Collectively, these results suggest that rotenone interferes with porcine oocyte maturation by inhibiting mitochondrial function.

High expression of CFTR in cumulus cells from mature oocytes is associated with high-quality of oocyte and subsequent embryonic development

OBJECTIVE

The purpose of this study was to explore the association of expression of cystic fibrosis transmembrane conductance regulator (CFTR) in cumulus cells (CCs) from mature oocytes with oocyte quality and embryonic development.

METHODS

A total of 338 infertile women who underwent ovarian stimulation cycle of oocyte retrieval in Zhejiang University School of Medicine were retrospectively enrolled in this study. The relative mRNA expression levels of CFTR, bone morphogenetic protein 15 (BMP15), and growth differentiation factor 9 (GDF9) in CCs were detected by qPCR technology. ROC curve was applied for the diagnosis of oocyte maturation. The serum levels of anti-Müllerian hormone (AMH), E2, follicle-stimulating hormone (FSH), luteinizing hormone (LH), and androstenedione were measured. Oocyte maturation rate, fertilization rate, cleavage rate, high-quality embryo formation rate, and implantation rate after embryo transfer were also determined.

RESULTS

The mRNA expression levels of CFTR in CCs were significantly increased in metaphase II (MII) oocytes compared to that in metaphase I (MI) or germinal vesicle (GV) oocytes. The ROC curve analysis illustrated that CFTR mRNA expression could efficiently discriminate MII oocytes from MI or GV oocytes (AUC = 0.954), and revealed that 0.695 RQU is the optimal cut-off value for diagnosis. So the cut-off value of 2-ΔΔCT = 0.70 was used to divide the patients into two groups: low- (n = 114) and high-CFTR group (n = 224). The mRNA expression of CFTR in CCs was positively correlated with the antral follicular count (AFC), number of oocytes retrieved, number of MII oocytes, serum E2 level on hCG day, and BMP15 and GDF9 expression in CCs. Under continuous stimulation with the same dose of recombinant follicle-stimulating hormone (rFSH), the number of follicles, average recovered oocytes, recovered oocytes, MII oocytes, as well as the oocyte recovery rate, fertilization rate, oocyte cleavage rate, high-quality embryo formation rate, and implantation rate were decreased in patients with lower CFTR.

CONCLUSIONS

This study suggests that CFTR expression in CCs is associated with the developmental potential of human oocytes.

N-Acetyl cysteine reduces the levels of reactive oxygen species and improves in vitro maturation of oocytes from medium-sized bovine antral follicles

This study aims to evaluate the effects of N-acetylcysteine (NAC) on bovine oocyte maturation, mitochondrial activity and transzonal projections (TZP), as well as on the levels of reactive oxygen species (ROS) and messenger RNA (mRNA) for catalase (CAT) superoxide dismutase (SOD), periredoxin-6 (Prdx6), glutathione peroxidase (GPx), growth and differentiation factor-9 (GDF9), histone H1Foo, cyclin B1 (CCNB1) and c-Mos. Bovine cumulus-oocyte complexes (COC) of medium-sized antral follicles (3.0-6.0 mm) were prematured in TCM-199 for 8 h at 38.5°C in 5% CO₂. After prematuration in the presence of forskolin and C-type natriuretic peptide, COCs were matured in TCM-199 alone or with 0.1, 0.5 or 2.5 mM NAC. Then, oocytes were classified according to the stage of chromatin. Furthermore, mitochondrial activity and intracellular levels of ROS and TZP were also evaluated. The levels of mRNAs for CAT, SOD, Prdx6, GPx, GDF9, H1Foo, CCNB1 and c-Mos were evaluated using real-time polymerase chain reaction (RT-PCR). The results showed that NAC significantly increased the percentages of oocytes with resumption of meiosis when compared with those oocytes matured in control medium. Oocytes had homogeneous mitochondrial distribution, and those cultured with 0.1 and 0.5 mM NAC had lower levels of ROS when compared with the control. In addition, 0.5 mM NAC reduced TZP and the levels of mRNA for CCNB1. In contrast, NAC did not influence the expression of CAT, GPx, Prdx6, SOD, GDF9, H1Foo, and c-Mos. In conclusion, 0.5 mM NAC reduced the levels of ROS, TZP and mRNA for CCNB1, and improved in vitro resumption of meiosis in oocytes from medium-sized bovine antral follicles.

