Thermal properties of ethylene glycol aqueous solutions

Analysis of the Nonequilibrium Phase Change Behaviors of the Cryoprotectant Solutions for Cryopreservation of Human Red Blood Cells with Low-Concentration Glycerol

Recently, we proposed a low-glycerol cryoprotectant formulation (consisting of 0.4 M trehalose and 5% glycerol) for cryopreservation of human red blood cells (RBCs), which greatly reduced the concentration of glycerol, minimized intracellular ice damage, and achieved high recovery. Although this study was successful in cellular experiments, the nonequilibrium phase transition behaviors of the cryoprotective agent solution have not been systematically analyzed. Therefore, it is essential to provide reliable thermodynamic data to substantiate the viability of this cryopreservation technique. In this study, the phase change behaviors and thermal properties of typical trehalose and/or glycerol solutions quenched in liquid nitrogen were investigated using differential scanning calorimetry and cryomicroscopy. It was found that the glass transition temperatures of both the trehalose aqueous solution (<1.0 M) and glycerol aqueous solution (<40% w/v) did not vary apparently with the concentration at low concentrations, while they increased significantly with increasing concentration at high concentrations. Moreover, it was revealed that the inhibitory effect of trehalose on ice growth was affected by glycerol. We further found that the addition of low concentrations of glycerol facilitates the partial glass transition of trehalose solutions at low concentrations. The results of this work provide reliable thermodynamic data to support the cryopreservation of human RBCs with unusually low concentrations of glycerol.

27 MHz constant field dielectric warming of kidneys cryopreserved by vitrification

Organs cryopreserved by vitrification are exposed to the lowest possible concentration of cryoprotectants for the least time necessary to successfully avoid ice formation. Faster cooling and warming rates enable lower concentrations and perfusion times, reducing toxicity. Since warming rates necessary to avoid ice formation during recovery from vitrification are typically faster than cooling rates necessary for vitrification, warming speed is a major determining factor for successful vitrification. Dielectric warming uses an oscillating electric field to directly heat water and cryoprotectant molecules inside organs to achieve warming that's faster and more uniform than can be achieved by heat conduction from the organ surface. This work studied 27 MHz dielectric warming of rabbit kidneys perfused with M22 vitrification solution. The 27 MHz frequency was chosen because its long wavelength and penetration depth are suitable for human organs, because it had an anticipated favorable temperature of maximum dielectric absorption in M22, and because it's an allocated frequency for industrial and amateur use with inexpensive amplifiers available. Previously vitrified kidneys were warmed from -100 °C by placement in a 27 MHz electric field formed between parallel capacitor plates in a resonant circuit. Power was varied during warming to maintain constant electric field amplitude between the plates. Maximum power absorption occurred near -70 °C, with a peak warming rate near 150 °C/min in 50 mL total volume with approximately 500 W power. After some optimization, it was possible to warm ∼13 g vitrified kidneys with unprecedentedly little injury from medullary ice formation and a favorable serum creatinine trend after transplant. Distinct behaviors of power absorption and system tuning observed as a function of temperature during warming are promising for non-invasive thermometry and future automated control of the warming process at even faster rates with user-defined temperature dependence.

Technologies for Vitrification Based Cryopreservation

Cryopreservation is a unique and practical method to facilitate extended access to biological materials. Because of this, cryopreservation of cells, tissues, and organs is essential to modern medical science, including cancer cell therapy, tissue engineering, transplantation, reproductive technologies, and bio-banking. Among diverse cryopreservation methods, significant focus has been placed on vitrification due to low cost and reduced protocol time. However, several factors, including the intracellular ice formation that is suppressed in the conventional cryopreservation method, restrict the achievement of this method. To enhance the viability and functionality of biological samples after storage, a large number of cryoprotocols and cryodevices have been developed and studied. Recently, new technologies have been investigated by considering the physical and thermodynamic aspects of cryopreservation in heat and mass transfer. In this review, we first present an overview of the physiochemical aspects of freezing in cryopreservation. Secondly, we present and catalog classical and novel approaches that seek to capitalize on these physicochemical effects. We conclude with the perspective that interdisciplinary studies provide pieces of the cryopreservation puzzle to achieve sustainability in the biospecimen supply chain.

Experiments and simulations demonstrating the rapid ultrasonic rewarming of frozen tissue cryovials

The development of methods to safely rewarm large cryopreserved biological samples remains a barrier to the widespread adoption of cryopreservation. Here, experiments and simulations were performed to demonstrate that ultrasound can increase rewarming rates relative to thermal conduction alone. An ultrasonic rewarming setup based on a custom 444 kHz tubular piezoelectric transducer was designed, characterized, and tested with 2 ml cryovials filled with frozen ground beef. Rewarming rates were characterized in the -20 °C to 5 °C range. Thermal conduction-based rewarming was compared to thermal conduction plus ultrasonic rewarming, demonstrating a tenfold increase in rewarming rate when ultrasound was applied. The maximum recorded rewarming rate with ultrasound was 57° C/min, approximately 2.5 times faster than with thermal conduction alone. Coupled acoustic and thermal simulations were developed and showed good agreement with the heating rates demonstrated experimentally and were also used to demonstrate spatial heating distributions with small (<3° C) temperature differentials throughout the sample when the sample was below 0° C. The experiments and simulations demonstrate the potential for ultrasonic cryovial rewarming with a possible application to large volume rewarming, as faster rewarming rates may improve the viability of cryopreserved tissues and reduce the time needed for cells to regain normal function.

