Use of membrane transport models to design cryopreservation procedures for oocytes

Oocyte cryopreservation is increasingly being used in reproductive technologies for conservation and breeding purposes. Further development of oocyte cryopreservation techniques requires interdisciplinary insights in the underlying principles of cryopreservation. This review aims to serve this purpose by: (1) highlighting that preservation strategies can be rationally designed, (2) presenting mechanistic insights in volume and osmotic stress responses associated with CPA loading strategies and cooling, and (3) giving a comprehensive listing of oocyte specific biophysical membrane characteristics and commonly used permeation model equations. It is shown how transport models can be used to simulate the behavior of oocytes during cryopreservation processing steps, i.e., during loading of cryoprotective agents (CPAs), cooling with freezing as well as vitrification, warming and CPA unloading. More specifically, using defined cellular and membrane characteristics, the responses of oocytes during CPA (un)loading were simulated in terms of temperature- and CPA type-and-concentration-dependent changes in cell volume and intracellular solute concentration. In addition, in order to determine the optimal cooling rate for slow programmable cooling cryopreservation, the freezing-induced cell volume response was simulated at various cooling rates to estimate rates with tolerable limits. For vitrification, special emphasis was on prediction of the timing of reaching osmotic tolerance limits during CPA exposure, and the need to use step-wise CPA addition/removal protocols. In conclusion, we present simulations and schematic illustrations that explain the timing of events during slow cooling cryopreservation as well as vitrification, important for rationally designing protocols taking into account how different CPA types, concentrations and temperatures affect the oocyte.

Electroporation-mediated genome editing in vitrified/warmed porcine zygotes obtained in vitro

Clustered regularly interspaced short palindromic repeats (CRISPR)-associated 9 (Cas9) system is the most efficient and widely used technology for genome editing in all sorts of organisms, including livestock animals. Here, we examined the feasibility of CRISPR/Cas9-derived genome editing (GE) in vitrified porcine zygotes, where the flexible planning of experiments in time and space is expected. OCT4 and CD46 genes were targeted, and the Cas9/sgRNA ribonucleoprotein complexes (RNP) were electroporated into zygotes at 2 h after warming. Vitrification or GE alone did not significantly reduce the developmental rates to the blastocyst stage. However, vitrification followed by GE significantly reduced blastocyst development. Sequencing analysis of the resultant blastocysts revealed efficient GE for both OCT4 (nonvitrified: 91.0%, vitrified: 95.1%) and CD46 (nonvitrified: 94.5%, vitrified: 93.2%), with no significant difference among them. Immunocytochemical analysis showed that GE-blastocysts lacked detectable proteins. They were smaller in size, and the cell numbers were significantly reduced compared with the control (p < 0.01). Finally, we demonstrated that double GE efficiently occurs (100%) when the OCT4-RNP and CD46-RNP are simultaneously introduced into zygotes after vitrification/warming. This is the first demonstration that vitrified porcine zygotes can be used in GE as efficiently as nonvitrified ones.

Vitrification of immature oocytes in pigs

Cryopreservation of oocytes is an important technology for the in vitro gene banking of female germplasm. Although slow freezing is not feasible, porcine oocytes survive vitrification at high rates. Cryopreservation at the germinal vesicle stage appears to be more advantageous than that at the metaphase-II stage. Several factors are considered to affect the success of vitrification and subsequent utilization of immature porcine oocytes such as the device, the protocols for cryoprotectant application, warming, and the post-warming culture. Although live piglets could be obtained from vitrified immature oocytes, their competence to develop to the blastocyst stage is still reduced compared to their non-vitrified counterparts, indicating that there is room for further improvement. Vitrified oocytes suffer various types of damage and alteration which may reduce their developmental ability. Some of these can recover to some extent during subsequent culture, such as the damage of the cytoskeleton and mitochondria. Others such as premature nuclear progression, DNA damage and epigenetic alterations will require further research to be clarified and addressed. To date, the practical application of oocyte vitrification in pigs has been confined to the gene banking of a few native breeds.

Recent advances of oocyte/embryo vitrification in mammals from rodents and large animals

Vitrification is a valuable technology that enables semipermanent preservation and long-distance or international transportation of genetically modified and native animals. In laboratory mice, vitrification maintains and transports embryos, and many institutions and companies sell vitrified embryos. In contrast, despite numerous papers reporting on vitrification in livestock over the past decade, practical implementation has yet to be achieved. However, with advances in genome editing technology, it is anticipated that the number of genetically modified domestic animals will increase, leading to a rise in demand for vitrification of oocytes and embryos. Here, we provide an objective overview of recent advancements in vitrification technology for livestock, drawing a comparison with the current developments in laboratory animals. Additionally, we explore the future prospects for vitrification in livestock, focusing on its potential benefits and drawbacks.

