Improved vitrification solutions based on the predictability of vitrification solution toxicity

Long-term preservation of complex engineered tissues and organs at cryogenic temperatures in the absence of ice has been prevented to date by the difficulty of discovering combinations of cryoprotectants that are both sufficiently non-toxic and sufficiently stable to allow viability to be maintained and ice formation to be avoided during slow cooling to the glass transition temperature and subsequent slow rewarming. A new theory of the origin of non-specific cryoprotectant toxicity was shown to account, in a rabbit renal cortical slice model, for the toxicities of 20 vitrification solutions and to permit the design of new solutions that are dramatically less toxic than previously known solutions for diverse biological systems. Unfertilized mouse ova vitrified with one of the new solutions were successfully fertilized and regained 80% of the absolute control (untreated) rate of development to blastocysts, whereas ova vitrified in VSDP, the best previous solution, developed to blastocysts at a rate only 30% of that of controls. Whole rabbit kidneys perfused at -3 degrees C with another new solution at a concentration of cryoprotectant (8.4M) that was previously 100% lethal at this temperature exhibited no damage after transplantation and immediate contralateral nephrectomy. It appears that cryoprotectant solutions that are composed to be at the minimum concentrations needed for vitrification at moderate cooling rates are toxic in direct proportion to the average strength of water hydrogen bonding by the polar groups on the permeating cryoprotectants in the solution. Vitrification solutions that are based on minimal perturbation of intracellular water appear to be superior and provide new hope that the successful vitrification of natural organs as well as tissue engineered or clonally produced organ and tissue replacements can be achieved.     

Effect of different cryoprotectants and vitrificant solutions on the hatching rate of turbot embryos (Scophthalmus maximus)

Vitrification could provide a promising tool for the cryopreservation of fish embryos. However, in order to achieve a vitrifiable medium, a high concentration of permeable cryoprotectants must be employed, and the incorporation of high molecular weight compounds should also be considered. The toxicity of these permeable and non-permeable agents has to be assessed, particularly when high concentrations are required. In the present study, permeable and non-permeable cryoprotectant toxicity was determined in turbot embryos at two development stages (F stage-tail bud and G stage-tail bud free). Embryos treated with pronase (2mg/ml, 10 min at 22 degrees C) were incubated in dimethyl sulfoxide (Me2SO), methanol (Meth.) or ethylene glycol (EG) in concentrations ranging from 0.5 to 6M for periods of 10 or 30 min, and in 5, 10, and 15% polyvinylpyrrolidone (PVP), 10, 15, and 20% sucrose or 0.1, 1, and 2% X-1000 for 2 min. The embryos were then washed well and incubated in seawater until hatching. The toxicity of permeable cryoprotectants increased with concentration and exposure time. There were no significant differences between permeable cryoprotectants. However, embryos tolerated higher concentrations of Me2SO than other cryoprotectants. Exposure to permeable cryoprotectants did not affect the hatching rate except at G stage with X-1000 treatment and 20% sucrose. Taking into account the cryoprotectant toxicity and the vitrification ability of cryoprotectant mixtures, three vitrification solutions (V1, V2, and V3), and one protocol for stepwise incorporation were designed. The tested solutions contained 5M Me2SO+2M Meth+1M EG plus 5% PVP, 10% sucrose or 2% X-1000. The hatching rate of embryos that had been exposed to the the vitrification solutions was analyzed and no significant differences were noticed compared with the controls. Our results demonstrate that turbot embryos can be subject to this cryoprotectant protocol without deleterious effect on the hatching rate.     

