Vitrification media: toxicity, permeability, and dielectric properties

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.

CPA toxicity screening of cryoprotective solutions in rat hearts

In clinical practice, donor hearts are transported on ice prior to transplant and discarded if cold ischemia time exceeds ∼5 h. Methods to extend these preservation times are critically needed, and ideally, this storage time would extend indefinitely, enabling improved donor-to-patient matching, organ utilization, and immune tolerance induction protocols. Previously, we demonstrated successful vitrification and rewarming of whole rat hearts without ice formation by perfusion-loading a cryoprotective agent (CPA) solution prior to vitrification. However, these hearts did not recover any beating even in controls with CPA loading/unloading alone, which points to the chemical toxicity of the cryoprotective solution (VS55 in Euro-Collins carrier solution) as the likely culprit. To address this, we compared the toxicity of another established CPA cocktail (VEG) to VS55 using ex situ rat heart perfusion. The CPA exposure time was 150 min, and the normothermic assessment time was 60 min. Using Celsior as the carrier, we observed partial recovery of function (atria-only beating) for both VS55 and VEG. Upon further analysis, we found that the VEG CPA cocktail resulted in 50 % lower LDH release than VS55 (N = 4, p = 0.017), suggesting VEG has lower toxicity than VS55. Celsior was a better carrier solution than alternatives such as UW, as CPA + Celsior-treated hearts spent less time in cardiac arrest (N = 4, p = 0.029). While we showed substantial improvement in cardiac function after exposure to vitrifiable concentrations of CPA by improving both the CPA and carrier solution formulation, further improvements will be required before we achieve healthy cryopreserved organs for transplant.

Ice Control during Cryopreservation of Heart Valves and Maintenance of Post-Warming Cell Viability

Heart valve cryopreservation was employed as a model for the development of complex tissue preservation methods based upon vitrification and nanowarming. Porcine heart valves were loaded with cryoprotectant formulations step wise and vitrified in 1−30 mL cryoprotectant formulations ± Fe nanoparticles ± 0.6 M disaccharides, cooled to −100 °C, and stored at −135 °C. Nanowarming was performed in a single ~100 s step by inductive heating within a magnetic field. Controls consisted of fresh and convection-warmed vitrified heart valves without nanoparticles. After washing, cell viability was assessed by metabolic assay. The nanowarmed leaflets were well preserved, with a viability similar to untreated fresh leaflets over several days post warming. The convection-warmed leaflet viability was not significantly different than that of the nanowarmed leaflets immediately after rewarming; however, a significantly higher nanowarmed leaflet viability (p < 0.05) was observed over time in vitro. In contrast, the associated artery and fibrous cardiac muscle were at best 75% viable, and viability decreased over time in vitro. Supplementation of lower concentration cryoprotectant formulations with disaccharides promoted viability. Thicker tissues benefited from longer-duration cryoprotectant loading steps. The best outcomes included a post-warming incubation step with α-tocopherol and an apoptosis inhibitor, Q-VD-OPH. This work demonstrates progress in the control of ice formation and cytotoxicity hurdles for the preservation of complex tissues.

Vitrification with microinjection of single seminiferous tubules: an efficient cryopreservation approach for limited testicular tissue

RESEARCH QUESTION

Is vitrification with microinjection of single seminiferous tubules an efficient cryopreservation approach for limited testicular tissue?

DESIGN

Testicular tissue from 10 patients with normal spermatogenesis were assigned to a fresh control group or one of the following cryopreservation procedures: uncontrolled slow freezing (USF) using either 1.5 or 2.1 M DMSO combined with sucrose and vitrification with or without single seminiferous tubules microinjection.

RESULTS

Single seminiferous tubules microinjected with cryoprotective agents (CPA) enhanced the penetration of CPA compared with CPA-treated testicular tissue fragments. Microinjection of seminiferous tubules (VLP) maintained tubule structural integrity and germ cell numbers, and reduced spermatogonial apoptosis after cryopreservation compared with vitrification without microinjection (apoptosis rate: VLP versus vitrification without microinjection, P = 0.047; VLP versus USF, P= 0.049). Freezing of single seminiferous tubules using 0.25-ml straws and traditional sperm freezing methods protected sperm retrieval and recovery rates, and the progressive motility index.

CONCLUSIONS

Vitrification of single seminiferous tubule with microinjection of low CPA concentration is an effective approach to testicular cryopreservation.