Importance of Antioxidant Supplementation during In Vitro Maturation of Mammalian Oocytes

The in vitro embryo production (IVEP) technique is widely used in the field of reproductive biology. In vitro maturation (IVM) is the first and most critical step of IVEP, during which, the oocyte is matured in an artificial maturation medium under strict laboratory conditions. Despite all of the progress in the field of IVEP, the quality of in vitro matured oocytes remains inferior to that of those matured in vivo. The accumulation of substantial amounts of reactive oxygen species (ROS) within oocytes during IVM has been regarded as one of the main factors altering oocyte quality. One of the most promising approaches to overcome ROS accumulation within oocytes is the supplementation of oocyte IVM medium with antioxidants. In this article, we discuss recent advancements depicting the adverse effects of ROS on mammalian oocytes. We also discuss the potential use of antioxidants and their effect on both oocyte quality and IVM rate.

Comparing the effects of a commercial and a prototype vitrification medium on meiotic spindle morphology and survival rate of mouse oocytes

OBJECTIVE

To compare oocyte survival and meiotic spindle normality between vitrified-warmed oocytes in a mouse embryo assay using Tvitri-4 or Ingámed vitrification media.

METHODS

C57BL/6 female mice aged 8-12 weeks were submitted to superovulation with pregnant mare's serum gonadotropin and human chorionic gonadotropin (hCG) for obtaining of in vivo matured oocytes. The oocytes were randomly distributed into one of the following three groups: CTR - control (fresh oocytes); ING - oocytes vitrified-warmed in a standard commercial kit supplied by Ingámed, and T4 - oocytes vitrified-warmed in the novel prototype Tvitri-4 medium. After warming and recovery culture, oocytes were assessed with respect to survival rate (SR) and both meiotic spindle morphology and chromosome alignment of each oocyte fixed in the sagittal position after immunostaining and analysis by confocal microscopy.

RESULTS

A total of 354 mature oocytes were vitrified in ING (n=178) and T4 (n=176), out of which 299 (85%) survived after warming. Oocyte survival rates were not statistically different (p=0.08) between ING (145/178=81.5%) and T4 (154/176=87.5%). Regarding meiotic normality, there were no significant changes in the proportion of oocytes with normal meiotic spindle morphology and chromosome structure between ING (52,2%) and T4 (63.4%) after warming (RR: 0.95, 95% CI: 0.92-1.607). When the meiotic normality was assessed using the CTR group as a reference in the analysis of relative risk, no significant differences were observed between T4 (63.4%) and CTR (70.5%) (RR: 0.95, 95% CI: 0.72-1.12). On the other hand, the percentage of oocytes retaining normal meiotic spindle morphology and chromosome configuration in ING (52.2%) was lower than in the CTR group (RR: 0.95, 95% CI: 0.57-0.97).

CONCLUSIONS

The novel prototype Tvitri-4 medium was efficient in preserve survival rate and meiotic spindle normality of oocytes and, with further verification, may be able to replace commercially available media in future clinical applications.

Glyphosate exposure deteriorates oocyte meiotic maturation via induction of organelle dysfunctions in pigs

Background

Recently, defects in mammalian oocytes maturation induced by environmental pollution results in the decreasing animal reproduction. Animal exposed to glyphosate is largely unavoidable because glyphosate is one of the most widely used herbicide worldwide due to its high-efficiency and broad-spectrum effects, which causes glyphosate an environmental contaminant found in soil, water and food. During the last few years, the growing and wider use of glyphosate has raised great concerns about its effects of reproductive toxicity. In this study, using porcine models, we investigated effects of glyphosate on organelle functions during oocyte meiosis.