Improved cryopreservation media formulation reduces costs of maintenance while preserving function of genetically modified insect cells

Insect cell lines are an invaluable resource that facilitate various fundamental and applied research programs. Genetically engineered insect cell lines, in particular, serve as a platform through which the function of heterologously expressed proteins can be studied. However, a barrier to more widespread utilization and distribution of insect cell lines, genetically modified or not, is the technical and operational challenge associated with traditional cryopreservation methods, including their dependence on the use of liquid nitrogen facilities, animal or human serum products, and relatively high concentrations of permeating cryoprotectants (e.g., DMSO). Recent innovations in cryopreservation technologies have produced reagents with improved abilities to effectively preserve mammalian cell lines for long periods in regular laboratory deep freezers without using serum products, but their effectiveness in preserving genetically engineered insect cell lines has not yet been evaluated. In this study, we engineered Sf9 cells to express a dopamine receptor and used them as a model for evaluating the efficacy of a novel cryopreservation medium product, C80EZ^®-INSECT, in not only preserving cell viability and proliferation efficiency but also maintaining the insect cell line's "functionality" after storage at -80°C. We found that the engineered Sf9 cells frozen using C80EZ^®-INSECT with 5% DMSO alone and stored at -80°C for 6 mo displayed higher viability and growth rates than cells frozen using traditional fetal bovine serum (FBS)-based cryopreservation media with 10% DMSO that were stored at -80°C or in liquid nitrogen for the same period of time. We also found that after 6 mo of storage at -80°C or in liquid nitrogen the cells retained a responsiveness to dopamine comparable to that of the initial cell line, regardless of the cryopreservation reagent used. These results suggest that, due to the unique characteristics of C80EZ^®-INSECT in preventing ice recrystallization and reducing ice crystal size and cellular apoptosis during cryostorage procedures, it is an effective cryopreservation reagent for genetically engineered Sf9 cells, and it practically eliminates the need for liquid nitrogen-based storage facilities and FBS-based cryopreservation formulations, as well as reduces the use of permeating cryoprotectants.

Supplemented phase diagrams for vitrification CPA cocktails: DP6, VS55 and M22

DP6, VS55 and M22 are the most commonly used cryoprotective agent (CPA) cocktails for vitrification experiments in tissues and organs. However, complete phase diagrams for the three CPAs are often unavailable or incomplete (only available for full strength CPAs) thereby hampering optimization of vitrification and rewarming procedures. In this paper, we used differential scanning calorimetry (DSC) to measure the transition temperatures including heterogeneous nucleation temperatures (T_het), glass transition temperatures (T_g), rewarming phase crystallization (devitrification and/or recrystallization) temperatures (T_d) and melting temperatures (T_m) while cooling or warming the CPA sample at 5 °C/min and plotted the obtained transition temperatures for different concentrations of CPAs into the phase diagrams. We also used cryomicroscopy cooling or warming the sample at the same rate to record the ice crystallization during the whole process, and we presented the cryomicroscopic images at the transition temperatures, which agreed with the DSC presented phenomena.

Vitrifying multiple embryos in different arrangements does not alter the cooling rate

Vitrification is the most common method of cryopreservation of gametes in fertility clinics due to its improved survival rates compared to slow freezing techniques. For the Open Cryotop® vitrification device, the number of oocytes, or embryos, mounted onto a single device can vary. In this work, a mathematical model is developed for the cooling of oocytes and embryos (samples). The model is solved computationally, to investigate whether varying the number of samples mounted onto the Open Cryotop® affects the cooling rates, and consequently the survival rates, of vitrified samples. Several realistic spatial arrangements of samples are examined, determining their temperature over time. In this way we quantify the effect of spatial arrangement on the cooling rate. Our results indicate that neither the spatial arrangement nor the number of mounted samples has a large effect on cooling rates, so long as the volume of the cryoprotectant remains minimal. The time taken for cooling is found to be on the order of half a second, or less, regardless of the spatial arrangement or number of mounted samples. Hence, rapid cooling can be achieved for any number or arrangement of samples, as long as device manufacturer guidelines are adhered to.

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.

Absence of the liquid-liquid phase transition in aqueous ionic liquids

The anomalies of supercooled water may be explained by an underlying liquid-liquid phase transition (LLPT) between high- and low-density states. Recently, its observation at 185 K was inferred using solutions containing aqueous ionic liquids at a solute mole fraction of x=0.156 [Woutersen et al., Science 359, 1127 (2018)10.1126/science.aao7049]. We employ x-ray diffraction, calorimetry, and dilatometry on these hydrazinium trifluoroacetate solutions at x=0.00-0.40 to show that the transition at 185 K is not related to a genuine LLPT of water. Continuous densification upon compression, continuous changes of halo position, and absence of thermal signatures for a high- to low-density transition rule out the possibility of an LLPT for x≥0.13. The data show that employing sophisticated solutions adds a layer of complexity that hampers extrapolation of the LLPT concept from one- to two-component systems. The possibility of an LLPT can only be probed for pure water or sufficiently dilute aqueous solutions.