Physical and biological aspects of renal vitrification

Cryopreservation would potentially very much facilitate the inventory control and distribution of laboratory-produced organs and tissues. Although simple freezing methods are effective for many simple tissues, bioartificial organs and complex tissue constructs may be unacceptably altered by ice formation and dissolution. Vitrification, in which the liquids in a living system are converted into the glassy state at low temperatures, provides a potential alternative to freezing that can in principle avoid ice formation altogether. The present report provides a brief overview of the problem of renal vitrification. We report here the detailed case history of a rabbit kidney that survived vitrification and subsequent transplantation, a case that demonstrates both the fundamental feasibility of complex system vitrification and the obstacles that must still be overcome, of which the chief one in the case of the kidney is adequate distribution of cryoprotectant to the renal medulla. Medullary equilibration can be monitored by monitoring urine concentrations of cryoprotectant, and urine flow rate correlates with vitrification solution viscosity and the speed of equilibration. By taking these factors into account and by using higher perfusion pressures as per the case of the kidney that survived vitrification, it is becoming possible to design protocols for equilibrating kidneys that protect against both devitrification and excessive cryoprotectant toxicity.     

Physical and biological aspects of renal vitrification

Cryopreservation would potentially very much facilitate the inventory control and distribution of laboratory-produced organs and tissues. Although simple freezing methods are effective for many simple tissues, bioartificial organs and complex tissue constructs may be unacceptably altered by ice formation and dissolution. Vitrification, in which the liquids in a living system are converted into the glassy state at low temperatures, provides a potential alternative to freezing that can in principle avoid ice formation altogether. The present report provides a brief overview of the problem of renal vitrification. We report here the detailed case history of a rabbit kidney that survived vitrification and subsequent transplantation, a case that demonstrates both the fundamental feasibility of complex system vitrification and the obstacles that must still be overcome, of which the chief one in the case of the kidney is adequate distribution of cryoprotectant to the renal medulla. Medullary equilibration can be monitored by monitoring urine concentrations of cryoprotectant, and urine flow rate correlates with vitrification solution viscosity and the speed of equilibration. By taking these factors into account and by using higher perfusion pressures as per the case of the kidney that survived vitrification, it is becoming possible to design protocols for equilibrating kidneys that protect against both devitrification and excessive cryoprotectant toxicity.     

Cryoprotectant agent toxicity in porcine articular chondrocytes

Large articular cartilage defects have proven difficult to treat and often result in osteoarthritis of the affected joint. Cryopreservation of articular cartilage can provide an increased supply of tissues for osteochondral allograft but cryoprotective agents are required; however, few studies have been performed on the toxicity of these agents. This study was designed to determine the order of toxicity of five commonly used cryoprotectant agents as well as interactions that occur between them. Isolated porcine articular chondrocytes were exposed to individual cryoprotectant agents and combinations of these agents at 1 M and 3 M concentrations for 5 min and 120 min. Cell viability was determined using membrane integrity dyes and a metabolic activity assay. Subsequently, a regression analysis based study was undertaken to extract the maximum amount of information from this data. Results of this study demonstrated that all 1 M solutions were minimally toxic. The 3 M solutions demonstrated varying toxicity after 120 min. Ethylene glycol and glycerol were less toxic than propylene glycol, dimethyl sulfoxide, and formamide. Combinations of cryoprotectant agents were less toxic than single cryoprotectant agents at the same concentration. This is the most comprehensive study investigating cryoprotectant agent toxicity in articular chondrocytes and has resulted in important information regarding the order of toxicity and interactions that occur between these agents.     

Cryoprotectant agent toxicity in porcine articular chondrocytes

Large articular cartilage defects have proven difficult to treat and often result in osteoarthritis of the affected joint. Cryopreservation of articular cartilage can provide an increased supply of tissues for osteochondral allograft but cryoprotective agents are required; however, few studies have been performed on the toxicity of these agents. This study was designed to determine the order of toxicity of five commonly used cryoprotectant agents as well as interactions that occur between them. Isolated porcine articular chondrocytes were exposed to individual cryoprotectant agents and combinations of these agents at 1 M and 3 M concentrations for 5 min and 120 min. Cell viability was determined using membrane integrity dyes and a metabolic activity assay. Subsequently, a regression analysis based study was undertaken to extract the maximum amount of information from this data. Results of this study demonstrated that all 1 M solutions were minimally toxic. The 3 M solutions demonstrated varying toxicity after 120 min. Ethylene glycol and glycerol were less toxic than propylene glycol, dimethyl sulfoxide, and formamide. Combinations of cryoprotectant agents were less toxic than single cryoprotectant agents at the same concentration. This is the most comprehensive study investigating cryoprotectant agent toxicity in articular chondrocytes and has resulted in important information regarding the order of toxicity and interactions that occur between these agents.     

