Clinical cryobiology of tissues: preservation of corneas

Analysis of Cryopreservation Protocols and Their Harmful Effects on the Endothelial Integrity of Human Corneas

Corneal cryopreservation can partially solve the worldwide concern regarding donor cornea shortage for keratoplasties. In this study, human corneas were cryopreserved using two standard cryopreservation protocols that are employed in the Tissue Bank of the Teresa Herrera Hospital (Spain) to store corneas for tectonic keratoplasties (TK protocol) and aortic valves (AV protocol), and two vitrification protocols, VS55 and DP6. Endothelial viability and general corneal state were evaluated to determine the protocol that provides the best results. The potential corneal cryopreservation protocol was studied in detail taking into consideration some cryopreservation-related variables and the endothelial integrity and stroma arrangement of the resulting cryopreserved corneas. TK corneas showed mostly viable endothelial cells, while the others showed few (AV) or none (DP6 and VS55). The corneal structure was well maintained in TK and AV corneas. TK corneas showed endothelial acellular areas surrounded by injured cells and a normal-like stromal fiber arrangement. Cryoprotectant solutions of the TK protocol presented an increasing osmolality and a physiological pH value. Cooling temperature rate of TK protocol was of 1 °C/min to −40 °C and 3 °C/min to −120 °C, and almost all of dimethyl sulfoxide left the tissue after washing. Future studies should be done changing cryopreservation-related variables of the TK protocol to store corneas of optical grade.

Tumor necrosis factor-alpha and interferon-gamma induce inflammasome-mediated corneal endothelial cell death

PURPOSE

Chronic corneal endothelial cell (CEC) loss results in corneal edema and vision loss in conditions such as pseudophakic bullous keratopathy (PBK), Fuchs' dystrophy, and corneal graft failure. Low CEC density has been associated with an elevation of intraocular pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α and interferon (INF)-γ. These cytokines are capable of triggering pyroptosis, a programmed cell death mechanism mediated by the inflammasome, prompting the activation of the pro-inflammatory cytokine interleukin (IL)-1β, the perpetuation of inflammation, and subsequent damage of corneal endothelial tissue. Therefore, the purpose of this study was to determine the deleterious contribution of the inflammasome and pyroptosis to CEC loss.

METHODS

CECs from human donor corneas were treated ex vivo with TNF-α and IFN-γ for 48 h. Levels of caspase-1 and IL-1β were then assayed by ELISA, and the expression of caspase-1 and gasdermin-D (GSDM-D) were confirmed by immunofluorescence. Endothelial cell damage was analyzed by a lactate dehydrogenase (LDH) release assay, and oxidative stress was determined by measuring the levels of reactive oxygen species (ROS) in the culture media.

RESULTS

Inflammasome activation and oxidative stress were elevated in CECs following exposure to TNF-α and IFN-γ, which resulted in cell death by pyroptosis as determined by LDH release which was inhibited by the caspase-1 inhibitor Ac-YVAD-cmk.

CONCLUSION

CEC death is induced by the pro-inflammatory cytokines TNF-α and IFN-γ, which contribute to inflammasome activation. Moreover, the inflammasome is a promising therapeutic target for the treatment of chronic CEC loss.

Cryopreservation of cornea: a low cooling rate improves functional survival of endothelium after freezing and thawing

AIM

To investigate the influence of low cooling rates on endothelial function and morphology of corneas frozen with propane-1,2-diol (PROH).

METHODS

Rabbit corneas, mounted on support rings, were exposed to 1.4mol/l (10% v/v) PROH, seeded to initiate freezing, and cooled at 0.2 or 1 degrees C/min to -80 degrees C. Corneas were frozen immersed in liquid or suspended in air. After being held overnight in liquid nitrogen, corneas were warmed at 1 or 20 degrees C/min. After stepwise removal of the cryoprotectant, the ability of the endothelium actively to control corneal hydration was monitored during normothermic perfusion. Morphology was assessed after staining with trypan blue and alizarin red S, and by specular microscopy during perfusion.

RESULTS

Functional survival was achieved only after slow cooling (0.2 degrees C/min) with the cornea immersed in the cryoprotectant medium, and rapid warming (20 degrees C/min). These conditions also gave the best morphology after freezing and thawing.