Results

The results showed glyphosate exposure disrupted porcine oocyte maturation. Expression levels of cumulus expansion-related genes were interfered, further indicating the meiotic defects. The damaging effects were mediated by destruction of mitochondrial distribution and functions, which induced ROS accumulation and oxidative stress, also indicated by the decreased mRNA expression of related antioxidant enzyme genes. We also found an interference of endoplasmic reticulum (ER) distribution, disturbance of Ca²⁺ homeostasis, as well as fluctuation of ER stress, showing with the reduced ER stress-related mRNA or protein expression, which could indicate the dysfunction of ER for protein processing and signal transduction in glyphosate-exposed oocytes. Moreover, glyphosate exposure induced the disruption of lysosome function for autophagy, showing with the decrease of LAMP2 expression and autophagy-related genes mRNA expression. Additionally, our data showed the distribution of Golgi apparatus and the functions of ribosome were disturbed after glyphosate exposure, which might affect protein synthesis and transport.

Conclusions

Collectively, our study showed that exposed to glyphosate could affect animal reproduction by compromising the quality of oocytes through its wide toxic effects on organelle functions.

Comparing the IVM laboratory outcomes between stimulated IVF with unstimulated natural cycles

Recently, more attention has been raised towards fertility preservation in women with cancer. One option is in vitro maturation (IVM) of the immature oocytes as there is not enough time for induction of an ovarian stimulation protocol. The aim was to compare the IVM laboratory outcomes between stimulated and unstimulated (natural) in vitro fertilization (IVF) cycles. In total, 234 immature oocytes collected from 15 cancer patients who underwent an IVM programme (natural IVM) and 23 IVF cycles with a controlled ovarian hyperstimulation protocol (stimulated IVM) were analyzed. The oocyte morphology, zona pellucida (ZP), and meiotic spindle presence were measured using PolScope technology. Also, the rates of oocyte maturation and fertilization were assessed in both groups. The IVM rate was higher in the stimulated cycle (P < 0.05), but the fertilization rate was insignificant in comparison with unstimulated cycles. There were no significant differences in the spindle visualization and ZP birefringence scoring between the groups (P > 0.05). The oocyte normal morphology was better in the stimulated cycle compared with the natural cycle (P < 0.05). In conclusion, IVM can be recommended for cancer patients as an alternative treatment when there is insufficient time for conventional IVF before chemotherapy initiation.

Effect of Passage Number of Conditioned Medium Collected from Equine Amniotic Fluid Mesenchymal Stem Cells: Porcine Oocyte Maturation and Embryo Development

Oocyte in vitro maturation (IVM) is the most important first step in in vitro embryo production. One prerequisite for the success of IVM in oocytes is to provide a rich culture microenvironment that meets the nutritional needs of developing oocytes. We applied different equine amniotic fluid mesenchymal stem cell conditioned medium (eAFMSC-CM) from passages 7, 18, and 27 to porcine oocytes during IVM to determine its effects on oocyte development and subsequent embryo development, specifically. The eAFMSC-CM from passage 7 (eAFMSC-CMp7) has a considerable impact on 9 genes: BAX, BCL2, SOD2, NRF2, TNFAIP6, PTGS2, HAS2, Cx37, and Cx43, which are associated with cumulus cell mediated oocyte maturation. GSH levels and distribution of mitochondrial and cortical granules were significantly increased in oocytes incubated with eAFMSC-CMp7. In addition, catalase and superoxide dismutase activities were high after IVM 44 h with eAFMSC-CMp7. After in vitro fertilization, blastocyst quality was significantly increased in the eAFMSC-CMp7 group compared to control. Lastly, the antioxidant effect of eAFMSC-CMp7 substantially regulated the expression of apoptosis, pluripotency related genes and decreased autophagy activity in blastocysts. Taken together, this study demonstrated that the eAFMSC-CMp7 enhanced the cytoplasmic maturation of oocytes and subsequent embryonic development by generating high antioxidant activity.

