Thermally defined cryomicroscopy and some applications on human leucocytes

An optical differential scanning calorimeter cryomicroscope

An optical-DSC system was designed, built, tested, calibrated and verified to incorporate into a single device the capability for simultaneous optical cryomicroscopy and differential scanning calorimetry (DSC). This instrument can be used to obtain both visual and thermal data for an individual specimen subjected to a defined freezing and thawing protocol with very little compromise in quality or range of data available in comparison with dedicated single instruments. Temperature and caloric calibrations were performed based on phase transition states in water, n-dodecane and n-decane. The instrument has proven effective for process analysis in living cells and in foodstuffs.

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.

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)

Quantitative cryomicroscopic analysis of intracellular freezing of granulocytes without cryoadditive

Purified human granulocytes were frozen in isotonic saline at different constant cooling rates down to -60 degrees C and subsequently thawed on the thermally defined cryostage of a cryomicroscope. Cells monitored on videotape were examined with respect to cooling rate threshold, type, and temperature of intracellular ice formation during cooling and recrystallization during warming. Two apparently different mechanisms of intracellular ice formation (iif) were distinguished during cooling, i.e., "twitching" (no visible ice front) and "darkening" (diffuse ice front). Both types of iif are related to cooling rate and hence also to dehydration. Cooling rate thresholds and temperatures of intracellular recrystallization were determined. It was found that twitching iif occurs just about 6.3 to 7.4 degrees C above the homogeneous nucleation temperature, suggesting that it might be catalyzed by nucleators present within the cells. Darkening iif, on the other hand, was observed at much higher temperatures, i.e., 23.4 to 28.3 degrees C above the homogeneous nucleation temperature, which could possibly indicate a nucleation induced by extracellular ice crystals (at a cooling rate of 30 degrees K/min, however, darkening iif was observed to occur at a temperature lower than that required for twitching iif). The proposed mechanisms of cryoinjury are related to membrane integrity measurements presented in M. W. Scheiwe, Ch. Körber, and S. Englich, Cryo-Letters, 5, 300-306, 1984.

Determination of the permeability of human lymphocytes with a microscope diffusion chamber

A diffusion chamber similar to that proposed by J.J. McGrath (J. Microsc., in press) was constructed which allows microscopic observation of osmotically induced volume changes of individual cells in small (microliter) sample volumes. The cells are kept fixed in position in the upper compartment of the chamber by means of a highly permeable membrane and exposed to a step-like change in concentration generated in the lower compartment. An electrical conductivity probe in the upper compartment was used to monitor the temporal change of salt concentration as experienced by the cells. The rise from isotonic to hypertonic can be approximated by an exponential function. Its time constant of tau = 2.08 sec seems to be mainly determined by the change in flushing solution as tau = 1.48 sec was measured with no membrane installed. With human lymphocytes, no loss of cell volume was detected before 5 sec, i.e., when 95% of the final concentration was reached extracellularly. A step change can hence be assumed when modeling exosmosis for determining the lymphocyte membrane permeability. The equations for coupled transport of water and salt were solved numerically and fitted to the experimental data. The results were also compared to various other transport models described in the literature. Human lymphocytes are almost ideally semipermeable with a hydraulic reference permeability of Lp = 4.23 X 10(-4) cm/sec (3.13 X 10(-3) micron X atm-1 X sec-1) at T = 23 degrees C. The temperature and concentration dependence are described by an activation energy Ea = 14.3 kJ/mol and a concentration coefficient alpha 2 = 0.261 osmol/kg. An osmotically inactive volume fraction of 36.9% was determined from the final cell volumes reached asymptotically after shrinkage.

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.

Intracellular freezing of human granulocytes

Human granulocyte suspensions were exposed to controlled freezing regimens on a cryomicroscope, and the incidence of intracellular freezing was measured as a function of cooling rate and extracellular nucleation temperature. The presence of intracellular ice was assessed by analysis of serially recorded images of the freeze-thaw process and by correlation with measured patterns of change in the cell volume. For granulocytes suspended in autologous plasma, a threshold was described for intracellular freezing as an empirical function of cooling rate (B) and extracellular nucleation temperature (Tn): B (degrees C/min) = 1.1 Tn (degrees C) + 12.3.

Thermally defined cryomicroscopy and thermodynamic analysis in lymphocyte freezing

A cryomicroscope is described which provides the possibility of quantifying the volume loss of cells during freezing, detection of intracellular ice formation during cooling and warming, as well as the determination of viability as function of (constant) cooling rates. The basic mechanisms occurring in cryopreservation have been studied with this system using the human lymphocyte suspended in pure saline as a biological model system; experimentally observed exosmosis during freezing is compared to predictions from a thermodynamic model. Cell volume loss during freezing has been determined experimentally for cooling rates of 2.4, 12, 48, and 120 degrees K/min. Exosmosis also was calculated corresponding to various assumptions regarding the concentration dependence of the hydraulic permeability of the cells. Further calculations of exosmosis are performed for determining the effects of the initial cell volume. The temperatures and transition cooling rate ranges of intracellular ice formation have been determined. On the basis of exosmosis and a lethal level of intracellular salt concentration, a hypothetical relative optimum of the cooling rate is theoretically predicted and compared to the experiments.

Basic investigations on the freezing of human lymphocytes

Human lymphocytes were frozen at constant cooling rates in the range 2.4 to 1000 degrees K/min without cryoadditive on the cold stage of a thermally defined cryomicroscope. The volume loss due to water efflux was quantified optically for the cooling rates 2.4, 12, 48, and 120 degrees K/min. The likelihood of the formation of intracellular ice was determined as function of the cooling rate. Intracellular crystallization temperatures were obtained for ice formation during both cooling and rewarming. A theoretical analysis of the cell volume loss during freezing was compared to the experimental data and used for an indirect determination of the water permeability of the cells. A relative optimum of the cooling rate is predicted theoretically under the assumption of a critical level of intracellular salt concentration near the eutectic temperature. The dependence of survival and cooling rate was determined cryomicroscopically by simultaneously applying the FDA/EB fluorescence viability test. The optimal cooling rate of about 35 degrees K/min was also found for 2-ml samples frozen within the range of cooling rates of interest. The results show that for freezing in physiological saline solution (1) the optimum of the cooling rate is theoretically predictable, (2) cryomicroscopical data are significant for freezing of samples of larger volume, and (3) the lethal type of intracellular crystallization is cooling rate dependent and distinguishable from innocuous types.

Osmotic response of individual cells during freezing. I. Experimental volume measurements

The osmotic response of yeast to freezing was measured as a function of cooling rate and degree of extracellular supercooling. Thirteen experimental trials were conducted on a cryomicroscope in which incremental size changes of individual cells were recorded photographically, and the corresponding volume variations were measured using a digital computer image analysis algorithm. Plots were obtained of normalized cell volume as a function of temperature. Cellular dehydration during freezing was progressively inhibited with increasing values of cooling rate and extracellular supercooling. Normalized cell volume changes were not a function of the relative initial cell size. A constant volume plateau occurred for conditions under which intracellular ice formation was expected.