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

Relevance of controlled cooling and freezing phases in T-cell cryopreservation

When cells are cryopreserved, they go through a freezing process with several distinct phases (i.e., cooling until nucleation, ice nucleation, ice crystal growth and cooling to a final temperature). Conventional cell freezing approaches often employ a single cooling rate to describe and optimize the entire freezing process, which neglects its complexity and does not provide insight into the effects of the different freezing phases. The aim of this work was to elucidate the impact of each freezing phase by varying different process parameters per phase. Hereto, spin freezing was used to freeze Jurkat T cells in either a Me2SO-based or Me2SO-free formulation. The cooling rates before ice nucleation and after total ice crystallization impacted cell viability, resulting in viability ranging from 26.7% to 52.8% for the Me2SO-free formulation, and 22.5%-42.6% for the Me2SO-based formulation. Interestingly, the degree of supercooling upon nucleation did not exhibit a significant effect on cell viability in this work. However, the rate of ice crystal formation emerged as a crucial factor, with viability ranging from 2.4% to 53.2% for the Me2SO-free formulation, and 0.3%-53.2% for the Me2SO-based formulation, depending on the freezing rate. A morphological study of the cells post-cryopreservation was performed using confocal microscopy, and it was found that cytoskeleton integrity and cell volume were impacted, depending on the formulation-process parameter combination. These findings underscore the importance of scrutinizing all cooling and freezing phases, as each phase impacted post-thaw viability in a distinct way, depending of the specific formulation used.

Rational synthesis of total damage during cryoprotectant equilibration: modelling and experimental validation of osmomechanical, temperature, and cytotoxic damage in sea urchin (Paracentrotus lividus) oocytes

Sea urchins (e.g., Paracentrotus lividus) are important for both aquaculture and as model species. Despite their importance, biobanking of urchin oocytes by cryopreservation is currently not possible. Optimized cryoprotectant loading may enable novel vitrification methods and thus successful cryopreservation of oocytes. One method for determining an optimized loading protocol uses membrane characteristics and models of damage, namely osmomechanical damage, temperature damage (e.g., chill injury) and cytotoxicity. Here we present and experimentally evaluate existing and novel models of these damage modalities as a function of time and temperature. In osmomechanical damage experiments, oocytes were exposed for 2 to 30 minutes in hypertonic NaCl or sucrose supplemented seawater or in hypotonic diluted seawater. In temperature damage experiments, oocytes were exposed to 1.7 °C, 10 °C, or 20 °C for 2 to 90 minutes. Cytotoxicity was investigated by exposing oocytes to solutions of Me2SO for 2 to 30 minutes. We identified a time-dependent osmotic damage model, a temperature-dependent damage model, and a temperature and time-dependent cytotoxicity model. We combined these models to estimate total damage during a cryoprotectant loading protocol and determined the optimal loading protocol for any given goal intracellular cryoprotectant concentration. Given our fitted models, we find sea urchin oocytes can only be loaded to 13% Me2SO v/v with about 50% survival. This synthesis of multiple damage modalities is the first of its kind and enables a novel approach to modelling cryoprotectant equilibration survival for cells in general.

Ovarian tissue cryopreservation and transplantation: a review on reactive oxygen species generation and antioxidant therapy

Cancer is the leading cause of death worldwide. Fortunately, the survival rate of cancer continues to rise, owing to advances in cancer treatments. However, these treatments are gonadotoxic and cause infertility. Ovarian tissue cryopreservation and transplantation (OTCT) is the most flexible option to preserve fertility in women and children with cancer. However, OTCT is associated with significant follicle loss and an accompanying short lifespan of the grafts. There has been a decade of research in cryopreservation-induced oxidative stress in single cells with significant successes in mitigating this major source of loss of viability. However, despite its success elsewhere and beyond a few promising experiments, little attention has been paid to this key aspect of OTCT-induced damage. As more and more clinical practices adopt OTCT for fertility preservation, it is a critical time to review oxidative stress as a cause of damage and to outline potential ameliorative interventions. Here we give an overview of the application of OTCT for female fertility preservation and existing challenges; clarify the potential contribution of oxidative stress in ovarian follicle loss; and highlight potential ability of antioxidant treatments to mitigate the OTCT-induced injuries that might be of interest to cryobiologists and reproductive clinicians.