In vitro reconstitution and characterisation of the oxidative D-xylose pathway for production of organic acids and alcohols

The oxidative d-xylose pathway, i.e. Dahms pathway, can be utilised to produce from cheap biomass raw material useful chemical intermediates. In vitro metabolic pathways offer a fast way to study the rate-limiting steps and find the most suitable enzymes for each reaction. We have constructed here in vitro multi-enzyme cascades leading from d-xylose or d-xylonolactone to ethylene glycol, glycolic acid and lactic acid, and use simple spectrophotometric assays for the read-out of the efficiency of these pathways. Based on our earlier results, we focussed particularly on the less studied xylonolactone ring opening (hydrolysis) reaction. The bacterial Caulobacter crescentus lactonase (Cc XylC), was shown to be a metal-dependent enzyme clearly improving the formation of d-xylonic acid at pH range from 6 to 8. The following dehydration reaction by the ILVD/EDD family d-xylonate dehydratase is a rate-limiting step in the pathway, and an effort was made to screen for novel enolase family d-xylonate dehydratases, however, no suitable replacing enzymes were found for this reaction. Concerning the oxidation of glycolaldehyde to glycolic acid, several enzyme candidates were also tested. Both Escherichia coli aldehyde dehydrogenase (Ec AldA) and Azospirillum brasilense α-ketoglutarate semialdehyde dehydrogenase (Ab AraE) proved to be suitable enzymes for this reaction.

Vitrification Assessment: Thermal Analysis of Cryoprotective Aqueous Solutions 1,2 Propanediol and Ethylene Glycol

Cryopreservation of viable cells and cell materials is being developed for biological and biopharmaceutical applications. The inhibition of ice formation during the cooling and warming phase of vitrified living biological samples is important for their survival. The tendency to form glasses (glass transition temperature, T_g) upon cooling in the vitrification solution and the stability of the amorphous state upon warming to determine the critical cooling rate (V_ccr) and critical warming rates (V_cwr) are evaluated. The study of thermal properties of ethylene glycol (EG) and 1,2-propanediol (PD) solutions were performed to improve vitrification through better understanding of their molecular mobility and viscosity. Two sets of aqueous solutions were tested. In group A, 35% EG (w/w) was added to different PD concentrations (5%, 10%, and 15%). In group B, 20% PD (w/w) was combined with varying concentrations of EG (20%, 24%, 27%, and 30%). Using the semiempirical model of Boutron, the values of V_ccr and V_cwr for group A were 10, 8, <2.5°C/min, and 1.65 × 10⁵, 678, 32°C/min, respectively. For group B, the values were 24, 10, <2.5, <2.5°C/min, and 9.5 × 10³, 144, 48, 7°C/min, respectively. While the values of V_ccr and V_cwr for 40% EG were 123 and 8.84 × 10⁵°C/min, respectively. The methyl group in PD enhanced the vitreous state, lowering the melting point. Adding a small concentration of PD (3%) to VM3 vitrification solution improved and increased the T_g and enhanced their thermal stability. Analyzing the thermal properties of cryoprotectant is useful when designing the cryopreservation protocols.

Thermal Study of Polyols for the Technological Application as Plasticizers in Food Industry

In this work is presented the complete thermal analysis of polyols by direct methods such as simultaneous thermogravimetric and differential thermal analyzer (TGA-DTA), differential scanning calorimetry (DSC), modulated DSC (MDSC), and supercooling MDSC. The different thermal events in the temperature range of 113–553 K were identified for glycerol (GL), ethylene glycol (EG), and propylene glycol (PG). Boiling temperature (T_B) decreased as GL > EG > PG, but increased with the heating rate. GL showed a complex thermal event at 191–199 K, identified as the glass transition temperature (T_g) and devitrification temperature (T_dv), and a liquid–liquid transition (T_L-L) at 215–221 K was identified as the supercooling temperature. EG showed several thermal events such as T_g and T_dv at 154 K, crystallization temperature (T_c) at 175 K, and melting temperature (T_m) at 255 K. PG also showed a complex thermal event (T_g and T_dv) at 167 K, a second devitrification at 193 K, and T_L-L at 245 K. For PG, crystallization was not observed, indicating that, during the cooling, the liquid remained as an amorphous solid.

Vitrification tendency and stability of DP6-based vitrification solutions for complex tissue cryopreservation

Vitrification tendency and stability of the amorphous state were analyzed by means of differential scanning calorimetry (DSC) for the vitrification solution DP6, with and without additional solutes to enhance ice suppression. This study is a part of an ongoing research effort to characterize the thermophysical and mechanical properties of DP6 and its derivatives, and their qualities as cryoprotective solutions. DP6 was determined to have a critical cooling rate necessary to ensure vitrification of 2.7 °C/min. The following additional solutions were tested: DP6 + 6% (2R, 3R) 2,3-butanediol, DP6 + 6% 1,3-cyclohexanediol, DP6 + 6% (0.175M) sucrose, DP6 + 12% PEG 400, and DP6 + 17.1% (0.5 M) sucrose. The additives decreased the critical cooling rate of the DP6 solution to rates below 1 °C/min that were not quantifiable by the DSC techniques used. The following critical warming rates necessary to avoid devitrification were identified for DP6 and the modified solutions, respectively: 189 °C/min, 5 °C/min, ≈ 1 °C/min, 15 °C/min, <1 °C/min, and <1 °C/min. Glass transition temperatures and melting temperatures were also measured. Sucrose was the least effective additive on a per mass basis, with 1,3-cyclohexanediol appearing to be the most effective additive for suppressing ice formation in DP6.