CONCLUSION

Cooling rates lower than those typically applied to cornea improved functional survival of the endothelium. This result is in accord with previous observations showing the benefit of low cooling rates for cell monolayers [CryoLetters 17 (1996) 213-218].

Vitrification of posterior corneal lamellae

Cryopreservation of corneas has not yet been established as a routine method. Unsatisfactory experimental results with conventional techniques prompted us to explore the possibilities of vitrification. The aim of the present study was to optimize the heat exchange between the corneal tissue and cooling medium by reducing the corneal tissue volume and using a suitable sample container. A further objective was to promote vitrification by developing a new device for rapid cooling to -140 degrees C, just below the vitrification temperature of the cryopreservation medium. Experiments were done using posterior lamellar discs from pig corneas with a diameter of 7.5 mm and a thickness of 250-350 microm. The volume of tissue to be vitrified was 88% lower with posterior corneal lamellae than with the previously used corneoscleral discs. A very thin-walled (0.05 mm), teflon-coated bag served as the sample container. Immersed in only 0.1 ml of the vitrification solution VS41a, the lamellae were cooled to a final storage temperature of -196 degrees C. After warming and organ-culturing for 24h, the endothelium was stained with trypan blue and alizarin red, to determine cell viability. Vitrification of corneal lamellae without apparent ice formation or cracking of the specimen was achieved. Despite the successful vitrification, only a maximum of 10% of the endothelial cells was vital after warming. Thus, the toxicity of the cryoprotective agents and the devitrification that occurred during the heating process require further optimization of the method.

Quantitative measurement and prediction of biophysical response during freezing in tissues

Cryopreservation and cryosurgery are important biomedical applications used to selectively preserve or destroy cellular systems through freezing. Studies using cryomicroscopy techniques, which allow the visualization of the freezing process in single cells, have shown that a drop in viability correlates with the extent of two biophysical events during the freezing process: (a) intracellular ice formation and (b) cellular dehydration. These same biophysical events operate in tissue systems; however, the inability to visualize and quantify the dynamics of the freezing process in tissues has hampered direct correlation of these events with freezing-induced changes in viability. This review highlights two new techniques that use freeze substitution and differential scanning calorimetry to provide dynamic freezing data in tissue. Characteristic dimensions and parameters extracted from these new data are then used in a predictive model of biophysical freezing response in several tissues, including liver and tumor. This approach promises to help guide improved design of both cryopreservation and cryosurgical applications of tissue freezing.

Cryopreservation studies with porcine corneas

PURPOSE

A new technique for the cryopreservation of rabbit corneas in 20% w/w dimethylsulfoxide, which has been shown to preserve significant structural and functional integrity of the endothelium, was tested in porcine corneas.

METHODS

The characteristics of uptake of dimethylsulfoxide into porcine corneas were measured using proton ( 1 H) nuclear magnetic resonance (NMR) spectroscopy. The effect on structural integrity of exposure to 20% w/w dimethylsulfoxide without freezing was first assessed using vital staining (acridine orange and propidium iodide), and optimum temperature conditions for addition and removal of the cryoprotectant were derived. The effects on structural integrity of cryopreservation in 15% and 20% w/w dimethylsulfoxide, and of reducing the degree of cell swelling during cryoprotectant removal following cryopreservation, were then evaluated.

RESULTS

The characteristics of uptake of dimethylsulfoxide from a 10% w/w solution fitted a single exponential, resulting in a maximum tissue concentration of 14.6% when the addition occurred on ice, and 18.5% when the addition took place at room temperature. The toxic effects of dimethylsulfoxide in porcine corneas were highly temperature dependent and only evident after removal of the cryoprotectant. Unlike rabbit corneas, cryopreservation of porcine corneas in 15% and 20% w/w dimethylsulfoxide induced substantial endothelial injury which was not improved by reducing the degree of cell swelling that occurred during removal of the cryoprotectant.

CONCLUSIONS

Porcine corneas were substantially more susceptible to the toxic effects of dimethyl sulfoxide, and to cryopreservation injury, than rabbit corneas. These results underline the importance of species variation in animal studies aimed at the cryopreservation of human tissue for transplantation.

