The assessment of endothelial integrity by scanning electron microscopy and fluorescein diacetate staining following treatment with cryoprotective additives

In Situ Assessment of Cell Viability

Cryobiologlcal studies of tissues often require the simultaneous assessment of tissue structure and in situ cellular function. Localization of damage during cryopreservation occurs as a consequence of tissue structure and morphology and as a result of biophysical constraints imposed by diffusion and heat transfer. This study used five experimental model tissue systems: cells in suspension, cells attached to a substrate, a monolayer of cells attached to a substrate, porcine corneas, and intact porcine articular cartilage to examine the efficacy of assessing cell recovery using a novel fluorescent stain (SYTO-13). A graded freezing protocol was used to induce varying degrees of tissue damage. Recovery was assessed in the different tissue model systems using SYTO with ethidium bromide, fluorescein diacetate (FDA) with ethidium bromide, and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT). In each of the tissue model systems, the SYTO/EB assessment technique was shown to be equally effective as the existing techniques for the determination of cell recovery. In addition, the properties of fluorescence intensity and rate of release for SYTO were significantly better than those obtained using FDA. Assessment of in situ cell viability was clearly demonstrated using porcine corneas and articular cartilage. The SYTO/EB assay is superior to the existing techniques used for the localization of cell damage in tissues after cryopreservation. © 1998 Elsevier Science Inc.

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 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.