Vitrification of porcine immature oocytes and zygotes results in different levels of DNA damage which reflects developmental competence to the blastocyst stage

The present study investigated the effects of vitrification of porcine oocytes either at the immature Germinal Vesicle (GV) stage before in vitro maturation (GV-stage oocytes) or at the pronuclear stage after in vitro maturation and fertilization (zygotes) on DNA integrity in relevance with their subsequent embryo development. Vitrification at the GV stage but not at the pronuclear stage significantly increased the abundance of double-strand breaks (DSBs) in the DNA measured by the relative fluorescence after γH2AX immunostaining. Treatment of GV-stage oocytes with cryoprotectant agents alone had no effect on DSB levels. When oocytes were vitrified at the GV stage and subjected to in vitro maturation and fertilization (Day 0) and embryo culture, significantly increased DSB levels were detected in subsequent cleavage-stage embryos which were associated with low cell numbers on Day 2, the upregulation of the RAD51 gene at the 4-8 cell stage (measured by RT-qPCR) and reduced developmental ability to the blastocyst stage when compared with the non-vitrified control. However, total cell numbers and percentages of apoptotic cells (measured by TUNEL) in resultant blastocysts were not different from those of the non-vitrified control. On the other hand, vitrification of zygotes had no effect on DSB levels and the expression of DNA-repair genes in resultant embryos, and their development did not differ from that of the non-vitrified control. These results indicate that during vitrification GV-stage oocytes are more susceptible to DNA damages than zygotes, which affects their subsequent development to the blastocyst stage.

Equilibration time with cryoprotectants, but not melatonin supplementation during in vitro maturation, affects viability and metaphase plate morphology of vitrified porcine mature oocytes

The aims of this study were to investigate the effects of different equilibration times with cryoprotectants on viability and metaphase plate morphology of vitrified-warmed porcine mature oocytes (Experiment 1) and to evaluate the effects of supplementation with 10^-9 M melatonin during in vitro maturation on these parameters (Experiment 2). In Experiment 1, 2,392 mature oocytes were vitrified using different equilibration times of oocytes with cryoprotectants (3, 10, 15, 20, 30, 40, 60 and 80 min). Fresh oocytes matured in vitro for 44 hr (n = 509) were used as controls. In Experiment 2, a total of 573 COCs were used. COCs were matured with 10^-9 M melatonin supplementation or without melatonin (control). Some oocytes from each group were vitrified with a 60-min equilibration time with cryoprotectants according to the results of Experiment 1. The remaining oocytes from each maturation group were used as fresh control groups. In both experiments, oocytes were stained with 2',7'-dichlorodihydrofuorescein diacetate and Hoechst 33342 to assess viability and metaphase plate morphology, respectively. Vitrification and warming affected (p < .01) oocyte viability compared with controls, which were all viable after 44 hr of IVM. In Experiment 1, the longer the equilibration time with cryoprotectants, the higher the viability. Oocytes equilibrated for 60 and 80 min had the highest (p < .05) viability and similar metaphase plate characteristics to the fresh control oocytes. In Experiment 2, supplementation with melatonin during in vitro maturation had no effect on oocyte viability or metaphase plate morphology of vitrified-warmed oocytes. In conclusion, under our experimental conditions, vitrified porcine mature oocytes equilibrated with cryoprotectants for 60 or 80 min exhibited the highest viability and similar metaphase plate characteristics to fresh controls. Furthermore, supplementation with 10^-9 M melatonin during in vitro maturation had no effect on these parameters.

Current status of assisted reproductive technologies in buffaloes

Buffaloes are raised by small farm holders primarily as source of draft power owing to its resistance to hot climate, disease, and stress conditions. Over the years, transformation of these animals from draft to dairy was deliberately carried out through genetic improvement program leading to the development of buffalo-based enterprises. Buffalo production is now getting more attention and interest from buffalo raisers due to its socioeconomic impact as well as its contribution to propelling the livestock industry in many developing countries. Reproduction of buffaloes, however, is confronted with huge challenge and concern as being generally less efficient to reproduce compared with cattle due to both intrinsic and extrinsic factors such as poor estrus manifestation, silent heat, marked seasonal infertility, postpartum anestrus, long calving interval, delayed puberty, inherently low number of primordial follicles in their ovaries, high incidence of atresia, and apoptosis. Assisted reproductive technologies (ARTs) are major interventions for the efficient utilization of follicle reserve in buffaloes. The present review focuses on estrus and ovulation synchronization for fixed time artificial insemination, in vitro embryo production, intracytoplasmic sperm injection, cryopreservation of oocytes and embryos, somatic cell nuclear transfer, the factors affecting utilization in various ARTs, and future perspectives in buffaloes.

Oocyte Cryopreservation in Domestic Animals and Humans: Principles, Techniques and Updated Outcomes

Oocyte cryopreservation plays important roles in basic research and the application of models for genetic preservation and in clinical situations. This technology provides long-term storage of gametes for genetic banking and subsequent use with other assisted reproductive technologies. Until recently, oocytes have remained the most difficult cell type to freeze, as the oocytes per se are large with limited surface area to cytoplasm ratio. They are also highly sensitive to damage during cryopreservation, and therefore the success rate of oocyte cryopreservation is generally poor when compared to noncryopreserved oocytes. Although advancement in oocyte cryopreservation has progressed rapidly for decades, the improvement of cryosurvival and clinical outcomes is still required. This review focuses on the principles, techniques, outcomes and prospects of oocyte cryopreservation in domestic animals and humans.