Physical problems with the vitrification of large biological systems

Vitrification is an attractive potential pathway to the successful cryopreservation of mature mammalian organs, but modern cryobiological research on vitrification to date has been devoted mostly to experiments with solutions and with biological systems ranging in diameter from about 6 through about 100 microns. The present paper focuses on concerns which are particularly relevant to large biological systems, i.e., those systems ranging in size from approximately 10 ml to approximately 1.5 liters. New qualitative data are provided on the effect of sample size on the probability of nucleation and the ultimate size of the resulting ice crystals as well as on the probability of fracture at or below Tg. Nucleation, crystal growth, and fracture depend on cooling velocity and the magnitude of thermal gradients in the sample, which in turn depend on sample size, geometry, and cooling technique (environmental thermal history and thermal uniformity). Quantitative data on thermal gradients, cooling rates, and fracture temperatures are provided as a function of sample size. The main conclusions are as follows. First, cooling rate (from about 0.2 to about 2.5 degrees C/min) has a profound influence on the temperature-dependent processes of nucleation and crystal growth in 47-50% (w/w) solutions of propylene glycol. Second, fracturing depends strongly on cooling rate and thermal uniformity and can be postponed to about 25 degrees C below Tg for a 482-ml sample if cooling is slow and uniform. Third, the presence of a carrier solution reduces the concentration of cryoprotectant needed for vitrification (CV). However, the CV of samples larger than about 10 ml is significantly higher than the CV of smaller samples whether a carrier solution is present or not.     

Japanese flounder (Paralichthys olivaceus) embryos are difficult to cryopreserve by vitrification

The first successful cryopreservation of fish embryos was reported in the Japanese flounder by vitrification [Chen and Tian, Theriogenology, 63, 1207-1219, 2005]. Since very high concentrations of cryoprotectants are needed for vitrification and fish embryos have a large volume, Japanese flounder embryos must have low sensitivity to cryoprotectant toxicity and high permeability to water and cryoprotectants. So, we investigated the sensitivity and the permeability of Japanese flounder embryos. In addition, we assessed the survival of flounder embryos after vitrification with solutions containing methanol and propylene glycol, following Chen and Tian's report. The embryos were relatively insensitive to the toxicity of individual cryoprotectants at lower concentrations, especially methanol and propylene glycol as their report. Although their permeability to water and cryoprotectants could not be measured from volume changes in cryoprotectant solutions, the embryos appeared to be permeable to methanol but less permeable to DMSO, ethylene glycol, and propylene glycol. Although vitrification solutions containing methanol and propylene glycol, which were used in Chen and Tian's report, were toxic to embryos, a small proportion of embryos did survived. However, when vitrified with the vitrification solutions, no embryos survived after warming. The embryos became opaque during cooling with liquid nitrogen, indicating the formation of intracellular ice during cooling. When embryos had been kept in vitrification solutions for 60 min after being treated with the vitrification solution, some remained transparent during cooling, but became opaque during warming. This suggests that dehydration and/or permeation by cryoprotectants were insufficient for vitrification of the embryos even after they had been over-treated with the vitrification solutions. Thus, Chen and Tian's cryopreservation method lacks general application to Japanese flounder embryos.     