Comparison of dilute and nondilute osmotic equilibrium models for erythrocytes

Successful cryopreservation requires the addition of cryoprotective agents (CPAs). The addition of permeating CPAs, such as glycerol, is associated with some risk to the cells and tissues. These risks are both related to the CPA themselves (CPA toxicity) and to the volume response of the cell (osmotic damage). To minimize the potential for damage during cryopreservation, mathematical models are often employed to understand the interactions between protocols and cell volume responses. In the literature, this volume response is usually captured using ideal and dilute approximations of chemical potential and osmolality, an approach that has been called into question for cells in high concentrations of CPAs. To address this, the relevance of non-ideal and non-dilute models has been explored in a number of cell types in the presence of permeating CPAs. However, it has not been explored in erythrocytes, which have a cytosolic hemoglobin content of more than 20% by volume and are cryopreserved in 40% glycerol. Because hemoglobin has been suggested to be a highly non-ideal solute, if the non-ideal and non-dilute transport model is relevant to any cells, it should be relevant to erythrocytes. Here we investigate the use, and accuracy, of both the dilute and non-dilute models in predicting cell volume changes during CPA equilibration in erythrocytes, and demonstrate that using published values for the non-ideal and non-dilute model, applied to erythrocytes, leads to model predictions inconsistent with experimental data, whereas dilute approximations align well with experimental data.

hMSCs in contact with DMSO for cryopreservation: experiments and modelling of osmotic injury and cytotoxic effect

In this work a combined analysis of osmotic injury and cytotoxic effect useful for the optimization of the cryopreservation process of a cell suspension is carried out. The case of human Mesenchymal Stem Cells (hMSCs) from Umbilical Cord Blood (UCB) in contact with DiMethyl SulfOxide (DMSO) acting as Cryo-Protectant Agent (CPA) is investigated from the experimental as well as the theoretical perspective. The experimental runs are conducted by suspending the cells in hypertonic solutions of DMSO at varying osmolality, system temperature and contact times; then, at room temperature, cells are pelleted by centrifugation and suspended back to isotonic conditions. Eventually cell count and viability are measured by means of a Coulter counter and flow-cytometer, respectively. Overall, a decrease of cell count and viability results when DMSO concentration, temperature and contact time increase. A novel mathematical model is developed and proposed to interpret measured data by dividing the cell population between viable and non-viable cells. The decrease of cell count is ascribed exclusively to the osmotic injury caused by expansion lysis: excessive swelling causes the burst of both viable as well as non-viable cells. On the other hand, the reduction of cell viability is ascribed only to cytotoxicity which gradually transforms viable cells into non-viable ones. A chemical reaction engineering approach is adopted to describe the dynamics of both phenomena: by following the kinetics of two chemical reactions during cell osmosis inside a closed system it is shown that the simultaneous reduction of cell count and viability may be successfully interpreted. The use of the Surface Area Regulation (SAR) model recently proposed by the authors allows one to avoid the setting in advance of fixed cell Osmotic Tolerance Limits (OTLs), as traditionally done in cryopreservation literature to circumvent the mathematical simulation of osmotic injury. Comparisons between experimental data and theoretical simulations are provided: first, a non-linear regression analysis is performed to evaluate unknown model parameters through a best-fitting procedure carried out in a sequential fashion; then, the proposed model is validated by full predictions of system behavior measured at operating conditions different from those used during the best-fit procedure. This article is protected by copyright. All rights reserved.