Effect of vitrification at different meiotic stages on epigenetic characteristics of bovine oocytes and subsequently developing embryos

Vitrification by the Cryotop method is frequently used for bovine oocyte cryopreservation. Nevertheless, vitrified oocytes still have reduced developmental competency compared with fresh counterparts. The objective of this study was to compare the effect of vitrification either at the germinal vesicle (GV) stage or at the metaphase II (MII) stage on epigenetic characteristics of bovine oocytes and subsequently developing embryos. Our results demonstrated that vitrification of oocytes at each meiotic stage significantly reduced blastocyst development after in vitro fertilization (IVF). However, vitrification at the GV stage resulted in higher blastocyst development than did vitrification at the MII stage. Irrespective of the meiotic stage, oocyte vitrification did not affect 5-methylcytosine (5mC) immunostaining intensity in oocyte DNA. However, at both stages, it caused a similar reduction of 5mC levels in DNA of subsequently developing blastocysts. Oocyte vitrification had no effect on the intensity of H3K9me3 and acH3K9 immunostaining in oocytes and subsequent blastocysts. The results suggest that irrespective of meiotic stage, oocyte vitrification alters global methylation in resultant embryos although such alteration in the oocytes was not detected. Oocyte vitrification might not influence histone acetylation and methylation in oocytes and resultant embryos. Vitrification at the immature stage was more advantageous for blastocyst development than at the mature stage.

Effects of Porcine Immature Oocyte Vitrification on Actin Microfilament Distribution and Chromatin Integrity During Early Embryo Development in vitro

Vitrification is mainly used to cryopreserve female gametes. This technique allows maintaining cell viability, functionality, and developmental potential at low temperatures into liquid nitrogen at −196°C. For this, the addition of cryoprotectant agents, which are substances that provide cell protection during cooling and warming, is required. However, they have been reported to be toxic, reducing oocyte viability, maturation, fertilization, and embryo development, possibly by altering cell cytoskeleton structure and chromatin. Previous studies have evaluated the effects of vitrification in the germinal vesicle, metaphase II oocytes, zygotes, and blastocysts, but the knowledge of its impact on their further embryo development is limited. Other studies have evaluated the role of actin microfilaments and chromatin, based on the fertilization and embryo development rates obtained, but not the direct evaluation of these structures in embryos produced from vitrified immature oocytes. Therefore, this study was designed to evaluate how the vitrification of porcine immature oocytes affects early embryo development by the evaluation of actin microfilament distribution and chromatin integrity. Results demonstrate that the damage generated by the vitrification of immature oocytes affects viability, maturation, and the distribution of actin microfilaments and chromatin integrity, observed in early embryos. Therefore, it is suggested that vitrification could affect oocyte repair mechanisms in those structures, being one of the mechanisms that explain the low embryo development rates after vitrification.

Cryopreservation and oxidative stress in porcine oocytes

Several vitrification protocols have been established for porcine oocytes so as to facilitate gene banking of female germplasm. Although live piglets have been successfully produced from pig oocytes vitrified at the germinal vesicle (GV) stage, the competence of vitrified oocytes to develop into the blastocyst stage is greatly compromised following cryopreservation. The focus of this review is to elucidate the impact of cryopreservation on the redox balance of pig oocytes, making special reference to the relevance of non-enzymatic and enzymatic antioxidant defences. Besides, the regulation of gene expression in response to oxidative stress is also considered. Finally, we discuss the effects of supplementing maturation and vitrification media with the exogenous non-enzymatic antioxidants that have hitherto yielded the most relevant results.

The effects of vitrification after equilibration in different concentrations of cryoprotectants on the survival and quality of bovine blastocysts

This study assessed the effects of cryoprotectant concentration during equilibration on the efficiency of bovine blastocyst vitrification and the expression of selected developmentally important genes. In vitro produced bovine blastocysts were equilibrated in either 7.5% ethylene glycol (EG) + 7.5% DMSO (Va group) or in 2% EG + 2% DMSO (Vb group) then vitrified on Cryotop® sheets in 16.5% EG + 16.5% DMSO + 0.5M sucrose. After warming, embryos were cultured for 48 hr. Re-expansion, hatching, and the numbers of total and membrane damaged cells were compared among vitrified groups and a control. There was no significant difference between the vitrified groups in survival, cell numbers and the extent of membrane damage. Vitrification increased the number of membrane-damaged cells in both groups, however, in a greater extent in the Vb group. Vitrification increased (p < .05) the expression of the HSP70 gene in Va but not in Vb embryos. The expression of IGF2R, SNRPN, HDAC1, DNMT3B, BAX, OCT4, and IFN-t genes were the same in control and vitrified groups. In conclusion, the concentration of cryoprotectants during equilibration did not affect survival rates; however, normal cell numbers could be maintained only by equilibration in 15% cryoprotectants which was associated with increased HSP70 expression.

