Efficient production of live offspring from mouse oocytes vitrified with a novel cryoprotective agent, carboxylated ε-poly-L-lysine

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.

Trehalose in Biomedical Cryopreservation-Properties, Mechanisms, Delivery Methods, Applications, Benefits, and Problems

Cells and tissues are the foundation of translational medicine. At present, one of the main technological obstacles is their preservation for long-term usage while maintaining adequate viability and function. Optimized storage techniques must be developed to make them safer to use in the clinic. Cryopreservation is the most common long-term preservation method to maintain the vitality and function of cells and tissues. But, the formation of ice crystals in cells and tissues is considered to be the main mechanism that could harm cells and tissues during freezing and thawing. To reduce the formation of ice crystals, cryoprotective agents (CPAs) must be added to the cells and tissues to achieve the cryoprotective effect. However, conventional cryopreservation of cells and tissues often needs to use toxic organic solvents as CPAs. As a result, cryopreserved cells and tissues may need to go through a time-consuming washing process to remove CPAs for further applications in translational medicine, and multiple valuable cells are potentially lost or killed. Currently, trehalose has been researched as a nontoxic CPA due to its cryoprotective ability and stability during cryopreservation. Nevertheless, trehalose is a nonpermeable CPA, and the lack of an effective intracellular trehalose delivery method has become the main obstacle to its use in cryopreservation. This article illustrated the properties, mechanisms, delivery methods, and applications of trehalose, summarized the benefits and limits of trehalose, and summed up the findings and research direction of trehalose in biomedical cryopreservation.

Carboxylated ε-Poly-l-lysine Improves Post-Thaw Quality, Mitochondrial Functions and Antioxidant Defense of Goat Cryopreserved Sperm

Carboxylated ε-poly-l-lysine (CPLL), a novel cryoprotectant, can protect the sperm membranes by inhibiting ice crystal formation during the cryopreservation process. The present study was conducted to investigate the consequence of CPLL supplementation on the post-thaw quality of cryopreserved goat sperm. For this, different doses (0, 0.5%, 1%, 1.5%, and 2%; v/v) of CPLL were added to the cryopreservation medium, and the motility, membrane and acrosome integrity, mitochondrial membrane potential (MMP), ATP level, ROS production, anti-oxidant defense system, malondialdehyde (MDA) level, and apoptosis in post-thaw sperm were evaluated. It was observed that the addition of 1% CPLL significantly (p < 0.05) increased the total motility, membrane integrity, acrosome integrity, and catalase (CAT) activity of post-thaw sperm compared to those of control and other CPLL doses. The ATP content was observed significantly (p < 0.05) higher in 0.5% and 1% CPLL, however, the SOD activity and progressive motility were significantly (p < 0.05) increased by adding CPLL at 1% and 1.5% level. Moreover, the addition of CPLL at 1% dose not only showed a lower percentage of apoptosis, but also significantly (p < 0.05) increased the MMP while reducing ROS production and MDA levels compared to those of other CPLL doses and/or control. Therefore, it is clear that the supplementation of 1% CPLL can remarkably improve the post-thaw goat sperm motility, membrane and acrosome integrity, antioxidant abundance, mitochondrial potentials, and ATP supply by protecting the sperm from cryodamage and undergoing apoptosis. These findings will provide novel insights into sperm cryobiology.

Assay-ready Cryopreserved Cell Monolayers Enabled by Macromolecular Cryoprotectants

Cell monolayers underpin the discovery and screening of new drugs and allow for fundamental studies of cell biology and disease. However, current cryopreservation technologies do not allow cells to be stored frozen while attached to tissue culture plastic. Hence, cells must be thawed from suspension, cultured for several days or weeks, and finally transferred into multiwell plates for the desired application. This inefficient process consumes significant time handling cells, rather than conducting biomedical research or other value-adding activities. Here, we demonstrate that a synthetic macromolecular cryoprotectant enables the routine, reproducible, and robust cryopreservation of biomedically important cell monolayers, within industry-standard tissue culture multiwell plates. The cells are simply thawed with media and placed in an incubator ready to use within 24 h. Post-thaw cell recovery values were >80% across three cell lines with low well-to-well variance. The cryopreserved cells retained healthy morphology, membrane integrity, proliferative capacity, and metabolic activity; showed marginal increases in apoptotic cells; and responded well to a toxicological challenge using doxorubicin. These discoveries confirm that the cells are “assay-ready” 24 h after thaw. Overall, we show that macromolecular cryoprotectants can address a long-standing cryobiological challenge and offers the potential to transform routine cell culture for biomedical discovery.