Bovine In Vitro Oocyte Maturation and Embryo Production Used as a Model for Testing Endocrine Disrupting Chemicals Eliciting Female Reproductive Toxicity With Diethylstilbestrol as a Showcase Compound

Endocrine disrupting chemicals (EDCs) can interfere with normal hormonal action and regulation. Exposure of women to EDCs has been associated with adverse reproductive health outcomes. The assays currently used to identify EDCs that elicit female reproductive toxicity lack screening tests that address effects on the maturation of oocytes, a process that enables them to be fertilized and develop into embryos. Here, a screening method employing the bovine model of in vitro oocyte maturation and embryo production is described. Endpoints explored address important events in oocyte maturation and developmental competence acquisition. To test the method, the effects of the known human EDC diethylstilbestrol (DES; an estrogen receptor agonist) were evaluated in a range of concentrations (10^–9 M, 10^–7 M, 10^–5 M). Bovine oocytes were exposed to DES during in vitro maturation (IVM) or embryos were exposed during in vitro embryo culture (IVC). The endpoints evaluated included nuclear maturation, mitochondrial redistribution, cumulus cell expansion, apoptosis, and steroidogenesis. DES-exposed oocytes were fertilized to record embryo cleavage and blastocyst rates to uncover effects on developmental competence. Similarly, the development of embryos exposed to DES during IVC was monitored to assess the impact on early embryo development. Exposure to 10^–9 M or 10^–7 M DES did not affect the endpoints addressing oocyte maturation or embryo development. However, there were considerable detrimental effects observed in oocytes exposed to 10^–5 M DES. Specifically, compared to vehicle-treated oocytes, there was a statistically significant reduction in nuclear maturation (3% vs 84%), cumulus expansion (2.8-fold vs 3.6-fold) and blastocyst rate (3% vs 32%). Additionally, progesterone and pregnenolone concentrations measured in IVM culture media were increased. The screening method described here shows that bovine oocytes were sensitive to the action of this particular chemical (i.e., DES), albeit at high concentrations. In principle, this method provides a valuable tool to assess the oocyte maturation process and early embryo development that can be used for reproductive toxicity screening and possibly EDC identification. Further studies should include EDCs with different mechanisms of action and additional endpoints to further demonstrate the applicability of the bovine oocyte model for chemical risk assessment purposes and EDC identification.

Aberrant Expression of Mitochondrial SAM Transporter SLC25A26 Impairs Oocyte Maturation and Early Development in Mice

The immature germinal vesicle (GV) oocytes proceed through metaphase I (MI) division, extrude the first polar body, and become mature metaphase II (MII) oocytes for fertilization which is followed by preimplantation and postimplantation development until birth. Slc25a26 is the gene encoding S-adenosylmethionine carrier (SAMC), a member of the mitochondrial carrier family. Its major function is to catalyze the uptake of S-adenosylmethionine (SAM) from cytosol into mitochondria, which is the only known mitochondrial SAM transporter. In the present study, we demonstrated that excessive SLC25A26 accumulation in mouse oocytes mimicked naturally aged oocytes and resulted in lower oocyte quality with decreased maturation rate and increased reactive oxygen species (ROS) by impairing mitochondrial function. Increased level of Slc25a26 gene impacted gene expression in mouse oocytes such as mt-Cytb which regulates mitochondrial respiratory chain. Furthermore, increased level of Slc25a26 gene in fertilized oocytes slightly compromised blastocyst formation, and Slc25a26 knockout mice displayed embryonic lethality around 10.5 dpc. Taken together, our results showed that Slc25a26 gene plays a critical role in oocyte maturation and early mouse development.

α-Ketoglutarate Improves Meiotic Maturation of Porcine Oocytes and Promotes the Development of PA Embryos, Potentially by Reducing Oxidative Stress through the Nrf2 Pathway

α-Ketoglutarate (α-KG) is a metabolite in the tricarboxylic acid cycle. It has a strong antioxidant function and can effectively prevent oxidative damage. Previous studies have shown that α-KG exists in porcine follicles, and its content gradually increases as the follicles grow and mature. However, the potential mechanism of supplementation of α-KG on porcine oocytes during in vitro maturation (IVM) has not yet been reported. The purpose of this study was to explore the effect of α-KG on the early embryonic development of pigs and the mechanisms underlying these effects. We found that α-KG can enhance the development of early pig embryos. Adding 20 μM α-KG to the in vitro culture medium significantly increased the rate of blastocyst formation and the total cell number. Compared with to that of the control group, apoptosis in blastocysts of the supplement group was significantly reduced. α-KG reduced the production of reactive oxygen species and glutathione levels in cells. α-KG not only improved the activity of mitochondria but also inhibited the occurrence of apoptosis. After supplementation with α-KG, pig embryo pluripotency-related genes (OCT4, NANOG, and SOX2) and antiapoptotic genes (Bcl2) were upregulated. In terms of mechanism, α-KG activates the Nrf2/ARE signaling pathway to regulate the expression of antioxidant-related targets, thus combating oxidative stress during the in vitro culture of oocytes. Activated Nrf2 promotes the transcription of Bcl2 genes and inhibits cell apoptosis. These results indicate that α-KG supplements have a beneficial effect on IVM by regulating oxidative stress during the IVM of porcine oocytes and can be used as a potential antioxidant for IVM of porcine oocytes.