Vitrification of porcine cumulus-oocyte complexes at the germinal vesicle stage does not trigger apoptosis in oocytes and early embryos, but activates anti-apoptotic Bcl-XL gene expression beyond the 4-cell stage

The aim of the present study was to clarify whether or not our vitrification procedure at the germinal vesicle (GV)-stage triggers the apoptotic cascade in oocytes and subsequent embryos. Immature porcine cumulus-oocyte complexes were either vitrified and warmed (vitrified group) or subjected to cryoprotectant agents (CPA group) or cultured without any treatment (control). Oocytes of all treatment groups were subjected to in vitro maturation (IVM), fertilization, and embryo culture. Apoptosis was assayed in live oocytes at the end of IVM culture and in cleavage-stage embryos after in vitro fertilization (IVF). We detected similar frequencies of DNA fragmentation, levels of caspase activity, phosphatidylserine externalization, and mRNA levels for pro-apoptotic Bax and CASP3 genes in oocytes at the end of IVM and in early embryos among all groups. However, in the vitrified group, the anti-apoptotic Bcl-XL gene was upregulated in 4–8 cell embryos, which caused an 8-fold significant increase in the Bcl-XL/Bax mRNA ratio compared with the control and CPA groups (P < 0.05). In conclusion, vitrification of porcine oocytes at the GV stage by our method did not trigger the apoptotic cascade in oocytes and subsequent embryos but triggered the upregulation of the anti-apoptotic Bcl-XL gene in embryos.

Cryopreservation of immature oocytes of the indigeneous Vietnamese Ban Pig

We report the cryopreservation of oocytes from Ban miniature pigs which are endemic in Vietnam. Immature cumulus-oocyte complexes were collected from antral follicles of 7-8 mo old female cyclic Ban pigs and vitrified in micro-drops. Oocyte morphology, lipid content, post-warming survival, nuclear maturation, and embryo development were compared to those of oocytes from commercially slaughtered Landrace × Large white hybrid pigs. The size of oocytes in the two breeds was similar. However, significantly lower amounts of intracellular lipid were detected in Ban oocytes. There was no difference (p > 0.05) between Ban and Landrace × Large white oocytes in percentages of post-warming survival (93.1 ± 3.4% vs. 70.7 ± 16.7%, respectively) and nuclear maturation after in vitro maturation (80.4 ± 5.1% vs. 90.0 ± 1.3% respectively). Similarly, cleavage (30.8 ± 7.8% vs. 10.3 ± 6.1%, respectively) and blastocyst development rates (9.4 ± 5.0% vs. 0.79 ± 0.79, respectively) were not different (p > 0.05) between vitrified Ban and Landrace × Large white oocytes after in vitro fertilization and embryo culture. In conclusion, high survival and maturation rates were achieved after vitrification of immature Ban oocytes and their cryo-tolerance was similar to that of Landrace × Large white oocytes, despite the difference in lipid content. We succeeded to generate reasonable rates of blastocysts from vitrified Ban oocytes by in vitro fertilization.

Effect of Cytochalasin B Treatment on the Improvement of Survival Rate in Vitrified Pig Oocyte

To improve survival rates of vitrified pig oocytes, the treatment of cytoskeletal stabilizer on an appropriate time is one of the possible approaches. However, the exact treatment timing and effect of cytoskeletal stabilizer such as cytochalasin B (CB) is not well known during oocyte vitrification procedures. Thus, the present study was conducted to determine optimal treatment timing of CB during vitrification and warming procedures. In experiment 1, the survival rates of the post-warming pig oocytes were analyzed by fluorescein diacetate (FDA) assays with 4 classifications. In results, post-warming oocytes showed significantly (p<0.05) decreased number of alive oocytes (31.8% vs. 86.4%) compared to fresh control. In detail, the significant difference (p<0.05) was found only in strong fluorescence (18.2% vs. 70.5%) not in intermediate fluorescence groups (13.6% vs. 15.9%). In experiment 2, CB was treated before (CB-Vitri) and after (Vitri-CB) vitrification. In results, group of Vitri-CB showed significantly (p<0.05) higher (91.6%) survival rates compared to group of CB-Vitri (83.7%), significantly (p<0.05) and comparable with group of Vitri Control (88.7%) by morphological inspection. In FDA assay results, group of Vitri-CB showed significantly (p<0.05) higher (44.2%) survival rates compared to groups of CB-Vitri (36.7%) and Vitri Control (35.1%). In conclusion, the increased survival rates of post-warming pig oocyte treated with Vitri-CB method are firstly described here. The main finding of present study is that the CB treatment during recovery could be helpful to refresh the post-warming pig oocyte resulting its improved survival rates.

Preincubation with glutathione ethyl ester improves the developmental competence of vitrified mouse oocytes

PURPOSE

Oocyte vitrification is currently used for human fertility preservation. However, vitrification damage is a problem caused by decreasing ooplasmic levels of glutathione (GSH). The GSH donor glutathione ethyl ester (GSH-OEt) can significantly increase the GSH content in oocytes. However, it is difficult to obtain oocyte from woman. To overcome this, we used mouse oocytes to replace human oocytes as a model of study.

