Thermally defined cryomicroscopy and thermodynamic analysis in lymphocyte freezing

Impact of drying and cooling rate on the survival of the desiccation-sensitive wheat pollen

Fast-drying and cooling induce fast intracellular water loss and reduced ice-crystal formation, which may promote the formation of intracellular glasses that might improve the likelihood of wheat pollen survival. Long-term storage of pollen is important for the fertilization of spatially or temporally isolated female parents, especially in hybrid breeding. Wheat pollen is dehydration-sensitive and rapidly loses viability after shedding. To preserve wheat pollen, we hypothesized that fast-drying and cooling rates would increase the rate of intracellular water content (WC) removal, decrease intracellular ice-crystal formation, and increase viability after exposure to ultra-low temperatures. Therefore, we compared slow air-drying with fast-drying (dry air flow) and found significant correlations between pollen WC and viability (r = 0.92, P < 0.001); significant differences in WCs after specific drying times; and comparable viabilities after drying to specific WCs. Fast-drying to WCs at which ice melting events were not detected (ΔH = 0 J mg^-1 DW, < 0.28 mg H₂O mg^-1 DW) reduced pollen viability to 1.2 ± 1.0%, but when drying to 0.39 mg H₂O mg^-1 DW, some viable pollen was detected (39.4 ± 17.9%). Fast cooling (150 °C min^-1) of fast-dried pollen to 0.91 ± 0.11 mg H₂O mg^-1 DW induced less and a delay of ice-crystal formation during cryomicroscopic-video-recordings compared to slow cooling (1 °C min^-1), but viability was low (4.5-6.1%) and comparable between cooling rates. Our data support that the combination of fast-drying and cooling rates may enable the survival of wheat pollen likely due to (1) a reduction of the time pollen would be exposed to drying-related deleterious biochemical changes and (2) an inhibition of intracellular ice-crystal formation, but additional research is needed to obtain higher pollen survival after cooling.

Comparison of non-ideal solution theories for multi-solute solutions in cryobiology and tabulation of required coefficients

Thermodynamic solution theories allow the prediction of chemical potentials in solutions of known composition. In cryobiology, such models are a critical component of many mathematical models that are used to simulate the biophysical processes occurring in cells and tissues during cryopreservation. A number of solution theories, both thermodynamically ideal and non-ideal, have been proposed for use with cryobiological solutions. In this work, we have evaluated two non-ideal solution theories for predicting water chemical potential (i.e. osmolality) in multi-solute solutions relevant to cryobiology: the Elliott et al. form of the multi-solute osmotic virial equation, and the Kleinhans and Mazur freezing point summation model. These two solution theories require fitting to only single-solute data, although they can make predictions in multi-solute solutions. The predictions of these non-ideal solution theories were compared to predictions made using ideal dilute assumptions and to available literature multi-solute experimental osmometric data. A single, consistent set of literature single-solute solution data was used to fit for the required solute-specific coefficients for each of the non-ideal models. Our results indicate that the two non-ideal solution theories have similar overall performance, and both give more accurate predictions than ideal models. These results can be used to select between the non-ideal models for a specific multi-solute solution, and the updated coefficients provided in this work can be used to make the desired predictions.

Polyvinylpyrrolidone (PVP) mitigates the damaging effects of intracellular ice formation in adult stem cells

The objective of this work was to assess the effect of 10% (w/v) polyvinylpyrrolidone (PVP) on the pattern of intracellular ice formation (IIF) in human adipose tissue derived adult stem cells (ASCs) in the absence of serum and other cryoprotective agents (CPAs). The freezing experiments were carried out using a fluorescence microscope equipped with a Linkam cooling stage using two cooling protocols. Both the cooling protocols had a common cooling ramp: cells were cooled from 20 degrees C to -8 degrees C at 20 degrees C/min and then further cooled to -13 degrees C at 1 degrees C/min. At this point we employed either cooling protocol 1: the cells were cooled from -13 degrees C to -40 degrees C at a pre-determined cooling rate of 1, 5, 10, 20, or 40 degrees C/min and then thawed back to 20 degrees C at 20 degrees C/min; or cooling protocol 2: the cells were re-warmed from -13 degrees C to -5 degrees C at 20 degrees C/min and then re-cooled at a pre-determined rate of 1, 5, 10, 20, or 40 degrees C/min to -40 degrees C. Almost all (>95%) of the ASCs frozen in 1x PBS and protocol 1 exhibited IIF. However, almost none (<5%) of the ASCs frozen in 1x PBS and protocol 2 exhibited IIF. Similarly, almost all (>95%) of the ASCs frozen in 10% PVP in PBS and protocol 1 exhibited IIF. However, ~0, ~40, ~47, ~67, and ~100% of the ASCs exhibited IIF when frozen in 10% PVP in PBS and utilizing protocol 2 at a cooling rate of 1, 5, 10, 20, or 40 degrees C/min, respectively.