Biochemical changes in the cytoplasm of bovine oocytes during the in vitro maturation process: a Raman microscopy study

The sequence and chronology of the main biochemical changes occurring in the cytoplasm of bovine oocytes during the in vitro maturation process were tracked by Raman microscopy applied to cells previously subjected to enzymatic digestion of the zona pellucida. Specific spectral markers for proteins, lipids and carbohydrates were used to evaluate the developmental status of the ooplasm at four different times. Spectral changes revealed that lipid accumulation was dominant during the first six hours of culture while protein content reached the average levels characteristic of mature oocytes within the last four hours of the maturation process. A time-dependent decrease in carbohydrates was also observed. Finally, the carbohydrate-to-protein (P₁₀₃₇/P₁₀₀₂) ratio proved to be sensitive enough to determine the cytoplasmic maturation state of bovine oocytes and promises to be useful in future research aimed at optimizing culture conditions through the promotion of protein accumulation in the ooplasm.

Effects of electromagnetic waves on oocyte maturation and embryonic development in pigs

Our living environment has been full of electromagnetic radiation (EMR) due to the prevailing electronic devices and equipment. Intermediate frequency electromagnetic field (IF-EMF) or waves constitute a significant part of EMR; therefore, an increasing number of household electrical appliances have become a source of IF-EMF, and concerns about IF-EMF on health are gaining more attention. However, little information is available about its impact on female reproductive traits, such as germ cell viability and early embryonic development, particularly at the cellular and molecular levels. In this study, we used porcine oocytes as a model system to explore the effect of IF-EMF at various intensities on the in vitro maturation (IVM) of oocytes and their subsequent embryonic development. Our results showed that no difference in oocyte maturation rates was detected among groups, but the cleavage and blastocyst rates of parthenotes derived from EMF-treated oocytes decreased with the weaker IF-EMF intensity (25 and 50 Gauss, G) groups compared to the control group (P < 0.05). For cytoplasmic maturation, the weaker IF-EMF intensity groups also showed a peripheral pattern of mitochondrial distribution resembling that of immature oocytes and increased autophagy activity. No obvious differences in cytoskeletal distribution and total cell numbers of blastocysts were investigated in the four IF-EMF treatments compared to those in the control group. Although the underlying mechanism associated with EMF effects on oocytes and embryos is still elusive, we have demonstrated that low intensity IF-EMF exerts harmful effects on porcine oocytes during the maturation stage, carrying over such effects to their subsequent embryonic development.

In Vitro Maturation of Oocytes Retrieved from Ovarian Tissue: Outcomes from Current Approaches and Future Perspectives

In vitro maturation (IVM) of transvaginally aspirated immature oocytes is an effective and safe assisted reproductive treatment for predicted or high responder patients. Currently, immature oocytes are also being collected from the contralateral ovary during laparoscopy/laparotomy and even ex vivo from the excised ovary or the spent media during ovarian tissue preparation prior to ovarian cortex cryopreservation. The first live births from in vitro-matured ovarian tissue oocytes (OTO-IVM) were reported after monophasic OTO-IVM, showing the ability to achieve mature OTO-IVM oocytes. However, fertilisations rates and further embryological developmental capacity appeared impaired. The introduction of a biphasic IVM, also called capacitation (CAPA)-IVM, has been a significant improvement of the oocytes maturation protocol. However, evidence on OTO-IVM is still scarce and validation of the first results is of utmost importance to confirm reproducibility, including the follow-up of OTO-IVM children. Differences between IVM and OTO-IVM should be well understood to provide realistic expectations to patients.