Production of ethylene glycol or glycolic acid from D-xylose in Saccharomyces cerevisiae

The important platform chemicals ethylene glycol and glycolic acid were produced via the oxidative D-xylose pathway in the yeast Saccharomyces cerevisiae. The expression of genes encoding D-xylose dehydrogenase (XylB) and D-xylonate dehydratase (XylD) from Caulobacter crescentus and YagE or YjhH aldolase and aldehyde dehydrogenase AldA from Escherichia coli enabled glycolic acid production from D-xylose up to 150 mg/L. In strains expressing only xylB and xylD, 29 mg/L 2-keto-3-deoxyxylonic acid [(S)-4,5-dihydroxy-2-oxopentanoic acid] (2K3DXA) was produced and D-xylonic acid accumulated to ca. 9 g/L. A significant amount of D-xylonic acid (ca. 14%) was converted to 3-deoxypentonic acid (3DPA), and also, 3,4-dihydroxybutyric acid was formed. 2K3DXA was further converted to glycolaldehyde when genes encoding by either YagE or YjhH aldolase from E. coli were expressed. Reduction of glycolaldehyde to ethylene glycol by an endogenous aldo-keto reductase activity resulted further in accumulation of ethylene glycol of 14 mg/L. The possibility of simultaneous production of lactic and glycolic acids was evaluated by expression of gene encoding lactate dehydrogenase ldhL from Lactobacillus helveticus together with aldA. Interestingly, this increased the accumulation of glycolic acid to 1 g/L. The D-xylonate dehydratase activity in yeast was notably low, possibly due to inefficient Fe-S cluster synthesis in the yeast cytosol, and leading to D-xylonic acid accumulation. The dehydratase activity was significantly improved by targeting its expression to mitochondria or by altering the Fe-S cluster metabolism of the cells with FRA2 deletion.

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.

Efficient long-term cryopreservation of pluripotent stem cells at -80 °C

Current long term cryopreservation of cell stocks routinely requires the use of liquid nitrogen (LN₂), because commonly used cryopreservation media containing cell membrane permeating cryoprotectants are thermally unstable when frozen at higher storage temperatures, e.g. -80 °C. This instability leads to ice recrystallization, causing progressive loss of cell viability over time under the storage conditions provided by most laboratory deep freezers. The dependency on LN₂ for cell storage significantly increases operational expense and raises issues related to impaired working efficiency and safety. Here we demonstrate that addition of Ficoll 70 to cryoprotectant solutions significantly improves system thermal stability at the working temperature (~-80 °C) of laboratory deep freezers. Moreover, a medium comprised of Ficoll 70 and dimethyl sulfoxide (DMSO) in presence or absence of fetal bovine serum (FBS) can provide reliable cryopreservation of various kinds of human and porcine pluripotent stem cells at -80 °C for periods that extend up to at least one year, with the post-thaw viability, plating efficiency, and full retention of pluripotent phenotype comparable to that achieved with LN₂ storage. These results illustrate the practicability of a promising long-term cryopreservation method that completely eliminates the need for LN₂.

Mathematically optimized cryoprotectant equilibration procedures for cryopreservation of human oocytes

Background

Simple and effective cryopreservation of human oocytes would have an enormous impact on the financial and ethical constraints of human assisted reproduction. Recently, studies have demonstrated the potential for cryopreservation in an ice-free glassy state by equilibrating oocytes with high concentrations of cryoprotectants (CPAs) and rapidly cooling to liquid nitrogen temperatures. A major difficulty with this approach is that the high concentrations required for the avoidance of crystal formation (vitrification) also increase the risk of osmotic and toxic damage. We recently described a mathematical optimization approach for designing CPA equilibration procedures that avoid osmotic damage and minimize toxicity, and we presented optimized procedures for human oocytes involving continuous changes in solution composition.

Methods

Here we adapt and refine our previous algorithm to predict piecewise-constant changes in extracellular solution concentrations in order to make the predicted procedures easier to implement. Importantly, we investigate the effects of using alternate equilibration endpoints on predicted protocol toxicity. Finally, we compare the resulting procedures to previously described experimental methods, as well as mathematically optimized procedures involving continuous changes in solution composition.

Results

For equilibration with CPA, our algorithm predicts an optimal first step consisting of exposure to a solution containing only water and CPA. This is predicted to cause the cells to initially shrink and then swell to the maximum cell volume limit. To reach the target intracellular CPA concentration, the cells are then induced to shrink to the minimum cell volume limit by exposure to a high CPA concentration. For post-thaw equilibration to remove CPA, the optimal procedures involve exposure to CPA-free solutions that are predicted to cause swelling to the maximum volume limit. The toxicity associated with these procedures is predicted to be much less than that of conventional procedures and comparable to that of the corresponding procedures with continuous changes in solution composition.