A simplified procedure to determine the optimal rate of freezing biological systems

The effect of several cell-level parameters on the predicted optimal cooling rate B(opt) of an arbitrary biological system has been studied using a well-defined water transport model. An extensive investigation of the water transport model revealed three key cell level parameters: reference permeability of the membrane to water L(pg), apparent activation energy E(Lp), and the ratio of the available surface area for water transport to the initial volume of intracellular water (SA/WV). We defined B(opt) as the "highest" cooling rate at which a predefined percent of the initial water volume is trapped inside the cell (values ranging from 5% to 80%) at a predefined end temperature (values ranging from -5 degrees C to -40 degrees C). Irrespective of the choice of the percent of initial water volume trapped and the end temperature, an exact and linear relationship exists between L(pg), SA/WV, and B(opt0. However, a nonlinear and inverse relationship is found between E(Lp) and B(opt). Remarkably, for a variety of biological systems a comparison of the published experimentally determined values of B(opt) agreed quite closely with numerically predicted B(opt) values when the model assumed 5% of initial water is trapped inside the cell at a temperature of -15 degrees C. This close agreement between the experimental and model predicted optimal cooling rates is used to develop a generic optimal cooling rate chart and a generic optimal cooling rate equation that greatly simplifies the prediction of the optimal rate of freezing of biological systems.

What happens in freezing bodies? Experimental study of histological tissue change caused by freezing injuries

In order to evaluate histological features of freezing damages to human tissue after death, we froze samples of liver and heart tissue to temperatures of -12 degrees C, -28 degrees C and -80 degrees C, and stored them for 24 and 72 h, respectively, at those temperatures. After thawing and routine preparation for histology, the samples were evaluated both by microscope and with an electronic image analyzer. In all cases, we found extended extracellular spaces and shrunken cells resulting from the freeze-thaw cycle. These features were more pronounced in tissues stored for longer durations. Such findings seem to be typical of tissue that has been frozen prior to examination. Two cases of dead bodies found outdoors at subzero temperatures demonstrate that formerly frozen and unfrozen tissues can be distinguished histologically. The findings are examined in relation to the fundamental laws of cryobiology.

Measurement of water transport during freezing in cell suspensions using a differential scanning calorimeter

A new technique using a differential scanning calorimeter (DSC) was developed to obtain dynamic and quantitative water transport data in cell suspensions during freezing. The model system investigated was a nonattached spherical lymphocyte (Epstein-Barr virus transformed, EBVT) human cell line. Data from the technique show that the initial heat release of a prenucleated sample containing osmotically active cells in media is greater than the final heat release of an identical sample of osmotically inactive or lysed cells in media. The total integrated magnitude of this difference, Deltaqdsc, was found to be proportional to the cytocrit and hence also to the supercooled water volume in the sample. Further, the normalized fractional integrated heat release difference as a function of temperature, Deltaq(T)dsc/Deltaqdsc, was shown to correlate with the amount of supercooled cellular water which had exosmosed from the cell as a function of subzero temperature at constant cooling rates of 5, 10, and 20 degrees C/min. Several important limitations of the technique are (1) that it requires a priori knowledge of geometric parameters such as the surface area, initial volume, and osmotically inactive cell volume and (2) that the technique alone cannot determine whether the heat released from supercooled cellular water is due to dehydration or intracellular ice formation. Cryomicroscopy was used to address these limitations. The initial cell volume and surface area were obtained directly whereas a Boyle-van't Hoff (BVH) plot was constructed to obtain the osmotically inactive cell volume Vb. Curve fitting the BVH data assuming linear osmometric behavior yielded Vb = 0.258V0; however, nonlinearity in the data suggests that the EBVT lymphocyte cells are not "ideal osmometers" at low subzero temperatures and created some uncertainty in the actual value of Vb. Cryomicroscopy further confirmed that dehydration was the predominant biophysical response of the cells over the range of cooling rates investigated. One notable exception occurred at a rate of 20 degrees C/min where evidence for intracellular ice formation due to a DSC measured heat release between -30 and -34 degrees C correlated with a higher end volume but no darkening of the cells during cryomicroscopy. For the cooling rate tested (5 degrees C/min) the cryomicroscopy data correlated statistically very well with the DSC water transport data. A model of water transport was fit to the DSC water transport data and the average (5, 10, and 20 degrees C/min) biophysical parameters for the EBVT lymphocytes were found to be Lpg = 0.10 micro m/min-atm, ELp = 15.5 kcal/mol. Finally, the decrease in heat release from osmotically active cells measured by the DSC during repetitive freezing and thawing was found to correlate strongly with the viability of the cells measured during identical freeze/thaw protocols with cryomicroscopy. This shows the additional ability of the technique to assess freeze/thaw injury. In summary, this DSC technique is a promising new approach for measuring water transport in cellular systems during freezing.