Exogenous Melatonin Ameliorates the Negative Effect of Osmotic Stress in Human and Bovine Ovarian Stromal Cells

Ovarian tissue cryopreservation transplantation (OTCT) is the most flexible option to preserve fertility in women and children with cancer. However, OTCT is associated with follicle loss and an accompanying short lifespan of the grafts. Cryopreservation-induced damage could be due to cryoprotective agent (CPA) toxicity and osmotic shock. Therefore, one way to avoid this damage is to maintain the cell volume within osmotic tolerance limits (OTLs). Here, we aimed to determine, for the first time, the OTLs of ovarian stromal cells (OSCs) and their relationship with reactive oxygen species (ROS) and mitochondrial respiratory chain activity (MRCA) of OSCs. We evaluated the effect of an optimal dose of melatonin on OTLs, viability, MRCA, ROS and total antioxidant capacity (TAC) of both human and bovine OSCs in plated and suspended cells. The OTLs of OSCs were between 200 and 375 mOsm/kg in bovine and between 150 and 500 mOsm/kg in human. Melatonin expands OTLs of OSCs. Furthermore, melatonin significantly reduced ROS and improved TAC, MRCA and viability. Due to the narrow osmotic window of OSCs, it is important to optimize the current protocols of OTCT to maintain enough alive stromal cells, which are necessary for follicle development and graft longevity. The addition of melatonin is a promising strategy for improved cryopreservation media.

Microfluidic measurement of individual cell membrane water permeability

This paper reports a microfluidic lab-on-chip for dynamic particle sizing and real time individual cell membrane permeability measurements. To achieve this, the device measures the impedance change of individual cells or particles at up to ten time points after mixing with different media, e.g. dimethyl sulfoxide or DI water, from separate inlets. These measurements are enabled by ten gold electrode pairs spread across a 20 mm long microchannel. The device measures impedance values within 0.26 s after mixing with other media, has a detection throughput of 150 samples/second, measures impedance values at all ten electrodes at this rate, and allows tracking of individual cell volume changes caused by cell osmosis in anisosmotic fluids over a 1.3 s postmixing timespan, facilitating accurate individual cell estimates of water permeability. The design and testing were performed using yeast cells (Saccharomyces cerevisiae). The relationship between volume and impedance in both polystyrene calibration beads as well as the volume-osmolality relationship in yeast were demonstrated. Moreover, we present the first noninvasive and non-optically-based water permeability measurements in individual cells.

Winter is coming: the future of cryopreservation

The preservative effects of low temperature on biological materials have been long recognised, and cryopreservation is now widely used in biomedicine, including in organ transplantation, regenerative medicine and drug discovery. The lack of organs for transplantation constitutes a major medical challenge, stemming largely from the inability to preserve donated organs until a suitable recipient is found. Here, we review the latest cryopreservation methods and applications. We describe the main challenges-scaling up to large volumes and complex tissues, preventing ice formation and mitigating cryoprotectant toxicity-discuss advantages and disadvantages of current methods and outline prospects for the future of the field.

Mathematical Modeling and Optimization of Cryopreservation in Single Cells

Cryobiology is a multiscale and interdisciplinary field. The scope and scale of interactions limit the gains that can be made by one theory or experiment alone. Because of this, modeling has played a critical role in both explaining cryobiological phenomena and predicting improved protocols. Modeling facilitates understanding of the biophysical and some of the biochemical mechanisms of damage during all phases of cryopreservation including CPA equilibration and cooling and warming. Moreover, as a tool for optimization of cryopreservation protocols, modeling has yielded many successes. Modern cryobiological modeling includes very detailed descriptions of the physical phenomena that occur during freezing, including ice growth kinetics and spatial gradients that define heat and mass transport models. Here we reduce the complexity and approach only a small but classic subset of these problems. Namely, here we describe the process of building and using a mathematical model of a cell in suspension where spatial homogeneity is assumed for all quantities. We define the models that describe the critical cell quantities used to describe optimal and suboptimal protocols and then give an overview of classical methods of how to determine optimal protocols using these models. We include practical considerations of modeling in cryobiology, including fitting transport models to cell volume data, performing optimization with cell volume constraints, and a look at expanding cost functions to cooling regimes.

