Cryopreservation of the common carotid artery of the rabbit: Optimization of dimethyl sulfoxide concentration and cooling rate

This paper describes the continuation of studies that demonstrated the suitability of CP-Tes solution as a medium for the introduction and removal of dimethyl sulfoxide in rabbit common carotid arteries and established the kinetics of cryoprotectant permeation in that tissue. In this paper we report the tolerance of rabbit common carotid artery to dimethyl sulfoxide, in concentrations up to 30% (w/w), using a technique of exposure that was designed to control osmotic stress. The maximum concentration achieved without damage was 15% (w/w). Vessels were then equilibrated with 15% dimethyl sulfoxide and cooled to -80 °C at 0.22, 0.69, 2.15, or 9.63 °C/min: they were then transferred to the gas phase of a liquid nitrogen refrigerator {temperature below -160 °C) for storage. Thawing was carried out in a 37 °C water bath. The optimum rate of cooling for these conditions was found to be 0.69 °C/min. The maximal recovery of contractile force in response to 10^-6 M norepinephrine was 30-40%; relaxation to acetylcholine (an endothelium-mediated function) was 80% of control, and an estimated 71% of endothelial cells survived with minimal ultrastructural change.

EVALUATION OF DMSO TRANSPORT IN HUMAN ARTICULAR CARTILAGE: VEHICLE SOLUTIONS AND EFFECTS ON CELL FUNCTION

Osteochondral allografting techniques are limited by the availability of suitable donor tissue; there is an urgent need for effective cryopreservation. A fundamental requirement is the need to establish initial conditions of exposure to cryoprotectant that the chondrocytes will tolerate and that load the tissue with an adequate concentration of cryoprotectant. Three vehicle solutions to transport DMSO into the tissue were studied. Knee joints were obtained from deceased donors with appropriate consent. Whole condyles were treated with 20% w/w DMSO in each of three vehicle solutions and chondrocyte function and tissue CPA content measured. The results showed that exposure to 20% DMSO in each vehicle solution for 2 hours at 0 degrees C was tolerated without loss of GAG synthetic activity. It was observed that penetration of DMSO increased little after 1 hour of CPA exposure at 0 degrees C but the final tissue concentration of CPA was markedly lower than that in the medium.

Application of a high-level peracetic acid disinfection protocol to re-process antibiotic disinfected skin allografts

Skin allografts, derived from cadaveric donors, are widely used for the treatment of burns and ulcers. Prior to use in clinical situations, these allografts are disinfected using a cocktail of antibiotics and then cryopreserved. Unfortunately, this antibiotic disinfection procedure fails to decontaminate a significant proportion and these contaminated grafts can not be used clinically. We have investigated whether it is possible to apply a second, more potent disinfection procedure to these contaminated grafts and effectively to re-process them for clinical use. Cadaveric skin grafts, treated with antibiotics and cryopreserved, were thawed and a peracetic acid (PAA) disinfection protocol applied. The grafts were then preserved in a high concentration of glycerol or propylene glycol, and properties thought to be essential for successful clinical performance assessed. The cytotoxicity of the grafts was assessed using both extract and contact assays; damage to the skin collagen was assessed using a collagenase susceptibility assay and the capacity of the grafts to elicit an inflammatory response in vitro was assessed by quantifying the production of the pro-inflammatory cytokine TNF-alpha by human peripheral blood mononuclear phagocytes. PAA disinfection, in conjunction with either glycerol or propylene glycol preservation, did not render the grafts cytotoxic, pro-inflammatory, or increase their susceptibility to collagenase digestion. The rates of penetration of glycerol and propylene glycol into the re-processed skin were comparable to those of fresh skin. This study has demonstrated that PAA disinfection combined with immersion in high concentrations of either glycerol or propylene glycol was an effective method for re-processing contaminated skin allografts, and may justify their clinical use.

Permeability characteristics and osmotic sensitivity of rhesus monkey (Macaca mulatta) oocytes

BACKGROUND

Permeability characteristics and sensitivity to osmotic shock are principal parameters that are important to derive procedures for the successful cryopreservation of mammalian oocytes.

METHODS AND RESULTS

The osmotically inactive volume of rhesus monkey oocytes was determined by measuring their volumes in the presence of hypertonic solutions of sucrose from 0.2 to 1.5 mol/l, compared with their volume in isotonic TALP-HEPES solution. Boyle-van't Hoff plots at infinite osmolality indicated that the non-osmotic volumes of immature and mature oocytes were 20 and 17% respectively. Osmotic responses of oocytes exposed to 1.0 mol/l solutions of glycerol, dimethylsulphoxide (DMSO) and ethylene glycol (EG) were determined. Rhesus monkey oocytes appeared to be less permeable to glycerol than to DMSO or to EG. Sensitivity of oocytes to osmotic shock was determined by exposing them to various solutions of EG (0.1 to 5.0 mol/l) and then abruptly diluting them into isotonic medium. Morphological survival, as measured by membrane integrity, of oocytes diluted out of EG depended significantly on the concentration of EG (P < 0.01).