In-straw cryoprotectant dilution of IVP bovine blastocysts vitrified in hand-pulled glass micropipettes

The aim of this study was to determine the influence of two ethylene glycol-based vitrification solutions on in vitro and in vivo survival after in-straw cryoprotectant dilution of vitrified in vitro-produced bovine embryos. Day-7 expanded blastocysts were selected according to diameter (> or = 180 microm) and osmotic characteristics and randomly assigned to one of three groups (i) VSa: vitrification in 40% EG+17.1% SUC+0.1% PVA; (ii) VSb: vitrification in 20% EG+20% DMSO; (iii) control: non-vitrified embryos. Vitrification was performed in hand-pulled glass micropipettes (GMP) and cryoprotectant dilution in 0.25 ml straws after warming in a plastic tube. Embryo viability was assessed by re-expansion and hatching rates after 72 h of IVC and by pregnancy rates after direct transfer of vitrified embryos. No differences in re-expansion rates were observed between vitrified groups after 24 h in culture (VSa=84.5%; VSb=94.8%). However, fewer VSa embryos (55.2%, P<0.05) hatched after 72 h than the VSb (75.8%) and control embryos (80.0%). To evaluate in vivo viability, vitrified embryos (VSa=20; VSb=21) were warmed under field conditions and individually transferred to synchronous recipients. Pregnancy rates (day 60) were similar between groups (VSa=20%; VSb=19%). Greater hatching rates occurred after 72 h of IVC for EG+DMSO than EG+SUC+PVA vitrification solutions. However, using a GMP vitrification container and in-tube warming, both solutions provided similar pregnancy rates after the in-straw cryoprotectant dilution and direct embryo transfer.     

Improved cryopreservation by diluted vitrification solution with supercooling-facilitating flavonol glycoside

The effect of kaempferol-7-O-glucoside (KF7G), one of the supercooling-facilitating flavonol glycosides which was originally found in deep supercooling xylem parenchyma cells of the katsura tree and was found to exhibit the highest level of supercooling-facilitating activity among reported substances, was examined for successful cryopreservation by vitrification procedures, with the aim of determining the possibility of using diluted vitrification solution (VS) to reduce cryoprotectant toxicity and also to inhibit nucleation at practical cooling and rewarming by the effect of supplemental KF7G. Examination was performed using shoot apices of cranberry and plant vitrification solution 2 (PVS2) with dilution. Vitrification procedures using the original concentration (100%) of PVS2 caused serious injury during treatment with PVS2 and resulted in no regrowth after cooling and rewarming (cryopreservation). Dilution of the concentration of PVS2 to 75% or 50% (with the same proportions of constituents) significantly reduced injury by PVS2 treatment, but regrowth was poor after cryopreservation. It is thought that dilution of PVS2 reduced injury by cryoprotectant toxicity, but such dilution caused nucleation during cooling and/or rewarming, resulting in poor survival. On the other hand, addition of 0.5 mg/ml (0.05% w/v) KF7G to the diluted PVS2 resulted in significantly (p < 0.05) higher regrowth rates after cryopreservation. It is thought that addition of supercooling-facilitating KF7G induced vitrification even in diluted PVS2 probably due to inhibition of ice nucleation during cooling and rewarming and consequently resulted in higher regrowth. The results of the present study indicate the possibility that concentrations of routinely used VSs can be reduced by adding supercooling-facilitating KF7G, by which more successful cryopreservation might be achieved for a wide variety of biological materials.     

Peculiarities of state diagrams of aqueous solutions of cryoprotective agents

The phase transitions in aqueous solutions of glycerol and PEO-1500 within the temperature range of +30 to −150 °C have been studied using the methods of thermoplastic analysis and volumetric scanning tensodilatometry. We present the revealed phenomenon of cluster cyrystallization of these solutions as well as principles of describing this phenomenon using state diagrams, containing the intervals of concentration corresponding to the existence of amorphous and cryocolloid fractions. We note that for the cryocolloid fraction, a low temperature association of molecules of cryoprotective agents leads the formation of ice nanocrystals either close to or directly inside the aggregations. These fractions exist in cooled cryoprotective solutions near the vitrification temperatures of the liquid phase and may contribute to the initiation of damaging events in cryopreserved biological systems. Our data may be helpful in explaining the peculiarities observed during crystallization of cryoprotective solutions and may further contribute to a broader understanding of the principles of protection and protocol optimization of biological materials at temperatures approaching vitrification.     