Effect of cryoprotectant concentration on bovine oocyte permeability and comparison of two membrane permeability modelling approaches

The plasma membrane permeability to water and cryoprotectant (CPA) significantly impacts vitrification efficiency of bovine oocytes. Our study was designed to determine the concentration-dependent permeability characteristics for immature (GV) and mature (MII) bovine oocytes in the presence of ethylene glycol (EG) and dimethyl sulphoxide (Me₂SO), and to compare two different modeling approaches: the two parameter (2P) model and a nondilute transport model. Membrane permeability parameters were determined by consecutively exposing oocytes to increasing concentrations of Me₂SO or EG. Higher water permeability was observed for MII oocytes than GV oocytes in the presence of both Me₂SO and EG, and in all cases the water permeability was observed to decrease as CPA concentration increased. At high CPA concentrations, the CPA permeability was similar for Me₂SO and EG, for both MII and GV oocytes, but at low concentrations the EG permeability of GV oocytes was substantially higher. Predictions of cell volume changes during CPA addition and removal indicate that accounting for the concentration dependence of permeability only has a modest effect, but there were substantial differences between the 2P model and the nondilute model during CPA removal, which may have implications for design of improved methods for bovine oocyte vitrification.

Cryopreservation of articular cartilage. Part 3: The liquidus-tracking method

Although it is relatively straightforward to cryopreserve living isolated chondrocytes, at the present time there is no satisfactory method to preserve surgical grafts between the time of procurement or manufacture and actual use. In earlier papers we have established that the cryoprotectants dimethyl sulphoxide or propylene glycol do penetrate into this tissue very rapidly. Chondrocytes are not unusually susceptible to osmotic stress; in fact they appear to be particularly resistant. It appears that damage is associated with the formation of ice per se, even at cooling rates that are optimal for the cryopreservation of isolated chondrocytes. We then showed that current methods of cartilage cryopreservation involve the nucleation and growth of ice crystals within the chondrons rather than ice being restricted to the surrounding acellular matrix. This finding established the need to avoid the crystallization of ice-in other words, vitrification. Song and his colleagues have published a vitrification method that is based on the use of one of Fahy's vitrification formulations. We confirmed the effectiveness of this method but found it to be very dependent on ultra rapid warming. However, we were able to develop a 'liquidus-tracking' method that completely avoids the crystallization of ice and does not require rapid warming. The ability of cartilage preserved in this way to incorporate sulphate into newly synthesized glycosaminoglycans (GAGs) approached 70% of that of fresh control cartilage. In this method the rates of cooling and warming can be very low, which is essential for any method that is to be used in Tissue Banks to process the bulky grafts that are required by orthopaedic surgeons. Work is continuing to refine this method for Tissue Bank use.

Tolerance of lamb and mouse oocytes to cryoprotectants during vitrification

SummaryMouse and lamb oocytes were vitrified with, or exposed to, different cryoprotectants and evaluated for their effects on their survival and developmental competence after in vitro fertilization (IVF) and activation treatments. Control oocytes remained untreated, whilst the remainder were exposed to three different combinations of vitrification solutions [dimethyl sulfoxide (DMSO) + ethylene glycol (EG), EG only, or propanediol (PROH) + EG] and either vitrified or left unfrozen (exposed groups). Oocytes in the control and vitrified groups underwent IVF and developmental competence was assessed to the blastocyst stage. In lambs, survival rate in vitrified oocytes was significantly lower than for oocytes in the exposed groups (P &lt;0.05). Blastocyst development was low in vitrified oocytes compared with controls (&lt;6% vs 38.9%, P &lt;0.01). Parthenogenetic activation was more prevalent in vitrified lamb oocytes compared with controls (P &lt;0.05). No evidence of zona pellucida hardening or cortical granule exocytosis could account for reduced fertilization rates in vitrified lamb oocytes. Mouse oocytes demonstrated a completely different response to lamb oocytes, with survival and parthenogenetic activation rates unaffected by the vitrification process. Treatment of mouse oocytes with DMSO + EG yielded significantly higher survival and cleavage rates than treatment with PROH + EG (87.8% and 51.7% vs 32.7% and 16.7% respectively, P &lt;0.01), however cleavage rate for vitrified oocytes remained lower than for the controls (51.7% vs 91.7%, P &lt;0.01) as did mean blastocyst cell number (33 ± 3.1 vs 42 ± 1.5, P &lt;0.05). From this study, it is clear that lamb and mouse show different tolerances to cryoprotectants commonly used in vitrification procedures, and careful selection and testing of species-compatible cryoprotectants is required when vitrifying oocytes to optimize survival and embryo development.

Concentration dependence of the cell membrane permeability to cryoprotectant and water and implications for design of methods for post-thaw washing of human erythrocytes

For more than fifty years the human red blood cell (RBC) has been a widely studied model for transmembrane mass transport. Existing literature spans myriad experimental designs with varying results and physiologic interpretations. In this review, we examine the kinetics and mechanisms of membrane transport in the context of RBC cryopreservation. We include a discussion of the pathways for water and glycerol permeation through the cell membrane and the implications for mathematical modeling of the membrane transport process. In particular, we examine the concentration dependence of water and glycerol transport and provide equations for estimating permeability parameters as a function of concentration based on a synthesis of literature data. This concentration-dependent transport model may allow for design of improved methods for post-thaw removal of glycerol from cryopreserved blood. More broadly, the consideration of the concentration dependence of membrane permeability parameters may be important for other cell types as well, especially for design of methods for equilibration with the highly concentrated solutions used for vitrification.