CONCLUSION

Determination of permeability characteristics and sensitivity to osmotic shock of rhesus oocytes will aid in the derivation of procedures for their cryopreservation.

The history and principles of cryopreservation

The ability of glycerol to protect cells from freezing injury was discovered accidentally. The subsequent development of cryopreservation techniques has had a huge impact in many fields, most notably in reproductive medicine. Freezing injury has been shown to have two components, direct damage from the ice crystals and secondary damage caused by the increase in concentration of solutes as progressively more ice is formed. Intracellular freezing is generally lethal but can be avoided by sufficiently slow cooling, and under usual conditions solute damage dominates. However, extracellular ice plays a major role in tissues. Cryoprotectants act primarily by reducing the amount of ice that is formed at any given subzero temperature. If sufficient cryoprotectant could be introduced, freezing would be avoided altogether and a glassy or vitreous state could be produced, but osmotic and toxic damage caused by the high concentrations of cryoprotectant that are required then become critical problems. The transport of cryoprotectants into and out of cells and tissues is sufficiently well understood to make optimization by calculation a practical possibility but direct experiment remains crucial to the development of other aspects of the cryopreservation process.

Processing of ovine cardiac valve allografts: 3. Implantation following antimicrobial treatment and preservation

It is known that a satisfactory clinical outcome can follow the implantation of cardiac valve allografts in spite of the loss of living cells in the tissue. If viable cells are not required for long term graft function, then effective disinfection of the tissue might become possible. In an earlier paper in this series we reported that peracetic acid (PAA) is an effective antimicrobial agent for the treatment of valve allografts; it was lethal to the cells but at a concentration of 0.21% had little effect on the mechanical properties or extracellular morphology of the valve leaflets. It was also found that PAA-treatment could be combined with storage in 85% glycerol at 4 degrees C, or cryopreservation with 10% Me(2)SO, without substantial further impairment of microscopic structure or mechanical properties. In this paper we describe the implantation of processed ovine aortic valves in the descending thoracic aorta of sheep. The experimental groups included control untreated valves and valves that had been treated with antibiotics or PAA and either cryopreserved, or stored in 85% glycerol. The recipient sheep showed good clinical appearances until the experiment was terminated at six months. The explanted grafts were examined by standard morphological and mechanical testing methods. The PAA-treated valves were clearly recognisable as valves: the leaflets had fair to medium morphology in both the unpreserved and the cryopreserved groups. All leaflets had a superficial overgrowth of cells. Microsatellite analysis for allelic differences were performed on samples of donor and recipient tissues using three markers of tissue source. Only one valve, which had been treated with PAA, revealed allelic differences between donor and recipient. It is suggested that DNA-fragments may have remained after the destruction of donor cells and six months of implantation: the overgrowing cells were almost certainly of recipient origin. We conclude that our experiments, in which PAA-treatment was combined with preservation, are sufficiently encouraging to justify further studies to refine the technique, but in our opinion they are not sufficient to justify a clinical trial at this time.

Processing of ovine cardiac valve allografts: 1. Effects of preservation method on structure and mechanical properties

It is essential to have some method of preservation of allograft valves during the time between procurement and implantation. Cryopreservation is the most commonly-used storage method today but it has the major disadvantage of high cost, and because its aim is to preserve living cells only relatively gentle antimicrobial treatments are used. This study addresses two interrelated questions: Is it necessary to maintain living donor cells in the tissue graft? Can more effective measures be used to reduce the risk of transmission of diseases, especially viral diseases, via human tissue grafts. In this paper, we report an investigation of four preservation methods that could be combined with more effective disinfection: cryopreservation with dimethyl sulphoxide, storage at approximately 4 degrees C in a high concentration of glycerol as used for the preservation of skin, snap-freezing by immersion in liquid nitrogen and vitrification. Snap freezing was mechanically damaging and vitrification proved to be impracticable but two methods, cryopreservation and storage in 85% glycerol, were judged worthy of further study. Cryopreservation was shown to maintain cellular viability and excellent microscopic structure with unchanged mechanical properties. The glycerol-preserved valves did not contain any living cells but the connective tissue matrix and mechanical properties were well preserved. The importance of living cells in allograft valves is uncertain. If living cells are unimportant then either method could be combined with more effective disinfection methods: in that case the simplicity and economy of the glycerol method would be advantageous. These questions are addressed in the two later papers in this series.

Processing of cardiac valve allografts: 2. Effects of antimicrobial treatment on sterility, structure and mechanical properties

This is the second in a series of papers that report experiments to investigate the properties required for effective tissue valve implants. This paper is concerned with investigations into alternative antimicrobial treatments and the effect these treatments produce on the structural and biomechanical properties of ovine aortic valves. Six treatments were studied: heat, peracetic acid (at two concentrations), chlorine dioxide, a surfactant cleaning agent and a solvent/detergent treatment. Samples of myocardial tissue were exposed to a mixed bacterial culture or one of three virus cultures and then decontaminated. Two of the six treatments (0.35% peracetic acid and heat) were effective in removing both bacterial and viral contamination, reducing levels of contamination by 2.5 to 3 logs, whilst a third (chlorine dioxide) was effective against viruses ( approximately 3 log reduction). Valves subjected to these treatments were examined by microscopy and measurements of mechanical properties were made. All three treatments seriously damaged endothelial cells and leaflet fibroblasts. Heat treatment also damaged connective tissue components (collagen and elastin) but these changes were not seen after chemical treatment. Mechanical testing confirmed severe damage following heat treatment but chemical treatment showed only minor effects on the elasticity of the leaflets and none on extensibility. These minor effects could be mitigated by exposure to a lower dose of peracetic acid and this treatment could be safely combined with cryopreservation or storage in 85% glycerol. Peracetic acid was the preferred disinfection method for use in the subsequent in vivo studies in sheep.

