Simulated physiological oocyte maturation (SPOM): a novel in vitro maturation system that substantially improves embryo yield and pregnancy outcomes

Impact of the near-physiological temperature on the in vitro maturation of bovine oocytes: A comparative proteomic approach

In vivo, the temperature inside preovulatory follicles of cows is approximately 1 °C lower than rectal temperature. However, standard bovine oocyte in vitro maturation (IVM) protocols use 38.5 °C based on rectal temperature. This study evaluated the effect of reducing IVM temperature to 37.5 °C on the proteomic profile of oocytes compared to the routine 38.5 °C. Nuclear maturation rate and cumulus cell (CC) expansion (30 COCs per group, 21 replicates) were assessed by observing the first polar body and using a subjective scoring method (0-4). Total nitrite concentrations in the culture medium were measured using the Griess method. Differential proteomics was performed using LC-MS/MS on pooled oocyte samples (500 matured oocytes per group, three replicates), followed by gene ontology enrichment, protein-protein interaction, and putative miRNA target analyses. No significant differences were observed between the groups in nuclear maturation, CC expansion, or nitrite concentration (P > 0.05). A total of 806 proteins were identified, with 7 up-regulated and 12 down-regulated in the treatment group compared to the control. Additionally, 12 proteins were unique to the control group, and 8 were unique to the treatment group. IVM at 37.5 °C resulted in the upregulation of proteins involved in protein folding and GTP binding, and the downregulation of enzymes with oxidoreductase activity and proteins involved in cytoskeletal fiber formation. Furthermore, 43 bovine miRNAs potentially regulating these genes (DES, HMOX2, KRT75, FARSA, IDH2, CARHSP1) were identified. We conclude that IVM of bovine oocytes at 37.5 °C induces significant proteomic changes without impacting nuclear maturation, cumulus cell expansion, or nitrite concentration in the IVM medium.

Recent advancements to increase success in assisted reproductive technologies in cattle

Assisted reproductive technologies (ART) are fundamental for cattle breeding and sustainable food production. Together with genomic selection, these technologies contribute to reducing the generation interval and accelerating genetic progress. In this paper, we discuss advancements in technologies used in the fertility evaluation of breeding animals, and the collection, processing, and preservation of the gametes. It is of utmost importance for the breeding industry to select dams and sires of the next generation as young as possible, as is the efficient and timely collection of gametes. There is a need for reliable and easily applicable methods to evaluate sexual maturity and fertility. Although gametes processing and preservation have been improved in recent decades, challenges are still encountered. The targeted use of sexed semen and beef semen has obliterated the production of surplus replacement heifers and bull calves from dairy breeds, markedly improving animal welfare and ethical considerations in production practices. Parallel with new technologies, many well-established technologies remain relevant, although with evolving applications. In vitro production (IVP) has become the predominant method of embryo production. Although fundamental improvements in IVP procedures have been established, the quality of IVP embryos remains inferior to their in vivo counterparts. Improvements to facilitate oocyte maturation and development of new culture systems, e.g. microfluidics, are presented in this paper. New non-invasive and objective tools are needed to select embryos for transfer. Cryopreservation of semen and embryos plays a pivotal role in the distribution of genetics, and we discuss the challenges and opportunities in this field. Finally, machine learning (ML) is gaining ground in agriculture and ART. This paper delves into the utilization of emerging technologies in ART, along with the current status, key challenges, and future prospects of ML in both research and practical applications within ART.

The Simulated Physiological Oocyte Maturation (SPOM) System Enhances Cytoplasmic Maturation and Oocyte Competence in Cattle

In vitro embryo production is a widely applied technique that allows the expansion of genetics and accelerated breeding programs. However, in cattle, this technique still needs improvement in order to reach quality and pregnancy rates comparable to in vivo-derived embryos. One of the limitations of this technique is related to in vitro maturation, where a heterogeneous population of oocytes is harvested from follicles and cultured in vitro in the presence of gonadotropic hormones to induce maturation. As a result, oocytes with different degrees of competence are obtained, resulting in a decrease in the quality and quantity of embryos obtained. A novel system based on the use of cyclic adenosine monophosphate (cAMP) modulators was developed to enhance bovine oocyte competence, although controversial results were obtained depending on the in vitro embryo production (IVP) system used in each laboratory. Thus, in the present work, we employed a reported cAMP protocol named Simulated Physiological Oocyte Maturation (SPOM) under our IVP system and analysed its effect on cytoplasmic maturation by measuring levels of stress-related genes and evaluating the activity and distribution of mitochondria as a marker for cytoplasmic maturation Moreover, we studied the effect of the cAMP treatment on nuclear maturation, cleavage, and blastocyst formation. Finally, we assessed the embryo quality by determining the hatching rates, total cell number per blastocyst, cryopreservation tolerance, and embryo implantation. We found that maturing oocytes in the presence of cAMP modulators did not affect nuclear maturation, although they changed the dynamic pattern of mitochondrial activity along maturation. Additionally, we found that oocytes subjected to cAMP modulators significantly improved blastocyst formation (15.5% vs. 22.2%, p < 0.05). Blastocysts derived from cAMP-treated oocytes did not improve cryopreservation tolerance but showed an increased hatching rate, a higher total cell number per blastocyst and, when transferred to hormonally synchronised recipients, produced pregnancies. These results reflect that the use of cAMP modulators during IVM results in competent oocytes that, after fertilisation, can develop in more blastocysts with a better quality than standard IVM conditions.

Improving the meiotic competence of small antral follicle-derived porcine oocytes by using dibutyryl-cAMP and melatonin

OBJECTIVE

We increased the nuclear maturation rate of antral follicle derived oocytes by using a pre-in vitro maturation (IVM) culture system and improved the developmental potential of these porcine pathenotes by supplementing with melatonin. Furthermore, we investigated the expression patterns of genes involved in cumulus expansion (HAS2, PTGS2, TNFAIP6, and PTX3) derived from small and medium antral follicles before and after oocyte maturation.

METHODS

Only the cumulus oocyte-complexes (COCs) derived from small antral follicles were induced with [Pre-SF(+)hCG] or without [Pre-SF(-)hCG] the addition of human chorionic gonadotropin (hCG) during the last 7 h of the pre-IVM period before undergoing the regular culture system. The mature oocytes were investigated on embryonic development after parthenogenetic activation (PA). Melatonin (10-7 M) was supplemented during in vitro culture (IVC) to improve the developmental potential of these porcine pathenotes.

RESULTS

A pre-IVM culture system with hCG added during the last 7 h of the pre-IVM period [Pre-SF(+)hCG] effectively supported small antral follicle-derived oocytes and increased their nuclear maturation rate. The oocytes derived from medium antral follicles exhibited the highest nuclear maturation rate in a regular culture system. Compared with oocytes cultured in a regular culture system, those cultured in the pre-IVM culture system exhibited considerable overexpression of HAS2, PTGS2, and TNFAIP6. Porcine embryos treated with melatonin during IVC exhibited markedly improved quality and developmental competence after PA. Notably, melatonin supplementation during the IVM period can reduce and increase the levels of intracellular reactive oxygen species (ROS) and glutathione (GSH), respectively.

CONCLUSION

Our findings indicate that the Pre-SF(+)hCG culture system increases the nuclear maturation rate of small antral follicle-derived oocytes and the expression of genes involved in cumulus expansion. Melatonin supplementation during IVC may improve the quality and increase the blastocyst formation rate of porcine embryos. In addition, it can reduce and increase the levels of ROS and GSH, respectively, in mature oocytes, thus affecting subsequent embryos.