Conclusions

The piecewise-constant procedures described in this study are experimentally facile and are predicted to be less toxic than conventional procedures for human oocyte cryopreservation. Moreover, the mathematical optimization approach described here will facilitate the design of cryopreservation procedures for other cell types.

Rationally optimized cryopreservation of multiple mouse embryonic stem cell lines: II--Mathematical prediction and experimental validation of optimal cryopreservation protocols

In Part I, we documented differences in cryopreservation success measured by membrane integrity in four mouse embryonic stem cell (mESC) lines from different genetic backgrounds (BALB/c, CBA, FVB, and 129R1), and we demonstrated a potential biophysical basis for these differences through a comparative study characterizing the membrane permeability characteristics and osmotic tolerance limits of each cell line. Here we use these values to predict optimal cryoprotectants, cooling rates, warming rates, and plunge temperatures. We subsequently verified these predictions experimentally for their effects on post-thaw recovery. From this study, we determined that a cryopreservation protocol utilizing 1M propylene glycol, a cooling rate of 1°C/minute, and plunging into liquid nitrogen at -41°C, combined with subsequent warming in a 22°C water bath with agitation, significantly improved post-thaw recovery for three of the four mESC lines, and did not diminish post-thaw recovery for our single exception. It is proposed that this protocol can be successfully applied to most mESC lines beyond those included within this study once the effect of propylene glycol on mESC gene expression, growth characteristics, and germ-line transmission has been determined. Mouse ESC lines with poor survival using current standard cryopreservation protocols or our proposed protocol can be optimized on a case-by-case basis using the method we have outlined over two papers. For our single exception, the CBA cell line, a cooling rate of 5°C/minute in the presence of 1.0M dimethyl sulfoxide or 1.0M propylene glycol, combined with plunge temperature of -80°C was optimal.

Rationally optimized cryopreservation of multiple mouse embryonic stem cell lines: I--Comparative fundamental cryobiology of multiple mouse embryonic stem cell lines and the implications for embryonic stem cell cryopreservation protocols

The post-thaw recovery of mouse embryonic stem cells (mESCs) is often assumed to be adequate with current methods. However as this publication will show, this recovery of viable cells actually varies significantly by genetic background. Therefore there is a need to improve the efficiency and reduce the variability of current mESC cryopreservation methods. To address this need, we employed the principles of fundamental cryobiology to improve the cryopreservation protocol of four mESC lines from different genetic backgrounds (BALB/c, CBA, FVB, and 129R1 mESCs) through a comparative study characterizing the membrane permeability characteristics and membrane integrity osmotic tolerance limits of each cell line. In the companion paper, these values were used to predict optimal cryoprotectants, cooling rates, warming rates, and plunge temperatures, and then these predicted optimal protocols were validated against standard freezing protocols.

Biosynthesis of ethylene glycol in Escherichia coli

Ethylene glycol (EG) is an important platform chemical with steadily expanding global demand. Its commercial production is currently limited to fossil resources; no biosynthesis route has been delineated. Herein, a biosynthesis route for EG production from D-xylose is reported. This route consists of four steps: D-xylose → D-xylonate → 2-dehydro-3-deoxy-D-pentonate → glycoaldehyde → EG. Respective enzymes, D-xylose dehydrogenase, D-xylonate dehydratase, 2-dehydro-3-deoxy-D-pentonate aldolase, and glycoaldehyde reductase, were assembled. The route was implemented in a metabolically engineered Escherichia coli, in which the D-xylose → D-xylulose reaction was prevented by disrupting the D-xylose isomerase gene. The most efficient construct produced 11.7 g L(-1) of EG from 40.0 g L(-1) of D-xylose. Glycolate is a carbon-competing by-product during EG production in E. coli; blockage of glycoaldehyde → glycolate reaction was also performed by disrupting the gene encoding aldehyde dehydrogenase, but from this approach, EG productivity was not improved but rather led to D-xylonate accumulation. To channel more carbon flux towards EG than the glycolate pathway, further systematic metabolic engineering and fermentation optimization studies are still required to improve EG productivity.

Effect of common cryoprotectants on critical warming rates and ice formation in aqueous solutions

Ice formation on warming is of comparable or greater importance to ice formation on cooling in determining survival of cryopreserved samples. Critical warming rates required for ice-free warming of vitrified aqueous solutions of glycerol, dimethyl sulfoxide, ethylene glycol, polyethylene glycol 200 and sucrose have been measured for warming rates of order 10-10⁴ K/s. Critical warming rates are typically one to three orders of magnitude larger than critical cooling rates. Warming rates vary strongly with cooling rates, perhaps due to the presence of small ice fractions in nominally vitrified samples. Critical warming and cooling rate data spanning orders of magnitude in rates provide rigorous tests of ice nucleation and growth models and their assumed input parameters. Current models with current best estimates for input parameters provide a reasonable account of critical warming rates for glycerol solutions at high concentrations/low rates, but overestimate both critical warming and cooling rates by orders of magnitude at lower concentrations and larger rates. In vitrification protocols, minimizing concentrations of potentially damaging cryoprotectants while minimizing ice formation will require ultrafast warming rates, as well as fast cooling rates to minimize the required warming rates.