Implications of variability in cell membrane permeability for design of methods to remove glycerol from frozen-thawed erythrocytes

In North America, red blood cells (RBCs) are currently cryopreserved in a solution of 40% glycerol. While glycerol is not inherently toxic to humans, it must be removed prior to transfusion to prevent intravascular osmotic hemolysis. The current deglycerolization procedure requires about 45 min per RBC unit. We previously presented predictions suggesting that glycerol could be safely removed from RBCs in less than 1 min. However, experimental evaluation of these methods resulted in much higher hemolysis than expected. Here we extend our previous study by considering both concentration-dependence of permeability and variability in permeability values in the mathematical optimization algorithm. To establish a model for the concentration dependence of glycerol permeability, we combined literature data with new measurements of permeability in the presence of 40% glycerol. To account for cell-dependent variability we scaled the concentration-dependent permeability model to define a permeability range for optimization. Methods designed using a range extending to 50% of the model-predicted glycerol permeability had a duration of less than 3 min and resulted in hemolysis ranging from 34% to 83%; hemolysis values were highly dependent on the blood donor. Extending the permeability range to 5% of the model-predicted value yielded a 30 min method that resulted in an average hemolysis of 12%. Our results suggest high variability in the glycerol permeability between donors and within a population of cells from the same donor. Such variability has broad implications for design of methods for equilibration of cells with cryoprotectants.

Survivability of rabbit amniotic fluid-derived mesenchymal stem cells post slow-freezing or vitrification

This work aimed to evaluate the effect of two distinct cryopreservation procedures - conventional slow-freezing and vitrification, on survivability and mesenchymal marker expression stability of rabbit amniotic fluid-derived mesenchymal stem cells (rAF-MSCs). Cells at passage 2 were slowly frozen, using 10% of dimethylsulfoxide, or vitrified, using 40% of ethylene glycol, 0.5 M sucrose and 18% Ficoll 70. After three months storage in liquid nitrogen, viability, chromosomal stability, ultrastructure, surface and intracellular marker expression and differentiation potential of cells were evaluated immediately post-thawing/warming and after additional culture for 48-72 h. Our results showed decreased (P ≤ 0.05) viability of cells post-thawing/warming. However, after additional culture, the viability was similar to those in fresh counterparts in both cryopreserved groups. Increase (P ≤ 0.05) in the population doubling time of vitrified cells was observed, while doubling time of slow-frozen cells remained similar to non-cryopreserved cells. No changes in karyotype (chromosomal numbers) were observed in frozen/vitrified AF-MSCs, and histological staining confirmed similar differentiation potential of fresh and frozen/vitrified cells. Analysis of mesenchymal marker expression by qPCR showed that both cryopreservation approaches significantly affected expression of CD73 and CD90 surface markers. These changes were not detected using flow cytometry. In summary, the conventional slow-freezing and vitrification are reliable and effective approaches for the cryopreservation of rabbit AF-MSCs. Nevertheless, our study confirmed affected expression of some mesenchymal markers following cryopreservation.

Osmotic behaviour of human mesenchymal stem cells: Implications for cryopreservation

Aimed at providing a contribution to the optimization of cryopreservation processes, the present work focuses on the osmotic behavior of human mesenchymal stem cells (hMSCs). Once isolated from the umbilical cord blood (UCB) of three different donors, hMSCs were characterized in terms of size distribution and their osmotic properties suitably evaluated through the exposure to hypertonic and isotonic aqueous solutions at three different temperatures. More specifically, inactive cell volume and cell permeability to water and di-methyl sulfoxide (DMSO) were measured, being cell size determined using impedance measurements under both equilibrium and dynamic conditions. Experimental findings indicate that positive cell volume excursions are limited by the apparent increase of inactive volume, which occurs during both the shrink-swell process following DMSO addition and the subsequent restoration of isotonic conditions in the presence of hypertonic solutions of impermeant or permeant solutes. Based on this evidence, hMSCs must be regarded as imperfect osmometers, and their osmotic behavior described within a scenario no longer compatible with the simple two-parameter model usually utilized in the literature. In this respect, the activation of mechano-sensitive ion-channels seemingly represents a reasonable hypothesis for rationalizing the observed osmotic behavior of hMSCs from UCB.

