The use of mineral oil during in vitro maturation, fertilization, and embryo culture does not impair the developmental competence of pig oocytes

Nobiletin as a novel agent to enhance porcine in vitro embryo development and quality

In vitro embryo production (IVP) is of great importance to the porcine industry, as well as for basic research and biomedical applications. Despite the large efforts made in laboratories worldwide to address suboptimal culture conditions, porcine IVP remains inefficient. Nobiletin (Nob, 5,6,7,8,3',4' hexamethoxyflavone) supplementation to in vitro culture (IVC) medium, enhances in vitro embryo development in various species. However, its impact on the quality and developmental capacity of in vitro-produced pig embryos is yet to be established. This study evaluated the effects of different concentrations (2.5 and 5 μM) of Nob during the early culture of in vitro-produced pig embryos on embryo developmental competence, mitochondrial activity, lipid content, intracellular Reactive Oxygen Species (ROS) and Glutathione (GSH) content, Total Cell Number (TCN) per blastocyst, and expression of genes related to embryo development, quality and oxidative stress. Embryos cultured in medium without Nob supplementation and in medium supplemented with 0.01 % dimethyl sulfoxide (DMSO-vehicle for Nob) constituted the Control and DMSO groups, respectively. Embryo development rates were evaluated on Days 2, 6 and 7 of IVC. Additionally, a representative group of embryos was selected to assess mitochondrial activity, lipid, ROS and GSH content (on Days 2 and 6 of IVC), TCN assessment and gene expression analyses (on Day 6 of IVC). No significant differences were observed in any of the parameters evaluated on Day 2 of IVC. In contrast, embryos cultured under the presence of Nob 2.5 showed higher developmental rates on Days 6 and 7 of IVC. In addition, Day 6 embryos showed increased mitochondrial activity, with decreased levels of ROS and GSH in the Nob 2.5 group compared to the other groups. Both Nob 2.5 and Nob 5 embryos showed higher TCN compared to the Control and DMSO groups. Furthermore, Nob 2.5 and Nob 5 upregulated the expression of Superoxide dismutase type 1 (SOD1) and Glucose-6-phosphate dehydrogenase (G6PDH) genes, which could help to counteract oxidative stress during IVC. In conclusion, the addition of Nob during the first 48 h of IVC increased porcine embryo development rates and enhanced their quality, including the upregulation of relevant genes that potentially improved the overall efficiency of the IVP system.

Cryopreservation of highly extended pig spermatozoa remodels its proteome and counteracts polyspermic fertilization in vitro

BACKGROUND

Currently, high polyspermy remains a significant obstacle to achieving optimal efficiency in in vitro fertilization (IVF) and in vitro embryo production (IVP) systems in pigs. Developing strategies that would prevent polyspermy is essential in overcoming this challenge and maximizing the potential of this reproductive biotechnology. Previous results have demonstrated that using boar spermatozoa subjected to a high-extension and reconcentration procedure and then cryopreserved resulted in significant improvements in IVF/IVP systems with high rates of monospermy and penetration.

OBJECTIVE

The aim of the present study was to unveil the molecular mechanisms that may underlie the changes in fertilization patterns exhibited by highly extended and cryopreserved boar spermatozoa.

MATERIALS AND METHODS

To achieve this goal, we used quantitative proteomic analysis (LC-ESI-MS/MS SWATH) to identify differentially abundant proteins (DAPs) between highly extended (HE) and conventionally (control; CT) cryopreserved boar spermatozoa. Prior to the analysis, we evaluated the in vitro post-thawing fertilizing ability of the sperm samples. The results demonstrated a remarkable improvement in monospermy and IVF efficiency when using HE spermatozoa in IVF compared with CT spermatozoa.

RESULTS

At the proteomic level, the combination of high-extension and cryopreservation had a significant impact on the frozen-thawed sperm proteome. A total of 45 proteins (24 downregulated and 21 upregulated) were identified as DAPs (FC > 1 or ≤1; p < 0.05) when compared with CT spermatozoa. Some of these proteins were primarily linked to metabolic processes and the structural composition of sperm cells. The dysregulation of these proteins may have a direct or indirect effect on essential sperm functions and significantly affect spermatozoa-oocyte interaction and, therefore, the sperm fertilization profile under in vitro conditions. While these findings are promising, further research is necessary to comprehend how the disturbance of specific proteins affects sperm fertilization ability.

