A theoretical model of intracellular devitrification

Cryopreservation of spermatogonia of the giant freshwater prawn, Macrobrachium rosenbergii, using slow and ultra-rapid freezing

The giant freshwater prawn, Macrobrachium rosenbergii, is an important species that is widely raised in tropical and subtropical regions. To develop techniques for preserving valuable traits in this species, we have devised protocols for cryopreserving spermatogonia-progenitor cells that give rise to spermatozoa through spermatogenesis-using slow freezing and ultra-rapid freezing techniques. Optimization of cryoprotectants and their concentrations revealed that 10 % dimethyl sulfoxide (ME2SO) is effective as a cryoprotectant in both methods. The optimal time for equilibration to the cryoprotectant before slow freezing was determined to be 15 min. Thawing temperature had a significant effect on cell recovery and viability, both being markedly higher when cells were thawed at 10 °C than at 27 °C. After long-term storage in liquid nitrogen, cells preserved by ultra-rapid freezing exhibited higher recovery and viability rates than those preserved using the slow freezing method, with recovery rates of up to 86.4 % ± 5.9 % at 6 months. Therefore, using 10 % ME2SO with the ultra-rapid freezing method is recommended as the optimal protocol for long-term cryopreservation of M. rosenbergii spermatogonia.

Critical points for optimizing long-term culture and neural differentiation capacity of rodent and human neural stem cells to facilitate translation into clinical settings

Despite several decades of research on the nature and functional properties of neural stem cells, which brought great advances in regenerative medicine, there is still a plethora of ambiguous protocols and interpretations linked to their applications. Here, we present a whole spectrum of protocol elements that should be standardized in order to obtain viable cell cultures and facilitate their translation into clinical settings. Additionally, this review also presents outstanding limitations and possible problems to be encountered when dealing with protocol optimization. Most importantly, we also outline the critical points that should be considered before starting any experiments utilizing neural stem cells or interpreting their results.

Cryopreservation of Germ Cells of Banana Shrimp (Fenneropenaeus merguiensis) and Black Tiger Shrimp (Penaeus monodon)

Germ cell cryopreservation has been used to preserve many fish species. However, this method has not been established for crustaceans; thus, we attempted to do this herein. The efficiency of slow freezing was compared to vitrification methods for germ cell cryopreservation in two types of marine shrimp, Fenneropenaeus merguiensis and Penaeus monodon. In situ hybridization with a vasa probe was used to identify germ cells. The effects of three cryoprotectants, dimethyl sulfoxide (DMSO), glycerol (GLY), and magnesium chloride (MgCl₂), on germ cell viability and recovery rate were compared at three concentrations (5%, 10%, and 15%). The effects of thawing temperature, including 10 and 27 °C, were also investigated. We discovered that 10% DMSO with the vitrification is suitable for preserving the germ cells of F. merguiensis for a long time, whereas 10% GLY with vitrification is suitable for P. monodon. Moreover, the most suitable thawing temperature was 10 °C for both species. This is the first report of germ cell cryopreservation in crustaceans. Thus, we provide evidence that crustacean germ cells can be preserved long-term in liquid nitrogen; this is the first step in the sustainable preservation of crustaceans, especially shrimp.

High-Speed Video Cryomicroscopy for Measurement of Intracellular Ice Formation Kinetics

Quantitative information about the kinetics and cumulative probability of intracellular ice formation is necessary to develop minimally damaging freezing procedures for the cryopreservation of cells and tissues. Conventional cryomicroscopic assays, which rely on indirect evidence of intracellular freezing (e.g., opacity changes in the cell cytoplasm), can yield significant errors in the estimated kinetics. In contrast, the formation and growth of intracellular ice crystals can be accurately detected using temporally resolved imaging methods (i.e., video recording at sub-millisecond resolution). Here, detailed methods for the setup and operation of a high-speed video cryomicroscope system are described, including protocols for imaging of intracellular ice crystallization events and stochastic analysis of the ice formation kinetics in a cell population. Recommendations are provided for temperature profile design, sample preparation, and configuration of the video acquisition parameters. Throughout this chapter, the protocols incorporate best practices that have been drawn from two decades of experience with high-speed video cryomicroscopy in our laboratory.