Common cryopreservation media deplete corneal endothelial cell plasma membrane Na+,K+ ATPase activity

This study describes the effects of three cryopreservation media on the specific activity of corneal endothelial plasma membrane Na+,K+ ATPase activity, a transporter required for the fluid pump in the cornea. Bovine corneal endothelial cell cultures were used as a model system for these studies. Cryopreserved primary cells were thawed and passaged once to increase cell number. The specific activity plasma membrane Na+,K+ ATPase activity was subsequently measured on 4-6 replicate cultures. One freeze/thaw cycle depleted the Na+,K+ ATPase specific activity of corneal endothelial cell cultures by approximately 90%, as compared to cells of equivalent passage which had not been cryopreserved. Cell morphology of the cryopreserved cultures was indistinguishable from that of control cultures. In other experiments, first passage cultures which had not been subjected to cryopreservation were incubated with a dimethyl sulfoxide-, glycerol-, or propane diol-based freezing medium and Na+,K+ ATPase was measured on plasma membranes subsequently isolated from the cultures. Incubation of cells with cryopreservation media in the absence of the freezing process also depleted Na+,K+ ATPase by approximately 90%. Radiolabeled ouabain was used to measure Na+,K+ ATPase sites on cell cultures pretreated with dimethyl sulfoxide-based freezing media. A 4 h treatment with DMSO-based freezing medium had no effect on ouabain binding; treatment for 18 h reduced binding by only 50%. Thus, the method used to assess pump function (determination of Na+,K+ ATPase specific activity versus ouabain binding) may provide conflicting data concerning the level of pump function cultured cells. The cryoprotectants present in many common media used to freeze tissue culture cells appear to inhibit corneal endothelial Na+,K+ ATPase. Since the fluid pump of corneal endothelial cells is coupled to Na+,K+ ATPase activity, care must be taken to insure that pump function is not impaired during cryopreservation of cell cultures.

Determination of the kinetics of permeation of dimethyl sulfoxide in isolated corneas

Corneal cryopreservation requires that endothelial cells remain viable and intercellular structure be preserved. High viability levels for cryopreserved endothelial cells have been achieved, but preserving intercellular structure, especially endothelial attachment to Descemet's membrane, has proved difficult. Cell detachment apparently is not caused by ice, suggesting osmotic or chemical mechanisms. Knowledge of the permeation kinetics of cryoprotectants (CPAs) into endothelial cells and stroma is essential for controlling osmotic and chemical activity and achieving adequate tissue permeation prior to cooling. Proton nuclear magnetic resonance (NMR) spectroscopy was used to assess the permeation of dimethyl sulfoxide (DMSO) into isolated rabbit corneas. Corneas with intact epithelia were exposed to isotonic medium or 2.0 mol/L DMSO for 60 min and subsequently transferred to 2.0 or 4.0 mol/L DMSO, respectively, at 22, 0, or -10 degrees C. DMSO concentration in the cornea was measured vs time. The Kedem-Katchalsky model was fitted to the data. Hydraulic permeability (m3/N.s) is 7.1 x 10(-13) + 216%-11% at 22 degrees C, 8.2 x 10(-13) + 235%-21% at 0 degree C, and 1.7 x 10(-14) + 19%-16% at -10 degrees C. The reflection coefficient is 1.0 + 2%-1% at 22 degrees C and 0 degree C, and 0.9 +/- 5% at -10 degrees C. Solute mobility (cm/s) is 5.9 x 10(-6) + 6%-11% at 22 degrees C, 3.1 x 10(-6) + 12%-11% at 0 degree C, and 5.0 x 10(-8) cm/s + 59%-40% at -10 degrees C.