Cryotop vitrification of large batches of pig embryos simultaneously provides excellent postwarming survival rates and minimal interference with gene expression

The most commonly used technique to vitrify pig embryos is the super open pulled straw (SOPS), where a maximum of 6 embryos can be vitrified simultaneously per device without compromising the minimum volume necessary for optimal preservation. Since optimal embryo transfer (ET) demands a transfer of 20-40 embryos per recipient, the customary use of SOPS complicates embryo warming and ET in field conditions. Such complications could be avoided when using the Cryotop® (OC) system, which has been proven to be an effective option for vitrifying at least 20 porcine embryos simultaneously. This study aimed to investigate the changes in the transcriptome of blastocysts caused by vitrification using both systems. In vivo-derived blastocysts were OC- (n = 60; 20 embryos/device) and SOPS- (n = 60; 4-6 embryos/device) vitrified and cultured for 24 h after warming. Nonvitrified blastocysts (n = 60) cultured for 24 h postcollection acted as controls. At the end of culture, 48 viable embryos from each group (6 pools of 8 embryos) were selected for microarray (GeneChip® Porcine Genome Array, P/N 900624, Affymetrix) analysis of differentially expressed genes (DEGs). The survival rate of embryos vitrified with the OC and SOPS systems (>97%) was similar to that of the control embryos (100%). Microarray analysis of each vitrification system compared to the control group showed 245 DEGs (89 downregulated and 156 upregulated) for the OC system and 210 (44 downregulated and 166 upregulated) for the SOPS system. Two pathways were enriched for the DEGs specifically altered in each vitrification system compared to the control (glycolysis/gluconeogenesis and carbon metabolism pathways for the OC system and amino sugar and nucleotide sugar metabolism and lysosome pathways in the SOPS group). The OC group showed 31 downregulated and 24 upregulated genes and two enriched pathways (mineral absorption and amino sugar and nucleotide sugar metabolism pathways) when compared to the SOPS group. In summary, vitrification with the OC system altered fewer genes related to apoptosis and activated genes related to cell proliferation. We conclude that vitrification with either the OC or SOPS system has a moderate to low effect on the transcriptome of in vivo-derived porcine blastocysts. Further investigation is needed to elucidate how the differences in the transcriptome of embryos vitrified with these systems affect their subsequent developmental ability after ET.

Exogenous Melatonin in the Culture Medium Does Not Affect the Development of In Vivo-Derived Pig Embryos but Substantially Improves the Quality of In Vitro-Produced Embryos

Cloned and transgenic pigs are relevant human disease models and serve as potential donors for regenerative medicine and xenotransplantation. These technologies demand oocytes and embryos of good quality. However, the current protocols for in vitro production (IVP) of pig embryos give reduced blastocyst efficiency and embryo quality compared to in vivo controls. This is likely due to culture conditions jeopardizing embryonic homeostasis including the effect of reactive oxygen species (ROS) influence. In this study, the antioxidant melatonin (1 nM) in the maturation medium, fertilization medium, or both media was ineffective in enhancing fertilization or embryonic development parameters of in vitro fertilized oocytes. Supplementation of melatonin in the fertilization medium also had no effect on sperm function. In contrast, the addition of melatonin to the embryo culture medium accelerated the timing of embryonic development and increased the percentages of cleaved embryos and presumed zygotes that developed to the blastocyst stage. Furthermore, it increased the number of inner mass cells and the inner mass cell/total cell number ratio per blastocyst while increasing intracellular glutathione and reducing ROS and DNA damage levels in embryos. Contrarily, the addition of melatonin to the embryo culture medium had no evident effect on in vivo-derived embryos, including the developmental capacity and the quality of in vivo-derived 4-cell embryos or the percentage of genome-edited in vivo-derived zygotes achieving the blastocyst stage. In conclusion, exogenous melatonin in the embryo culture medium enhances the development and quality of in vitro-derived embryos but not in in vivo-derived embryos. Exogenous melatonin is thus recommended during embryo culture of oocytes matured and fertilized in vitro for improving porcine IVP efficiency.

Use of mineral oil in IVF culture systems: physico-chemical aspects, management, and safety

PURPOSE

The study aims to summarize current knowledge on the use of oil in embryo culture systems, with a focus on proper management of different types of oil and possible impact on culture systems.