Utility of carboxylated poly L-lysine for the liquid storage of bottlenose dolphin semen

Assisted reproduction techniques are required to maintain a genetically diverse captive population of bottlenose dolphins. These techniques include semen preservation, and liquid storage has been proposed as a suitable alternative to cryopreservation, but the optimum conditions, in terms of temperature, duration, and media, have yet to be fully established. The aim of this study, therefore, was to determine the optimum temperature for the liquid storage of dolphin semen during a 14-day period and the usefulness of carboxylated poly-L-lysine (CPLL) as an additive to the semen extender used for the liquid storage. The semen was collected from a mature male dolphin housed at the Kagoshima Aquarium, Japan, transferred into a Beltsville (BF5F) extender, and analyzed for motility and characteristics after five-fold dilution. The optimum temperature was determined by evaluating sperm viability after liquid storage at 4, 17, or 36 °C, and the usefulness of CPLL was evaluated at concentrations of 0%, 0.5%, 1.0%, 2.0%, and 3.0% (v/v) at the optimum temperature. Sperm stored at 4 ℃ had a greater motility maintenance compared with samples stored at 17 or 36 ℃. The most efficacious storage regimen at various time points occurred when there was addition of CPLL at 1.0% (v/v) in terms of sperm motility and other relevant determinations, with this storage approach having greater efficacy that samples stored without CPLL. The most efficacious processes for preserving bottlenose dolphin sperm functions is storage at 4 °C and with there being semen extender supplementation of 1% CPLL.

Antifreeze proteins for low-temperature preservation in reproductive medicine: A systematic review over the last three decades

Antifreeze proteins (AFPs) are synthesized by diverse non-mammalian species, allowing them to survive in severely cold environments. Since the 1990s, the scientific literature reports their use for low-temperature preservation of germplasm. The aim of this systematic review was to compile available scientific evidence regarding the use of AFP for low-temperature preservation of several reproductive specimens. Internet databases were consulted using the terms: "antifreeze protein" OR "AFP" OR "antifreeze glycoprotein" OR "AFGP" OR "ice-binding protein" OR "IBP" OR "thermal hysteresis protein" AND "cryopreservation". From 56 articles, 87 experiments testing AFPs in low-temperature preservation of gametes, embryos or reproductive tissues/cells were fully analyzed and outcomes were annotated. A positive outcome was considered as a statistically significant improvement on any parameter evaluated after low-temperature preservation with AFP, whereas a negative outcome included worsening of any evaluated parameter, in comparison to untreated groups or groups treated with a lower concentration of AFP. The findings indicated that research on the use of AFP as a cryoprotectant for reproductive specimens has increased markedly over the past decade. Some experiments reported both positive and negative results, which depended, on AFP concentration in the preservation media. Variation in the outcomes associated with species was also observed. Among the 66 experiments conducted in mammals, 77.3% resulted in positive, and 28.8% in negative outcomes after the use of AFP. In fishes, positive and negative outcomes were observed in 71.4% and 33.3% of 21 experiments, respectively. Most positive outcomes included preserving cell post-warming survival. The beneficial effect of AFP supports its use in cryobiological approaches used in human and veterinary medicines and animal protein industry. Moreover, combination of different AFP types, or AFP with antioxidants, or even the use of AFP-biosimilar, comprise some promising approaches to be further explored in cryopreservation.

Carboxylated Poly-l-Lysine as a Macromolecular Cryoprotective Agent Enables the Development of Defined and Xeno-Free Human Sperm Cryopreservation Reagents

In human sperm cryopreservation, test yolk buffer and human serum albumin have been used as permeating macromolecular-weight cryoprotectants. In clinical reproductive medicine, human serum albumin is frequently used because of low risks of zoonoses and allergic reactions. However, the risk of allogeneic infectious diseases exists, and the supply may be unstable because human serum albumin is derived from human blood. Therefore, the development of xeno-free human sperm cryopreservative reagents that could overcome the aforementioned problems is warranted. We succeeded in developing a new xeno-free and defined sperm cryopreservation reagent containing glycerol, carboxylated poly-l-lysine, and raffinose. The cryopreservation reagent was not significantly different in terms of sperm motility, viability, and DNA fragmentation and was comparable in performance to a commercial cryopreservation reagent containing human serum albumin. Moreover, the addition of saccharides was essential for its long-term storage. These results may help elucidate the unknown function of macromolecular-weight permeating cryoprotective agents.