FTIR Analysis of Molecular Changes Associated with Warming Injury in Cryopreserved Leukocytes

In this article, we explored the effects of cooling rate, dimethyl sulfoxide (DMSO) concentration, and thawing protocol on the post-thaw viability of frozen human white blood cells (WBCs). Different cooling rates (1, 2, 5, 10, 20, and 50 °C/min) at two DMSO concentrations (5 and 10% v/v) were tested as the samples were cooled to -120 °C. Frozen samples were thawed following either a fast (100 °C/min) or slow (2 °C/min) warming protocol applied in either a single stage or in two stages interrupted by a 6 min hold at -40, -50, -60, -70, or -80 °C. The highest post-thaw viability was obtained when WBCs were cooled at 2 °C/min in a 5% DMSO solution and warmed at the fastest rate (100 °C/min) without any interruption. Post-thaw viability decreased when the warming rate was reduced or when rapid warming was interrupted by a hold at a temperature below -60 °C. To elucidate the mechanisms of warming injury in addition to the biological response, several key interfacial and molecular phenomena require greater understanding; thus, we used Fourier transform infrared (FTIR) spectroscopy to investigate the roles of molecular structure and conformation in damage to cryopreserved WBCs during warming. During warming, FTIR spectra revealed the accumulation of cellular protein and lipid membrane damage below -60 °C if the samples were thawed slowly at 2 °C/min. The results presented here suggest that irreversible alterations of biomolecular structure are correlated with cell injury during warming; these deleterious effects appeared to be caused by one or more low-temperature kinetic processes, consistent with eutectic formation/melting and/or devitrification in the intracellular milieu.

The role of preservation in the variability of regenerative medicine products

Regenerative medicine (RM) has the potential to restore or establish normal function of cells, tissues and organs that have been lost due to age, disease or injury. It is common for the site of raw material collection, site of manufacture and site of clinical use to be different for RM products, and at the same time cells must remain viable and functional during transportation among different sites. Freezing products down to cryogenic temperatures along with cold chain transportation has become an effective method of preserving RM products. The quality of RM products along this supply chain represents the cumulative effects of all of the processing steps and all of the reagents used in the process. A variety of sources of variability in the preservation of RM products can result in both cell losses and greater variability in the quality of RM products. The purpose of this article is to review the sources of variability in the preservation process as well as the methods by which variability can be controlled or avoided.

Hydrogel Encapsulation Facilitates Rapid-Cooling Cryopreservation of Stem Cell-Laden Core-Shell Microcapsules as Cell-Biomaterial Constructs

Core-shell structured stem cell microencapsulation in hydrogel has wide applications in tissue engineering, regenerative medicine, and cell-based therapies because it offers an ideal immunoisolative microenvironment for cell delivery and 3D culture. Long-term storage of such microcapsules as cell-biomaterial constructs by cryopreservation is an enabling technology for their wide distribution and ready availability for clinical transplantation. However, most of the existing studies focus on cryopreservation of single cells or cells in microcapsules without a core-shell structure (i.e., hydrogel beads). The goal of this study is to achieve cryopreservation of stem cells encapsulated in core-shell microcapsules as cell-biomaterial constructs or biocomposites. To this end, a capillary microfluidics-based core-shell alginate hydrogel encapsulation technology is developed to produce porcine adipose-derived stem cell-laden microcapsules for vitreous cryopreservation with very low concentration (2 mol L-1 ) of cell membrane penetrating cryoprotective agents (CPAs) by suppressing ice formation. This may provide a low-CPA and cost-effective approach for vitreous cryopreservation of "ready-to-use" stem cell-biomaterial constructs, facilitating their off-the-shelf availability and widespread applications.