Use of assisted reproductive technologies (ARTs) to shorten the generational interval in ruminants: current status and perspectives

The challenges posed by climate change and increasing world population are stimulating renewed efforts for improving the sustainability of animal production. To meet such challenges, the contribution of genomic selection approaches, in combination with assisted reproductive technologies (ARTs), to spreading and preserving animal genetics is essential. The largest increase in genetic gain can be achieved by shortening the generation interval. This review provides an overview of the current status and progress of advanced ARTs that could be applied to reduce the generation time in both female and male of domestic ruminants. In females, the use of juvenile in vitro embryo transfer (JIVET) enables to generate offspring after the transfer of in vitro produced embryos derived from oocytes of prepubertal genetically superior donors reducing the generational interval and acceleration genetic gain. The current challenge is increasing in vitro embryo production (IVEP) from prepubertal derived oocytes which is still low and variable. The two main factors limiting IVEP success are the intrinsic quality of prepubertal oocytes and the culture systems for in vitro maturation (IVM). In males, advancements in ARTs are providing new strategies to in vitro propagate spermatogonia and differentiate them into mature sperm or even to recapitulate the whole process of spermatogenesis from embryonic stem cells. Moreover, the successful use of immature cells, such as round spermatids, for intracytoplasmic injection (ROSI) and IVEP could allow to complete the entire process in few months. However, these approaches have been successfully applied to human and mouse whereas only a few studies have been published in ruminants and results are still controversial. This is also dependent on the efficiency of ROSI that is limited by the current isolation and selection protocols of round spermatids. In conclusion, the current efforts for improving these reproductive methodologies could lead toward a significant reduction of the generational interval in livestock animals that could have a considerable impact on agriculture sustainability.

Plasma-derived extracellular vesicles improve mice embryo development

BACKGROUND

To investigate the effect of plasma-derived extracellular vesicles (EVs) or conventional medium in fertilization and early embryo development rate in mice.

METHODS AND RESULTS

MII oocytes (matured in vivo or in vitro conditions) were obtained from female mice. The extracellular vesicles were isolated by ultracentrifugation of plasma and were analyzed and measured for size and morphology by dynamic light scattering (DLS) and transmission electron microscopy (TEM). By western blotting analysis, the EVs proteins markers such as CD82 protein and heat shock protein 90 (HSP90) were investigated. Incorporating DiI-labeled EVs within the oocyte cytoplasm was visible at 23 h in oocyte cytoplasm. Also, the effective proteins in the early reproductive process were determined in isolated EVs by western blotting. These EVs had a positive effect on the fertilization rate (P < 0.05). The early embryo development (8 cell, morula and blastocyst stages) was higher in groups supplemented with EVs (P < 0.01).

CONCLUSION

Our findings showed that supplementing in vitro maturation media with EVs derived- plasma was beneficial for mice's embryo development.

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.

Meiotic and developmental competence of growing pig oocytes derived from small antral follicles is enhanced in culture medium containing FGF2, LIF, and IGF1 (FLI medium)

BACKGROUND

Oocytes of large animal species isolated from small ovarian follicles (< 2 mm) are less competent to support early embryonic development after in vitro maturation and fertilization than their counterparts isolated from medium-sized and preovulatory follicles. This study aimed to assess the effect of a new maturation medium containing FGF2, LIF, and IGF1 (FLI medium) on the meiotic and developmental competence of pig cumulus-oocytes complexes (COCs) derived from the small and medium-sized follicles.

METHODS

The growing oocytes were isolated from 1 to 2 (small follicle; SF) and the fully-grown ones from 3 to 6 (large follicle; LF) mm follicles and matured in a control M199 medium with gonadotropins and EGF and the FLI medium enriched by the triplet of growth factors. The matured oocytes were parthenogenetically activated and cultured to the blastocyst stage. Chromatin configuration before and during the culture and MAP kinase activity were assessed in the oocytes. Finally, the expression of cumulus cell genes previously identified as markers of oocyte quality was assessed.

RESULTS

The maturation and blastocyst rates of oocytes gained from LF were significantly higher than that from SF in the control medium. In contrast, similar proportions of oocytes from LF and SF completed meiosis and developed to blastocysts when cultured in FLI. Most of the oocytes freshly isolated from SF possessed germinal vesicles with fine filaments of chromatin (GV0) or chromatin surrounding the nucleolus (GVI; 30%); the oocytes from LF were mainly in GVI (or GVII) exhibiting a few small lumps of chromatin beneath the nuclear membrane. When cultured in the FLI medium for 16 h, an acceleration of the course of maturation in oocytes both from SF and LF compared to the control medium was observed and a remarkable synchrony in the course of chromatin remodeling was noticed in oocytes from SF and LF.

CONCLUSIONS

This work demonstrates that the enrichment of culture medium by FGF2, LIF, and IGF1 can enhance the meiotic and developmental competence of not only fully-grown, but also growing pig oocytes and significantly thus expanding the number of oocytes available for various assisted reproductive technology applications.

The effect of L-carnitine supplementation during in vitro maturation on oocyte maturation and somatic cloned embryo development

The quality of the recipient cytoplasm was reported as a crucial factor in maintaining the vitality of SCNT embryos and SCNT efficiency for dairy cows. Compared with oocytes matured in vivo, oocytes matured in vitro showed abnormal accumulation and metabolism of cytoplasmic lipids. L-carnitine treatment was found to control fatty acid transport into the mitochondrial β-oxidation pathway, which improved the process of lipid metabolism. The results of this study show that 0.5 mg/ml L-carnitine significantly reduced the cytoplasmic lipid content relative to control. No significant difference was observed in the rate of oocyte nuclear maturation, but the in vitro developmental competence of SCNT embryos was improved in terms of increased blastocyst production and lower apoptotic index in the L-carnitine treatment group. In addition, the pregnancy rate with SCNT embryos in the treatment group was significantly higher than in the control group. In conclusion, the present study demonstrated that adding L-carnitine to the maturation culture medium could improve the developmental competence of SCNT embryos both in vitro and in vivo by reducing the lipid content of the recipient cytoplasm.

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.

Lipid modulation during IVM increases the metabolism and improves the cryosurvival of cat oocytes

This study investigated the time course of lipid accumulation during IVM and assessed the role of lipid modulators added during IVM on lipid content, nuclear maturation, oxidative stress, mitochondrial activity, gene expression, and cryosurvival of cat oocytes. First, the lipid content of immature COCs was compared to those subjected to different IVM duration times (24, 28, and 32 h). Then, the lipid content was investigated after the use of different lipid modulators [conjugated linoleic acid (CLA), forskolin (FSK), l-carnitine (LC)]. Subsequently, both the CONTROL group and MIX 18 (CLA+FSK+LC) were compared regarding nuclear maturation, mitochondrial activity, reactive oxygen 19 species (ROS), and glutathione (GSH) levels, to the expression of SDHA, GDF9, BMP15, ZAR-1, 20 PRDX1, SIRT1, and SIRT3 genes (normalized by ACTB and YWHAZ genes); and to vitrification and 21 post-warming viability assessment. When not using any lipid modulator, an increase (P < 0.05) in lipid content could be observed after 28 h of IVM. The MIX group showed the greatest (P < 0.05) reduction in oocyte lipid content after 28 h of IVM. No difference (P > 0.05) was observed in the MII rate in the CONTROL (45%) and MIX (41%) groups and in mitochondrial activity ((1.00 ± 0.35 A U vs 1.19 ± 0.14 A U). Although ROS and GSH levels were higher (P < 0.05) in MIX than in CONTROL, the redox balance (ROS/GSH) was greater (P < 0.05) in the latter (C:1.00 ± 0.20b vs M:0.26 ± 0.06 a A.U). The GDF9, HSP70, PRDX1, and SIRT1 transcripts were downregulated (P < 0.05) in MIX-oocytes, compared to the CONTROL. After vitrification, MIX (74%) presented a higher (P < 0.05) viability compared to control (53%). In conclusion, MIX can reduce the total lipid content and improve viability after cryopreservation, however, it seems to affect the oocyte metabolism in a way that still needs to be better understood in the cat biological model.

The follicle-stimulating hormone triggers rapid changes in mitochondrial structure and function in porcine cumulus cells

Oocyte maturation is a key process during which the female germ cell undergoes resumption of meiosis and completes its preparation for embryonic development including cytoplasmic and epigenetic maturation. The cumulus cells directly surrounding the oocyte are involved in this process by transferring essential metabolites, such as pyruvate, to the oocyte. This process is controlled by cyclic adenosine monophosphate (cAMP)-dependent mechanisms recruited downstream of follicle-stimulating hormone (FSH) signaling in cumulus cells. As mitochondria have a critical but poorly understood contribution to this process, we defined the effects of FSH and high cAMP concentrations on mitochondrial dynamics and function in porcine cumulus cells. During in vitro maturation (IVM) of cumulus-oocyte complexes (COCs), we observed an FSH-dependent mitochondrial elongation shortly after stimulation that led to mitochondrial fragmentation 24 h later. Importantly, mitochondrial elongation was accompanied by decreased mitochondrial activity and a switch to glycolysis. During a pre-IVM culture step increasing intracellular cAMP, mitochondrial fragmentation was prevented. Altogether, the results demonstrate that FSH triggers rapid changes in mitochondrial structure and function in COCs involving cAMP.