Principles of Ice-Free Cryopreservation by Vitrification

Vitrification is an alternative to cryopreservation by freezing that enables hydrated living cells to be cooled to cryogenic temperatures in the absence of ice. Vitrification simplifies and frequently improves cryopreservation because it eliminates mechanical injury from ice, eliminates the need to find optimal cooling and warming rates, eliminates the importance of differing optimal cooling and warming rates for cells in mixed cell type populations, eliminates the need to find a frequently imperfect compromise between solution effects injury and intracellular ice formation, and can enable chilling injury to be "outrun" by using rapid cooling without a risk of intracellular ice formation. On the other hand, vitrification requires much higher concentrations of cryoprotectants than cryopreservation by freezing, which introduces greater risks of both osmotic damage and cryoprotectant toxicity. Fortunately, a large number of remedies for the latter problem have been discovered over the past 35 years, and osmotic damage can in most cases be eliminated or adequately controlled by paying careful attention to cryoprotectant introduction and washout techniques. Vitrification therefore has the potential to enable the superior and convenient cryopreservation of a wide range of biological systems (including molecules, cells, tissues, organs, and even some whole organisms), and it is also increasingly recognized as a successful strategy for surviving harsh environmental conditions in nature. But the potential of vitrification is sometimes limited by an insufficient understanding of the complex physical and biological principles involved, and therefore a better understanding may not only help to improve present outcomes but may also point the way to new strategies that may be yet more successful in the future. This chapter accordingly describes the basic principles of vitrification and indicates the broad potential biological relevance of this alternative method of cryopreservation.

Highly successful production of viable mice derived from vitrified germinal vesicle oocytes

The vitrification of immature germinal vesicle (GV) oocytes is an important way to preserve genetic resources and female fertility. However, it is well known that cryopreserved GV oocytes have very poor developmental ability and that further improvement in this technique is needed. We previously reported the successful vitrification of matured mouse oocytes with enclosed cumulus cells using the calcium-free vitrification solution supplemented with ethylene glycol (EG) by the minimal volume cooling (MVC) method. In this study, we investigated whether our method is applicable to the vitrification of mouse oocytes at the GV stage (GV oocytes). Following maturation and fertilization in vitro, vitrified GV oocytes showed high survival (94.3 ± 2.0%) and maturation (94.3 ± 2.1%) rates. Although the fertilization and blastocyst rates of vitrified oocytes (fertilization: 46.6 ± 4.9% and blastocyst: 46.6 ± 3.0%) were significantly lower than those of fresh oocytes (fertilization: 73.0 ± 7.1% and blastocyst: 71.6 ± 8.0%) (P < 0.01), there were no differences in the ability to develop to term between fresh oocytes (50.0 ± 8.4%) and vitrified oocytes (37.5 ± 4.6%) (P > 0.05). In conclusion, we here show, for the first time, the efficient production of live mice derived from vitrified GV oocytes.

Carboxylated ε-poly-L-lysine, a cryoprotective agent, is an effective partner of ethylene glycol for the vitrification of embryos at various preimplantation stages

It has been known that different protocols are used for embryo preservation at different stages due to different sensitivity to the physical and physiological stress caused by vitrification. In this study, we developed a common vitrification protocol using carboxlated ε-poly-l-lysine (COOH-PLL), a new cryoprotective agent for the vitrification of mouse embryos at different stages. The IVF-derived Crl:CD1(ICR) x B6D2F1/Crl pronuclear, 2-cell, 4-cell, and 8-cell, morula and blastocyst stage embryos were vitrified with 15% (v/v) ethylene glycol (EG) and 10% (w/v) COOH-PLL (E15P15) or 15% (v/v) EG and 15% (v/v) dimethyl sulfoxide (E15D15) using the minimal volume cooling method. The survival of vitrified embryos from pronuclear to blastocyst stages was equivalent between E15P15 and E15D15 groups. However, the rate of development to blastocysts was significantly lower in E15P15 than E15D15. The rates of survival and development to blastocysts were dramatically improved by a slight modification of EG and COOH-PLL concentrations (E20P10). After transferring 17 (E20P10) and 15 (E15D15) vitrified/warmed blastocysts, 8 and 7 pups were obtained (47.1% and 46.7%, respectively). Taken together, these results indicate that our vitrification protocol is appropriate for the vitrification of mouse embryos at different stages.