The development of the cell cryopreservation protocol with controlled rate thawing

Thawing in the water bath is often considered as a standard procedure. The thermal history of samples thawed in this way is poorly controlled, but cryopreservation and banking of cell-based products require standardization, automation and safety of all the technological stages including thawing. The programmable freezers allow implementation of the controlled cooling as well as the controlled thawing. As the cell damage occurs during the phase transformation that takes place in the cryoprotectant medium in the process of freezing-thawing, the choice of warming rates within the temperature intervals of transformations is very important. The goal of the study was to investigate the influence of warming rates within the intervals of the phase transformations in the DMSO-based cryoprotectant medium on the cell recovery and to develop a cryopreservation protocol with controlled cooling and warming rates. The temperature intervals of phase transformations such as melting of the eutectic mixture of the cryoprotectant solution (MEMCS), melting of the eutectic salt solution (MESS), melting of the main ice mass (MMIM), recrystallization before MEMCS, recrystallization before MESS and recrystallization before MMIM were determined by thermo-mechanical analysis. The biological experiments were performed on the rat testicular interstitial cells (TIC). The highest levels of the cell recovery and metabolic activity after cryopreservation were obtained using the protocol with the high (20 °C/min) warming rate in the temperature intervals of crystallization of the eutectics as well as recrystallizations and the low (1 °C/min) warming rate in the temperature intervals of melting of the eutectics as well as MMIM. The total cell recovery was 65.3 ± 2.1 %, the recovery of the 3-beta-HSD-positive (Leydig) cells was 82.9 ± 1.8 %, the MTT staining was 32.5 ± 0.9 % versus 42.1 ± 1.7 %; 57.4 ± 2.1 % and 24.0 ± 1.1 % respectively, when compared to the thawing in the water bath.

Noninvasive Quality Control of Cryopreserved Samples

We present a novel noninvasive technology for quality control in biobanking. We implemented a contactless optical in situ method with a remote detection unit. The method detects physical and chemical changes by emission spectroscopy. In the present study, ice formation in a vitrified sample is revealed by Raman scattering. The technology allows us to monitor sample quality during cold storage and to assess the sample state after preservation, storage, or transport without the need for thawing.

Measurement of intracellular ice formation kinetics by high-speed video cryomicroscopy

Quantitative information about the kinetics and cumulative probability of intracellular ice formation is necessary to develop minimally damaging freezing procedures for the cryopreservation of cells and tissue. Conventional cryomicroscopic assays, which rely on indirect evidence of intracellular freezing (e.g., opacity changes in the cell cytoplasm), can yield significant errors in the estimated kinetics. In contrast, the formation and growth of intracellular ice crystals can be accurately detected using temporally resolved imaging methods (i.e., video recording at sub-millisecond resolution). Here, detailed methods for the setup and operation of a high-speed video cryomicroscope system are described, including protocols for imaging of intracellular ice crystallization events, and stochastic analysis of the ice formation kinetics in a cell population. Recommendations are provided for temperature profile design, sample preparation, and configuration of the video acquisition parameters. Throughout this chapter, the protocols incorporate best practices that have been drawn from over a decade of experience with high-speed video cryomicroscopy in our laboratory.

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.

Novel Approaches to the Cryopreservation of Human Spermatozoa: History and Development of the Spermatozoa Vitrification Technology

Cryobiology is very intensively applied in reproductive and veterinary medicine for preservation of gametes, embryos and reproductive tissues. Sub-zero temperatures combined with appropriate cryoprotective agents preserve the physiological and reproductive functions of the cells making long-term storage possible without loss of viability. With the use of cryoprotective agents it has become possible to develop cryopreservation techniques, such as the slow conventional freezing and vitrification that are in use in the present times. In slow controlled-rate conventional freezing extracellular ice crystals are formed whereas in vitrification no ice crystals are formed. Glass formation is compatible with the survival of the cell and the preservation of its intracellular structures provided the type(s) and concentrations of cryoprotectant used are not chemo- or osmotoxic. However, irrespective of the type of cooling method employed the cryosurvival of cells and tissues is influenced by the size and maturity of cells, amounts of intracellular water, quality and quantity of intracellular lipids, type of cells, their function and morphology. The intracellular milieu of cryopreserved cells and tissues remain less understood. The application of nanotechnology may help reveal and help advance our knowledge of the cryobiological principles involved in cryosurvival. At this moment the methods of cryopreservation that merit further investigation are vitrification and lyophilization. Vitrification is cheap if reagents are prepared in-house and the procedure can be performed rapidly. It has been successfully applied for gametes and embryos (of different stages of development), and reproductive cells/tissues, somatic cells and stem cells. However, vitrification is more demanding technically and requires operation and storage at sub-zero temperatures. On the other hand lyophilization deserves further investigation because it is a cheaper form of cryopreservation that may enable cryostorage at less demanding temperatures of 4°C and may even allow transport at ambient temperature. These possibilities are explored in this review.