The role of L-carnitine in bovine embryo metabolism. A review of the effect of supplementation with a metabolic modulator on in vitro embryo production

Early embryo development is driven first by the maternal RNAs and proteins accumulated during the oocyte's cytoplasmic maturation and then after the embryo genome activation. In mammalian cells, ATP generation occurs via oxidative pathways or by glycolysis, whereas in embryonic stem cells, the consumption of glucose, pyruvate, lipids, and amino acids results in ATP synthesis. Although the bovine embryo has energy reserves in glycogen and lipids, the glycogen concentration is deficient. Conversely, lipids represent the most abundant energy reservoir of bovine embryos, where lipid droplets-containing triacylglycerols are the main fatty acid stores. Oocytes of many mammalian species contain comparatively high amounts of lipids stored as droplets in the ooplasm. L-carnitine has been described as a cofactor that facilitates the mobilization of fatty acids present in the oocyte's cytoplasm into the mitochondria to facilitate β-oxidation processes. However, the L-carnitine effects by addition to media in the in vitro produced embryos on the quality are highly disputed and contradictory by different researchers. This review's objective was to explore the effect that the addition of L-carnitine on culture media could have on the overall bovine embryo production in vitro, from the oocyte metabolism to the modulation of gene expression in the developing embryos.

Differential Transcript Profiles in Cumulus-Oocyte Complexes Originating from Pre-Ovulatory Follicles of Varied Physiological Maturity in Beef Cows

Small dominant follicle diameter at induced ovulation, but not at spontaneous ovulation, decreased pregnancy rate, fertilization rate, and day seven embryo quality in beef cows. We hypothesized that the physiological status of the follicle at GnRH-induced ovulation has a direct effect on the transcriptome of the Cumulus-Oocyte complex, thereby affecting oocyte competence and subsequent embryo development. The objective of this study was to determine if the transcriptome of oocytes and associated cumulus cells (CC) differed among small (≤11.7 mm) and large follicles (≥12.7 mm) exposed to a GnRH-induced gonadotropin surge and follicles (11.7–14.0 mm) exposed to an endogenous gonadotropin surge (spontaneous follicles). RNA sequencing data, from pools of four oocytes or their corresponding CC, revealed 69, 94, and 83 differentially expressed gene transcripts (DEG) among oocyte pools from small versus large, small versus spontaneous, and large versus spontaneous follicle classifications, respectively. An additional 128, 98, and 80 DEG were identified among small versus large, small versus spontaneous, and large versus spontaneous follicle CC pools, respectively. The biological pathway “oxidative phosphorylation” was significantly enriched with DEG from small versus spontaneous follicle oocyte pools (FDR < 0.01); whereas the glycolytic pathway was significantly enriched with DEG from CC pools obtained from large versus small follicles (FDR < 0.01). These findings collectively suggest that altered carbohydrate metabolism within the Cumulus-Oocyte complex likely contributes to the decreased competency of oocytes from small pre-ovulatory follicles exposed to an exogenous GnRH-induced gonadotropin surge.

Conditions to optimise the developmental competence of immature equine oocytes

Optimising the developmental potential of immature equine oocytes and invitro-produced (IVP) embryos was explored through modifications of established media and holding temperature. In Experiment 1, delaying spontaneous resumption of meiosis through the process of simulated physiological oocyte maturation with the addition of the adenylate cyclase activator forskolin (50µM) and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (100µM) to overnight holding medium before maturation improved blastocyst production (P<0.05). In Experiment 2, the blastocyst production rate was increased significantly when cumulin (100ng mL-1) was added to the overnight holding or culture media (P<0.05). In Experiment 3, immature oocytes held overnight at 16°C before maturation had improved developmental competence than those held at 20°C and 5°C (P<0.05). There was no difference between maturation rates, but blastocyst formation per cleaved oocyte was significantly greater in oocytes held overnight at 16°C than at 20°C or 5°C. Furthermore, blastocyst formation per recovered oocyte and per fertilised oocyte was greater when oocytes were held before maturation at 16°C than at 5°C (P<0.05). In Experiment 4, the addition of sodium ascorbate (AC; 50µg mL-1) to the maturation and/or culture media of oocytes and IVP embryos did not improve blastocyst production, but did appear to lower cleavage rates compared with oocytes and embryos cultured without AC.