Cryopreservation of Mammalian Oocytes: Slow Cooling and Vitrification as Successful Methods for Cryogenic Storage

Two basic methods for the laboratory-focused cryopreservation of mammalian oocytes are described, based on work with murine oocytes. One method uses a relatively low concentration of the cryoprotectant propanediol plus sucrose and requires controlled rate cooling equipment to achieve a slow cooling rate. This method has also produced live births from cryopreserved human oocytes. The second method, which is described here, employs a high concentration of the cryoprotectant dimethyl sulfoxide plus a low concentration of polyethylene glycol. This is a vitrification method, which involves ultra-rapid cooling by plunging standard straws into liquid nitrogen vapor, hence avoiding the need for specialized equipment, but requires technical ability to manipulate the oocytes quickly in the highly concentrated cryoprotectant solutions. Murine oocytes that have been vitrified using this technique have resulted in live births. Vitrification using other cryoprotectant mixtures is now a popular clinically accepted method for cryobanking of human oocytes.

Molecular Design of Polyampholytes for Vitrification-Induced Preservation of Three-Dimensional Cell Constructs without Using Liquid Nitrogen

Current slow-freezing methods are too inefficient for cryopreservation of three-dimensional (3D) tissue constructs. Additionally, conventional vitrification methods use liquid nitrogen, which is inconvenient and increases the chance of cross-contamination. Herein, we have developed polyampholytes with various degrees of hydrophobicity and showed that they could successfully vitrify cell constructs including spheroids and cell monolayers without using liquid nitrogen. The polyampholytes prevented ice crystallization during both cooling and warming, demonstrating their potential to prevent freezing-induced damage. Monolayers and spheroids vitrified in the presence of polyampholytes yielded high viabilities post-thawing with monolayers vitrified with PLL-DMGA exhibiting more than 90% viability. Moreover, spheroids vitrified in the presence of polyampholytes retained their fusibilities, thus revealing the propensity of these polyampholytes to stabilize 3D cell constructs. This study is expected to open new avenues for the development of off-the-shelf tissue engineering constructs that can be prepared and preserved until needed.

Post-Thaw Culture and Measurement of Total Cell Recovery Is Crucial in the Evaluation of New Macromolecular Cryoprotectants

The storage and transport of cells is a fundamental technology which underpins cell biology, biomaterials research, and emerging cell-based therapies. Inspired by antifreeze and ice-binding proteins in extremophiles, macromolecular (polymer) cryoprotectants are emerging as exciting biomaterials to enable the reduction and/or replacement of conventional cryoprotective agents such as DMSO. Here, we critically study post-thaw cellular outcomes upon addition of macromolecular cryoprotectants to provide unambiguous evidence that post-thaw culturing time and a mixture of assays are essential to claim a positive outcome. In particular, we observe that only measuring the viability of recovered cells gives false positives, even with non-cryoprotective polymers. Several systems gave apparently high viability but very low total cell recovery, which could be reported as a success but in practical applications would not be useful. Post-thaw culture time is also shown to be crucial to enable apoptosis to set in. Using this approach we demonstrate that polyampholytes (a rapidly emerging class of cryoprotectants) improve post-thaw outcomes across both measures, compared to poly(ethylene glycol), which can give false positives when only viability and short post-thaw time scales are considered. This work will help guide the discovery of new macromolecular cryoprotectants and ensure materials which only give positive results under limited outcomes can be quickly identified and removed.