METHODS

Oocytes from B6D2F1 mice were preincubated for 30 min with 2.5 mmol/L GSH-OEt (GSH-OEt group), without GSH-OEt preincubation before vitrification (control vitrification group) or in nonvitrified oocytes (fresh group). After thawing, oocytes were fertilized for evaluating the developmental competence of embryos in vitro and in vivo. Immunofluorescence, Polscope equipment and quantitative reverse transcription polymerase chain reaction (RT-qPCR) were used to analyze damage, including mitochondrial distribution, reactive oxygen species (ROS) levels, spindle morphology, and gene expression levels (Bcl-2, BAX, and MnSOD).

RESULTS

The rates of fertilization, 3-4 cell, blastocyst formation and expanded blastocysts were significantly higher (p < 0.05) in the GSH-OEt group (90.4%; 91.1%; 88.9% and 63.0%) than in the control (80.0%; 81.4%; 77.7% and 50.5%). Provided embryos overcame the 2-cell block and developed to the blastocyst stage, birth rates of all groups were similar. Vitrification altered mitochondrial distribution, increased ROS levels, and caused abnormal spindle morphology; GSH-OEt preincubation could improve such damage. RT-qPCR showed that the expression of Bcl-2 was lower in the control group compared with the GSH-OEt group; BAX and MnSoD expression levels were higher in the control group than in the GSH-OEt group (p < 0.05).

CONCLUSIONS

The beneficial effect of GSH-OEt preincubation occurred before the 2-cell stage.

Successful vitrification of pronuclear-stage pig embryos with a novel cryoprotective agent, carboxylated ε-poly-L-lysine

Vitrification is a powerful tool for the efficient production of offspring derived from cryopreserved oocytes or embryos in mammalian species including domestic animals. Genome editing technologies such as transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPR)/ CRISPR-associated (Cas)9 are now available even for domestic species, suggesting that the vitrification of embryos at the pronuclear stage (PN) will be more important because they could provide genomic host cells to be targeted by TALENs or CRISPR/Cas9. Although we reported the successful production of piglets derived from vitrified PN embryos by a solid-surface vitrification method with glutathione supplementation, further improvements are required. The cryoprotective agent (CPA) carboxylated ε-poly-L-lysine (COOH-PLL) was introduced in 2009. COOH-PLL reduces the physical and physiological damage caused by cryopreservation in mammalian stem cells and the vitrification of mouse oocytes and embryos. Those results suggested that vitrification of COOH-PLL may help improve the developmental ability of pig embryos vitrified at the PN stage. However, it remains unclear whether COOH-PLL is available as a CPA for the vitrification of embryos in domestic species. In this study, we evaluated COOH-PLL as a CPA with ethylene glycol (EG) and Cryotop as a device for the vitrification of PN pig embryos. Exposure to vitrification solution supplemented with COOH-PLL up to 30% did not decrease developmental ability to the 2-cell stage and the blastocyst stage. After warming, most of the vitrified embryos survived regardless of the concentration of COOH-PLL (76.0 ± 11.8% to 91.8 ± 4.6%). However, the vitrified embryos without COOH-PLL showed a lower development rate up to the blastocyst stage (1.3 ± 1.0%) compared to the fresh embryos (28.4 ± 5.0%) (p<0.05). In contrast, supplementation of 20% (w/v) COOH-PLL in the vitrification solution dramatically improved the developmental ability to blastocysts of the vitrified embryos (19.4 ± 4.6%) compared to those without COOH-PLL (p<0.05). After the transfer of embryos vitrified with 30% (v/v) EG and 20% (w/v) COOH-PLL, we successfully obtained 15 piglets from 8 recipients. Taken together, our present findings demonstrate for the first time that COOH-PLL is an effective CPA for embryo vitrification in the pig. COOH-PLL is a promising CPA for further improvements in the vitrification of oocytes and embryos in mammalian species.

Vitrification of porcine immature oocytes: Association of equilibration manners with warming procedures, and permeating cryoprotectants effects under two temperatures