Where should the cooling rate be determined in an extended freezing sample?

Computer simulations have been performed to calculate the transient cooling and solidification process at different locations in a flat plate-shaped freezing container filled with isotonic solution (0.9 wt% NaCl in water). The one-dimensional model accounts for the influences of the external cooling conditions, the container wall, the freezing bag, and the nonplanar solidification of an aqueous solution. The cooling rate was found to increase within the sample from the portions adjacent to the inner surface of the bag toward the center. An important result for cryobiological experiments was the fact that the geometrical center of the sample, a commonly used location for the determination of cooling rate, is not representative for the entire volume of the sample. Even worse, the calculations have shown that the center is often the least representative place. As an alternative, the most suitable location for cooling rate measurements has been determined. With the assumed surface cooling and geometry conditions an optimum location at x/(d/2) approximately 2/3 (x is the space coordinate, originating at the inner surface of the freezing bag; d is the sample thickness) has been calculated, i.e., a distance of one-third away from the center and two-thirds from the inside surface of the sample container. Admitting a 50% range of variation, the cooling rate measured at this point represents at least 80% of the entire sample volume. The survival signature, i.e., the functional dependence of cell survival from cooling rate (determined at a single location), for a fictitious cell kind is also influenced by the location of temperature determination: the "optimum" cooling rate seems to be shifted, and the shape of the signature is changed depending on the location where the cooling rate is determined.

Intracellular ice formation: cryomicroscopical observation and calorimetric measurement

The formation of ice crystals within biological cells is generally deleterious and results in a severe loss of cellular viability and function. With the aim of circumventing this lethal event, the mechanisms of nucleation and their dependence on governing parameters such as temperature, cooling rate and solute and/or additive concentration, and the correlation with the osmotically induced water transport across the cell membrane were investigated. Quantitative low-temperature light microscopy was used for this purpose as it offers the major advantage of studying the dynamics of the involved processes. To substantiate further the visual observations of the morphological changes associated with intracellular ice formation, supplementary studies by differential scanning calorimetry (DSC) were performed under comparable conditions to measure the quantity of water actually transformed into the crystalline state due to the evolution of latent heat. Human lymphocytes were used as a biological model cell. In particular it could be shown that the twitching type of intracellular ice formation which is evident but difficult to observe under the cryomicroscope can be attributed to a liquid-solid phase change within the cells as determined by DSC. Good agreement was obtained between the results measured by both techniques with respect to the following dependencies of governing parameters: the fraction of cells exhibiting intracellular ice determined as a function of the cooling rate shows a sharp demarcation zone with an increase from 0 to 100% at about the same threshold cooling rate. On the other hand, the temperatures at which intracellular ice forms were found to be only weakly dependent on the cooling rate. With respect to the effect of cryo-additive concentration at a fixed value of the cooling rate, the crystallization temperatures were seen to decrease with concentration. The DSC results may hence be regarded as a validation of the microscopic observations.

Redefining cooling rate in terms of ice front velocity and thermal gradient: first evidence of relevance to freezing injury of lymphocytes