Polyampholytes as Emerging Macromolecular Cryoprotectants

Cellular cryopreservation is a platform technology which underpins cell biology, biochemistry, biomaterials, diagnostics, and the cold chain for emerging cell-based therapies. This technique relies on effective methods for banking and shipping to avoid the need for continuous cell culture. The most common method to achieve cryopreservation is to use large volumes of organic solvent cryoprotective agents which can promote either a vitreous (ice free) phase or dehydrate and protect the cells. These methods are very successful but are not perfect: not all cell types can be cryopreserved and recovered, and the cells do not always retain their phenotype and function post-thaw. This Perspective will introduce polyampholytes as emerging macromolecular cryoprotective agents and demonstrate they have the potential to impact a range of fields from cell-based therapies to basic cell biology and may be able to improve, or replace, current solvent-based cryoprotective agents. Polyampholytes have been shown to be remarkable (mammalian cell) cryopreservation enhancers, but their mechanism of action is unclear, which may include membrane protection, solvent replacement, or a yet unknown protective mechanism, but it seems the modulation of ice growth (recrystallization) may only play a minor role in their function, unlike other macromolecular cryoprotectants. This Perspective will discuss their synthesis and summarize the state-of-the-art, including hypotheses of how they function, to introduce this exciting area of biomacromolecular science.

Effect of carboxylated poly l-Lysine as a cryoprotectant on post-thaw quality and in vivo fertility of Nili Ravi buffalo (Bubalus bubalis) bull semen

Buffalo bull sperm are more prone to cryo-injuries. Glycerol being the most common permeable cryoprotectant exerts cytotoxic effects on sperm which cause a reduction in fertility. Thus, the exploration of new cryoprotectant is needed. For this purpose, we investigated the effect of carboxylated poly l-Lysine (CPLL) as cryoprotectant used with different concentrations of glycerol on post-thaw sperm motility, kinematics, plasma membrane integrity, mitochondrial membrane potential (MMP), lipid peroxidation (LPO), catalase concentration and in vivo fertility of Nili Ravi buffalo bull semen. In experiment 1, semen samples (n = 15, bulls = 3) were diluted with Tris-citrate-egg yolk extender containing different concentration of CPLL [0% (C0), 0.25% (C0.25), 0.5% (C0.5), 0.75% (C0.75), 1% (C1)]. Each concentration of CPLL was added in extender containing either 7% (G7) or 5% (G5) glycerol. Diluted semen samples were cooled and cryopreserved using standard procedures. Post-thaw total and progressive motility, plasma membrane integrity, acrosome integrity, and MMP were found higher (P < 0.05) in group (G5C0.75) containing 0.75% CPLL and 5% glycerol as compared to the control group (G7C0) and other groups while LPO was recorded lower (P < 0.05) in the same group (G5C0.75). In experiment 2, in vivo fertility was compared between G5C0.75 (5% Glycerol+ 0.75% CPLL; depicted better post-thaw quality) and control group G7C0. Buffaloes were inseminated after 24 h of onset of estrus. Pregnancy diagnosis was performed per rectum at least 60 days post insemination. The fertility rates [56% (58/102) vs. 36% (37/103)] were higher (P < 0.05) in G5C0.75 as compared to the control group G7C0. Based upon these results, this study concludes that the addition of 0.75% CPLL in combination with 5% glycerol in freezing extender improves the post-thaw structure, function and in vivo fertility of Nili Ravi buffalo bull semen.

Microfluidic method reduces osmotic stress injury to oocytes during cryoprotectant addition and removal processes in porcine oocytes

Oocyte cryopreservation is an important technology in assisted reproduction and fertility preservation. However, the developmental potential of cryopreserved oocyte remains poor. Osmotic stress injury (OSI) during cryoprotectants (CPAs) loading and unloading steps has critical impact on successful cryopreservation. In order to minimize OSI to oocytes, a microfluidic device was designed and fabricated to achieve continuous CPA concentration change. MII porcine oocytes were loaded and unloaded CPAs with step-wise and microfluidic methods, oocyte volume changes were recorded and compared, loading and unloading duration of microfluidic methods were optimized. The survival and developmental rate of treated oocytes in step-wise and microfluidic linear methods were also evaluated. The results showed that oocyte volume changes with microfluidic method were obviously less than step-wise method, and the survival, cleavage and blastocyst rate of oocytes were 95.3%, 64.4%, and 19.4%, respectively, which were significantly higher than the traditional step-wise method (79.4%, 43.6%, and 9.7%) (p < 0.05). In conclusion, microfluidic device can effectively reduce the osmotic damage to oocytes and improve the survival rate and developmental rate of oocytes, which may provide a new path for oocyte cryopreservation.