Corneal tolerance of vitrifiable concentrations of glycerol

Equilibration of corneas with sufficiently high concentrations of cryoprotectants to inhibit potentially damaging ice formation during cryopreservation has not yet been achieved. This study examined the effects on the structure and function of rabbit corneal endothelium of the low toxicity cryoprotectant glycerol. Corneas were exposed to concentrations ranging from 2.0 to 6.8 M glycerol in a Hepes-buffered Ringer's solution containing glutathione, adenosine, 5 mM sodium bicarbonate and 6% w/v bovine serum albumin. Endothelial function was assessed by monitoring corneal thickness during perfusion of the endothelial surface at 34 degrees C for 6 h. Endothelial structure was observed using specular microscopy during perfusion and scanning electron microscopy after perfusion. Corneas tolerated exposure to 2.0 and 3.4 M glycerol for 20 min at 4 and -5 degrees C, respectively. Tolerance of 4.8 M glycerol for 10 min at -10 degrees C was improved by decreasing the dilution temperatures. Ten-minute exposure to 6.1 and 6.8 M glycerol was tolerated at -15 degrees C. In all cases corneas initially showed signs of damage but endothelial function was regained following structural repair. Corneas exposed to 6.8 M glycerol and cooled below the glass transition temperature were nonfunctional after warming. Ice formation during warming was believed to be the cause of injury.

Corneal tolerance of vitrifiable concentrations of propane-1,2-diol

The merit of corneal cryopreservation by vitrification as opposed to conventional freezing is the avoidance of ice damage which is believed to disrupt the integrity of the corneal endothelium resulting in loss of corneal transparency. The cornea must be equilibrated with high concentrations of cryoprotectant in order to achieve vitrification at practicable cooling rates. In an earlier study, corneas were exposed to 3.4 mol/liter propane-1,2-diol (Rich and Armitage (1990) Cryobiology 27, 42-54). The present study exposed rabbit corneas to concentrations of propane-1,2-diol between 3.4 and 5.4 mol/liter in a Hepes-buffered Ringer's solution containing glutathione, adenosine, 5 mmol/liter sodium bicarbonate, 6% (w/v) bovine serum albumin, and 2.5% (w/v) dextran sulfate. Dextran sulfate was as effective as chondroitin sulfate at improving endothelial tolerance of 3.4 mol/liter propane-1,2-diol. This beneficial effect may be linked to the polyanionic nature of these molecules. Corneas exposed to 5.4 mol/liter propane-1,2-diol were cooled in liquid nitrogen vapor at a temperature of -140 degrees C for 2 h. Warming was achieved by direct transfer to a dilution solution at -10 degrees C. Endothelial function was assessed by monitoring corneal thickness during perfusion of the endothelial surface at 34 degrees C for 6 h. Endothelial structure was observed by specular microscopy during the perfusion and by scanning electron microscopy after perfusion. Corneas tolerated exposure to 3.4 mol/liter propane-1,2-diol for 20 min at 0 degrees C and to 4.1 mol/liter for 10 min at -10 degrees C. Exposure to 4.8 and 5.4 mol/liter for 10 min at -10 degrees C caused endothelial damage, although a degree of endothelial function was retained. Function following exposure to 5.4 mol/liter was improved by reducing the temperature of exposure to -15 degrees C. Corneas cooled after exposure to 5.4 mol/liter propane-1,2-diol for 10 min at -15 degrees C apparently vitrified, but devitrified on warming. The corneas swelled to such an extent during perfusion that the endothelium could not be viewed by specular microscopy, subsequent scanning electron microscopy showed a severely disrupted endothelium.

Propane-1,2-diol as a potential component of a vitrification solution for corneas

Any method of cryopreservation of the cornea must maintain integrity of the corneal endothelium, a monolayer of cells on the inner surface of the cornea that controls corneal hydration and keeps the cornea thin and transparent. During freezing, the formation of ice damages the endothelium, and vitrification has been suggested as a means of achieving ice-free cryopreservation of the cornea. To achieve vitrification at practicable cooling rates, tissues must be equilibrated with high concentrations of cryoprotectants. In this study, the effects of propane-1,2-diol on the structure and function of rabbit corneal endothelium were studied. Corneas were exposed to concentrations of propane-1,2-diol ranging from 10 to 30% v/v in a Hepes-buffered Ringer's solution containing glutathione, adenosine, 5 mmol/liter sodium bicarbonate, and 6% w/v bovine serum albumin. Endothelial function was assessed by monitoring corneal thickness during perfusion of the endothelial surface at 34 degrees C for 6 hr. Exposure to 10-15% v/v propane-1,2-diol was well tolerated for 20 min at 4 degrees C when the cryoprotectant was removed in steps or by sucrose dilution. However, exposure to 25% v/v propane-1,2-diol for 20 min at 0 or -5 degrees C was consistently tolerated only when 2.5% w/v chondroitin sulfate was included in the vehicle solution. Exposure to 30% v/v propane-1,2-diol was harmful at -5 and -10 degrees C. The endothelial damage following exposure to 30% v/v propane-1,2-diol was probably the result of a toxic effect rather than osmotic stress. Although 25% v/v propane-1,2-diol does not vitrify at cooling rates that are practicable for corneas, it could at this concentration form a major component of a vitrification solution comprising a mixture of cryoprotectants.