The aim of this study was to evaluate the association of equilibration manners with warming procedures, and the different permeating cryoprotectants (pCPAs) effects under two temperatures, in terms of survival, maturation and subsequent parthenogenetic development of porcine immature oocytes after Cryotop vitrification. In Experiment 1, oocytes were equilibrated by exposure to 5% (v/v) ethylene glycol (EG) for 10 min (EM1) or stepwise to 7.5% (v/v) and 15% (v/v) EG for 2.5 min respectively (EM2). Warming procedures were performed in 1.0 M sucrose for 1 min, then in 0.5 and 0.25 M sucrose for 2.5 min respectively (WP1), or in 0.5, 0.25 and 0.125 M sucrose each step for 2 min (WP2), or in 0.25, 0.125 and 0.063 M sucrose each step for 2 min (WP3). After 2 h of warming, the survival rate of oocytes treated by EM1 and WP1 was significantly higher (P < 0.05) than that of the other groups. Moreover, a similar proportion of survival and nuclear maturation in all vitrified groups was obtained after completion of the IVM. No significant difference in blastocyst development was observed among vitrified groups except the group treated by EM2 and WP3. In Experiment 2, oocytes were vitrified by using EG alone, EG combined with dimethyl sulphoxide (EG + DMSO) or propylene glycol (EG + PROH) as pCPAs under 25 °C and 39 °C. The percentages of cryosurvival and nuclear maturation were similar in all vitrified groups. Under 25 °C, the embryo development and total cell numbers of blastocysts were not significantly different among EG, EG + DMSO and EG + PROH groups. However, the application of EG + PROH at 39 °C resulted in significantly decreased both cleavage and blastocyst formation rates. In conclusion, our data showed that equilibration manner and warming procedure affect the cryosurvival of porcine immature oocytes, and the combination of pCPAs cannot give a better cryopreservation outcome whether 25 °C or 39 °C. Notably, the Cryotop vitrification accompanied by our modified strategy for porcine immature oocytes could achieve high survival and respectable blastocyst production.

Contribution of in vitro systems to preservation and utilization of porcine genetic resources

Historically, the conservation or preservation of mammalian genetic resources, especially farm animals, has been conducted under in situ conditions by maintaining living individuals as "livestock." However, systems for laboratory in vitro embryo production using gametes such as spermatozoa and oocytes are now available, in addition to ex situ preservation methods for mammalian genetic resources. One of these methods is the cryopreservation of gametes, embryos, and gonadal tissues. In pigs, freezing of sperm is the most reliable and well-established method for this purpose. On the other hand, cryopreservation of female gametes (oocytes) and gonadal tissues-usually by vitrification-has been associated with very low efficacies. Recently, in our laboratory, some research themes related to this issue have been pursued. We have been focusing on advances in porcine in vitro embryo production systems, and here, we introduce recent data on the vitrification of porcine immature oocytes and gonadal tissues followed by their xenografting into host mice to produce gametes.

Optimization of cryoprotectant treatment for the vitrification of immature cumulus-enclosed porcine oocytes: comparison of sugars, combinations of permeating cryoprotectants and equilibration regimens

Our aim was to optimize the cryoprotectant treatment for the preservation of immature porcine cumulus-oocyte complexes (COCs) by solid surface vitrification. In each experiment, the vitrification solution consisted of 50 mg/ml polyvinyl pyrrolidone, 0.3 M of the actual sugar and in total 35% (v/v) of the actual permeating cryoprotectant (pCPA) combination. After warming, the COCs were subjected to in vitro maturation, fertilization and embryo culture. In Experiment 1, trehalose and sucrose were equally effective during vitrification and warming in terms of facilitating oocyte survival and subsequent embryo development. In Experiment 2, when equilibration was performed at 38.5 C in a total of 4% (v/v) pCPA for 15 min, the combination of ethylene glycol and propylene glycol (EG + PG = 1:1) was superior to EG and dimethyl sulfoxide (EG + DMSO = 1:1) in terms of oocyte survival after vitrification and the quality of resultant blastocysts. In Experiment 3, equilibration in 4% (v/v) pCPA for 15 min before vitrification was superior to that in 15% (v/v) CPA for 5 min for achievement of high survival rates irrespective of the pCPA combination used. In Experiment 4, when equilibration was performed in 4% EG + PG for 5 min, 15 min or 25 min, there was no difference in oocyte survival and subsequent embryo development after vitrification and warming; however, the developmental competence of cleaved embryos was tendentiously reduced when equilibration was performed for 25 min. In conclusion, trehalose and sucrose were equally effective in facilitating vitrification, and the optimum pCPA treatment was 5–15 min equilibration in 4% (v/v) of EG + PG followed by vitrification in 35% (v/v) EG + PG.

Generation of live piglets from cryopreserved oocytes for the first time using a defined system for in vitro embryo production

We report the successful piglet production from cryopreserved oocytes for the first time by using a simple, high capacity vitrification protocol for preservation and a defined system for in vitro embryo production. Immature cumulus-oocyte complexes (COCs) from prepubertal gilts were vitrified in microdrops and stored in liquid nitrogen. After warming, COCs were subjected to in vitro maturation (IVM), fertilization (IVF), and subsequent culture (IVC). Adjusting warmplate temperature to 42°C during warming prevented temperature drops in a medium below 34.0°C and significantly increased the percentage of oocyte survival and thus blastocyst yields obtained from total vitrified oocytes compared with that of warming at 38°C (87.1% vs 66.9% and 4.4% vs 2.7%, respectively). Nuclear maturation and fertilization of oocytes were not affected by vitrification and warming temperature. Blastocyst development on day 7 (day 0 = IVF) of the surviving oocytes after warming at 38°C and 42°C was not different but lower (P<0.05) than those of non-vitrified control oocytes (4.6%, 5.2% and 17.9%, respectively). However, blastocyst cell numbers in the control and vitrified groups were similar irrespective of warming temperature. Omitting porcine follicular fluid (pFF) from IVM medium (POM) did not affect maturation, fertilization and embryo development of vitrified-warmed oocytes. Transfer of blastocysts obtained on day 5 from vitrified oocytes matured either with or without pFF into 4 recipients (2 for each group) resulted in 4 pregnancies and the delivery of a total of 18 piglets. In conclusion, optimization of warming temperature was a key factor for achieving high survival rates, and surviving oocytes could be utilized in vitro using defined media. Using these modifications, live piglets could be obtained from cryopreserved oocytes for the first time.