Statistical prediction of the vitrifiability and glass stability of multi-component cryoprotective agent solutions

Long-term biologic storage of articular cartilage has proven elusive due to cellular degradation over time or acute damage during attempts at cryopreservation. Vitrification is one option that may result in successful cryopreservation but difficulty with cryoprotective agent (CPA) toxicity at high concentrations of a single cryoprotectant has hindered development of successful protocols. This study was designed to determine the vitrifiability and glass stability of solutions containing combinations of commonly used CPAs and to document CPA interactions that occur. One hundred and sixty-four multi-CPA combination solutions of 6-9 M were evaluated for vitrifiability and glass stability using direct visualization after immersion in liquid nitrogen for 30 min and upon warming. Binary and ordinal logistic regression analysis was used to statistically analyze each CPA for its ability to vitrify and its effect on glass stability in multi-component CPA solutions. Propylene glycol had the greatest incremental contribution to vitrification while formamide had the least contribution. A threshold was established whereby the ability of a solution to vitrify could be determined by calculation. Glass stability was not as clearly defined due to variability in the results; however, contributions of interactions between CPAs to the glass stability of solutions were determined. This study provided values that predict if a solution will vitrify. Furthermore, the glass stability of solutions containing multiple CPAs do not behave as linear additions of binary solutions and interactions between CPAs have a significant effect on the glass stability of these solutions. These variables should be considered when designing vitrification solutions.     

First successful vitrification of salmonid ovarian tissue

Due to a lack of cryopreservation protocols for fish eggs and embryos, alternative techniques which will enable storage of female genetic resources are crucial for future development of reproduction management in conservation biology and aquaculture. Experiments were conducted to develop an optimal vitrification protocol for cryopreservation of brown trout Salmo trutta juvenile ovarian tissue. Needle immersed vitrification (NIV) method was used where ovaries were pinned on an acupuncture needle, passaged through equilibration and vitrification solutions containing different combinations and concentrations of methanol (MeOH), propylene glycol (PG) and dimethyl sulfoxide (Me₂SO) and subsequently plunged into liquid nitrogen. Vitrification solutions containing equal cryoprotectant concentrations (3M Me₂SO and 3M PG) yielded the highest oogonia survival rates (up to 40%) and qualitatively and quantitatively unaltered perinucleolar follicles. The method developed for brown trout could be applied to the conservation of female genetic resources of other salmonid species, including endangered and endemic species or populations.     

Numerical investigations of transient heat transfer characteristics and vitrification tendencies in ultra-fast cell cooling processes

During freezing, cells are often damaged directly or indirectly by ice formation. Vitrification is an alternative approach to cryopreservation that avoids ice formation. The common method to achieve vitrification is to use relatively high concentrations of cryoprotectant agents (CPA) in combination with a relatively slow cooling rate. However, high concentrations of CPAs have potentially damaging toxic and/or osmotic effects on cells. Therefore, establishing methods to achieve vitrification with lower concentrations of CPAs through ultra-fast cooling rates would be advantageous in these aspects. These ultra-fast cooling rates can be realized by a cooling system with an ultra-high heat transfer coefficient (h) between the sample and coolant. The oscillating motion heat pipe (OHP), a novel cooling device utilizing the pressure change to excite the oscillation motion of the liquid plugs and vapor bubbles, can significantly increase h and may fulfill this aim. The current investigation was designed to numerically study the effects of different values of h on the transient heat transfer characteristics and vitrification tendencies of the cell suspension during the cooling processes in an ultra-thin straw (100 microm in diameter). The transient temperature distribution, the cooling rate and the volume ratio (x) of the ice quantity to the maximum crystallizable ice of the suspension were calculated. From these numerical results, it is concluded that the ultra-high h (>10(4) W/m2 K) obtained by OHPs could facilitate vitrification by efficiently decreasing x as well as the time to pass through the dangerous temperature region where the maximum ice formation happens. For comparison, OHPs can decrease both of the parameters to less than 20% of those from the widely used open pulled straw methods. Therefore, the OHP method will be a promising approach to improving vitrification tendencies of CPA solutions and could also decrease the required concentration of CPAs for vitrification, both of which are of great importance for the successful cryopreservation of cells by vitrification.     