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.

Cryoprotectant Toxicity: Facts, Issues, and Questions

High levels of penetrating cryoprotectants (CPAs) can eliminate ice formation during cryopreservation of cells, tissues, and organs to cryogenic temperatures. But CPAs become increasingly toxic as concentration increases. Many strategies have been attempted to overcome the problem of eliminating ice while minimizing toxicity, such as attempting to optimize cooling and warming rates, or attempting to optimize time of adding individual CPAs during cooling. Because strategies currently used are not adequate, CPA toxicity remains the greatest obstacle to cryopreservation. CPA toxicity stands in the way of cryogenic cryopreservation of human organs, a procedure that has the potential to save many lives. This review attempts to describe what is known about CPA toxicity, theories of CPA toxicity, and strategies to reduce CPA toxicity. Critical analysis and suggestions are also included.

Open pulled straw vitrification and slow freezing of sheep IVF embryos using different cryoprotectants

The aim of the present study was to evaluate the post-thaw survival and hatching rates of sheep blastocysts using different cryoprotectants. In Experiment 1, Day 6 sheep embryos were cryopreserved by a slow freezing protocol using 10% ethylene glycol (EG), 10% dimethyl sulfoxide (DMSO) or a mixture of 5% EG and 5% DMSO. Hatching rates were higher in the 10% EG group than in the 10% DMSO or EG + DMSO groups (30% vs 18% and 20%, respectively). In Experiment 2, embryos were cryopreserved by open pulled straw (OPS) vitrification using either 33% EG, 33% DMSO or a mixture of 16.5% EG + 16.5% DMSO. Re-expansion and hatching rates in the EG + DMSO group (79.16% and 52.74%, respectively) were higher than those in the EG group (64.28% and 30.02%, respectively), whereas the outcomes for the DMSO group were the lowest (45.18% and 8.6%, respectively). In Experiment 3, embryos were cryopreserved by OPS vitrification using either 40% EG, 40% DMSO or a mixture of 20% EG + 20% DMSO. Re-expansion and hatching rates were highest in the EG group than in the EG + DMSO and DMSO groups (92.16% vs 76.30% and 55.84% re-expansion, respectively; and 65.78% vs 45.55% and 14.46% hatching, respectively). In conclusion, OPS vitrification was found to be more efficient for cryopreservation of in vitro-developed sheep embryos than traditional freezing.

The cryobiology of spermatozoa

The impact of successful cryopreservation of spermatozoa can be found in many fields, including agriculture, laboratory animal medicine, and human assisted reproduction, providing a cost-effective and efficient method to preserve genetic material for decades. The success of any cryobiologic protocol depends critically on understanding the fundamentals that underlie the process. In this review, we summarize the biophysical fundamentals critical to much of the research in sperm cryobiology, provide a synopsis of the development of sperm cryobiology as a discipline, and present the current state and directions for future research in sperm cryobiology in the three major areas outlined above-agriculture, laboratory animal medicine, and human clinical assisted reproduction. There is much room for new research, both empiric and fundamental, in all areas, including refinement of mathematical models, optimization of cryoprotective agent addition and removal procedures for spermatozoa from many species, development of effective, efficient, and facile cryopreservation protocols and freezing containers for agricultural sperm cryopreservation, and tailoring cryopreservation protocols for individual human samples.

Cryopreservation of articular cartilage

Cryopreservation has numerous practical applications in medicine, biotechnology, agriculture, forestry, aquaculture and biodiversity conservation, with huge potentials for biological cell and tissue banking. A specific tissue of interest for cryopreservation is the articular cartilage of the human knee joint for two major reasons: (1) clinically, there exists an untapped potential for cryopreserved cartilage to be used in surgical repair/reconstruction/replacement of injured joints because of the limited availability of fresh donor tissue and, (2) scientifically, successful cryopreservation of cartilage, an avascular tissue with only one cell type, is considered a stepping stone for transition from biobanking cell suspensions and small tissue slices to larger and more complicated tissues. For more than 50years, a great deal of effort has been directed toward understanding and overcoming the challenges of cartilage preservation. In this article, we focus mainly on studies that led to the finding that vitrification is an appropriate approach toward successful preservation of cartilage. This is followed by a review of the studies on the main challenges of vitrification, i.e. toxicity and diffusion, and the novel approaches to overcome these challenges such as liquidus tracking, diffusion modeling, and cryoprotective agent cocktails, which have resulted in the recent advancements in the field.