Tolerance of human corneal endothelium to glycerol

As an initial step in the development of a method for corneal cryopreservation by vitrification, we attempted to establish the maximum concentration of glycerol to which human corneal endothelium could be exposed at 4 degrees C for 15 min without damage. Damage was defined as an increase in mean endothelial cell size or the inability to maintain corneal thickness for 1 week after exposure to glycerol. Using a system for long-term corneal perfusion, we perfused 24 paired human corneas with glycerol at 4 degrees C. The concentration of glycerol increased at a rate of 20% (w/v) (2.2 M) per hour until the desired maximum concentration was reached for that cornea, stabilized for 15 min, and then decreased at the same rate. The corneas were then perfused at 37 degrees C with Dulbecco's medium at a rate of 5 microliters/min under 18 mm Hg intracameral pressure for 7 days with daily measurements of corneal thickness. Endothelial morphology was examined by specular microscopy and by scanning electron microscopy. After 7 days of perfusion at 37 degrees C, there was a statistically significant direct relationship between the maximum concentration of glycerol to which the experimental eyes had been exposed and the increase in mean endothelial cell size. The mean endothelial cell size increased in corneas exposed to glycerol concentrations of 40, 50, and 60% (w/v), but did not differ significantly from baseline measurements in the corneas exposed to 30% glycerol or less. Thus, there was no detectable damage to human corneas exposed to 30% (w/v) (3.3 M) glycerol in this system. Tolerance of higher concentrations may be achieved by changes in the rates of addition and removal of glycerol or in the composition of the perfusate.

Hypothermic preservation of corneas in a hyperkalaemic solution (CPTES): II. Extended storage in the presence of chondroitin sulphate

Periods of preservation for donor corneas, even for short times, are necessary to facilitate optimum conditions in penetrating keratoplasty. However, current techniques for corneal storage at low temperatures may not provide optimal conditions for maintaining tissue integrity. In particular, the ionic composition of the storage medium has received little attention since it has been assumed throughout that the normal complement of ions in tissue culture media will also be suitable for preservation at reduced temperatures. This study extends our previous investigations on the merits of using CPTES (corneal-potassium-TES), a potassium-rich balanced salt solution containing an impermeant anionic pH buffer (TES), as a storage solution specifically designed to prevent the loss of intracellular potassium and minimise endothelial cell swelling during the time that the normal regulatory processes are switched off. The effect of adding the natural polymer chondroitin sulphate (CS) as a colloid osmotic agent to the hyperkalaemic storage medium is now examined. Corneas stored in CPTES containing 2.5% chondroitin sulphate retained a very high level of structural and functional integrity after three, five, and seven days storage at 0 degrees C; furthermore, stromal swelling was restricted to only 21%. All corneas stored in CPTES + 2.5% CS showed active endothelial function by thinning efficiently at rates that were greater than those previously reported for rabbit corneas stored for similar lengths of time in either M-K medium or K-sol. The zwitterionic buffers TES and HEPES were interchangeable in the hyperkalaemic solution and were non-toxic to corneal endothelium at a concentration of 100 mM. These compounds offer excellent pH buffering in bicarbonate-free medium.