A fresh start for IVM: capacitating the oocyte for development using pre-IVM

BACKGROUND

While oocyte IVM is practiced sporadically it has not achieved widespread clinical practice globally. However, recently there have been some seminal advances in our understanding of basic aspects of oocyte biology and ovulation from animal studies that have led to novel approaches to IVM. A significant recent advance in IVM technology is the use of biphasic IVM approaches. These involve the collection of immature oocytes from small antral follicles from minimally stimulated patients/animals (without hCG-priming) and an ∼24 h pre-culture of oocytes in an advanced culture system ('pre-IVM') prior to IVM, followed by routine IVF procedures. If safe and efficacious, this novel procedure may stand to make a significant impact on human ART practices.

OBJECTIVE AND RATIONALE

The objectives of this review are to examine the major scientific advances in ovarian biology with a unique focus on the development of pre-IVM methodologies, to provide an insight into biphasic IVM procedures, and to report on outcomes from animal and clinical human data, including safety data. The potential future impact of biphasic IVM on ART practice is discussed.

SEARCH METHODS

Peer review original and review articles were selected from PubMed and Web of Science searches for this narrative review. Searches were performed using the following keywords: oocyte IVM, pre-IVM, biphasic IVM, CAPA-IVM, hCG-triggered/primed IVM, natural cycle IVF/M, ex-vivo IVM, OTO-IVM, oocyte maturation, meiotic competence, oocyte developmental competence, oocyte capacitation, follicle size, cumulus cell (CC), granulosa cell, COC, gap-junction communication, trans-zonal process, cAMP and IVM, cGMP and IVM, CNP and IVM, EGF-like peptide and IVM, minimal stimulation ART, PCOS.

OUTCOMES

Minimizing gonadotrophin use means IVM oocytes will be collected from small antral (pre-dominant) follicles containing oocytes that are still developing. Standard IVM yields suboptimal clinical outcomes using such oocytes, whereas pre-IVM aims to continue the oocyte's development ex vivo, prior to IVM. Pre-IVM achieves this by eliciting profound cellular changes in the oocyte's CCs, which continue to meet the oocyte's developmental needs during the pre-IVM phase. The literature contains 25 years of animal research on various pre-IVM and biphasic IVM procedures, which serves as a large knowledge base for new approaches to human IVM. A pre-IVM procedure based on c-type natriuretic peptide (named 'capacitation-IVM' (CAPA-IVM)) has undergone pre-clinical human safety and efficacy trials and its adoption into clinical practice resulted in healthy live birth rates not different from conventional IVF.

WIDER IMPLICATIONS

Over many decades, improvements in clinical IVM have been gradual and incremental but there has likely been a turning of the tide in the past few years, with landmark discoveries in animal oocyte biology finally making their way into clinical practice leading to improved outcomes for patients. Demonstration of favorable clinical results with CAPA-IVM, as the first clinically tested biphasic IVM system, has led to renewed interest in IVM as an alternative, low-intervention, low-cost, safe, patient-friendly ART approach, and especially for patients with PCOS. The same new approach is being used as part of fertility preservation in patients with cancer and holds promise for social oocyte freezing.

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.

Supplementation with fibroblast growth factor 7 during in vitro maturation of porcine cumulus-oocyte complexes improves oocyte maturation and early embryonic development

In vitro generation of porcine embryos is an indispensable method in the realms of both agriculture and biomedicine. Nonetheless, the extant procedures encounter substantial obstacles pertaining to both the caliber and efficacy of the produced embryos, necessitating extensive research to in vitro maturation (IVM), the seminal commencement phase. One potentially fruitful approach may lie in refining the media and supplements composition utilized for oocyte maturation. Fibroblast growth factor-7 (FGF7), alternatively termed keratinocyte growth factor, is a theca-derived cytokine integral to folliculogenesis. This study aimed to examine the ramifications of supplementing FGF7 during the IVM phase. To determine the FGF7 location and its receptor in porcine ovaries, immunohistochemistry was executed based on follicle size categories (1-2, 3-6, and 7-9 mm). Regardless of follicle size, it was determined that FGF7 was expressed in theca and granulosa cells (GCs), whereas the FGF7 receptor was only expressed in the GCs of the larger follicles. During the IVM process, the maturation medium was supplied with various concentrations of FGF7, aiming to mature porcine cumulus-oocyte complexes (COCs). The data indicated a significant augmentation in the nuclear maturation rate only within the group treated with 10 ng/mL of FGF7 (p < 0.05). Post-IVM, the oocytes diameter exhibited a significant expansion in all groups that received FGF7 supplementation (p < 0.05). Additionally, all FGF7-supplemented groups exhibited a substantial elevation in intracellular glutathione levels, coupled with a noticeable reduction in reactive oxygen species levels (p < 0.05). With respect to gene expressions related to apoptosis, FGF7 treatment elicited a downregulation of pro-apoptotic genes and an upregulation of anti-apoptotic genes. The expression of genes associated with antioxidants underwent a significant enhancement (p < 0.05). In terms of the FGF7 signaling pathway-associated genes, there was a significant elevation in the mRNA expression of ERK1, ERK2, c-kit, and KITLG (p < 0.05). Remarkably, the group of 10 ng/mL of FGF7 demonstrated an appreciable uptick in the blastocyst formation rate during embryonic development post-parthenogenetic activation (p < 0.05). In conclusion, the FGF7 supplementation during IVM substantially augments the quality of matured oocytes and facilitates the subsequent development of parthenogenetically activated embryos. These results offer fresh perspectives on improved maturation and following in vitro evolution of porcine oocytes.

Effects of lactate, super-GDF9, and low oxygen tension during bi-phasic in vitro maturation on the bioenergetic profiles of mouse cumulus-oocyte complex†

In vitro maturation (IVM) is an alternative assisted reproductive technology with reduced hormone-related side effects and treatment burden compared to conventional IVF. Capacitation (CAPA)-IVM is a bi-phasic IVM system with improved clinical outcomes compared to standard monophasic IVM. Yet, CAPA-IVM efficiency compared to conventional IVF is still suboptimal in terms of producing utilizable blastocysts. Previously, we have shown that CAPA-IVM leads to a precocious increase in cumulus cell (CC) glycolytic activity during cytoplasmic maturation. In the current study, considering the fundamental importance of CCs for oocyte maturation and cumulus-oocyte complex (COC) microenvironment, we further analyzed the bioenergetic profiles of maturing CAPA-IVM COCs. Through a multi-step approach, we (i) explored mitochondrial function of the in vivo and CAPA-IVM matured COCs through real-time metabolic analysis with Seahorse analyzer, and to improve COC metabolism (ii) supplemented the culture media with lactate and/or super-GDF9 (an engineered form of growth differentiation factor 9) and (iii) reduced culture oxygen tension. Our results indicated that the pre-IVM step is delicate and prone to culture-related disruptions. Lactate and/or super-GDF9 supplementations failed to eliminate pre-IVM-induced stress on COC glucose metabolism and mitochondrial respiration. However, when performing pre-IVM culture under 5% oxygen tension, CAPA-IVM COCs showed similar bioenergetic profiles compared to in vivo matured counterparts. This is the first study providing real-time metabolic analysis of the COCs from a bi-phasic IVM system. The currently used analytical approach provides the quantitative measures and the rational basis to further improve IVM culture requirements.

The developmental potential of mature oocytes derived from rescue in vitro maturation

OBJECTIVE

To examine the developmental competence of immature oocytes in stimulated cycles, that matured after rescue in vitro maturation (IVM) compared with their sibling in vivo matured oocytes.

DESIGN

Retrospective cohort study.

SETTING

IVF clinic.

PATIENTS

A total of 182 patients underwent 200 controlled ovarian stimulation cycles with intracytoplasmic sperm injection cycles in which immature oocytes were retrieved and at least one mature oocyte was obtained through rescue IVM.

INTERVENTION

In vitro culture of immature germinal vesicle (GV) and metaphase I (MI) oocytes, retrieved in stimulated cycles.