Supplementary cryoprotective effect of carboxylated ε-poly-l-lysine during vitrification of rat pancreatic islets

This study was designed to investigate whether cryosurvival of rat pancreatic islets can be improved by carboxylated ε-poly-l-lysine (CPLL). Islets isolated from Wistar × Brown-Norway F1 rats (101-200 μm in diameter) were cryopreserved in three vitrification solutions containing ethylene glycol (EG; 30%, v/v) and CPLL (0%, 10%, or 20%, v/v) by Cryotop^® protocol (10 islets per device). The post-warm survival rate of the islets vitrified in the presence of 20% CPLL (74%), assessed by FDA/PI double staining, was higher than those in 0% and 10% CPLL (65% and 66%, respectively). Decreased EG concentrations (10% and 20%) in the presence of 20% CPLL resulted in impaired post-warm islet survival rates (50% and 64%, respectively). Value of stimulus index (SI) for 20 mM/3 mM glucose-stimulated insulin secretion was 4.1 in islets vitrified-warmed in the presence of 30% EG and 20% CPLL, which was comparable with those in fresh control islets and vitrified islets in 30% EG alone (4.1 and 4.4, respectively). A large number of islets (50 islets per device) could be cryopreserved in the presence of 30% EG and 20% CPLL by using nylon mesh as the device, without considerable loss of post-warm survival (68%) and SI value (3.7). In conclusion, supplementation of antifreeze 20% CPLL was effective in improving the post-warm survival of isolated rat pancreatic islets when vitrification solution containing 30% EG was used.

Synthetic Polyampholytes as Macromolecular Cryoprotective Agents

A series of polyampholytes based on different molar ratios on N, N-dimethylaminopropyl methacrylamide (DMAPMA), acrylic acid (AA), and optionally, N- tert-butylacrylamide ( t-BuAAm), were prepared by free radical copolymerization, and tested as DMSO-free cryoprotective agents for 3T3 fibroblast cells by using a standard freeze-rethaw protocol. Polybetaines prepared by reaction of DMAPMA homo and copolymers with 1,3-propane sultone were used as additional controls. Results showed strong effects of copolymer composition, molecular weight, polymer and NaCl concentrations, on post-thaw cell viability. Binary (DMAPMA/AA) copolymers showed best post-thaw cell viability of 70% at a 30/70 mol % ratio of DMAPMA/AA, which increased to 90% upon introduction of 9 mol % t-BuAAm while maintaining the 30/70 mol % cation/anion ratio. The use of acrylamide linkages in DMAPMA ensures absence of hydrolytic loss of cationic side chains. These polyampholytes were found to decrease ice crystal size and to form a polymer-rich, ice-free layer around cells, reducing damage from intercellular ice crystals during both freezing and thawing steps. These polyampholytes also dehydrate cells during freezing, which helps protect cells from intracellular ice damage. While cell viability immediately after thawing was high, subsequent culturing revealed poor attachment and long-term viability, which is attributed to residual cell damage from intracellular ice formation. Addition of 2 wt % DMSO or 1% BSA to the polymer-based freeze medium was found to mitigate this damage and result in post-thaw viabilities matching those achieved with 10 wt % DMSO.

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.

Current status and future direction of cryopreservation of camelid embryos

Over the past 3 decades, and similar to the horse industry, fresh embryo transfer has been widely practiced on large commercial scales in different camelid species, especially the dromedary camel and alpaca. However, the inability to cryopreserve embryos significantly reduces its broader application, and as such limits the capacity to utilize elite genetic resources internationally. In addition, cryopreservation of the semen of camelids is also difficult, suggesting an extreme sensitivity of the germplasm to cooling and freezing. As a result, genetic resources of camelids must continue to be maintained as living collections of animals. Due to concerns over disease outbreaks such as that of the highly pathogenic Middle East Respiratory Syndrome in the Middle East and Asia, there is an urgent need to establish an effective gene banking system for camelid species, especially the camel. The current review compares and summarizes recent progress in the field of camelid embryo cryopreservation, identifying four possible reasons for the slow development of an effective protocol and describing eight future directions to improve the current protocols. At the same time, the results of a recent dromedary camel embryo transfer study which produced a high morphologic integrity and survival rate of Open Pulled Straw-vitrified embryos are also discussed.