Quantitative investigations on the effects of exposure durations to the combined cryoprotective agents on mouse oocyte vitrification procedures

Vitrification by using two-step exposures to combined cryoprotective agents (CPAs) has become one of the most common methods for oocyte cryopreservation. By quantitatively examining the status of oocytes during CPA additions and dilutions, we can analyze the degree of the associated osmotic damages. The osmotic responses of mouse MII oocyte in the presence of the combined CPAs (ethylene glycol, EG, and dimethyl sulfoxide, DMSO) were recorded and analyzed. A two-parameter model was used in the curve-fitting calculation to determine the values of hydraulic conductivity (L(p)) and permeability (P(s)) to the combined CPAs at 25°C and 37°C. The effects of exposure durations and the exposure temperatures on the cryopreservation in terms of frozen-thawed cell survival rates and subsequent development were examined in a series of cryopreservation experiments. Mouse MII oocytes were exposed to pretreatment solution (PTS) and vitrification solution (VS) at specific temperatures. The PTS used in our experiment was 10% EG and 10% DMSO dissolved in modified PBS (mPBS), and the VS was EDFS30 (15% EG, 15% DMSO, 3 × 10(-3) M Ficoll, and 0.35 M sucrose in mPBS).The accumulative osmotic damage (AOD) and intracellular CPA concentrations were calculated under the different cryopreservation conditions, and for the first time, the quantitative interactions between survival rates, subsequent development rates, and values of AOD were investigated.

Cryopreservation of mammalian embryos: Advancement of putting life on hold

Rodent transgenesis and human-assisted reproductive programs involve multistep handling of preimplantation embryos. The efficacy of production and quality of results from conventionally scheduled programs are limited by temporal constraints other than the quality and quantities of embryos per se. The emergence of vitrification, a water ice-free cryopreservation technique, as a reliable way to arrest further growth of preimplantation embryos, provides an option to eliminate the time constraint. In this article, current and potential applications of cryopreservation to facilitate laboratory animal experiments, colony management, and human-assisted reproductive programs are reviewed. Carrier devices developed for vitrification in the last two decades are compared with an emphasis on their physical properties that infer cooling rate of samples and sterility assurance. Biological impacts of improved cryopreservation on preimplantation embryos are also discussed.

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.

Thermodynamic aspects of vitrification

Vitrification is a process in which a liquid begins to behave as a solid during cooling without any substantial change in molecular arrangement or thermodynamic state variables. The physical phenomenon of vitrification is relevant to both cryopreservation by freezing, in which cells survive in glass between ice crystals, and cryopreservation by vitrification in which a whole sample is vitrified. The change from liquid to solid behavior is called the glass transition. It is coincident with liquid viscosity reaching 10(13) Poise during cooling, which corresponds to a shear stress relaxation time of several minutes. The glass transition can be understood on a molecular level as a loss of rotational and translational degrees of freedom over a particular measurement timescale, leaving only bond vibration within a fixed molecular structure. Reduced freedom of molecular movement results in decreased heat capacity and thermal expansivity in glass relative to the liquid state. In cryoprotectant solutions, the change from liquid to solid properties happens over a approximately 10 degrees C temperature interval centered on a glass transition temperature, typically near -120 degrees C (+/-10 degrees C) for solutions used for vitrification. Loss of freedom to quickly rearrange molecular position causes liquids to depart from thermodynamic equilibrium as they turn into a glass during vitrification. Residual molecular mobility below the glass transition temperature allows glass to very slowly contract, release heat, and decrease entropy during relaxation toward equilibrium. Although diffusion is practically non-existent below the glass transition temperature, small local movements of molecules related to relaxation have consequences for cryobiology. In particular, ice nucleation in supercooled vitrification solutions occurs at remarkable speed until at least 15 degrees C below the glass transition temperature.

Human oocyte vitrification: the permeability of metaphase II oocytes to water and ethylene glycol and the appliance toward vitrification

OBJECTIVE

To determine the permeability of human metaphase II oocytes to ethylene glycol and water in the presence of ethylene glycol, and to use this information to develop a method to vitrify human oocytes.

DESIGN

An incomplete randomized block design.

SETTING

A university-affiliated assisted reproductive center.

PATIENT(S)

Women undergoing assisted reproduction in the Center for Reproductive Medicine at Shandong University.

INTERVENTION(S)

Oocytes were exposed to 1.0 molar ethylene glycol in a single step and photographed during subsequent volume excursions.

MAIN OUTCOME MEASURE(S)

A two-parameter model was employed to estimate the permeability to water and ethylene glycol.

RESULT(S)

Water permeability ranged from 0.15 to 1.17 microm/(min.atm), and ethylene glycol permeability ranged from 1.5 to 30 microm/min between 7 degrees C at 36 degrees C. The activation energies for water and ethylene glycol permeability were 14.42 Kcal/mol and 21.20 Kcal/mol, respectively.