METHODS

PubMed was used to search the MEDLINE database for peer-reviewed English-language original articles and reviews concerning the use of oil in embryo culture systems. Searches were performed by adopting "embryo," "culture media," "oil," and "contaminants" as main terms. The most relevant publications were assessed and discussed critically.

RESULTS

Oils used in IVF are complex mixtures of straight-chain hydrocarbons, cyclic and aromatic hydrocarbons, and unsaturated hydrocarbons, whose precise composition influences their chemical and physical properties. Possible presence of contaminants suggests their storage at 4 °C in the dark to prevent peroxidation. Washing, generally performed by manufacturers prior to commercialization, may remove trace chemical contaminants. Oils reduce evaporation from culture media at rates depending on their chemical physical properties, culture system parameters, and incubator atmosphere. Contaminants - mainly metal ion and plastic components derived from refinement processes and storage - can pass to the aqueous phase of culture systems and affect embryo development.

CONCLUSIONS

Oils are essential components of culture systems. Their original quality and composition, storage, handling, and use can affect embryo development with significant efficiency and safety implications.

Evaluation of a small volume oil-free in vitro production system for bovine embryos

Background:

Embryos are usually produced in culture systems with an oil overlay, which conveys protection against the evaporation of water and microbial contamination. The oil can also release toxic substances and absorb essential components, such as hormones, which adversely affect the quality of the oocytes and the development of embryos in vitro.

Objective:

The aim of this study was to validate an oil‐free bovine in vitro production (IVP) system.

Method:

Cumulus–oocyte complexes collected from abattoir‐derived ovaries were matured, fertilized and cultured employing a standard system. The quantity of medium in both groups (with and without an oil overlay) and throughout all stages of IVP was maintained at a volume of 100 μl. The oil group was covered with paraffin oil. The maturation stage of oocytes was assessed using fluorescence staining after 24 hr and developmental stages of embryos were evaluated on day 8. The expanded day 8 blastocysts were assessed by live–dead staining.

Results:

Oocytes matured in the absence of an oil overlay had significantly higher maturation rates when compared against matured oocytes in medium with an oil overlay. Steroid concentration is higher in medium after maturation without oil cover. The developmental rate was significantly higher after culture without oil overlay. The total cell number and the live–dead ratio was not significantly different. The osmolality did not differ between both groups during maturation and slightly decreased during culture without oil.

Conclusion:

Based on the current study, bovine oil‐free IVP systems can be suggested as an alternative to oil‐covered medium.

High pre-freezing sperm dilution improves monospermy without affecting the penetration rate in porcine IVF

The high incidence of polyspermy is still an unresolved problem for the production of in vitro-produced porcine embryos. In this work, we modified the usual sperm processing sequence for in vitro fertilization (IVF), and the spermatozoa from four boars were frozen directly at a low sperm concentration of 20 × 10⁶ sperm/mL (high pre-freezing sperm dilution group; F20), thawed and processed for IVF in three replicates. Spermatozoa from the same boars frozen at a conventional concentration (1000 × 10⁶ sperm/mL) were used as the control group. The post-thaw sperm quality evaluation demonstrated that despite there being no differences in the percentage of motile spermatozoa between groups, the proportion of live spermatozoa with intact acrosomes was significantly higher in the F20 group than in the control. The in vitro penetration rate was also similar between groups; however, the co-incubation of oocytes with F20 sperm increased monospermy, IVF efficiency, cleavage rate and the efficiency of blastocyst formation compared with the results for oocytes co-incubated with control spermatozoa. These results indicate, for the first time, that a high pre-freezing sperm dilution increases monospermy without affecting penetration rates, thereby increasing blastocyst formation.