Status of cryopreservation of embryos from domestic animals.

The discovery of glycerol as an effective cryoprotectant for spermatozoa led to research on cryopreservation of embryos. The first successful offspring from frozen-thawed embryos were reported in the mouse and later in other laboratory animals. Subsequently, these techniques were applied to domestic animals. Research in cryopreservation techniques have included studies concerning the type and concentration of cryoprotectant, cooling and freezing rates, seeding and plunging temperatures, thawing temperatures and rates, and methods of cryoprotectant removal. To date, successful results based on pregnancy rates have been obtained with cryopreserved cow, sheep, goat, and horse embryos but no success has been reported in swine. Post-thaw embryo survival has been shown to be dependent on the initial embryo quality, developmental stage, and species. The freezing techniques most frequently used in research and by commercial companies are identified as "equilibrium" cryopreservation. In this technique the embryos are placed in a concentrated glycerol solution (1.4 M in PBS supplemented with BSA) at room temperature and the glycerol is allowed to equilibrate for a 20-min period. During the cooling process the straws are seeded (-4 to -7 degrees C) and cooling is continued at a rate of 0.3 to 0.5 degree C/min to -30 degrees C when bovine embryos may be plunged into LN2. Sheep embryos are successfully frozen with ethylene glycol (1.5 M) or DMSO (1.5 M) rather than with glycerol. Horse embryos have been frozen in 0.5 rather than 0.25 cc straws but with cooling rates and seeding and plunging temperatures similar to those used with bovine embryos. Swine embryos have shown a high sensitivity to temperature and cryoprotectants probably due to their high lipid content and a temperature decrease to 15 or 10 degrees C causes a dramatic increase in the percentage of degenerated embryos. However, a recent study has shown that hatched pig blastocysts survived exposure below 15 degrees C. Recent research has shown that embryos may also be frozen by a "nonequilibrium" method. This rapid freezing by vitrification consists of dehydration of the embryo at room temperature by a very highly concentrated vitrification media (3.5 to 4.0 M) and a very rapid freeze that avoids the formation of ice allowing the solution to change from a liquid to a glassy state. Vitrification solutions consist of combinations of sucrose, glycerol, and propylene glycol. With this technique, 50% pregnancy rates have been reported with the bovine blastocyst.     

Improved cryopreservation by diluted vitrification solution with supercooling-facilitating flavonol glycoside

The effect of kaempferol-7-O-glucoside (KF7G), one of the supercooling-facilitating flavonol glycosides which was originally found in deep supercooling xylem parenchyma cells of the katsura tree and was found to exhibit the highest level of supercooling-facilitating activity among reported substances, was examined for successful cryopreservation by vitrification procedures, with the aim of determining the possibility of using diluted vitrification solution (VS) to reduce cryoprotectant toxicity and also to inhibit nucleation at practical cooling and rewarming by the effect of supplemental KF7G. Examination was performed using shoot apices of cranberry and plant vitrification solution 2 (PVS2) with dilution. Vitrification procedures using the original concentration (100%) of PVS2 caused serious injury during treatment with PVS2 and resulted in no regrowth after cooling and rewarming (cryopreservation). Dilution of the concentration of PVS2 to 75% or 50% (with the same proportions of constituents) significantly reduced injury by PVS2 treatment, but regrowth was poor after cryopreservation. It is thought that dilution of PVS2 reduced injury by cryoprotectant toxicity, but such dilution caused nucleation during cooling and/or rewarming, resulting in poor survival. On the other hand, addition of 0.5 mg/ml (0.05% w/v) KF7G to the diluted PVS2 resulted in significantly (p < 0.05) higher regrowth rates after cryopreservation. It is thought that addition of supercooling-facilitating KF7G induced vitrification even in diluted PVS2 probably due to inhibition of ice nucleation during cooling and rewarming and consequently resulted in higher regrowth. The results of the present study indicate the possibility that concentrations of routinely used VSs can be reduced by adding supercooling-facilitating KF7G, by which more successful cryopreservation might be achieved for a wide variety of biological materials.     