Effect of zinc chloride and sodium selenite supplementation on in vitro maturation, oxidative biomarkers, and gene expression in buffalo (Bubalus bubalis) oocytes

This study examined the effects of zinc chloride (ZnCl2) and sodium selenite (Na2SeO3) supplementation in maturation medium on in vitro maturation (IVM) rate, oxidative biomarkers and gene expression in buffalo oocytes. Ovaries from a slaughterhouse were aspirated and good quality cumulus-oocyte complexes (COCs) with at least four layers of compact cumulus cells and evenly granulated dark ooplasm were selected. COCs were randomly allocated during IVM (22 h) to one of four treatment groups: (1) control maturation medium (basic medium), or basic medium supplemented with (2) ZnCl2 (1.5 µg/ml), (3) Na2SeO3 (5 µg/l), or (4) ZnCl2 + Na2SeO3 (1.5 µg/ml + 5 µg/l, respectively). Oocytes were denuded after 22 h of IVM in the first four replicates. Specimens were fixed and stained to evaluate the stage of nuclear maturation. The spent medium was collected for biochemical assays of total antioxidant capacity (TAC), malondialdehyde (MDA) and hydrogen peroxide concentrations. A second four replicates were used for COCs for RNA extraction. The expression levels of antioxidant (SOD1, GPX4, CAT and PRDX1), antiapoptotic (BCL2 and BCL-XL) and proapoptotic (BAX and BID) genes were measured. Supplementation with ZnCl2 and Na2SeO3 during IVM increased the ratio of oocytes reaching metaphase II at 22 h, increased TAC and decreased MDA and H2O2 concentrations in the maturation medium (P < 0.05). Moreover, beneficial effects were associated with complementary changes in expression patterns of antioxidative, antiapoptotic and proapoptotic genes, suggesting lower oxidative stress and apoptosis. Supplementation medium with zinc chloride and sodium selenite improves the maturation rate, reduces oxidative stress and increases expression levels of antioxidative and antiapoptotic genes.

Treatment of in vitro-Matured Bovine Oocytes With Tauroursodeoxycholic Acid Modulates the Oxidative Stress Signaling Pathway

In several species, oocyte and embryo competence are improved by the addition of endoplasmic reticulum (ER) stress inhibitors to in vitro maturation (IVM) medium and/or in vitro culture (IVC) medium. This study aimed to evaluate the effects of three concentrations of tauroursodeoxycholic acid (TUDCA; 50, 200, and 1,000 μM), a chemical chaperone for relieving ER stress, during IVM of bovine cumulus-oocyte complexes (COCs) for 24 h. Treated oocytes were analyzed for nuclear maturation, reactive oxygen species (ROS) production, mitochondrial activity, and abundance of target transcripts. In addition, the number of pronuclei in oocytes was evaluated after 18-20 h of insemination, and the rates of blastocyst and hatched blastocyst formation were evaluated after 7 and 8/9 days of culture, respectively. We further evaluated the transcript abundance of embryonic quality markers. Our findings showed that supplementation of IVM medium with 200 μM of TUDCA decreased ROS production and increased abundance of transcripts related to antioxidant activity in oocytes (CAT, GPX1, and HMOX1) and embryos (GPX1 and PRDX3). Interestingly, high concentration of TUDCA (1,000 μM) was toxic to oocytes, reducing the nuclear maturation rate, decreasing mitochondrial activity, and increasing the abundance of ER stress (HSPA5) and cellular apoptosis (CASP3 and CD40) related transcripts. The results of this study suggest that treatment with 200 μM of TUDCA is associated with a greater resistance to oxidative stress and indirectly with ER stress relief in bovine oocytes.