MAIN OUTCOME MEASURES

Fertilization rate, cleavage rate, blastulation rate, ploidy of embryos evaluated using preimplantation genetic testing for aneuploidy, morphokinetic parameters and pregnancy outcomes.

RESULTS

In total, 2,288 oocytes were retrieved from 200 cycles. After denudation, 1,056 of the oocytes (46% ± 16%) were classified as metaphase II (MII). A total of 333/375 (89%) of MI oocytes and 292/540 (54%) of GV oocytes matured overnight and underwent intracytoplasmic sperm injection. The fertilization rates of matured oocytes from MI rescue IVM (R-MI) and from GV rescue IVM (R-GV) were comparable with those of their sibling MII oocytes (71% vs. 66%; 66% vs. 63%, respectively). Early cleavage rates (80% ± 35% vs. 92% ± 20%; 80% ± 42% vs. 95% ± 28%, respectively) and blastulation rates (32 ± 40% vs. 62 ± 33%; 24 ± 37% vs. 60 ± 35%, respectively) were significantly decreased in rescue IVM matured oocytes (R-oocytes)-derived zygotes, but the blastocyst (BL) euploidy rate and "good quality" BL rate were comparable with those of MII sibling-derived embryos. In addition, rescue IVM embryos showed significantly higher levels of multinucleation at the 2- and 4-cell stages, as well as higher rates of zygote direct cleavage from one to 3 to 4 cells. Overall, 21 transfers of rescue IVM embryos resulted in 3 healthy live births.

CONCLUSIONS

For patients with a low maturation rate and/or low numbers of mature oocytes at retrieval, rescue IVM may contribute more competent oocytes and additional viable BLs for transfer from the same stimulation cycle, maximizing the chances for pregnancy and live birth.

Effects of cyclic adenosine monophosphate modulating agents during oocyte pre-maturation and the role of melatonin on in vitro maturation of bovine cumulus-oocyte complexes

This study investigated the effects of cyclic adenosine monophosphate modulating during cumulus-oocyte complexes (COCs) pre-maturation and the role of melatonin on in vitro maturation (IVM) of bovine COCs. In experiment one, COCs were pre-matured for 8 h in control medium or with 3-isobutyl-1-methylxanthine (IBMX) and forskolin, IBMX and C-type natriuretic peptide, c-type natriuretic peptide and forskolin or IBMX, forskolin and c-type natriuretic peptide. Then, meiotic progression was evaluated. In experiment two, COCs were pre-matured, followed by IVM in control medium alone or with 10-6, 10-7 or 10-8 M melatonin. After IVM, chromatin configuration, transzonal projections (TZPs), reactive oxygen species, mitochondrial distribution, ultrastructure and mRNA expression for antioxidant enzymes were evaluated. In experiment 1, COCs pre-matured with both C-type natriuretic peptide and forskolin or C-type natriuretic peptide, forskolin and IBMX had lower meiotic resumption rate when compared to control. Considering that IBMX had not an additional effect to potentiate inhibition of meiotic resumption, a combination of C-type natriuretic peptide and forskolin was chosen. In experiment 2, COCs matured with 10-8 M melatonin had greater rates of meiotic resumption when compared to the other treatments (P < 0.05). The COCs matured with 10-7 or 10-8 M melatonin had greater mitochondrial activity (P < 0.05), while those matured with 10-6 or 10-8 M of melatonin had greater levels of TZPs. Ultrastructure of oocyte and cumulus cells after IVM with melatonin was relatively well preserved. COCs matured with 10-8 M melatonin increased mRNA expression for superoxide dismutase (SOD) and catalase (CAT) (P < 0.05), when compared to non-cultured and pre-matured COCs, respectively. In conclusion, bovine COC pre-maturation with C-type natriuretic peptide and forskolin, followed by IVM with 10-8 M melatonin improves meiotic resumption rates, TZPs, mitochondrial distribution and mRNA expression for SOD and CAT.

A Redesigned Method for CNP-Synchronized In Vitro Maturation Inhibits Oxidative Stress and Apoptosis in Cumulus-Oocyte Complexes and Improves the Developmental Potential of Porcine Oocytes

In vitro embryo production depends on high-quality oocytes. Compared with in vivo matured oocytes, in vitro oocytes undergo precocious meiotic resumption, thus compromising oocyte quality. C-type natriuretic peptide (CNP) is a follicular factor maintaining meiotic arrest. Thus, CNP-pretreatment has been widely used to improve the in vitro maturation (IVM) of oocytes in many species. However, the efficacy of this strategy has remained unsatisfactory in porcine oocytes. Here, by determining the functional concentration and dynamics of CNP in inhibiting spontaneous meiotic resumption, we improved the current IVM system of porcine oocytes. Our results indicate that although the beneficial effect of the CNP pre-IVM strategy is common among species, the detailed method may be largely divergent among them and needs to be redesigned specifically for each one. Focusing on the overlooked role of cumulus cells surrounding the oocytes, we also explore the mechanisms relevant to their beneficial effect. In addition to oocytes per se, the enhanced anti-apoptotic and anti-oxidative gene expression in cumulus cells may contribute considerably to improved oocyte quality. These findings not only emphasize the importance of screening the technical parameters of the CNP pre-IVM strategy for specific species, but also highlight the critical supporting role of cumulus cells in this promising strategy.

Pre-maturational culture promotes the developmental competence of bovine oocytes derived from an 8-day in vitro growth culture system

In this study, we evaluated the effects of pre-maturational culture (pre-IVM) on the developmental competence of bovine oocytes derived from an 8-day in vitro growth (IVG) culture system. IVG oocytes were subjected to 5 h pre-IVM before in vitro maturation, followed by in vitro fertilization (IVF). The proportion of oocytes that progressed to the germinal vesicle breakdown stage was similar between groups with and without pre-IVM. Although metaphase II oocytes and cleavage rates after IVF were similar regardless of pre-IVM culture, the blastocyst rate was significantly higher in the group with pre-IVM (22.5%) than without pre-IVM (11.0%, P < 0.05). In conclusion, pre-IVM culture improved the developmental competence of bovine oocytes derived from an 8-day IVG system.

The spectrum of in vitro maturation in clinical practice: the current insight

In vitro oocyte maturation (IVM) has been used worldwide. Despite the long-term implementation, the uptake of this procedure to complement current in vitro fertilization (IVF) remains low. The main reason is likely due to the non-synchronization of protocol and definition criteria, leading to difficulty in collective proper outcome data worldwide and, thus, lack of understanding of the exact IVM procedure. The review aims to consolidate the current clinical practice of IVM by dissecting relevant publications to be tailored for a current spectrum of clinical practice. Nevertheless, the background theories of oocyte maturation were also explored to provide a comprehensive understanding of the basis of IVM theories. Additional discussion of other potential uses of IVM in the future, such as in ovarian tissue cryopreservation known as OTO-IVM for fertility preservation and among women with diminished ovarian reserve, was also explored. Otherwise, future collaboration among all IVM centers is paramount for better collection of clinical data to provide valid recommendations for IVM in clinical practice, especially in molecular integrity and possible DNA alteration if present for IVM offspring outcome safety purposes.

Developmental Hurdles That Can Compromise Pregnancy during the First Month of Gestation in Cattle

Several key developmental events are associated with early embryonic pregnancy losses in beef and dairy cows. These developmental problems are observed at a greater frequency in pregnancies generated from in-vitro-produced bovine embryos. This review describes critical problems that arise during oocyte maturation, fertilization, early embryonic development, compaction and blastulation, embryonic cell lineage specification, elongation, gastrulation, and placentation. Additionally, discussed are potential remediation strategies, but unfortunately, corrective actions are not available for several of the problems being discussed. Further research is needed to produce bovine embryos that have a greater likelihood of surviving to term.

Morphokinetic parameters of mouse oocyte meiotic maturation and cumulus expansion are not affected by reproductive age or ploidy status

INTRODUCTION

Morphokinetic analysis using a closed time-lapse monitoring system (EmbryoScope + ™) provides quantitative metrics of meiotic progression and cumulus expansion. The goal of this study was to use a physiologic aging mouse model, in which egg aneuploidy levels increase, to determine whether there are age-dependent differences in morphokinetic parameters of oocyte maturation.