Vitrification of pre-pubertal ovine cumulus-oocyte complexes: effect of cytochalasin B pre-treatment

The aim of this study was to evaluate the effect of cytochalasin B (CCB) pre-treatment before vitrification on ability of immature oocytes from lamb ovaries to progress until metaphase II (MII) stage after vitrification/warming procedure. Cumulus-oocyte complexes (COCs) were obtained from ovaries of lambs, from 80 to 90 days old, collected from a local slaughterhouse. Before vitrification, COCs were randomly distributed in two experimental groups corresponding to the incubation with or without 7.5 microg/ml CCB for 30 min. In order to study cryoprotectant and CCB pre-treatment toxicity (toxicity test), oocytes were exposed to cryoprotectants, with or without CCB pre-treatment, but without plunging into N2 liquid. Vitrification solution was composed by 4.48 M EG plus 3.50 M DMSO supplemented with 0.25 M sucrose. Two-step addition was performed. After vitrification or toxicity test, COCs were matured in bicarbonate-buffered TCM 199 containing 10% foetal calf serum and 10 ng/ml epidermal growth factor. A sample of COCs was directly in vitro matured (control group). Rates of MII oocytes of toxicity groups both, with or without CCB pre-treatment were lower than control group (41.1-50.0 versus 79.9, respectively; P<0.05). After vitrification, a lower number of oocytes progressed to MII stage in comparison with non-vitrification groups (P<0.05). In vitrified groups both with or without CCB pre-treatment 8.0 and 12.7%, respectively, of immature oocytes reached MII stage by the end of in vitro maturation culture. No effect of CCB was observed, either in the toxicity or vitrified groups. In conclusion, no effect of CCB pre-treatment before vitrification was detected in this study with immature oocytes of pre-pubertal sheep. More studies are needed in order to increase ovine oocyte survival after vitrification.     

Thermal performance of quartz capillaries for vitrification

In this paper we report the thermal behavior of a new approach for vitrification. Thermal performance of traditional open pulled straws is compared with a new technique based on the combined use of quartz capillaries with slush nitrogen. This new method of vitrification achieved ultrafast cooling rates of 250,000 °C/min. As a result, a much lower concentration of cryoprotectant was needed to reach vitrification. In fact, a cryoprotectant solution typically used in oocyte slow freezing protocols was shown to remain transparent after cooling to liquid nitrogen temperatures indicating apparent “vitrification”. This approach offers a new and very promising technique for vitrification of cells using low levels of cryoprotectants.     

Physical aspects of vitrification in aqueous solutions

Vitrification is the process by which a liquid solidifies at temperatures usually far below the normal freezing point, but without the formation of any crystalline phase. The liquid has formed a glass. Occasionally, when the liquid consists of a solution, one of the components freezes to form a crystalline phase during cooling, but the remainder of the solution vitrifies. The product is then a partially crystallized glass. Glass formation, as a feature of aqueous solutions, either of the whole solution or of the remaining fraction after crystallization of ice, is discussed. We focus on the general principles involved in glass formation and also discuss in detail the effect of pressure on the nucleation and vitrification of the solution. In particular we look at the physical processes involved as well as the chemical aspects of the solutes which can be used in vitrifiable aqueous solutions. In any application of vitrification of aqueous solutions the material properties of the resultant glass are also important; these are also briefly considered. Recent work concerning the nature of the devitrification (crystallization during warming) event at high pressures is detailed.