Paraquat Reduces the Female Fertility by Impairing the Oocyte Maturation in Mice

Paraquat (PQ) is a widely used non-selective and oxidizing herbicide in farmland, orchards, flower nursery, and grassland. Overuse of PQ will accumulate in the body and affect the reproduction in mammals. In this study, we found that PQ could reduce the female fertility by oral administration for 21 days in mice. PQ exposure could impair the nuclear maturation by perturbing the spindle assembly and kinetochore–microtubule attachment to cause the misaligned chromosomes during meiosis. In the meantime, PQ exposure disturbed the mitochondrial distribution and enhanced the level of reactive oxygen species and early apoptosis, which thereby deteriorated the early embryo development. Also, PQ administration could cause some changes in epigenetic modifications such as the level of H3K9me2 and H3K27me3. Therefore, PQ administration reduces the female fertility by impairing the nuclear and cytoplasmic maturation of oocytes in mice.

Heat stress reduces maturation and developmental capacity in bovine oocytes

The ovarian pool of follicles, and their enclosed oocytes, is highly sensitive to hyperthermia. Heat-induced changes in small antral follicles can later manifest as impaired follicle development and compromised competence of the enclosed oocytes to undergo maturation, fertilisation and further development into an embryo. This review describes the main changes documented so far that underlie the oocyte damage. The review discusses some cellular and molecular mechanisms by which heat stress compromises oocyte developmental competence, such as impairment of nuclear and cytoplasmic maturation and mitochondrial function, changes in the expression of both nuclear and mitochondrial transcripts and the induction of apoptosis. The review emphasises that although the oocyte is exposed to heat stress, changes are also evident in the developed embryo. Moreover, the effect of heat stress is not limited to the summer; it carries over to the cold autumn, as manifest by impaired steroid production, low oocyte competence and reduced fertility. The spontaneous recovery of oocytes from the end of the summer through the autumn until the beginning of winter suggests that only subpopulations of follicles, rather than the entire ovarian reserve, are damaged upon heat exposure.

Comparison of the Morphology and Developmental Potential of Oocytes Obtained from Prepubertal and Adult Domestic and Wild Cats

Simple Summary

This study was conducted with the aim of determining the morphological similarities and developmental potential of oocytes obtained from adult and prepubertal domestic cats (Felis catus) and wild cats. The results of our research showed that ovaries obtained from prepubertal felids may be a rich source of good quality oocytes that are competent for in vitro maturation and able to reach blastocyst stage after in vitro fertilization. The results are important in the context of the possibility of using oocytes from prepubertal felids threatened with extinction in conservation programs based on assisted reproductive techniques (ART).

Abstract

The aim of the study was to compare the morphology and developmental potential of oocytes obtained from adult and prepubertal domestic cats (Felis catus) and wild cats (Lynx lynx, Leptailurus serval, Felis manul, Panthera tigris altaica). The average number of oocytes obtained from an adult domestic cat was 23 ± 11, which was significantly lower than from kittens (43 ± 29). A similar number of oocytes was derived from adult Pallas’s cats (28 ± 8), and serval (30). The lowest number of oocytes was collected from the lynx (5 ± 3). No oocytes were obtained from newborn Amur tiger while in the case of older domestic and Pallas’s cat and lynx kittens (1–3 months) 43, 48 and 41 oocytes were collected, respectively. Significant differences (p < 0.001) were observed between the number of oocytes with dark cytoplasm from adult and prepubertal animals of all analyzed species. The diameter of oocytes from adult and prepubertal animals was similar in all species, and was on average 161 ± 4 µm for oocytes with dark cytoplasm and 150 ± 18 µm for oocytes with light cytoplasm. In all species, oocytes with light cytoplasm were significantly smaller (p < 0.05) than dark ones, and their population was more diverse. Results of in vitro maturation of the domestic and wild cat′s oocytes obtained from adult and prepubertal females were similar (47–52%). The cleavage rate after in vitro fertilization (IVF) was lower for prepubertal than adult domestic cats (42 vs. 51%; p < 0.05%). Moreover, we observed differences in the quantity (28 vs. 39%; p < 0.05) and quality of blastocysts and even greater problems with hatching blastocysts from prepubertal kittens (8 vs. 19%; p < 0.001). More blastomeres were detected in blastocysts of adult cats. They also demonstrated significantly higher number of inner cell mass (ICM) (p < 0.001) and higher number of trophoblast cells (TE) (p < 0.05).