METHODS

Denuded oocytes and intact cumulus-oocyte complexes (COCs) were isolated from reproductively young and old mice and in vitro matured in the EmbryoScope + ™. Morphokinetic parameters of meiotic progression and cumulus expansion were evaluated, compared between reproductively young and old mice, and correlated with egg ploidy status.

RESULTS

Oocytes from reproductively old mice were smaller than young counterparts in terms of GV area (446.42 ± 4.15 vs. 416.79 ± 5.24 µm2, p < 0.0001) and oocyte area (4195.71 ± 33.10 vs. 4081.62 ± 41.04 µm2, p < 0.05). In addition, the aneuploidy incidence was higher in eggs with advanced reproductive age (24-27% vs. 8-9%, p < 0.05). There were no differences in the morphokinetic parameters of oocyte maturation between oocytes from reproductively young and old mice with respect to time to germinal vesicle breakdown (GVBD) (1.03 ± 0.03 vs. 1.01 ± 0.04 h), polar body extrusion (PBE) (8.56 ± 0.11 vs. 8.52 ± 0.15 h), duration of meiosis I (7.58 ± 0.10 vs. 7.48 ± 0.11 h), and kinetics of cumulus expansion (0.093 ± 0.002 vs. 0.089 ± 0.003 µm/min). All morphokinetic parameters of oocyte maturation were similar between euploid and aneuploid eggs irrespective of age.

CONCLUSION

There is no association between age or ploidy and the morphokinetics of mouse oocyte in vitro maturation (IVM). Future studies are needed to evaluate whether there is an association between morphokinetic dynamics of mouse IVM and embryo developmental competence.

The combination of rolipram and cilostamide improved the developmental competence of cloned porcine embryos

In vitro maturation of porcine oocytes is characterized by asynchronous cytoplasmic and nuclear maturation, leading to less competent oocytes supporting embryo development. The purpose of this study was to evaluate the combined effect of rolipram and cilostamide as cyclic Adenine monophosphate (cAMP) modulators to find the maximum cAMP levels that temporarily arrest meiosis. We determined the optimal time to maintain functional gap junction communication during pre-in vitro maturation to be four hours. Oocyte competence was evaluated by the level of glutathione, reactive oxygen species, meiotic progression, and gene expression. We evaluated embryonic developmental competence after parthenogenetic activation and somatic cell nuclear transfer. The combined treatment group showed significantly higher glutathione and lower reactive oxygen species levels and a higher maturation rate than the control and single treatment groups. Cleavage and blastocyst formation rates in parthenogenetic activation and somatic cell nuclear transfer embryos were higher in two-phase in vitro maturation than in the other groups. The relative levels of BMP15and GDF9 expression were increased in two-phase in vitro maturation. Somatic cell nuclear transfer blastocysts from two-phase in vitro maturation oocytes showed a lower level of expression of apoptotic genes than the control, indicating better pre-implantation developmental competence. The combination of rolipram and cilostamide resulted in optimal synchrony of cytoplasmic and nuclear maturation in porcine in vitro matured oocytes and there by enhanced the developmental competence of pre-implantation embryos.

IVM Advances for Early Antral Follicle-Enclosed Oocytes Coupling Reproductive Tissue Engineering to Inductive Influences of Human Chorionic Gonadotropin and Ovarian Surface Epithelium Coculture

In vitro maturation (IVM) is not a routine assisted reproductive technology (ART) for oocytes collected from early antral (EA) follicles, a large source of potentially available gametes. Despite substantial improvements in IVM in the past decade, the outcomes remain low for EA-derived oocytes due to their reduced developmental competences. To optimize IVM for ovine EA-derived oocytes, a three-dimensional (3D) scaffold-mediated follicle-enclosed oocytes (FEO) system was compared with a validated cumulus-oocyte complex (COC) protocol. Gonadotropin stimulation (eCG and/or hCG) and/or somatic cell coculture (ovarian vs. extraovarian-cell source) were supplied to both systems. The maturation rate and parthenogenetic activation were significantly improved by combining hCG stimulation with ovarian surface epithelium (OSE) cells coculture exclusively on the FEO system. Based on the data, the paracrine factors released specifically from OSE enhanced the hCG-triggering of oocyte maturation mechanisms by acting through the mural compartment (positive effect on FEO and not on COC) by stimulating the EGFR signaling. Overall, the FEO system performed on a developed reproductive scaffold proved feasible and reliable in promoting a synergic cytoplasmatic and nuclear maturation, offering a novel cultural strategy to widen the availability of mature gametes for ART.

Progress toward species-tailored prematuration approaches in carnivores

In the past four decades, the bovine model has been highly informative and inspiring to assisted reproductive technologies (ART) in other species. Most of the recent advances in ART have come from studies in cattle, particularly those unveiling the importance of several processes that must be recapitulated in vitro to ensure the proper development of the oocyte. The maintenance of structural and functional communications between the cumulus cells and the oocyte and a well-orchestrated chromatin remodeling with the gradual silencing of transcriptional activity represent essential processes for the progressive acquisition of oocyte developmental competence. These markers are now considered the milestones of physiological approaches to increase the efficiency of reproductive technologies. Different in vitro approaches have been proposed. In particular, the so-called "pre-IVM" or "prematuration" is a culture step performed before in vitro maturation (IVM) to support the completion of the oocyte differentiation process. Although these attempts only partially improved the embryo quality and yield, they currently represent a proof of principle that oocytes retrieved from an ovary or an ovarian batch shouldn't be treated as a whole and that tailored approaches can be developed for culturing competent oocytes in several species, including humans. An advancement in ART's efficiency would be desirable in carnivores, where the success is still limited. Since the progress in reproductive medicine has often come from comparative studies, this review highlights aspects that have been critical in other species and how they may be extended to carnivores.

Quantitative morphokinetic parameters identify novel dynamics of oocyte meiotic maturation and cumulus expansion†

Meiotic maturation and cumulus expansion are essential for the generation of a developmentally competent gamete, and both processes can be recapitulated in vitro. We used a closed time-lapse incubator (EmbryoScope+™) to establish morphokinetic parameters of meiotic progression and cumulus expansion in mice and correlated these outcomes with egg ploidy. The average time to germinal vesicle breakdown (GVBD), time to first polar body extrusion (PBE), and duration of meiosis I were 0.91 ± 0.01, 8.82 ± 0.06, and 7.93 ± 0.06 h, respectively. The overall rate of cumulus layer expansion was 0.091 ± 0.002 μm/min, and the velocity of expansion peaked during the first 8 h of in vitro maturation (IVM) and then slowed. IVM of oocytes exposed to Nocodazole, a microtubule disrupting agent, and cumulus oocyte complexes (COCs) to 4-methylumbelliferone, a hyaluronan synthesis inhibitor, resulted in a dose-dependent perturbation of morphokinetics, thereby validating the system. The incidence of euploidy following IVM was >90% for both denuded oocytes and intact COCs. No differences were observed between euploid and aneuploid eggs with respect to time to GVBD (0.90 ± 0.22 vs. 0.97 ± 0.19 h), time to PBE (8.89 ± 0.98 vs. 9.10 ± 1.42 h), duration of meiosis I (8.01 ± 0.91 vs. 8.13 ± 1.38 h), and overall rate and kinetics of cumulus expansion (0.089 ± 0.02 vs 0.088 ± 0.03 μm/min) (P > 0.05). These morphokinetic parameters provide novel quantitative and non-invasive metrics for the evaluation of meiotic maturation and cumulus expansion and will enable screening compounds that modulate these processes.

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.

Sequential IVM by CNP preincubation and cooperating of PGE2 with AREG enhances developmental competence of SCNT reconstructs in goat

Developmental competence of in vitro matured cumulus oocyte complexes (COCs) in conventional IVM (C.IVM) is lower than in vivo maturated COCs and is related to unsynchronized nuclear and cytoplasmic maturation. To overcome this dearth, COCs can be exposed to granulosa secreted factors in a two-step system. Therefore, in the first experiment, 1000 nM of C-type natriuretic peptide for 8 h was determined (CAPA), as the best time and concentration to retain oocytes in germinal vesicle stage. This condition, also reduces lipid droplets and increases the expression of ATGL and PLIN2 involved in lipolysis and lipogenesis, respectively. In the second experiment, maturation was stimulated with prostaglandin E2 and amphiregulin for 18 h (CAPA-IVM), and their optimal concentrations based on blastocyst formation rates through in vitro fertilization (IVF) were determined as 1 and 600 nM, respectively. In the third experiment, the in vitro and in vivo developmental competency of SCNT embryos in CAPA-IVM group were determined. Despite similar blastocyst formation rates in IVF and SCNT between CAPA-IVM and C.IVM, the quality of blastocysts were quality was higher in CAPA-IVM, which reflected itself, as higher ICM/TE ratio and also expression of NANOG in SCNT blastocysts. Pregnancy rate, live births rate and SCNT efficiency were not significant between CAPA-IVM and C.IVM groups. Therefore, CAPA-IVM can improve the developmental competency of SCNT derived embryos.