Reduced nucleic acid methylation impairs meiotic maturation and developmental potency of pig oocytes

Oocyte meiosis is a complex process coordinated by multiple endocrinal and molecular circuits. Recently, N⁶-methyladenosine (m⁶A) epigenetic modification on RNA is revealed to be important for meiotic maturation. However, the molecular mechanism of how m⁶A modification exerts its effect on oocyte maturation is largely unknown. Here, we showed that endogenous m⁶A writers (Mettl3 and Wtap) and eraser (Fto) elevated their transcript levels during meiotic maturation of pig oocytes. From germinal vesicle (GV) to metaphase II (MII) stages, global m⁶A level significantly increased, and existed mostly in ooplasm. Methyl donor (betaine, 16 mM) treatment of porcine cumulus-oocyte complexes (COCs) during in vitro maturation (IVM) significantly boosted nucleic acid m⁶A level within oocytes, but unchanged meiotic process and oocyte subsequent development. By contrast, methylation inhibitor (cycloleucine, 20 mM) reduced nucleic acid m⁶A level, and significantly decreased the germinal vesicle breakdown (GVBD) rate, the extrusion rate of the first polar body, and the cleavage and blastocyst rates of parthenotes. In addition, in cycloleucine-treated oocytes Wtap increased but Lin28 decreased their abundances significantly, along with the higher incidence of spindle defects and chromosome misalignment. Furthermore, pT161-CDK1 protein level in pig oocytes was confirmed to be decreased after cycloleucine treatment for 24 h. Taken together, chemical induced reduction of nucleic acid m⁶A methylation during pig oocyte meiosis could impair meiotic maturation and subsequent development potency, possibly through down-regulating pluripotency marker Lin28 mRNA abundance and disturbing MPF-regulated chromosome/spindle organization.

Comparison of the Oocyte Quality Derived from Two-Dimensional Follicle Culture Methods and Developmental Competence of In Vitro Grown and Matured Oocytes

In vitro follicle growth (IVFG) is an emerging fertility preservation technique, which can obtain fertilizable oocytes from an in vitro culture system in female. This study aimed to compare efficiency of the most widely used two-dimensional follicle culture methods [with or without oil layer (O+ or O- group)]. Preantral follicles were isolated from mice and randomly assigned. Follicles were cultured for 10 days and cumulus-oocyte complexes harvested 16-18 hours after hCG treatment. Follicle and oocyte growth, hormones in spent medium, meiotic spindle localization, expression of reactive oxygen species (ROS), mitochondrial activity, and gene expression were evaluated. In follicle growth, survival, pseudoantral cavity formation, ovulation, and oocyte maturation were also significantly higher in O+ group than O- group. Hormone production was significantly higher in follicles cultured in O+ than O-. There were no significant differences in mRNA expression related to development. On the other hand, the level of ROS was increased while the mitochondrial activity of in vitro grown matured oocyte was less than in vivo matured oocytes. In conclusion, follicle culture with O+ group appears to be superior to the culture in O- group in terms of follicle growth, development, oocyte growth, maturation, and microorganelles in oocyte.

Importance of oil overlay for production of porcine embryos in vitro

Technologies to edit the zygote genome have revolutionized biomedical research not only for the creation of animal models for the study of human disease but also for the generation of functional human cells and tissues through interspecies blastocyst complementation technology. The pig is the ideal species for these purposes due to its great similarity in anatomy and physiology to humans. Emerging biotechnologies require the use of oocytes and/or embryos of good quality, which might be obtained using in vitro production (IVP) techniques. However, the current porcine embryo IVP systems are still suboptimal and result in low monospermic fertilization and blastocyst formation rates and poor embryo quality. During recent years, intensive investigations have been performed to evaluate the influence of specific compounds on gametes and embryos and to avoid the use of undefined supplements (serum and serum derivate) in the incubation media. However, little consideration has been given to the use of the mineral oil (MO) to overlay incubation droplets, which, albeit being a routine component of the IVP systems, is a totally undefined and thus problematic product for the safety of gametes and embryos. In this review, we provide an overview on the advantages and disadvantages of using MO to cover the incubation media. We also review one important concern in IVP laboratories: the use of oils containing undetected contamination. Finally, we discuss the effects of different types of oils on the in vitro embryo production outcomes and the transfer of compounds from oil into the culture media.