Permeation of dimethyl sulfoxide into articular cartilage at subzero temperatures

Osteochondral allografting has been proved to be a useful method to treat diseased or damaged areas of joint surfaces. Operational long-term stocks of grafts which supply a buffer between procurement and utilization would contribute to the commercialization or industrialization of this technology. Vitrification has been thought to be a promising method for successful preservation of articular cartilage (AC), but high concentration cryoprotectants (CPAs) are used which may cause high cellular toxicity. An effective way to reduce CPA toxicity is to increase CPA concentration gradually while the temperature is lowered. Understanding the mechanism of CPA permeation at subzero temperatures is important for designing the cryopreservation protocol. In this research, the permeation of dimethyl sulfoxide (Me(2)SO) in ovine AC at subzero temperatures was studied experimentally. Pretreated AC discs were exposed in Me(2)SO solutions for different time (0, 5, 15, 30, 50, 80, and 120 min) at three temperature levels (-10, -20, and -30 °C). The Me(2)SO concentration within the tissue was determined by ultraviolet (UV) spectrophotometry. The diffusion coefficients were estimated to be 0.85×10(-6), 0.48×10(-6), and 0.27×10(-6) cm(2)/s at -10, -20, and -30 °C, respectively, and the corresponding activation energy was 29.23 kJ/mol. Numerical simulation was performed to compare two Me(2)SO addition protocols, and the results demonstrated that the total loading duration could be effectively reduced with the knowledge of permeation kinetics.

Emerging technologies in medical applications of minimum volume vitrification

Cell/tissue biopreservation has broad public health and socio-economic impact affecting millions of lives. Cryopreservation technologies provide an efficient way to preserve cells and tissues targeting the clinic for applications including reproductive medicine and organ transplantation. Among these technologies, vitrification has displayed significant improvement in post-thaw cell viability and function by eliminating harmful effects of ice crystal formation compared to the traditional slow freezing methods. However, high cryoprotectant agent concentrations are required, which induces toxicity and osmotic stress to cells and tissues. It has been shown that vitrification using small sample volumes (i.e., <1 µl) significantly increases cooling rates and hence reduces the required cryoprotectant agent levels. Recently, emerging nano- and micro-scale technologies have shown potential to manipulate picoliter to nanoliter sample sizes. Therefore, the synergistic integration of nanoscale technologies with cryogenics has the potential to improve biopreservation methods.

Comparison and avoidance of toxicity of penetrating cryoprotectants

The objective of this study was to elucidate the toxicity of widely used penetrating cryoprotective agents (CPAs) to mammalian oocytes. To this end, mouse metaphase II (M II) oocytes were exposed to 1.5 M solutions of dimethylsulfoxide (DMSO), ethylene glycol (EG), or propanediol (PROH) prepared in phosphate buffered saline (PBS) containing 10% fetal bovine serum. To address the time- and temperature-dependence of the CPA toxicity, M II oocytes were exposed to the aforementioned CPAs at room temperature (RT, ∼23°C) and 37°C for 15 or 30 minutes. Subsequently, the toxicity of each CPA was evaluated by examining post-exposure survival, fertilization, embryonic development, chromosomal abnormalities, and parthenogenetic activation of treated oocytes. Untreated oocytes served as controls. Exposure of MII oocytes to 1.5 M DMSO or 1.5 M EG at RT for 15 min did not adversely affect any of the evaluated criteria. In contrast, 1.5 M PROH induced a significant increase in oocyte degeneration (54.2%) and parthenogenetic activation (16%) under same conditions. When the CPA exposure was performed at 37°C, the toxic effect of PROH further increased, resulting in lower survival (15%) and no fertilization while the toxicity of DMSO and EG was still insignificant. Nevertheless, it was possible to completely avoid the toxicity of PROH by decreasing its concentration to 0.75 M and combining it with 0.75 M DMSO to bring the total CPA concentration to a cryoprotective level. Moreover, combining lower concentrations (i.e., 0.75 M) of PROH and DMSO significantly improved the cryosurvival of MII oocytes compared to the equivalent concentration of DMSO alone. Taken together, our results suggest that from the perspective of CPA toxicity, DMSO and EG are safer to use in slow cooling protocols while a lower concentration of PROH can be combined with another CPA to avoid its toxicity and to improve the cryosurvival as well.

On-chip characterization of cryoprotective agent mixtures using an EWOD-based digital microfluidic device

For tissue engineering and regenerative medicine, cryopreservation, a technique for preserving biomaterials in the frozen state with cryoprotective agents (CPAs), is critically important for preserving engineered tissues (ETs) as well as cells necessary to create ETs. As more diverse ETs are produced using various cell types, CPAs and corresponding freeze/thaw (F/T) protocols need to be developed cell/tissue-type specifically. This is because CPAs and F/T protocols that have been successful for one cell/tissue type have proven to be difficult to adapt to other cell/tissue types. The most critical barrier to address this challenge is the inability to screen and identify CPA or CPA mixtures efficiently. In this paper, we developed an "electro-wetting-on-dielectic" (EWOD) based digital microfluidic platform to characterize and screen CPA mixtures cell-type specifically. The feasibility of the EWOD platform was demonstrated by characterizing and optimizing a mixture of dimethlysulfoxide (DMSO) and PBS for human breast cancer cell line as model CPA mixture and cell line. The developed platform multiplexed droplets of DMSO and PBS to create an array of DMSO-PBS mixtures, and mapped the phase change diagram of the mixture. After loading cell suspensions on the platform, the mixture was further screened on-chip for toxicity and cryoprotection. The results were discussed to illustrate the capabilities and limitations of the EWOD platform for cell and tissue-type specific optimization of CPA mixtures and F/T protocols.