Cryopreservation: Vitrification and Controlled Rate Cooling

Cryopreservation is the application of low temperatures to preserve the structural and functional integrity of cells and tissues. Conventional cooling protocols allow ice to form and solute concentrations to rise during the cryopreservation process. The damage caused by the rise in solute concentration can be mitigated by the use of compounds known as cryoprotectants. Such compounds protect cells from the consequences of slow cooling injury, allowing them to be cooled at cooling rates which avoid the lethal effects of intracellular ice. An alternative to conventional cooling is vitrification. Vitrification methods incorporate cryoprotectants at sufficiently high concentrations to prevent ice crystallization so that the system forms an amorphous glass thus avoiding the damaging effects caused by conventional slow cooling. However, vitrification too can impose damaging consequences on cells as the cryoprotectant concentrations required to vitrify cells at lower cooling rates are potentially, and often, harmful. While these concentrations can be lowered to nontoxic levels, if the cells are ultra-rapidly cooled, the resulting metastable system can lead to damage through devitrification and growth of ice during subsequent storage and rewarming if not appropriately handled.The commercial and clinical application of stem cells requires robust and reproducible cryopreservation protocols and appropriate long-term, low-temperature storage conditions to provide reliable master and working cell banks. Though current Good Manufacturing Practice (cGMP) compliant methods for the derivation and banking of clinical grade pluripotent stem cells exist and stem cell lines suitable for clinical applications are available, current cryopreservation protocols, whether for vitrification or conventional slow freezing, remain suboptimal. Apart from the resultant loss of valuable product that suboptimal cryopreservation engenders, there is a danger that such processes will impose a selective pressure on the cells selecting out a nonrepresentative, freeze-resistant subpopulation. Optimizing this process requires knowledge of the fundamental processes that occur during the freezing of cellular systems, the mechanisms of damage and methods for avoiding them. This chapter draws together the knowledge of cryopreservation gained in other systems with the current state-of-the-art for embryonic and induced pluripotent stem cell preservation in an attempt to provide the background for future attempts to optimize cryopreservation protocols.

Mode of action of cryoprotectants for sperm preservation

Sperm cryopreservation facilitates storage and transport for use in artificial reproduction technologies. Cryopreservation processing, however, exposes cells to stress resulting in cellular damage compromising sperm function. Cryoprotective agents are needed to minimize cryopreservation injury, but at higher concentration they are toxic to cells. In this review, we describe cryoinjury mechanisms, and modes of action of different types of cryoprotective agents. Furthermore, measures are discussed how to minimize toxic effects caused by adding and removing cryoprotective agents. Cryoprotective agents can be divided into permeating and non-permeating agents. Permeating agents such as glycerol can move across cellular membranes and modulate the rate and extent of cellular dehydration during freezing-induced membrane phase transitions. Permeating protectants provide intracellular protection because they are preferentially excluded from the surface of biomolecules thereby stabilizing the native state. Non-permeating agents can be divided into osmotically active smaller molecules and osmotically inactive macromolecules. Both, permeating and non-permeating protectants form a protective glassy state during freezing preserving biomolecular and cellular structures. Freezing extenders for sperm contain salts, buffer compounds, sugars, proteins and lipids, and typically contain glycerol as the main permeating cryoprotective agent providing intracellular protection. Non-permeating protectants including sugars and proteins are used as bulking agents and to increase the glass transition temperature of the freezing extender. Ultra-heat-treated milk and egg yolk are frequently added as membrane modifying agents to enhance the inherent sperm cryostability. The protocol how to use and add cryoprotectants is a compromise between their beneficial and potentially detrimental effects.

Toxicity Minimized Cryoprotectant Addition and Removal Procedures for Adherent Endothelial Cells

Ice-free cryopreservation, known as vitrification, is an appealing approach for banking of adherent cells and tissues because it prevents dissociation and morphological damage that may result from ice crystal formation. However, current vitrification methods are often limited by the cytotoxicity of the concentrated cryoprotective agent (CPA) solutions that are required to suppress ice formation. Recently, we described a mathematical strategy for identifying minimally toxic CPA equilibration procedures based on the minimization of a toxicity cost function. Here we provide direct experimental support for the feasibility of these methods when applied to adherent endothelial cells. We first developed a concentration- and temperature-dependent toxicity cost function by exposing the cells to a range of glycerol concentrations at 21°C and 37°C, and fitting the resulting viability data to a first order cell death model. This cost function was then numerically minimized in our state constrained optimization routine to determine addition and removal procedures for 17 molal (mol/kg water) glycerol solutions. Using these predicted optimal procedures, we obtained 81% recovery after exposure to vitrification solutions, as well as successful vitrification with the relatively slow cooling and warming rates of 50°C/min and 130°C/min. In comparison, conventional multistep CPA equilibration procedures resulted in much lower cell yields of about 10%. Our results demonstrate the potential for rational design of minimally toxic vitrification procedures and pave the way for extension of our optimization approach to other adherent cell types as well as more complex systems such as tissues and organs.