CONCLUSION(S)

Despite the lower permeability of human metaphase II oocytes to ethylene glycol compared with previously published values for propylene glycol and dimethylsulfoxide, methods to add and remove human oocytes with a vitrifiable concentration of ethylene glycol can be designed that prevent excessive osmotic stress and minimize exposure to high concentrations of this compound.

An improved cryopreservation method for a mouse embryonic stem cell line

Embryonic stem (ES) cell lines including the C57BL/6 genetic background are central to projects such as the Knock-Out Mouse Project, North American Conditional Mouse Mutagenesis Program, and European Conditional Mouse Mutagenesis Program, which seek to create thousands of mutant mouse strains using ES cells for the production of human disease models in biomedical research. Crucial to the success of these programs is the ability to efficiently cryopreserve these mutant cell lines for storage and transport. Although the ability to successfully cryopreserve mouse ES cells is often assumed to be adequate, the percent post-thaw recovery of viable cells varies greatly among genetic backgrounds and individual cell lines within a genetic background. Therefore, there is a need to improve the efficiency and reduce the variability of current mouse ES cell cryopreservation methods. To address this need, we employed the principles of fundamental cryobiology to improve the cryopreservation protocol of a C57BL/6 mouse ES cell line by characterizing the membrane permeability characteristics and osmotic tolerance limits. These values were used to predict optimal cooling rates, warming rates, and type of cryoprotectant, which were then verified experimentally. The resulting protocol, generated through this hypothesis-driven approach, resulted in a 2-fold increase in percent post-thaw recovery of membrane-intact ES cells as compared to the standard freezing protocol, as measured by propidium iodide exclusion. Additionally, our fundamental cryobiological approach to improving cryopreservation protocols provides a model system by which additional cryopreservation protocols may be improved in future research for both mouse and human ES cell lines.

Towards whole sheep ovary cryopreservation

Cryopreservation of ovarian tissue aims to assist young women who require treatments that may lead to sterility or infertility. Cryopreservation procedures should therefore be as simple and efficient as possible. This study investigates rapid cooling outcomes for whole sheep ovaries. Ovaries were perfused with VS4 via the ovarian artery, and cooled by quenching in liquid nitrogen in less than a minute (estimated cooling rate above 300 degrees C/min till the vitreous transition temperature). The ovaries were rewarmed in two stages: slow warming (12-16 degrees C/min from -196 to -133 degrees C) in liquid nitrogen vapour, followed by rapid thawing in a 45 degrees C water bath at about 200 degrees C/min. DSC measurements showed that under these cryopreservation conditions VS4 would vitrify, but that VS4 perfused ovarian cortex fragments did not vitrify, but formed ice (around 18.4%). Immediately following rewarming, a dye exclusion test indicated that 61.4+/-2.2% of small follicles were viable while histological analysis showed that 48+/-3.8% of the primordial follicles were normal. It remains to be clarified whether follicle survival rates will increase if conditions allowing complete tissue vitrification were used.

Rapidly cooled horse spermatozoa: Loss of viability is due to osmotic imbalance during thawing, not intracellular ice formation

The cellular damage that spermatozoa encounter at rapid rates of cooling has often been attributed to the formation of intracellular ice. However, no direct evidence of intracellular ice has been presented. An alternative mechanism has been proposed by Morris (2006) that cell damage is a result of an osmotic imbalance encountered during thawing. This paper examines whether intracellular ice forms during rapid cooling or if an alternative mechanism is present. Horse spermatozoa were cooled at a range of cooling rates from 0.3 to 3,000 degrees C/min in the presence of a cryoprotectant. The ultrastructure of the samples was examined by Cryo Scanning Electron Microscopy (CryoSEM) and freeze substitution, to determine whether intracellular ice formed and to examine alternative mechanisms of cell injury during rapid cooling. No intracellular ice formation was detected at any cooling rate. Differential scanning Calorimetry (DSC) was employed to examine the amount of ice formed at different rate of cooling. It is concluded that cell damage to horse spermatozoa, at cooling rates of up to 3,000 degrees C/min, is not caused by intracellular ice formation. Spermatozoa that have been cooled at high rates are subjected to an osmotic shock when they are thawed.

Effect of antibiotics on thermodynamic properties of freezing media in rabbit species: A first calorimetric approach

Semen freezing is an effective and safe solution for the cryopreservation of animal genetic resources and for the diffusion of the genetic progress. Actually, these techniques are not yet under control for the rabbit species partly because methods are not clearly defined. Thus, the aim of this work is to study the effect of antibiotics (Penicillin G, Streptomycin) routinely used in freezing semen on the thermodynamic properties of freezing media mainly used in rabbit species. Measurements realized by differential scanning calorimetry show that these antibiotics may change the temperature of crystallization and the quantity of ice formed in the freezing media considered. Our calorimetric approach underlined that the composition and the properties of the cryoprotective solutions should be studied more precisely.