Assessment of freezing procedures for rat immature testicular tissue

Fertility preservation has been included in the management of childhood cancer treatment. Cryopreservation of immature testicular tissue is the only available solution for pre-pubertal boys. Different freezing protocols have been developed in several species but without a clearly identified procedure. We tried to evaluate several protocols for cryopreservation of rat immature testicular tissue. Twelve different freezing protocols using different (i) cryoprotectant (dimethylsulphoxide [DMSO] or 1,2-propanediol [PROH]), (ii) cryoprotectant concentration (1.5M or 3M), (iii) equilibration time (30 or 60 min), (iv) equilibration temperature (4 °C or room temperature), (v) size of testicular fragment (7.5mg or 15 mg), (vi) package (straws or cryovials), were compared using cord morphological damage evaluation. A testicular tissue piece of 7.5mg cryopreserved in cryovial using 1.5M DMSO, an equilibration time of 30 min at 4 °C showed fewer morphological alterations than the other protocols tested. The selected freezing protocol was able to maintain rat immature testicular tissue architecture, functionality after testicular pieces organotypic culture, and could be proposed in a human application.

Cytotoxicity effects of cryoprotectants as single-component and cocktail vitrification solutions

Cryoprotectant (CPA) cytotoxicity constitutes a challenge in developing cryopreservation protocols, specifically in vitrification where high CPA concentrations are necessary to achieve the ice-free, vitreous state. Few cytotoxicity studies have investigated vitrification-relevant concentrations of CPAs, and the benefits and disadvantages of cocktail solutions and of incorporating non-permeating solutes have not been fully evaluated. In this study, we address these issues by determining the cytotoxicity kinetics for dimethylsulfoxide (Me(2)SO) and 1,2-propanediol (PD) on alginate-encapsulated βTC-tet mouse insulinomas for a range of concentrations and temperatures. Cytotoxicity kinetics were also determined for two cocktails, DPS (3M Me(2)SO+3M PD+0.5M sucrose) and PEG400 (1M Me(2)SO+5M PD+0.34M poly(ethylene)glycol with M.W. of 400). PD was found to be more cytotoxic than Me(2)SO at higher concentrations and temperatures. This was reflected in PEG400 being more cytotoxic at room temperature than PEG400 at 4°C or DPS at either temperature. Addition of non-permeating solutes increased the cytotoxicity of cocktails. Furthermore, results indicate that CPA cytotoxicity may not be additive and that combining CPAs may increase cytotoxicity synergistically. Finally, when comparing cytotoxic effects towards encapsulated HepG2 and βTC-tet cells, and towards βTC-tet cells in capsules and in monolayers, CPAs appear more cytotoxic towards cells with higher metabolic activity. The incorporation of these results in the rational design of CPA addition/removal processes in vitrification is discussed.

Whole sheep ovary cryopreservation: evaluation of a slow freezing protocol with dimethylsulphoxide

PURPOSE

To evaluate a slow freezing method for whole ovary cryopreservation by evaluating effects of added cryoprotectant.

METHODS

Sheep ovaries were isolated during surgery, flushed with either Ringer-Acetate or dimethylsulphoxide and cryopreserved by slow freezing. After rapid thawing, viability was assessed by ovarian in vitro perfusion, cell culture, histology and fluorescent live-dead assay.

RESULTS

Production of cyclic AMP and progesterone was slightly higher in the dimethylsulphoxide group. Cultured ovarian cells from dimethylsulphoxide-preserved ovaries secreted larger amounts of progesterone than cells from Ringer-Acetate preserved. Light microscopy of ovarian biopsies obtained after perfusion, revealed well-preserved tissue in the dimethysulphoxide group but not in the Ringer-Acetate group. The density of small follicles and ovarian cell viability were higher in dimethysulphoxide ovaries compared to Ringer-Acetate ovaries.

CONCLUSIONS

Equilibrium with its protective effect can be achieved by slow freezing protocol, with an additional protective effect by the presence of dimethylsulphoxide.