A freezing process and the resulting injury or survival of biological cells is commonly characterized in terms of the cooling rate, B. Under certain circumstances, the cooling rate can be expressed as B = G.v, where G denotes the thermal gradient at the ice-liquid interface and v its velocity, respectively. To determine the influence of G and v on the morphology of the ice-liquid interface and on cell survival, a gradient freezing stage was designed. Flat capillaries could be pushed with constant velocity from a warm to a cold heat reservoir. With this setup both parameters, G and v, are independently adjustable and the resulting process of directional solidification can be observed dynamically in a light microscope. Human lymphocytes in phosphate-buffered saline with 10 vol% of dimethyl sulfoxide were used as biological test material. Viability was assessed by a membrane integrity test with fluorescein diacetate and ethidium bromide. All cells were cooled down to a final temperature of -196 degrees C and then rapidly thawed. The results obtained with this technique show that the viability determined after freezing and thawing with a certain cooling rate, B = G.v, may vary considerably depending on the imposed values of the thermal gradient, G, and the ice front velocity, v. In addition, the data seem to suggest that, first, the maximum viability which can be reached is governed by the cooling rate, and, second, this maximum for a given cooling rate could be achieved by establishing small temperature gradients and high interface velocities (about 30 degrees K/cm and 500 microns/sec, respectively, for the range of values of G and v tested).

Phenomena at the advancing ice-liquid interface: solutes, particles and biological cells

Ice formation in aqueous solutions and suspensions involves a number of significant changes and processes in the residual liquid. The resulting effects were described concerning the redistribution of dissolved salts, the behaviour of gaseous solutes and bubble formation, the rejection and entrapment of second-phase particles. This set of conditions is also experienced by biological cells subjected to freezing. The influences of ice formation in that respect and their relevance for cryopreservation were considered as well. A model of transient heat conduction and solute diffusion with a planar ice front, propagating through a system of finite length was found to be in good agreement with measured salt concentration profiles. The spacing of the subsequently developing columnar solidification pattern was of the same order of magnitude as the pertubation wavelengths predicted from the stability criterion. Non-planar solidification of binary salt solutions was described by a pure heat transfer model under the assumption of local thermodynamic equilibrium. The rejection of gaseous solutes and the resulting gas concentration profile ahead of a planar ice front has been estimated by means of a test bubble method, yielding a distribution coefficient of 0.05 for oxygen. The nucleation of gas bubbles has been observed to occur at slightly less than 20-fold supersaturation. The subsequent radial growth of the bubbles obeys a square-root time dependence as expected from a diffusion controlled model until the still expanding bubbles become engulfed by the advancing ice-liquid interface. The maximum bubble radii decrease for increasing ice front velocities. The transition between repulsion and entrapment of spherical latex particles by an advancing planar ice-front has been characterized by a critical value of the velocity of the solidification interface. The critical velocity is inversely proportional to the particle radius as suggested by models assuming an undisturbed ice front. The increase of the critical velocity for increasing thermal gradients shows good agreement with a theoretically predicted square-root type of dependence. Critical velocities have also been measured for yeast and red blood cells. The effect of freezing on biological cells has been analyzed for human lymphocytes and erythrocytes. The reduction of cell volume observed during non-planar freezing agrees reasonably well with shrinkage curves calculated from a water transport model. The probability of intracellular ice formation has been characterized by threshold cooling rates above which the amount of water remaining within the cell is sufficient for crystallization.(ABSTRACT TRUNCATED AT 400 WORDS)

Low temperature light microscopy and its application to study freezing in aqueous solutions and biological cell suspensions

The freezing of biological cell suspensions can be understood in terms of ice formation in the external suspension medium and the cellular reactions to the changing environment. Cryomicroscopy allows a quantitative analysis of both categories of phenomena. Besides freezing stages of appropriate thermal design, the components used for that purpose include a microcomputer (PSI 80) based control system, an image analysis system (Intellect 100) and a spectrophotometer (MPV compact). The investigation of extracellular ice formation is focused on the following effects: The redistribution of solutes in the residual liquid and the resulting concentration profiles are determined photometrically or densitometrically. The transitions between various morphologies of the ice-liquid phase boundary (planar-cellular-dendritic) can be related to interface instability theories. With respect to solute segregation, the studies also involve the formation of bubbles from supersaturated gaseous solutes and freezing potentials resulting from the differential incorporation of cations and anions into the solid phase. The interaction between particles or cells and the advancing ice front is determined from critical interface velocities marking the transition between repulsion and entrapment. The effects of freezing on biological cells are studied mainly with blood cells, especially lymphocytes. The water efflux due to osmotical gradients across the membrane yields volume shrinkage curves which are recorded and analysed from video images for various cooling rates. Beyond a certain threshold cooling rate, intracellular ice starts to form, and different crystallization morphologies can be detected. The intracellular crystallization temperatures depend on cooling and warming rates as well as on the presence of penetrating cryoadditives. A fluorescence viability is used to determine the percentage of damaged cells immediately after thawing.