Cryopreservation of spin-dried mammalian cells

This study reports an alternative approach to achieve vitrification where cells are pre-desiccated prior to cooling to cryogenic temperatures for storage. Chinese Hamster Ovary (CHO) cells suspended in a trehalose solution were rapidly and uniformly desiccated to a low moisture content (<0.12 g of water per g of dry weight) using a spin-drying technique. Trehalose was also introduced into the cells using a high-capacity trehalose transporter (TRET1). Fourier Transform Infrared Spectroscopy (FTIR) was used to examine the uniformity of water concentration distribution in the spin-dried samples. 62% of the cells were shown to survive spin-drying in the presence of trehalose following immediate rehydration. The spin-dried samples were stored in liquid nitrogen (LN(2)) at a vitrified state. It was shown that following re-warming to room temperature and re-hydration with a fully complemented cell culture medium, 51% of the spin-dried and vitrified cells survived and demonstrated normal growth characteristics. Spin-drying is a novel strategy that can be used to improve cryopreservation outcome by promoting rapid vitrification.

Review of biomaterial thermal property measurements in the cryogenic regime and their use for prediction of equilibrium and non-equilibrium freezing applications in cryobiology

It is well accepted in cryobiology that the temperature history and cooling rates experienced in biomaterials during freezing procedures correlate strongly with biological outcome. Therefore, heat transfer measurement and prediction in the cryogenic regime is central to the field. Although direct measurement of temperature history (i.e. heat transfer) can be performed, accuracy is usually achieved only for local measurements within a given system and cannot be readily generalized to another system without the aid of predictive models. The accuracy of these models rely upon thermal properties which are known to be highly dependent on temperature, and in the case of significant cryoprotectant loading, also on crystallized fraction. In this work, we review the available thermal properties of biomaterials in the cryogenic regime. The review shows a lack of properties for many biomaterials in the subzero temperature domain, and especially for systems with cryoprotective agents. Unfortunately, use of values from the limited data available (usually only down to -40 degrees C) lead to an underestimation of thermal property change (i.e. conductivity rise and specific heat drop due to ice crystallization) with lower temperatures. Conversely, use of surrogate values based solely on ice thermal properties lead to an overestimation of thermal property change for most biomaterials. Additionally, recent work extending the range of available thermal properties to -150 degrees C has shown that the thermal conductivity will drop in both PBS and tissue (liver) due to amorphous/glassy phases (versus crystalline) of biomaterials with the addition of cryoprotective additives such as glycerol. Thus, we investigated the implications of using approximated or constant property values versus measured temperature-dependent values for predicting temperature history during freezing in PBS (phosphate-buffered saline) and porcine liver with and without cryoprotectants (glycerol). Using measured property values (thermal conductivity, specific heat, and latent heat of phase change) of porcine liver, a standard was created which showed that values based on surrogate ice properties under-predicted cooling times, while constant properties (i.e. based on limited data reported near the freezing point) over-predicted cooling times. Additionally, a new iterative numerical method that accommodates non-equilibrium cooling effects as a function of time and position (i.e. crystallization versus amorphous phase) was used to predict temperature history during freezing in glycerol loaded systems. Results indicate that in addition to the increase in cooling times due to the lowering of thermal diffusivity with more glycerol, non-equilibrium effects such as the prevention of maximal crystallization (i.e. amorphous phases) will further increase required cooling times. It was also found that the amplified effect of non-equilibrium cooling and crystallization with system size prevents the thermal history to be described with non-dimensional lengths, such as was possible under equilibrium cooling. These results affirm the need to use accurate thermal properties that incorporate temperature dependence and crystallized fraction. Further studies are needed to extract thermal properties of other important biomaterials in the subzero temperature domain and to develop accurate numerical methods which take into account non-equilibrium cooling events encountered in cryobiology when partial or total vitrification occurs.