Cryopreservation of primate embryonic stem cells with chemically-defined solution without Me2SO

Human embryonic stem (hES) cells are expected to be useful in the fields of regenerative medicine and tissue engineering due to their pluripotency. Therefore, it is necessary to establish highly efficient and reliable methods for the cryopreservation of hES cells. We have cryopreserved cynomolgus and human ES cells by the vitrification method, using a chemically-defined dimethyl sulfoxide (Me(2)SO)-free and serum-free medium composed of Euro-Collins solution as a base medium and 40% (v/v) ethylene glycol (EG) and 10% (w/v) polyethylene glycol (PEG) as cryoprotectants. When the vitrification and the cryoprotectants were combined, the recovery ratio of hES cells was 22.9+/-7.7%, compared to 0.4+/-0.2% when the conventional slow-freezing method was used. After the cryopreservation and thawing cycle, hES cells were easily cultured and expressed undifferentiated cell markers such as Nanog, Oct-4, SSEA-4, and alkaline phosphatase activity after several subculturing steps. We also found that the pluripotency of hES cells was maintained, as demonstrated by teratoma formation of ES cells transplanted into severe combined immunodeficient (SCID) mice. Thus, we conclude that we have successfully cryopreserved primate ES cells with high efficiency using a Me(2)SO-free, chemically-defined medium.

Vitrification Successfully Preserves Hepatocyte Spheroids

This is the first report on low-temperature preservation of self-assembled cell aggregates by vitrification, which is both a time- and cost-effective technology. We developed an effective protocol for vitrification (ice-free cryopreservation) of hepatocyte spheroids that employs rapid stepwise exposure to cryoprotectants (10.5 min) at room temperature and direct immersion into liquid nitrogen (-196°C). For this, three vitrification solutions (VS) were formulated and their effects on vitrified-warmed spheroids were examined. Cryopreservation using ethylene glycol (EG)-sucrose VS showed excellent preservation capability whereby highly preserved cell viability and integrity of vitrified spheroids were observed, through confocal and scanning electron microscopy imaging, when compared to untreated control. The metabolic functions of EG-sucrose VS-cryopreserved spheroids, as assessed by urea production and albumin secretion, were not significantly different from those of control within the same day of observation. In both the vitrification and control groups, albumin secretion was consistently high, ranging from 47.57 ± 14.39 to 70.38 ± 11.29 μg/106 cells and from 56.84 ± 14.48 to 71.79 ± 16.65 μg/106 cells, respectively, and urea production gradually increased through the culture period. The efficacy of vitrification procedure in preserving the functional ability of hepatocyte spheroids was not improved by introduction of a second penetrating cryoprotectant, 1,2-propanediol (PD). Spheroids cryopreserved with EG-PD-sucrose VS showed maintained cell viability; however, in continuous culture, levels of both metabolic functions were lower than those cryopreserved with EG-sucrose VS. EG-PD VS, in which nonpenetrating cryoprotectant (sucrose) was excluded, provided poor protection to spheroids during cryopreservation. This study demonstrated that sucrose plays an important role in the effective vitrification of self-assembled cell aggregates. In a broad view, the excellent results obtained suggest that the developed vitrification strategy, which is an alternative to freezing, may be effectively used as a platform technology in the field of cell transplantation.

Optimized preservation of extracellular matrix in cardiac tissues: implications for long-term graft durability

BACKGROUND

Cryopreservation of human tissues, particularly heart valves, is widespread in clinical practice although the effects of this process on underlying tissue structures and its potential impact on valve durability have been poorly studied. Multiphoton imaging and second-harmonic generation (SHG) microscopy permit high-resolution, noninvasive analysis of living tissues at a subcellular level. In the present study we used these novel imaging modalities to compare the effects of vitreous and frozen cryopreservation on the extracellular matrix (ECM) of cardiac tissues.

METHODS

Conventional histology, electron microscopy, and multiphoton imaging to obtain autofluorescence and SHG images were performed on cardiac tissues to characterize the ECM in fresh, vitrified, and frozen cryopreserved tissues.

RESULTS

Autofluorescence and particularly SHG images revealed that conventional frozen cryopreservation of cardiac valves, when compared with fresh or vitrified tissues, leads to the loss of normal ECM structures in valve leaflets. Similar results were found in all other cardiac tissues suggesting that structural deterioration of the ECM is a common consequence of frozen cryopreservation.

CONCLUSIONS

Our results demonstrate that conventional cryopreservation, when compared with fresh or vitrified tissues, causes more destruction of normal ECM structure, which might contribute to eventual graft dysfunction. Whether vitrification preservation will translate into greater durability or less valve failure will need to be determined.