Recent progress in cryopreservation of bovine oocytes

Principle of oocyte cryoinjury is first overviewed and then research history of cryopreservation using bovine oocytes is summarized for the last two decades with a few special references to recent progresses. Various types of cryodevices have been developed to accelerate the cooling rate and applied to the oocytes from large domestic species enriched with cytoplasmic lipid droplets. Two recent approaches include the qualitative improvement of IVM oocytes prior to the vitrification and the short-term recovery culture of vitrified-warmed oocytes prior to the subsequent IVF. Supplementation of L-carnitine to IVM medium of bovine oocytes has been reported to reduce the amount of cytoplasmic lipid droplets and improve the cryotolerance of the oocytes, but it is still controversial whether the positive effect of L-carnitine is reproducible. Incidence of multiple aster formation, a possible cause for low developmental potential of vitrified-warmed bovine oocytes, was inhibited by a short-term culture of the postwarm oocytes in the presence of Rho-associated coiled-coil kinase (ROCK) inhibitor. Use of an antioxidant α-tocopherol, instead of the ROCK inhibitor, also supported the revivability of the postwarm bovine oocytes. Further improvements of the vitrification procedure, combined with pre- and postvitrification chemical treatment, would overcome the high sensitivity of bovine oocytes to cryopreservation.

Effects of resveratrol on vitrified porcine oocytes

Vitrified MII porcine oocytes are characterized by reduced developmental competence, associated with the activation of the apoptotic pathway. Resveratrol (R), a polyphenolic compound present in several vegetal sources, has been reported to exert, among all its other biological effects, an antiapoptotic one. The aim of this study was to determine the effects of R (2  µM) on the apoptotic status of porcine oocytes vitrified by Cryotop method, evaluating phosphatidylserine (PS) exteriorization and caspases activation. R was added during IVM (A); 2 h postwarming incubation (B); vitrification/warming and 2 h postwarming incubation (C); all previous phases (D). Data on PS exteriorization showed, in each treated group, a significantly higher (P < 0.05) percentage of live nonapoptotic oocytes as compared with CTR; moreover, the percentage of live apoptotic oocytes was significantly (P < 0.05) lower in all R-treated groups relative to CTR. The results on caspase activation showed a tendency to an increase of viable oocytes with inactive caspases in B, C, and D, while a significant (P < 0.05) increase in A compared to CTR was recorded. These data demonstrate that R supplementation in various phases of IVM and vitrification/warming procedure can modulate the apoptotic process, improving the resistance of porcine oocytes to cryopreservation-induced damage.

Comparison of cytoskeletal integrity, fertilization and developmental competence of oocytes vitrified before or after in vitro maturation in a porcine model

Aim of the study was to investigate the effect of vitrification on viability, cytoskeletal integrity and in vitro developmental competence after in vitro fertilization (IVF) of oocytes vitrified before or after in vitro maturation (IVM) using a pig model. Oocytes from abattoir-derived porcine ovaries were vitrified at either the germinal vesicle (GV) or metaphase II (MII) stage by modified solid surface vitrification (SSV). Oocyte viability was evaluated by stereomicroscopic observation whereas their nuclear stage and morphology of microtubules and F-actin were observed by confocal microscopy after immunostaining. Fertilization was assessed by orcein staining. The survival rate after vitrification was higher for MII-stage than for GV-stage oocytes. However, the ability of surviving oocytes to reach the MII stage after vitrification at the GV stage (GV-vitrified oocytes) was similar to that of control oocytes. Furthermore, after IVM, GV-vitrified oocytes had better spindle and F-actin integrity than oocytes vitrified at the MII stage (MII-vitrified oocytes). In accordance with this result, GV-vitrified oocytes had better ability to extrude the second polar body and support male pronucleus formation after in vitro fertilization (IVF), in comparison to MII-vitrified oocytes. Fertilization rates did not differ among groups. Finally, the ability of GV-vitrified oocytes to develop into embryos was superior to that of MII-vitrified oocytes. However, both vitrified groups showed reduced blastocyst development compared with the control group. In conclusion vitrification of porcine oocytes at the GV stage is advantageous in conferring better cytoskeletal organization and competence to develop to the blastocyst stage in comparison with vitrification at the MII stage.