Plant vitrification solution 2 lowers water content and alters freezing behavior in shoot tips during cryoprotection

Plant shoot tips do not survive exposure to liquid nitrogen temperatures without cryoprotective treatments. Some cryoprotectant solutions, such as plant vitrification solution 2 (PVS2), dehydrate cells and decrease lethal ice formation, but the extent of dehydration and the effect on water freezing properties are not known. We examined the effect of a PVS2 cryoprotection protocol on the water content and phase behavior of mint and garlic shoot tips using differential scanning calorimetry. The temperature and enthalpy of water melting transitions in unprotected and recovering shoot tips were comparable to dilute aqueous solutions. Exposure to PVS2 changed the behavior of water in shoot tips: enthalpy of melting transitions decreased to about 40 J g H2O(-1) (compared to 333 J g H2O(-1) for pure H2O), amount of unfrozen water increased to approximately 0.7 g H2O g dry mass(-1) (compared to approximately 0.4 g H2Og dry mass(-1) for unprotected shoot tips), and a glass transition (T(g)) at -115 degrees C was apparent. Evaporative drying at room temperature was slower in PVS2-treated shoot tips compared to shoot tips receiving no cryoprotection treatments. We quantified the extent that ethylene glycol and dimethyl sulfoxide components permeate into shoot tips and replace some of the water. Since T(g) in PVS2-treated shoot tips occurs at -115 degrees C, mechanisms other than glass formation prevent freezing at temperatures between 0 and -115 degrees C. Protection is likely a result of controlled dehydration or altered thermal properties of intracellular water. A comparison of thermodynamic measurements for cryoprotection solutions in diverse plant systems will identify efficacy among cryopreservation protocols.

Follicular viability and histological assessment after cryopreservation of whole sheep ovaries with vascular pedicle by vitrification

OBJECTIVE

To evaluate a cryopreservation technique by vitrification of whole ovaries with their vascular pedicle in sheep, by using two cryoprotectant solutions.

DESIGN

Animal study.

SETTING

Fertility clinic in a university teaching hospital.

ANIMAL(S)

Five to 6-month-old ewes.

INTERVENTION(S)

Whole sheep ovaries with their vascular pedicles were collected at the slaughterhouse and prepared for cryoprotectant toxicity tests and freezing procedures.

MAIN OUTCOME MEASURE(S)

Follicle viability assessment by trypan blue test and histological examination of ovary and vessel structure.

RESULT(S)

No statistically significant difference in follicle viability or normal primordial follicle rates were observed between ovaries exposed or nonexposed to cryoprotectant solutions. Nor was any statistically significant difference observed before and after vitrification with the two cryoprotectant solutions. The decrease in the number of primordial follicles was smaller when frozen-thawed ovaries were treated with VS4 solution containing dimethyl sulfoxide, formamide, and propylene glycol. There were fewer nuclear anomalies and general follicular anomalies with the VS4 solution. Pedicle fractures occurred in most ovaries during thawing (11/15).

CONCLUSION(S)

Cryopreservation of whole ovary by vitrification appears a promising technique in reproductive medicine. The best histologic results were obtained with the VS4 cryoprotectant. Further studies are required to overcome vitrified ovarian vessel fracture.

Parasite cryopreservation by vitrification

Parasitic protozoa and helminths and parasitic/vector insects each have distinct requirements for cryopreservation. Most parasitic protozoa respond to cryopreservation stresses similarly to other single cell suspensions, but few species are currently routinely cryopreserved by protocols specifically designed for vitrification. With slow equilibrium cooling, some protozoa osmotically dehydrated by solutes concentrated in the residual unfrozen fraction will survive by vitrifying. Several species of helminths, together with insect embryos cannot be cryopreserved by slow cooling protocols and have an absolute requirement for vitrification. Studies incorporating slow cooling and stepped cooling of both protozoa and helminths, particularly the intraerythrocytic stages of malaria and the schistosomula larvae of Schistosoma mansoni have aided in the design of vitrification protocols for parasites. For helminths, the most widely used cryopreservation protocol, originally successful for cryopreserving S. mansoni schistosomula, consists of the addition of ethanediol in two steps, followed by rapid cooling (approximately 5100 degrees C min(-1)) to -196 degrees C. This technique exploits the temperature-dependent differential in permeability of the cryoprotectant additive (CPA) to first permeate into the organism at 37 degrees C followed by a dehydration-mediated internal CPA increase in concentration resulting from incubation in a second higher CPA concentration at 0 degree C. Samples are rapidly warmed/diluted (approximately 14,000 degrees C min(-1)) to recover the organisms from liquid nitrogen storage. Variations on this technique have also been successful in cryopreserving the larvae and adult worms of filariae, muscle stage larvae of Trichinella spp., the infective stages of gastro-intestinal nematode parasites and insect embryos. Other protocols where the dehydration step precedes CPA addition have been used to cryopreserve entomogenous nematode larvae by vitrification. Techniques that utilize high concentrations of CPA cocktails and slower cooling, developed for the vitrification of mammalian embryos, have been applied to the cryopreservation of parasitic protozoa, but with limited success to date. Where cryopreservation by classical slow cooling methods is possible, vitrification has enhanced the levels of survival obtained. And vitrification has enabled the successful cryopreservation of those parasitic species not able to be cryopreserved by traditional methods. Since a limited number of parasitic organisms has been cryopreserved using vitrification protocols, there is considerable scope for further improvement in the cryopreservation techniques used for many parasitic species.