Cryoprotectant delivery and removal from murine insulinomas at vitrification-relevant concentrations

Development of optimal cryopreservation protocols requires delivery and removal of cryoprotective agents (CPAs) in such a way that negative osmotic and cytotoxic effects on cells are minimized. This is especially true for vitrification, where high CPA concentrations are employed. In this study, we report on the determination of cell membrane permeability parameters for water (L(p)) and solute (P(s)), and on the design and experimental verification of CPA addition and removal protocols at vitrification-relevant concentrations for a murine insulinoma cell line, betaTC-tet cells. Using membrane permeability values and osmotic tolerance limits, mathematical modeling and computer simulations were used to design CPA addition and removal protocols at high concentrations. The cytotoxic effects of CPAs were also evaluated. Cells were able to tolerate the addition and removal of 2.5M dimethyl sulfoxide (DMSO) and 2.5M 1,2 propanediol (PD) in single steps, but required multi-step addition and removal with 3.0M DMSO, 3.0M PD, and a vitrification-relevant concentration of 3.0M DMSO+3.0M PD. Cytotoxicity studies revealed that betaTC-tet cells were able to tolerate the presence of single component 6.0M DMSO and 6.0M PD and to a lesser extent 3.0M DMSO+3.0M PD. These results determine the time and concentration domain of CPA exposure that cells can tolerate and are essential for designing cryopreservation protocols for free cells as well as cells in engineered tissues.

Cryopreservation of articular cartilage. Part 3: the liquidus-tracking method

Although it is relatively straightforward to cryopreserve living isolated chondrocytes, at the present time there is no satisfactory method to preserve surgical grafts between the time of procurement or manufacture and actual use. In earlier papers we have established that the cryoprotectants dimethyl sulphoxide or propylene glycol do penetrate into this tissue very rapidly. Chondrocytes are not unusually susceptible to osmotic stress; in fact they appear to be particularly resistant. It appears that damage is associated with the formation of ice per se, even at cooling rates that are optimal for the cryopreservation of isolated chondrocytes. We then showed that current methods of cartilage cryopreservation involve the nucleation and growth of ice crystals within the chondrons rather than ice being restricted to the surrounding acellular matrix. This finding established the need to avoid the crystallization of ice--in other words, vitrification. Song and his colleagues have published a vitrification method that is based on the use of one of Fahy's vitrification formulations. We confirmed the effectiveness of this method but found it to be very dependent on ultra rapid warming. However, we were able to develop a 'liquidus-tracking' method that completely avoids the crystallization of ice and does not require rapid warming. The ability of cartilage preserved in this way to incorporate sulphate into newly synthesized glycosaminoglycans (GAGs) approached 70% of that of fresh control cartilage. In this method the rates of cooling and warming can be very low, which is essential for any method that is to be used in Tissue Banks to process the bulky grafts that are required by orthopaedic surgeons. Work is continuing to refine this method for Tissue Bank use.

Development of a single mode electromagnetic resonant cavity for rewarming of cryopreserved biomaterials

An electromagnetic (EM) heating system is developed to achieve the rapid and uniform warming of cryopreserved biomaterials. Using the heating system, a rectangular resonant cavity is excited in TE101 mode at frequencies near 434 MHz. In experiments, a spherical phantom of biomaterial with a diameter of 36 mm is placed at the center of the cavity. The phantom is first cooled down to about -80 degrees C within the cavity and then thawed by EM absorption. Results show that EM warming can produce much higher warming rate than conventional water-bath warming method. The spatial temperature distribution in the phantom during EM warming is also more uniform than that during the water-bath warming.

Human oocyte cryopreservation and the fate of cortical granules

OBJECTIVE

To examine the effect of the commonly used oocyte cryopreservation protocol on the cortical granules (CGs) of human immature germinal vesicle (GV) and mature metaphase II (MII) oocytes.

DESIGN

Laboratory study.

SETTING

IVF unit.

INTERVENTION(S)

Unfertilized, intracytoplasmic sperm injected (ICSI) oocytes, and immature oocytes were cryopreserved using a slow freezing-rapid thawing program with 1,2-propanediol (PROH) as a cryoprotectant.

MAIN OUTCOME MEASURE(S)

Cortical granule exocytosis (CGE) was assessed by either confocal microscopy or transmission electron microscopy (TEM).

RESULT(S)

The survival rates of frozen-thawed oocytes (mature and immature) were significantly lower compared with zygotes. Both mature and immature oocytes exhibited increased fluorescence after cryopreservation, indicating the occurrence of CGE. Mere exposure of oocytes to cryoprotectants induced CGE of 70% the value of control zygotes. The TEM revealed a drastic reduction in the amount of CGs at the cortex of frozen-thawed GV and MII oocytes, as well as appearance of vesicles in the ooplasm.

CONCLUSION(S)

The commonly used PROH freezing protocol for human oocytes resulted in extensive CGE. This finding explains why ICSI is needed to achieve fertilization of frozen-thawed human oocytes.

Cryopreservation of articular cartilage. Part 1: Conventional cryopreservation methods

There is increasing interest in the possibility of treating diseased or damaged areas of synovial joint surfaces by grafts of healthy allogeneic cartilage. Such grafts could be obtained from cadaver tissue donors or in the future they might be manufactured by 'tissue engineering' methods. Cartilage is an avascular tissue and hence is immunologically privileged but to take advantage of this is the graft must contain living cells. Preservation methods that achieve this are required to build up operational stocks of grafts, to provide a buffer between procurement and use, and to enable living grafts of a practical size to be provided at the right time for patient and surgeon. Review of the literature shows that it has been relatively straightforward to cryopreserve living isolated chondrocytes, but at the present time there is no satisfactory method to preserve cartilage between the time of procurement or manufacture and surgical use. In this paper, we review the relevant literature and we confirm that isolated ovine chondrocytes in suspension can be effectively cryopreserved by standard methods yet the survival of chondrocytes in situ in cartilage tissue is inadequate and extremely variable.