Kinetics and activation energy of recrystallization of intracellular ice in mouse oocytes subjected to interrupted rapid cooling

Intracellular ice formation (IIF) is almost invariably lethal. In most cases, it results from the too rapid cooling of cells to below -40 degrees C, but in some cases it is manifested, not during cooling, but during warming when cell water that vitrified during cooling first devitrifies and then recrystallizes during warming. Recently, Mazur et al. [P. Mazur, I.L. Pinn, F.W. Kleinhans, Intracellular ice formation in mouse oocytes subjected to interrupted rapid cooling, Cryobiology 55 (2007) 158-166] dealt with one such case in mouse oocytes. It involved rapidly cooling the oocytes to -25 degrees C, holding them 10 min, rapidly cooling them to -70 degrees C, and warming them slowly until thawed. No IIF occurred during cooling but intracellular freezing, as evidenced by blackening of the cells, became detectable at -56 degrees C during warming and was complete by -46 degrees C. The present study differs in that the oocytes were warmed rapidly from -70 degrees C to temperatures between -65 and -50 degrees C and held for 3-60 min. This permitted us to determine the rate of blackening as function of temperature. That in turn allowed us to calculate the activation energy (E(a)) for the blackening process; namely, 27.5 kcal/mol. This translates to about a quadrupling of the blackening rate for every 5 degrees C rise in temperature. These data then allowed us to compute the degree of blackening as a function of temperature for oocytes warmed at rates ranging from 10 to 10,000 degrees C/min. A 10-fold increase in warming rate increased the temperature at which a given degree of blackening occurred by 8 degrees C. These findings have significant implications both for cryobiology and cryo-electron microscopy.

Biopreservation of red blood cells: past, present, and future

Preservation and long-term storage of red blood cells (RBCs) is needed to ensure a readily available, safe blood supply for transfusion medicine. Effective preservation procedures are required at various steps in the production of a RBC product including testing, inventory, quality control, and product distribution. Biopreservation is the process of maintaining the integrity and functionality of cells held outside the native environment for extended storage times. The biopreservation of RBCs for clinical use can be categorized based on the techniques used to achieve biologic stability and ensure a viable state after long-term storage. This paper will review the history, science, current practices, and emerging technologies of current RBC biopreservation approaches: hypothermic storage, cryopreservation, and lyophilization.

Biopreservation of cells and engineered tissues

The development of effective preservation and long-term storage techniques is a critical requirement for the successful clinical and commercial application of emerging cell-based technologies. Biopreservation is the process of preserving the integrity and functionality of cells, tissues and organs held outside the native environment for extended storage times. Biopreservation can be categorized into four different areas on the basis of the techniques used to achieve biological stability and to ensure a viable state following long-term storage. These include in vitro culture, hypothermic storage, cryopreservation and desiccation. In this chapter, an overview of these four techniques is presented with an emphasis on the recent developments that have been made using these technologies for the biopreservation of cells and engineered tissues.

Cryoprotectant-Free Cryopreservation of Human Spermatozoa by Vitrification and Freezing in Vapor: Effect on Motility, DNA Integrity, and Fertilization Ability

Human spermatozoa can be successfully cryopreserved avoiding the use of cryoprotectants through vitrification at very high cooling rates (up to 7.2 x 10(5) degrees C/min). This is achieved by directly plunging a copper cryoloop loaded with a sperm suspension into liquid nitrogen. After storage, vitrified spermatozoa are instantly thawed by melting in an agitated, warm medium. The goal of the present study was to compare the quality of spermatozoa cryopreserved using this rapid vitrification method with that of spermatozoa cooled relatively slowly by preexposure of the loaded cryoloop to liquid nitrogen vapor (-160 degrees C) with speed in the range 150-250 degrees C/min) before immersion into liquid nitrogen. Both cooling modes led to comparable results in terms of the motility, fertilization ability, and DNA integrity of the warmed spermatozoa. In both cases, instant thawing by melting in a warm medium was essential for successful cryopreservation. Our findings suggest that optimal regimes for the cryoprotectant-free cryopreservation of spermatozoa need not be restricted to very fast cooling before storage in liquid nitrogen, a wide range of cooling rates being acceptable. Herein, we discuss the implications of this finding in the light of the physics of extra- and intracellular vitrification.