Hypothermic preservation of corneas in a hyperkalaemic solution (CPTES): I. Short-term storage in the absence of colloid osmotic agents

Preservation solutions for short-term storage of isolated donor corneas for use in penetrating keratoplasty have all been based on tissue culture medium, on the assumption that media designed to maintain the viability of cells at physiological temperatures will also provide suitable conditions for preservation at reduced temperatures. But for hypothermic preservation of some other tissues and organs, when ionic pumps are inhibited, it is unnecessary to support metabolism, and beneficial control of ion and water distribution between intra- and extracellular compartments is achieved by storage in appropriately formulated 'intracellular-type' solutions. We have therefore designed a solution that will restrict ionic imbalances and minimise endothelial cell swelling in corneas during exposure at reduced temperatures. This potassium-rich solution contains the biological pH buffer TES as an impermeant anion and is designated CPTES (corneal-potassium-TES). The structural and functional integrity of rabbit corneas stored at 0 degrees C in CPTES, without the addition of colloid osmotic agents, is compared with that of corneas stored in glutathione bicarbonate Ringers' solution (GBR), an 'extracellular-type' medium formulated for the maintenance of endothelial integrity during in-vitro perfusion at 34 degrees C. Corneas swelled significantly less during storage in CPTES than in GBR and could be stored for five days before reaching the same degree of hydration as corneas stored for only three days in GBR. Gross structural integrity and endothelial ultrastructure were maintained during storage for three and five days in CPTES. The rate of thinning of corneas stored in CPTES was significantly greater than in comparable groups of corneas stored in GBR. However, the efficient dehydration of corneas stored in CPTES was always preceded during perfusion by a brief period of additional swelling which was shown to be an osmotic response during the elution of the buffer compound TES that had permeated the stroma during storage. The omission of calcium or the addition of adenosine and glutathione to the CPTES preservation medium had no detectable effect on the integrity of the endothelium, but the omission of bicarbonate was beneficial, producing significantly higher rates of stromal thinning during normothermic perfusion. Additional benefits for extending storage by including colloid osmotic agents are described in a companion paper.

Survival of corneal endothelium following exposure to a vitrification solution

Corneal endothelium, a monolayer of cells lining the inner surface of the cornea, is particularly susceptible to freezing injury. Ice formation damages the structural and functional integrity of the endothelium, and this results in a loss of corneal transparency. Instead of freezing, an alternative method of cryopreservation is vitrification, which avoids damage associated with ice formation. Vitrification at practicable cooling rates, however, requires exposure of tissues to very high concentrations of cryoprotectants, and this can cause damage through chemical toxicity and osmotic stress. The effects of a vitrification solution (VS1) containing 2.62 mol/liter (20.5%, w/v) dimethyl sulfoxide, 2.62 mol/liter (15.5%, w/v) acetamide, 1.32 mol/liter (10%, w/v) propane-1,2-diol, and 6% (w/v) polyethylene glycol were studied on corneal endothelium. Endothelial function was assessed by monitoring corneal thickness during 6 hr of perfusion at 35 degrees C with a Ringer solution supplemented with glutathione and adenosine. Various dilutions of the vitrification solution were introduced and removed in a stepwise manner to mitigate osmotic stress. Survival of endothelium after exposure to VS1 or a solution containing 90% of the cryoprotectant concentrations in VS1 (90% VS1) was dependent on the duration of exposure, the temperature of exposure, and the dilution protocol. The basic dilution protocol was performed at 25 degrees C: corneas were transferred from 90% VS1 or VS1 into 50% VS1 for 15 min, followed by 25% VS1 for 15 min and finally into isosmotic Ringer solution. Using this protocol, corneal endothelium survived exposure to 90% VS1 for 15 min at -5 degrees C, but 5 min in VS1 at -5 degrees C was harmful and resulted in some very large and misshapen endothelial cells. This damage was not ameliorated by using a sucrose dilution technique; but endothelial function was improved when the temperature of exposure to VS1 was reduced from -5 to -10 degrees C. Exposure to VS1 for 5 min at -5 degrees C was well tolerated, however, when the temperature of the first dilution step into 50% VS1 was reduced from 25 to 0 degree C. The large, misshapen cells were not observed under these conditions nor after exposure to VS1 at -10 degrees C. These results suggested that damage was the result of cryoprotectant toxicity rather than osmotic stress. Thus, corneal endothelium survived exposure to two solutions of cryoprotectants, namely, 90% VS1 and VS1, that were sufficiently concentrated to vitrify. Whether corneas can be cooled fast enough in these solutions to achieve vitrification and warmed fast enough to avoid devitrification remains to be determined.