Nuclear maturation and embryo development of porcine oocytes vitrified by cryotop: effect of different stages of in vitro maturation

The present study was designed to evaluate the viability, meiotic competence and subsequent development of porcine oocytes vitrified using the cryotop method at different stages of in vitro maturation (IVM). Cumulus-oocyte complexes (COCs) were cultured in IVM medium supplemented with 1mM dibutyryl cAMP (dbcAMP) for 22 h and then for an additional 22 h without dbcAMP in the medium. Germinal vesicle (GV), germinal vesicle breakdown (GVBD), metaphase I (MI), anaphase I/telophase I (AI/TI) and metaphase II (MII) were found to occur predominantly at 0-22, 26, 32, 38 and 44 h of IVM, respectively. Oocytes were exposed to cryoprotectant (CPA) or vitrified after different durations of IVM (0, 22, 26, 32, 38 and 44 h). After CPA exposure and vitrification, surviving oocytes that were treated before completion of the 44 h maturation period were placed back into IVM medium for the remaining maturation period, and matured oocytes were incubated for 2h. CPA treatment did not affect the viability of oocytes matured for 26, 32, 38 or 44 h, but significantly decreased survival rate of oocytes matured for 0 or 22 h. CPA treatment had no effect on the ability of surviving oocytes to develop to the MII stage regardless of the stage during IVM; however, blastocyst formation following PA was severely lower (P<0.05) than that in the control. At 2h post-warming, the survival rates of oocytes vitrified at 26, 32, 38 and 44 h of IVM were similar but were higher (P<0.05) than those of oocytes vitrified at 0 or 22 h of IVM. The MII rates of surviving oocytes vitrified at 0 and 38 h of IVM did not differ from the control and were higher (P<0.05) than those of oocytes vitrified at 22, 26 or 32 h of IVM. After parthenogenetic activation (PA), both cleavage and blastocyst rates of vitrified oocytes matured for 22, 26, 32, 38 and 44 h did not differ, but all were lower (P<0.05) than those matured 0 h. In conclusion, our data indicate that survival, nuclear maturation and subsequent development of porcine oocytes may be affected by their stage of maturation at the time of vitrification; a higher percentage of blastocyst formation can be obtained from GV oocytes vitrified before the onset of maturation.

Comparison of ethylene glycol and propylene glycol for the vitrification of immature porcine oocytes

Our aim was to optimize a cryoprotectant treatment for vitrification of immature porcine cumulus-oocyte complexes (COCs). Immature COCs were vitrified either in 35% ethylene glycol (EG), 35% propylene glycol (PG) or a combination of 17.5% EG and 17.5% PG. After warming, the COCs were in vitro matured (IVM), and surviving oocytes were in vitro fertilized (IVF) and cultured. The mean survival rate of vitrified oocytes in 35% PG (73.9%) was higher (P<0.05) than that in 35% EG (27.8%). Oocyte maturation rates did not differ among vitrified and non-vitrified control groups. Blastocyst formation in the vitrified EG group (10.8%) was higher (P<0.05) than that in the vitrified PG group (2.0%) but was lower than that in the control group (25.0%). Treatment of oocytes with 35% of each cryoprotectant without vitrification revealed a higher toxicity of PG on subsequent blastocyst development compared with EG. The combination of EG and PG resulted in 42.6% survival after vitrification. The maturation and fertilization rates of the surviving oocytes were similar in the vitrified, control and toxicity control (TC; treated with EG+PG combination without cooling) groups. Blastocyst development in the vitrified group was lower (P<0.05) than that in the control and TC groups, which in turn had similar development rates (10.7%, 18.1% and 23.3%, respectively). In conclusion, 35% PG enabled a higher oocyte survival rate after vitrification compared with 35% EG. However, PG was greatly toxic to oocytes. The combination of 17.5% EG and 17.5% PG yielded reasonable survival rates without toxic effects on embryo development.

Vitrification procedure decreases inositol 1,4,5-trisphophate receptor expression, resulting in low fertility of pig oocytes

Although cryopreservation of mammalian oocytes is an important technology, it is well known that unfertilized oocytes, especially in pigs, are highly sensitive to low temperature and that cryopreserved oocytes show low fertility and developmental ability. The aim of the present study was to clarify why porcine in vitro matured (IVM) oocytes at the metaphase II (MII) stage showed low fertility and developmental ability after vitrification. In vitro matured cumulus oocyte complexes (COCs) were vitrified with Cryotop and then evaluated for fertility through in vitro fertilization (IVF). Although sperm-penetrated oocytes were observed to some extent (30-40%), the rate of pronuclear formation was low (9%) and none of them progressed to the two-cell stage. The results suggest that activation ability of cryopreserved oocytes was decreased by vitrification. We examined the localization and expression level of the type 1 inositol 1,4,5 trisphosphate receptor (IP3 R1), the channel responsible for Ca(2+) release during IVF in porcine oocytes. Localization of IP3 R1 close to the plasma membrane and total expression level of IP3 R1 protein were both decreased by vitrification. In conclusion, our present study indicates that vitrified-warmed porcine COCs showed a high survival rate but low fertility after IVF. This low fertility seems to be due to the decrease in IP3 R1 by the vitrification procedure.