Modeling and optimization of cryopreservation

Modeling plays a critical role in understanding the biophysical processes behind cryopreservation. It facilitates understanding of the biophysical and some of the biochemical mechanisms of damage during all phases of cryopreservation including CPA equilibration, cooling, and warming. Modeling also provides a tool for optimization of cryopreservation protocols and has yielded a number of successes in this regard. While modern cryobiological modeling includes very detailed descriptions of the physical phenomena that occur during freezing, including ice growth kinetics and spatial gradients that define heat and mass transport models, here we reduce the complexity and approach only a small but classic subset of these problems. Namely, here we describe the process of building and using a mathematical model of a cell in suspension where spatial homogeneity is assumed for all quantities. We define the models that describe the critical cell quantities used to describe optimal and suboptimal protocols and then give an overview of classical methods of how to determine optimal protocols using these models.

Foundations of modeling in cryobiology-I: concentration, Gibbs energy, and chemical potential relationships

Mathematical modeling plays an enormously important role in understanding the behavior of cells, tissues, and organs undergoing cryopreservation. Uses of these models range from explanation of phenomena, exploration of potential theories of damage or success, development of equipment, and refinement of optimal cryopreservation/cryoablation strategies. Over the last half century there has been a considerable amount of work in bio-heat and mass-transport, and these models and theories have been readily and repeatedly applied to cryobiology with much success. However, there are significant gaps between experimental and theoretical results that suggest missing links in models. One source for these potential gaps is that cryobiology is at the intersection of several very challenging aspects of transport theory: it couples multi-component, moving boundary, multiphase solutions that interact through a semipermeable elastic membrane with multicomponent solutions in a second time-varying domain, during a two-hundred Kelvin temperature change with multi-molar concentration gradients and multi-atmosphere pressure changes. In order to better identify potential sources of error, and to point to future directions in modeling and experimental research, we present a three part series to build from first principles a theory of coupled heat and mass transport in cryobiological systems accounting for all of these effects. The hope of this series is that by presenting and justifying all steps, conclusions may be made about the importance of key assumptions, perhaps pointing to areas of future research or model development, but importantly, lending weight to standard simplification arguments that are often made in heat and mass transport. In this first part, we review concentration variable relationships, their impact on choices for Gibbs energy models, and their impact on chemical potentials.

Membrane permeability of the human pluripotent stem cells to Me₂SO, glycerol and 1,2-propanediol

Due to the unlimited capacity of self-renew and ability to differentiate into derivatives of three germ layers, human pluripotent stem cells, including human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs), have a great potential in regenerative medicine. A major challenge we are facing during the long-term storage of human pluripotent stem cells with the conventional slow cooling rate is the low cell recovery rate after cryopreservation which cannot meet the requirements for the future clinical applications. Evaluating the cell membrane permeability and the corresponding activation energy of hESCs and hiPSCs for water and different cryoprotective agents (CPA), including dimethyl sulfoxide (Me2SO), 1,2-propandiol and glycerol, is important for facilitating the development of cryopreservation protocol to enhance cell recovery after the cryopreservation. The osmotically inactive volume of hESCs and hiPSCs determined using the Boyle-van't Hoff model was 0.32V0 and 0.42V0, respectively. The membrane permeability was assessed from the volume changes of cells exposed to Me2SO, 1,2-propanediol and glycerol at the temperatures ranging from 8 to 30°C. These results showed the biophysical differences between hESCs and hiPSCs. Their activation energy for water and CPAs extrapolated from the Arrhenius relationship indicated that the water transport was probably not through the channel-mediated mechanism.

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.