Physiological parameters related to oocyte nuclear differentiation for the improvement of IVM/IVF outcomes in women and cattle

In vitro maturation (IVM) has been applied in numerous different contexts and strategies in humans and animals, but in both cases it represents a challenge still far from being overcome. Despite the large dataset produced over the last two decades on the mechanisms that govern antral follicular development and oocyte metabolism and differentiation, IVM outcomes are still unsatisfactory. This review specifically focuses on data concerning the potential consequences of using supraphysiological levels of FSH during IVM, as well as on the regulation of oocyte chromatin dynamics and its utility as a potential marker of oocyte developmental competence. Taken together, the data revisited herein indicate that a significant improvement in IVM efficacy may be provided by the integration of pre-OPU patient-specific protocols preparing the oocyte population for IVM and more physiological culture systems mimicking more precisely the follicular environment that would be experienced by the recovered oocytes until completion of metaphase II.

Signaling mechanisms and their regulation during in vivo or in vitro maturation of mammalian oocytes

In vitro fertilization (IVF) is currently one of the most effective methods of infertility treatment. An alternative to commonly used ovarian hyperstimulation can become extracorporeal maturation of oocytes (in vitro maturation; IVM). Fertilization and normal development of the embryo depends on the cytoplasmic, nuclear and genomic maturity of the oocyte. The microenvironment of the ovarian follicle and maternal signals, which mediate bidirectional communication between granulosa, cumulus and oocyte cells, influence the growth, maturation and acquisition of oocyte development capability. During oogenesis in mammals, the meiosis is inhibited in the oocyte at the prophase I of the meiotic division due to the high cAMP level. This level is maintained by the activity of C-type natriuretic peptide (CNP, NPPC) produced by granulosa cells. The CNP binds to the NPR2 receptor in cumulus cells and is responsible for the production of cyclic guanosine monophosphate (cGMP). The cGMP penetrating into the oocyte through gap junctions inhibits phosphodiesterase 3A (PDE3A), preventing cAMP hydrolysis responsible for low MPF activity. The LH surge during the reproductive cycle reduces the activity of the CNP/NPR2 complex, which results in a decrease in cGMP levels in cumulus cells and consequently in the oocyte. Reduced cGMP concentration unblocks the hydrolytic activity of PDE3A, which decreases cAMP level inside the oocyte. This leads to the activation of MPF and resumption of meiosis. The latest IVM methods called SPOM, NFSOM or CAPA IVM consist of two steps: prematuration and maturation itself. Taking into account the role of cAMP in inhibiting and then unblocking the maturation of oocytes, they have led to a significant progress in terms of the percentage of mature oocytes in vitro and the proportion of properly developed embryos in both animals and humans.

Present state and future outlook for the application of in vitro oocyte maturation (IVM) in human infertility treatment

In vitro oocyte maturation (IVM) is an assisted reproductive technology in which a meiotically immature oocyte (prophase I or germinal vesicle stage) is recovered from an antral follicle and matured in vitro prior to fertilization. This technology, although in widespread use in domestic livestock, is not typically implemented during human IVF cycles. This review examines how IVM is currently used in the clinical setting, including the various ways IVM is defined in practice. The role of IVM in patient care, and the major challenges for implementation are described. Efficiency and safety are critically explored. The role of IVM in oncofertility will also be discussed. Finally, the outlook for the future of clinical IVM is considered.

Effect of cumulin and super-GDF9 in standard and biphasic mouse IVM

PURPOSE

In vitro maturation (IVM) is a technology that generates mature oocytes following culture of immature cumulus-oocyte complexes (COC) in vitro. IVM is characterized by minimal patient stimulation, making it attractive for certain patient groups. Recently, a biphasic IVM system, capacitation (CAPA)-IVM, has shown improved clinical outcomes relative to standard IVM; however, it remains less efficient than IVF. This study assessed whether supplementation of CAPA-IVM culture media with the novel TGFβ superfamily proteins cumulin and super-GDF9 improves subsequent mouse embryo development.

METHODS

Immature mouse COCs were cultured by standard IVM or biphasic IVM ± cumulin or super-GDF9.

RESULTS

Both cumulin and super-GDF9 in standard IVM significantly improved day-6 blastocyst rate (53.9% control, 73.6% cumulin, 70.4% super-GDF9; p = 0.006; n = 382-406 oocytes). Cumulin or super-GDF9 in CAPA-IVM did not alter embryo yield or blastocyst cell allocation in an unstimulated model. Moreover, cumulin did not alter these outcomes in a mild PMSG stimulation model. Cumulin in CAPA-IVM significantly increased cumulus cell expression of cumulus expansion genes (Ptgs2, Ptx3, Adamts1, Gfat2) and decreased Lhr expression relative to control. However, cumulin-induced mRNA expression of cumulus cell (Ptgs2, Ptx3) and oocyte genes (Gdf9, Bmp15, Oct4, Stella) in CAPA-IVM remained significantly lower than that of in vivo matured cells.

CONCLUSION

Cumulin did not provide an additional beneficial effect in biphasic IVM in terms of blastocyst yield and cell allocation; however in standard IVM, cumulin and super-GDF9 significantly improve oocyte developmental competence.

The Role of MAPK3/1 and AKT in the Acquisition of High Meiotic and Developmental Competence of Porcine Oocytes Cultured In Vitro in FLI Medium

The developmental potential of porcine oocytes cultured in vitro was remarkably enhanced in a medium containing FGF2, LIF and IGF1 (FLI) when compared to a medium supplemented with gonadotropins and EGF (control). We analyzed the molecular background of the enhanced oocyte quality by comparing the time course of MAPK3/1 and AKT activation, and the expression of genes controlled by these kinases in cumulus-oocyte complexes (COCs) cultured in FLI and the control medium. The pattern of MAPK3/1 activation in COCs was very similar in both media, except for a robust increase in MAPK3/1 phosphorylation during the first hour of culture in the FLI medium. The COCs cultured in the FLI medium exhibited significantly higher activity of AKT than in the control medium from the beginning up to 16 h of culture; afterwards a deregulation of AKT activity occurred in the FLI medium, which was not observed in the control medium. The expression of cumulus cell genes controlled by both kinases was also modulated in the FLI medium, and in particular the genes related to cumulus-expansion, signaling, apoptosis, antioxidants, cell-to-cell communication, proliferation, and translation were significantly overexpressed. Collectively, these data indicate that both MAPK3/1 and AKT are implicated in the enhanced quality of oocytes cultured in FLI medium.

Biological roles of l-carnitine in oocyte and early embryo development

Poor oocyte quality is responsible for female infertility. Multiple studies have been carried out to find supplements to enhance oocyte quality and mitigate infertility problems. l-carnitine and its derivatives have diverse roles in developing oocytes and early embryos. This review focuses on the in vitro and in vivo studies that using l-carnitine alone or in combination with other supplements for oocyte quality enhancement. The key roles of l-carnitine in oocyte quality and embryo growth were summarized, and the underlying mechanism was also elucidated. l-carnitine helps in the lipid metabolism process by controlling the transfer of fatty acids to mitochondria for β-oxidation. l-carnitine modulates glucose metabolism and enhances respiratory chain enzyme activity. Furthermore, it acts as an antioxidant to prevent oxidative damage and inhibit apoptosis, a signal in response to oxidative stress. Results show the potential of l-carnitine as a potential agent in assisted reproductive technology to improve oocyte quality and the subsequent embryonic development.