Cryopreservation of Stem Cells Using Trehalose: Evaluation of the Method Using a Human Hematopoietic Cell Line

While stem cell cryopreservation methods have been optimized using dimethylsulfoxide (DMSO), the established techniques are not optimal when applied to unfertilized human embryonic cells. In addition, important questions remain regarding the toxicity and characteristics of DMSO for treatment of stem cells for clinical use. The objective of this study was to establish an optimal method for cryopreservation of stem cells using low concentrations (0.2 M) of trehalose, a nontoxic disaccharide of glucose, which possesses excellent protective characteristics, in place of current methods utilizing high concentrations (1-2 M) of DMSO. A human hematopoietic cell line was used in this investigation as a surrogate for human stem cells. Trehalose was loaded into cells using a genetically engineered mutant of the pore-forming protein alpha-hemolysin from Staphylococcus aureus. This method results in a nonselective pore equipped with a metal-actuated switch that is sensitive to extracellular zinc concentrations, thus permitting controlled loading of trehalose. Preliminary experiments characterized the effects of poration on TF-1 cells and established optimal conditions for trehalose loading and cell survival. TF-1 cells were frozen at 1 degrees C/min to -80 degrees C with and without intra- and extracellular trehalose. Following storage at -80 degrees C for 1 week, cells were thawed and evaluated for viability, differentiation capacity, and clonogenic activity in comparison to cells frozen with DMSO. Predictably, cells frozen without any protective agent did not survive freezing. Colony-forming units (CFU) generated from cells frozen with intra- and extracellular trehalose, however, were comparable in size, morphology, and number to those generated by cells frozen in DMSO. There was no observable alteration in phenotypic markers of differentiation in either trehalose- or DMSO-treated cells. These data demonstrate that low concentrations of trehalose can protect hematopoietic progenitors from freezing injury and support the concept that trehalose may be useful for freezing embryonic stem cells and other primitive stem cells for therapeutic and investigational use.

Vitrification of mammalian spermatozoa in the absence of cryoprotectants: from past practical difficulties to present success

The use of cryoprotective agents for the conventional cryopreservation of human spermatozoa, oocytes, zygotes, early cleavage stage embryos and blastocysts is an integral part of almost every human IVF programme. Moreover, the cryopreservation of these types of cells by direct plunging into liquid nitrogen usually requires high cryoprotectant concentrations with consequent cytotoxic effects. This review covers the history of this problem, and in this light offers an explanation, through physico-chemical concepts, for one of the most recent developments in this area: the recovery of motile and potent spermatozoa after cryoprotectant-free vitrification.

Effects of cooling and warming rate to and from -70 degrees C, and effect of further cooling from -70 to -196 degrees C on the motility of mouse spermatozoa

We have previously reported high survival in mouse sperm frozen at 21 degrees C/min to -70 degrees C in a solution containing 18% raffinose in 0.25 x PBS (400 mOsm) and then warmed rapidly at approximately 2000 degrees C/min, especially under lowered oxygen tensions induced by Oxyrase, a bacterial membrane preparation. The best survival rates were obtained in the absence of glycerol. The first concern of the present study was to determine the effects of the cooling rate on the survival of sperm suspended in this medium. The sperm were cooled to -70 degrees C at rates ranging from 0.3 to 530 degrees C/min. The survival curve was an inverted "U" shape, with the highest motility occurring between 27 and 130 degrees C/min. Survival decreased precipitously at higher cooling rates. Decreasing the warming rate, however, decreased survivals at all cooling rates. The motility depression with slow warming was especially evident in sperm cooled at the optimal rates. This fact is consistent with our current view that the frozen medium surrounding sperm cells is in a metastable state, perhaps partly vitrified as a result of the high concentrations of sugar. The decimation of sperm cooled more rapidly than optimum (>130 degrees C/min), even with rapid warming, is consistent with the induction of considerable quantities of intracellular ice at these rates. When glycerol was added to the above medium, motilities were also dependent on the cooling rate, but they tended to be substantially lower than those obtained in the absence of glycerol. The minimum temperature in the above experiments was -70 degrees C. When sperm were frozen to -70 degrees C at optimum rates, lowering the temperature to -196 degrees C had no adverse effect.