Corneal preservation

Significant advances in corneal preservation have been made over the past decade. The introduction of chondroitin sulfate-containing media for use at 4 degrees C allows storage of corneas for up to ten days prior to transplantation. Organ culture techniques have also been improved with the addition of chondroitin sulfate. There has been an increase in our understanding of preservation using McCarey-Kaufman medium. Studies have been published that compare these methods and help the clinician decide which method to use based on objective data. Also, our understanding of existing methods of evaluating endothelial viability has increased and new methods have been developed. Finally, the acquired immunodeficiency syndrome is having an increasing effect on eye-banking and the supply of donor corneas.

Effects of osmotic stress on rabbit corneal endothelium

The effects of osmotic stress on corneal endothelium were investigated by exposing rabbit corneas to anisosmotic conditions, and then perfusing the corneas with isosmotic glutathione bicarbonate Ringer solution for 4 hr at 35 degrees C. During the perfusion, endothelial function was assessed by measuring corneal thickness with a specular microscope. After perfusion, the corneas were prepared for scanning and transmission electron microscopy. Endothelial ultrastructure and function were well maintained after exposure to a wide range of osmolality (0.12-2.7 osmol/kg), but this tolerance of osmotic stress was dependent both on the duration and the temperature of exposure to the anisosmotic conditions. Exposure to an osmolality of 2.7 osmol/kg for 15 min at 23 or 37 degrees C resulted in gross damage to the endothelium when the hyperosmotic agent was sodium chloride. This damage was not the result of increased osmolality per se nor cellular shrinkage because the endothelium tolerated exposure to a sucrose solution of the same osmolality for 15 min at 37 degrees C. The detrimental effect of sodium chloride, however, was mitigated by reducing the temperature of exposure to 0 degrees C or reducing the duration of exposure to 5 min. These results suggest that endothelial cells become more tolerant of high electrolyte concentrations with reducing temperature, and this could be an important factor in the survival of the endothelium in corneal cryopreservation. The results also help to define the limits of osmotic shrinkage and swelling tolerated by endothelial cells. This will be of value in overcoming the detrimental osmotic effects associated with the addition and, in particular, the removal of cryoprotectants.

Cryopreservation of the rabbit cornea: freezing with dimethyl sulphoxide in air or in medium

There have been considerable difficulties in developing a satisfactory method for the cryopreservation of corneas. In this paper we describe the effect of two variables that appear to influence the effectiveness of preservation, the concentration of cryoprotectant and the medium that surrounds the cornea during freezing. Rabbit corneas were exposed to the cryoprotectant dimethyl sulphoxide (Me2SO) in concentrations of 1 or 2 mol/l in a high-potassium well-buffered solution, and then cooled to -196 degrees C either in air or surrounded by 5 ml of the Me2SO solution. After storage at -196 degrees C, thawing and removal of the Me2SO, survival was assessed by electron microscopy and measurement of stromal thickness during perfusion on the specular microscope. The least degree of damage was observed when corneas were equilibrated with 1M Me2SO and frozen in air. The evidence suggests that 2M may be an excessive concentration of Me2SO in this system and that damage to the stroma may be reduced by freezing the cornea in air rather than surrounded by the Me2SO solution.

Osmotic properties of the rabbit corneal endothelium and their relevance to cryopreservation

The process of cryopreservation subjects cells to gross changes in the composition of the solution that surrounds them, changes that cause the cells first to shrink and then to swell by an osmotic mechanism. Empirical methods have been developed that permit many cells to survive freezing and thawing, but the cornea, which is crucially dependent upon the function of its endothelial monolayer, has proved quite refractory. In this paper we explore the osmotic response of the corneal endothelium of the rabbit to solutions ranging in osmolality from 0.25 to 8.6 X isotonic. Boyle van't Hoff behavior was observed between 0.43 and 8.6 X isotonic, and there was an apparent nonosmotic volume of 33.6%. However, ultrastructural damage was observed at the limits of this range, and it appeared that the tolerated range was 0.64-4.4 X isotonic. We show the extent to which dimethyl sulfoxide (Me2SO) would be expected to moderate changes in volume during freezing and suggest that its initial concentration should be at least 2M to prevent excessive shrinkage. We also show that cell swelling during removal of Me2SO is especially likely to be hazardous.