The role of forskolin as a lipolytic stimulator during in vitro oocyte maturation and the in vitro embryo production of livestock

Cryopreservation is a modern technique which assists in the preservation of genetic material from oocytes and embryos for a long time. However, elevated vulnerability to cryopreservation due to the large accumulation of intracellular lipids within oocytes or embryos avoids success of this method. These lipids remain the main crucial factor limiting survival rates of oocytes and embryos after thawing. Lipid ingathering in the oocyte cytoplasm augments lipid peroxidation (LPO) and oxidative stress increases the apoptosis process, declines the viability after thawing, declines cytoskeleton actin filament injuries, lowers the blastocyst rates and reduces cryotolerance in the early stages of embryo development. There have been several attempts to reduce the ingathering of intracellular lipids in oocytes or embryos during the cryopreservation process, in that way enhancing the competence of cryopreserved oocytes or embryos and increasing their viability. One of the most applied agents for chemical delipidation is forskolin. Forskolin exhibited a possible part in improving the oocytes cryopreservation through stimulating cyclic adenosine monophosphate (cAMP) production. The main purpose of cAMP modulation is to provide energy to sustain the mammalian oocytes´ meiotic arrest. The purpose of the existing article is to assess and offer more evidence concerning the forskolin utilization as a modulator of cAMP during the cryopreservation of oocytes and its influence on meiosis completion and the reorganization of cytoplasm, which are prerequisites for the development of oocytes in addition to the contribution to fertilization and subsequently, the development of embryos.

Approaches to oocyte meiotic arrest in vitro and impact on oocyte developmental competence

Oocytes are maintained in a state of meiotic arrest following the first meiotic division until ovulation is triggered. Within the antral follicle, meiotic arrest is actively suppressed in a process facilitated by the cyclic nucleotides cGMP and cAMP. If removed from this inhibitory follicular environment and cultured in vitro, mammalian oocytes undergo spontaneous meiotic resumption in the absence of the usual stimulatory follicular stimuli, leading to asynchronicity with oocyte cytoplasmic maturation and lower developmental competence. For more than 50 years, pharmacological agents have been used to attenuate oocyte germinal vesicle (GV) breakdown in vitro. Agents which increase intra-oocyte cAMP or prevent its degradation have been predominantly used, however agents such as kinase and protein synthesis inhibitors have also been trialled. Twenty years of research demonstrates that maintaining GV arrest for a period before in vitro maturation (IVM) improves oocyte developmental competence, and is likely attributed to maintenance of bidirectional communication with cumulus cells leading to improved oocyte metabolic function. However, outcomes are influenced by various factors including the mode of action of the modulators, dose, treatment duration, species, and the degree of hormonal priming of the oocyte donor. Cyclic GMP and/or cAMP modulation in a prematuration step (called pre-IVM) prior to IVM has shown the greatest consistency in improving oocyte developmental competence, whereas kinase and protein synthesis inhibitors have proven less effective at improving IVM outcomes. Such pre-IVM approaches have shown potential to alter current use of artificial reproductive technologies in medical and veterinary practice.

cAMP Modulators before In Vitro Maturation Decrease DNA Damage and Boost Developmental Potential of Sheep Oocytes

Simple Summary

Oocyte in vitro maturation has massive potential for the generation of great numbers of embryos for research and for the application of assisted reproductive technologies, such as in vitro embryo production. However, the developmental ability of in vitro matured oocytes is lower than those matured in vivo. Here, incubating the oocytes with cAMP modulating agents for two hours before in vitro maturation decreased oocyte DNA damage and increased the number of embryos generated after in vitro fertilization. The present findings could help to develop new methods to improve the quality and developmental potential of in vitro matured oocytes.

Abstract

To date, the underlying mechanisms by which cAMP modulators act during in vitro maturation to improve oocyte developmental competence are poorly understood. Here, we sought to fill this knowledge gap by evaluating the use of phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) and adenylyl cyclase activator forskolin during a culture period of 2 h before in vitro maturation (pre-IVM) on the nuclear and cytoplasmic maturation features in essential organelles, cumulus cells activity, and in vitro developmental potential of sheep oocytes. Results showed that pre-IVM treatment significantly decreased (p < 0.05) the DNA damage of mature oocytes (pre-IVM = 2.08% ± 3.51% vs. control = 20.58% ± 3.51%) and increased (p ≤ 0.05) expanded blastocyst rates compared to the control (from the total of oocytes: pre-IVM = 23.89% ± 1.47% vs. control = 18.22% ± 1.47%, and from the cleaved embryos: pre-IVM = 45.16% ± 1.73% vs. control = 32.88% ± 1.73%). Considering that oocytes are highly vulnerable to the accumulation of DNA damage because of exposure to in vitro culture conditions, our results suggest that the modulation of intra-oocyte cAMP levels with forskolin and IBMX before IVM might afford oocytes a more effective DNA repair mechanism to overcome damage obstacles and ultimately improve developmental competence. This previously unappreciated action of cAMP modulators could help to develop improved methods for assisted reproduction technologies in animal and clinical research.

Effects of epidermal growth factor and progesterone on oocyte meiotic resumption and the expression of maturation-related transcripts during prematuration of oocytes from small and medium-sized bovine antral follicles

This study evaluated the effects of epidermal growth factor (EGF) and progesterone (P4) on growth, the resumption of meiosis and expression of eukaryotic translation initiation factor 4E(eIF4E), poly(A)-specific ribonuclease (PARN), oocyte-specific histone H1 (H1FOO), oocyte maturation factor Mos (cMOS), growth differentiation factor-9 (GDF9) and cyclin B1 (CCNB1) mRNA in oocytes from small and medium-sized antral follicles after prematuration and maturation invitro. Oocytes from small (<2.0mm) and medium (3.0-6.0mm) antral follicles were cultured in medium containing EGF (10ng mL-1), P4 (100 µM) or both. After culture, growth rate, resumption of meiosis and eIF4E, PARN, H1FOO, cMOS, GDF9 and CCNB1 mRNA levels were evaluated. P4 increased cMOS, H1FOO and CCNB1 mRNA levels after the culture of oocytes from small antral follicles, and EGF increased CCNB1 mRNA levels in these oocytes. In the medium-sized antral follicles, P4 alone or in combination with EGF increased oocyte diameter after prematuration invitro. In these oocytes, the presence of either EGF or P4 in the culture medium increased cMOS mRNA levels. In conclusion, P4 increases cMOS, H1FOO and CCNB1 mRNA levels after the culture of oocytes from small antral follicles. P4 and the combination of EGF and P4 promote the growth of oocytes from medium-sized antral follicles, and both EGF and P4 increase cMOS mRNA levels.

Comparison of in vitro maturation and in vitro fertilization for polycystic ovary syndrome patients: a systematic review and meta-analysis

BACKGROUND

There are two common treatments for polycystic ovary syndrome (PCOS): in vitro fertilization (IVF) and in vitro maturation (IVM). Our study aimed to assess the clinical effects and safety of IVM versus IVF for PCOS.

METHODS

We searched randomized controlled trials and retrospective cohort studies comparing IVM versus IVF for PCOS. Data were extracted from eligible studies. We sought to evaluate fertilization rate, clinical pregnancy rate, live birth rate, and miscarriage. Results were expressed as risk ratio (RR) with 95% confidence intervals (CIs).

RESULTS

Eight studies with a total of 1,579 patients were included in the present study. According to the heterogeneity analysis, there were no differences between the IVM group and IVF group in terms of fertilization rate, clinical pregnancy rate and miscarriage. Additionally, the IVF group had a higher live birth rate than the IVM group (overall P=0.0007). Sensitivity analysis and funnel plot showed that our study was robust and based on the funnel plot this article had low publication bias.

DISCUSSION

The findings of the present study indicated that IVM had similar clinical effects compared with IVF in patients with PCOS. However, IVM might be a suitable option for PCOS in terms of cost and successful pregnancy rate.