The overlaying oil type influences in vitro embryo production: differences in composition and compound transfer into incubation medium between oils

The oil overlay micro-drop system is widely used for cultures of mammalian gametes and embryos. We evaluated hereby the effects of two unaltered commercial oils- Sigma mineral oil (S-MO) and Nidoil paraffin oil (N-PO)-on in vitro embryo production (IVP) outcomes using a pig model. The results showed that while either oil apparently did not affect oocyte maturation and fertilization rates, S-MO negatively affected embryo cleavage rates, blastocyst formation rates, and, consequently, total blastocyst efficiency of the system. No differences in the oxidation state were found between the oils or culture media incubated under S-MO or N-PO. Although both oils slightly differed in elemental composition, there were no differences in the concentrations of elements between fresh media and media incubated under oils. By contrast, we demonstrated clear oil-type differences in both the composition of volatile organic compounds (VOC) and the transfer of some of these VOC´s (straight-chain alkanes and pentanal and 1,3-diethyl benzene) to the culture medium, which could have influenced embryonic development.

Development of In Vitro Embryo Production System Using Collagen Matrix Gel Attached with Vascular Endothelial Growth Factor Derived from Interleukin-1 Beta-Treated Porcine Endometrial Tissue

The aim of this study was to establish an embryo culture system using collagen gel attached with vascular endothelial growth factor (VEGF) derived from interleukin-1 beta (IL-1β)-treated endometrial tissues from pigs. Endometria were separated from the porcine uterus at the follicular phase of the estrous cycle and were cultured with IL-1β. The collagen gels coincubated with IL-1β-treated endometria (C, without endometrial tissue; CE, with endometrial tissue; and CEI, IL-1β-treated endometrial tissue) were used for embryo culture. We found that, compared with the comparable figures in the control group, prostaglandin synthase-2 (PTGS-2) mRNA was increased in IL-1β-treated endometrial tissue (p < 0.05). The VEGF protein was not observed in collagen gel coincubated without endometrial tissue (C); however, it was detected in collagen gels coincubated with endometrial tissue (CE and CEI). The embryo cleavage rates and blastocyst formation did not differ among the treatment groups. The proportion of blastocysts did not differ among the groups. However, the number of blastocyst cells was significantly (p < 0.05) higher in the CEI group than in the other groups. These results clarify the effects of the intrauterine environment on preimplantation embryos and may be useful in research on the effects of extracellular matrix- and cytokine-treated endometrial tissue on embryo development.

Dimethyl Sulfoxide Perturbs Cell Cycle Progression and Spindle Organization in Porcine Meiotic Oocytes

Meiotic maturation of mammalian oocytes is a precisely orchestrated and complex process. Dimethyl sulfoxide (DMSO), a widely used solvent, drug, and cryoprotectant, is capable of disturbing asymmetric cytokinesis of oocyte meiosis in mice. However, in pigs, DMSO’s effect on oocyte meiosis still remains unknown. We aimed to evaluate if DMSO treatment will affect porcine oocyte meiosis and the underlying molecular changes as well. Interestingly, we did not observe the formation of the large first polar body and symmetric division for porcine oocytes treated with DMSO, contrary to findings reported in mice. 3% DMSO treatment could inhibit cumulus expansion, increase nuclear abnormality, disturb spindle organization, decrease reactive oxygen species level, and elevate mitochondrial membrane potential of porcine oocytes. There was no effect on germinal vesicle breakdown rate regardless of DMSO concentration. 3% DMSO treatment did not affect expression of genes involved in spindle organization (Bub1 and Mad2) and apoptosis (NF-κB, Pten, Bcl2, Caspase3 and Caspase9), however, it significantly decreased expression levels of pluripotency genes (Oct4, Sox2 and Lin28) in mature oocytes. Therefore, we demonstrated that disturbed cumulus expansion, chromosome alignment, spindle organization and pluripotency gene expression could be responsible for DMSO-induced porcine oocyte meiotic arrest and the lower capacity of subsequent embryo development. Our results provide new insights on DMSO’s effect on porcine oocyte meiosis and raise safety concerns over DMSO’s usage on female reproduction in both farm animals and humans.

Rapid and noninvasive technique to assess the metabolomics profile of bovine embryos produced in vitro by Raman spectroscopy

Morphological assessments are used to select embryos with the highest implantation potential, however it is still very limited. The development of new technologies, such as Raman spectroscopy have improved quantitative and qualitative analysis, and consequently led to a better characterization of embryos and improvements on the prediction of their potential. Therefore, we propose a method based on the conventional in vitro culture system of bovine embryos, and the subsequent analysis of the culture media drops by Raman spectroscopy. Our results obtained by PCA analysis clearly showed a separation of the spectral profiles from culture media drops with and without embryos.