The Structural and Cellular Viability in Cryopreserved Rabbit Carotid Arteries

OBJECTIVE

We investigated the histological and mechanical changes in addition to viable cellular recovery in cryopreserved blood vessels.

MATERIALS AND METHODS

Rabbit carotids were cryopreserved in a cryoprotective medium containing 1.5 M of 1,2-propanediol (PD) and then were thawed slowly in an ice bag that had been precooled in liquid nitrogen. Fresh carotids were used as the control. The fresh and freeze-thawed arteries were cultured for the growth of vascular smooth muscle cells (VSMCs). The freeze-thawed arterial tissues were perfused in vitro for 6, 12, or 24 h, respectively, to assess the integrity of carotid walls and the mechanical properties.

RESULTS

The results showed that it took almost the same time (24 approximately 36 h) for the VSMCs of the PD-cryopreserved arteries to regenerate as those from the fresh arteries. Their growing speeds also were similar. On the contrary, Me2SO-cryopreserved (1.5 M) arteries were unable to regenerate VSMCs in culture. After freeze-thawing, the mechanical properties decreased significantly (P < 0.003 for elastic modulus and P < 0.001 for fracture strength). After in vitro perfusion of the freeze-thawed carotid arteries, all of the survived endothelial cells fell off, and some of the VSMCs denaturalized or necrosed. The internal elastic fibers and collagen showed various degrees of cracking. The mechanical properties were decreased (P < 0.05).

CONCLUSION

Our findings demonstrate that the PD-containing cryoprotective medium can preserve regenerative capacity of VSMCs, which makes it a useful technique for viable VSMC recovery. However, the freeze-thawing process and the in vitro perfusion caused serious disruption in the arterial mechanical properties, rendering the cryopreserved blood vessels less useful for vessel reconstruction.

The role of vitrification techniques of cryopreservation in reproductive medicine

Traditional cryopreservation methods allow ice to form and solute concentrations to rise during the preservation process: both ice and high solute concentrations can cause damage. Cryoprotectants are highly soluble, permeating compounds of low toxicity; they reduce the amount of ice that crystallises at any given temperature and thereby limit the solute concentration factor. Vitrification methods use cryoprotectant concentrations that are sufficient to prevent the crystallisation of ice altogether: the material solidifies as an amorphous glass and both ice and solute concentration are avoided. However, the concentrations of cryoprotectant required are very high indeed and therefore are potentially, and often actually, harmful to cells. Optimisation of the temperature and the rate of introduction and removal of such high cryoprotectant concentrations are critical. The necessary concentration can be lowered if very rapid cooling, and even more rapid warming, are used. This paper draws on experience in other fields of cryobiology to discuss these basic phenomena and to consider the place of vitrification techniques in the cryopreservation of human gametes, embryos and gonads.

An inverse approach to determine solute and solvent permeability parameters in artificial tissues

This study presents a generic numerical model to simulate the coupled solute and solvent transport in tissue sections during addition and removal of chemical additives or cryoprotective agents (CPA; dimethylsulfoxide or DMSO). Osmotic responses of various tissue cells within the artificial tissue are predicted by the numerical model with three model parameters: Permeability of the tissue cell membrane to water (Lp), permeability of the tissue cell membrane to the solute or CPA (omega), and the diffusion coefficient of the solute or CPA in the extracellular space (D). By fitting the model results with published experimental data on solute/water concentrations at various locations within an artificial tissue, we were able to determine the permeability parameters of artificial tissue cells in the presence of 1.538 M DMSO. Lp and omega were determined at three different locations within the artificial tissue assuming a constant value of solute diffusivity (D = 1.0 x 10(-9) m2/s). The best fit values of Lp ranged from 0.59 x 10(-14) to 4.22 x 10(-14) m3/N-s while omega ranged from 0 to 6.6 x 10(-13) mol/N-s. Based on these values of Lp and omega, the solute reflection coefficient, sigma = 1 - omegav(-)CPA/Lp, ranged from 0.9923 to 1.0. The relative values of omega and sigma suggest that the artificial tissue cells are relatively impermeable to DMSO (or omega approximately 0 and sigma approximately 1.0). This observation was used to modify our model to predict the values of Lp and D assuming omega = 0 and sigma = 1.0. The best fit values of Lp ranged from 640 x 10(-14) to 2.1 x 10(-14) m3/N-s while D ranged from 0.63 x 10(-9) to 1.52 x 10(-9) m2/s. The permeability parameters obtained in the present study represent the first such effort for artificial tissues.