Simulated physiological oocyte maturation (SPOM) improves developmental competence of in vitro produced goat embryos

The effect of simulated physiological oocyte maturation on the developmental competence, reactive oxygen species production and apoptosis rate of in vitro produced goat embryos were studied in the present experiment. Oocytes and spermatozoa were recovered from ovaries and epididymis, respectively, procured from a local small animal abattoir. The oocytes aspirated from the ovaries were allocated into two groups, control (subjected to routine in vitro maturation, fertilization and culture) and simulated physiological oocyte maturation (SPOM) group (subjected to prematuration, followed by routine in vitro maturation, fertilization and culture). The SPOM group showed a significantly (p < 0.05) higher maturation and blastocyst rates (90.60 ± 0.46% and 29.09 ± 2.59%, respectively) as compared to the control group (85.29 ± 0.98% and 24.09 ± 1.08%). The intensity of reactive oxygen species of the embryos in the control group (14.98 ± 0.83 pixels/embryo) was significantly (p < 0.05) higher than the SPOM group (9.60 ± 0.76 pixels/embryo). The apoptosis rate was also significantly (p < 0.05) higher in the embryos of the control group (9.18 ± 1.07%) as compared to the SPOM group (5.71 ± 0.90%). In conclusion, the simulated physiological oocyte maturation system significantly increases the developmental competence of the oocytes and decreases the intensity of reactive oxygen species and embryonic apoptosis in abattoir derived goat embryos.

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.

Recreating the Follicular Environment: A Customized Approach for In Vitro Culture of Bovine Oocytes Based on the Origin and Differentiation State

The mammalian ovary is a large source of oocytes organized into follicles at various stages of folliculogenesis. However, only a limited number of them can be used for in vitro embryo production (IVEP), while most have yet to complete growth and development to attain full meiotic and embryonic developmental competence. While the in vitro growth of primordial follicles in the ovarian cortex has the potential to produce mature oocytes, it is still at an experimental stage. The population of early antral follicles (EAFs), instead, may represent a reserve of oocytes close to completing the growth phase, which might be more easily exploited in vitro and could increase the number of female gametes dedicated to IVEP.Here we present in vitro culture strategies that have been developed utilizing physiological parameters to support the specific needs of oocytes at distinct stages of differentiation, in order to expand the source of female gametes for IVEP by maximizing the attainment of fertilizable oocytes. Furthermore, these culture systems provide powerful tools to dissect the molecular processes that direct the final differentiation of the mammalian oocyte.

Reversing complete mechanical transzonal projections disruption during mouse in vitro follicle culture with unaltered oocyte competence†

In vitro oocyte growth is widely studied as an alternative fertility preservation approach. Several animal models are used to generate extensive information on this complex process regulated by the constant and dynamic interaction between the oocyte and its somatic compartment throughout follicle growth and maturation. A two-dimensional attachment mouse secondary follicle culture system was used to assess the oocyte's capacity to overcome disconnection from its somatic companions at different developmental stages for final competence acquisition. To test this, complete mechanical denudation of oocytes from preantral (PA) and early antral (EA) follicles was performed. Established endpoints were the oocyte's potential to reconnect with somatic cells and the impact of connectivity disruption on mature oocyte quality. This study proves that oocytes from PA and EA cultured mouse follicles can overcome complete denudation, restoring likely functional transzonal projections with no significant differences in meiotic and developmental competence compared with those from intact cultured follicles. These novel findings constitute good premises for developing successful strategies to rescue human oocyte competence in the context of in vitro culture approaches such as nonhuman chorionic gonadotropin triggered in vitro maturation.

Histone deacetylase inhibitor during in vitro maturation decreases developmental capacity of bovine oocytes

This study aimed to evaluate the effect of scriptaid during pre-maturation (PIVM) and/or maturation (IVM) on developmental competence of bovine oocytes. Cumulus-oocyte complexes (COCs) were submitted to PIVM for 6 h in the presence or absence of scriptaid. COCs were distributed into five groups: T1-IVM for 22 h, T2-PIVM for 6 h and IVM for 22 h, T3-PIVM with scriptaid for 6 h and IVM for 22 h, T4-PIVM for 6 h and IVM with scriptaid for 22 h, and T5-PIVM with scriptaid for 6 h and IVM with scriptaid for 22 h. Nuclear maturation, gene expression, cumulus cells (CCs) expansion, and embryo development and quality were evaluated. At the end of maturation, all groups presented the majority of oocytes in MII (P>0.05). Only HAT1 gene was differentially expressed (P<0.01) in oocytes with different treatments. Regarding embryo development at D7, T4 (23%) and T5 (18%) had lower blastocyst rate (P<0.05) than the other treatments (T1 = 35%, T2 = 37% and T3 = 32%). No effect was observed when scriptaid in PIVM was used in less competent oocytes (P>0.05). In conclusion, presence of scriptaid in PIVM and/or IVM did not improve developmental competence or embryo quality.

In vitro growth culture in a medium with reduced sodium chloride improves maturation and developmental competence of pig oocytes derived from small antral follicles

The objective of this study was to evaluate the effects of reducing the sodium chloride content in in vitro growth (IVG) medium to 61.6 mM on in vitro maturation (IVM) and embryonic development of pig oocytes derived from small antral follicles (SAF) less than 3 mm in diameter. SAF oocytes were cultured for 2 days to induce IVG in alpha-minimal essential medium with 108 mM NaCl (αMEM-108) or porcine zygote medium (PZM) containing 61.6 mM (PZM-61.6) or 108 mM (PZM-108) NaCl. These media were further supplemented with 1 mM dibutyryl cyclic adenosine monophosphate (dbcAMP) and 10% (v/v) fetal bovine serum. After IVG culture, oocytes were matured for 44 h in our standard IVM medium. The IVG culture in PZM-61.6 significantly increased nuclear maturation (88.0 ± 2.2%) of SAF oocytes compared to that in PZM-108 (77.3 ± 3.9%) or αMEM-108 (75.9 ± 3.8%). After parthenogenesis (PA), the proportions of blastocysts, based on the number of metaphase II (MII) oocytes, induced for PA were not different among IVG oocytes cultured in PZM-61.6 (50.2 ± 3.0%), PZM-108 (46.8 ± 2.9%), or αMEM-108 (45.6 ± 2.9%). The IVM oocytes derived from IVG in PZM-61.6 showed increased perivitelline space (PVS) (12.1 ± 0.6 μm) and intra-oocyte glutathione (GSH) content (1.19 ± 0.04 pixels/oocyte) compared to PVS (8.0 ± 0.5 and 7.4 ± 0.4 μm) and GSH (1.03 ± 0.04 and 1.00 ± 0.04 pixels/oocyte) of oocytes derived from PZM-108 and αMEM-108, respectively. The IVG culture in PZM-61.6 stimulated meiotic resumption after IVG and faster nuclear progression after IVM than that in αMEM-108. After somatic cell nuclear transfer (SCNT), the blastocyst formation of SAF oocytes grown in PZM-61.6 (17.8 ± 3.3%) was higher than that of oocytes grown in PZM-108 (7.5 ± 2.7%) but not different from that of oocytes in αMEM-108 (11.4 ± 3.4%). Regardless of the different osmotic pressures, nuclear maturation was significantly increased by IVG culture in PZM with reduced NaCl (86.8 ± 2.3 and 84.9 ± 4.2% in PZM-61.6 and PZM-61.6 with sorbitol, respectively) than in PZM-108 (70.5 ± 3.4%). Blastocyst formation was not affected by the differences in NaCl content and osmotic pressure of the IVG medium, whereas the mean number of cells in blastocysts was significantly higher following IVG culture in PZM-61.6 than in the other groups. In conclusion, the results demonstrate that, following SCNT in pigs, IVG culture of SAF oocytes in a medium with a reduced NaCl concentration stimulates oocyte maturation and improves subsequent embryonic development.

The Epigenetics of Gametes and Early Embryos and Potential Long-Range Consequences in Livestock Species-Filling in the Picture With Epigenomic Analyses

The epigenome is dynamic and forged by epigenetic mechanisms, such as DNA methylation, histone modifications, chromatin remodeling, and non-coding RNA species. Increasing lines of evidence support the concept that certain acquired traits are derived from environmental exposure during early embryonic and fetal development, i.e., fetal programming, and can even be "memorized" in the germline as epigenetic information and transmitted to future generations. Advances in technology are now driving the global profiling and precise editing of germline and embryonic epigenomes, thereby improving our understanding of epigenetic regulation and inheritance. These achievements open new avenues for the development of technologies or potential management interventions to counteract adverse conditions or improve performance in livestock species. In this article, we review the epigenetic analyses (DNA methylation, histone modification, chromatin remodeling, and non-coding RNAs) of germ cells and embryos in mammalian livestock species (cattle, sheep, goats, and pigs) and the epigenetic determinants of gamete and embryo viability. We also discuss the effects of parental environmental exposures on the epigenetics of gametes and the early embryo, and evidence for transgenerational inheritance in livestock.