Differences in the requirements for cryopreservation of porcine aortic smooth muscle and endothelial cells

One of the basic requirements for the production of tissue-engineered constructs is an effective means of storing both the constructs and the cells that will be used to make them. This paper reports on the cryopreservation of porcine aortic smooth muscle and endothelial cells intended for the production of model vascular constructs. We first determined the cell volume, nonosmotic volume, and the permeability parameters for water and the cryoprotectant dimethyl sulfoxide (Me(2)SO) in these cells at 2-4 degrees and 22 degrees C. The following results were obtained: Table unavailable in HTML format. Using a cell culture assay, both cell types were shown to tolerate threefold changes in cell volume, in either direction, without significant injury. Although these data suggested that single-step methods for the introduction and removal of 10% w/w Me(2)SO should be effective, an additional mannitol dilution step was adopted in order to reduce the time required for removal of the Me(2)SO. Following cooling at 0.3, 1, or 10 degrees C/min and storage at less than -160 degrees C, the survival of porcine aortic smooth muscle cell suspensions, measured by a cell culture assay, was inversely related to cooling rate; at 0.3 degrees C/min, recovery was >80%. The survival rate for aortic endothelial cells was directly related to cooling rate over the range tested and was >80% at 10 degrees C/min.

The current status of tissue cryopreservation

Cryopreservation plays an important role in tissue banking and will assume even greater importance when tissue engineering becomes an everyday reality. For some tissue grafts, living cells are unnecessary and adequate preservation methods are usually available. For other tissues living and functioning cells are needed and preservation methods are much less advanced. The basic requirements for cell recovery can usually be defined if a few basic biophysical properties of the cell are known and some standard measurements of the effect of cryobiological variables are carried out. The problems in tissue cryopreservation are not usually due to difficulties in preserving the living cells per se, but arise from the properties of the integrated cell/matrix systems upon which tissue function almost always depends. Some examples of such difficulties are described. It is concluded that the formation of ice, through both direct and indirect effects, is probably fundamental to these difficulties, and this is why vitrification seems to be the most likely way forward. However, two major problems still to be overcome are cryoprotectant toxicity and recrystallization during rewarming. Less obvious, and certainly less well understood is chilling injury - damage caused by reduction in temperature per se; this may yet turn out to be of fundamental importance.

Rapid electromagnetic warming of cells and tissues

We describe a system for thawing frozen cell suspensions and tissues by electromagnetic absorption. A 25-ml sample is heated in a cylindrical resonant cavity, which is excited in three modes all close to 434 MHz. Maximum warming rates are over 10 degrees C/s (600 C/min), and a frozen sample may be brought from -65 degrees C to room temperature in < 30 s, with final spatial differences of < 20 degrees C. Samples may be frozen externally, or cooled within the cavity at typically 1 degree C/min. We have also used the resonant cavity to measure the permittivity and conductivity of the sample at temperatures from -83 degrees C to +8 degrees C. By measuring the heat capacity of the sample, we have calculated the power deposited in it as a function of its temperature. The system is currently being used to investigate the effect of warming rate on cell survival.

Osmotically inactive volume, hydraulic conductivity, and permeability to dimethyl sulphoxide of human mature oocytes

Controlled ovarian stimulation during an in vitro fertilization cycle usually produces large numbers of oocytes and, consequently, it is likely that more embryos will be generated than can be transferred in a given cycle. It is desirable to freeze-bank surplus oocytes before insemination to avoid the ethical and legal complications of disposing of or storing embryos. Although many attempts have been made to cryopreserve human oocytes, to date, post-thaw survival has been poor, and viable pregnancies after in vitro fertilization have been rare. A possible explanation for the lack of success is that the freezing methods have been adapted from animal studies but have not been optimized for the human oocyte. In this study, video microscopy was used to determine the volumetric responses of mature human oocytes to changes in osmolarity during preparation for freezing. A Boyle van't Hoff plot of data collected in static experiments with fresh human oocytes gave a value of 0.19 +/- 0.01 (mean +/- SEM) for the osmotically inactive volume. Dynamic measurements during exposure to dimethyl sulphoxide at room temperature (22 degrees C) were analysed by a two-parameter transport model and produced values of 1.30 x 10(-6) cm atm-1 s-1 for the hydraulic conductivity of the plasma membrane and 3.15 x 10(-5) cm s-1 for dimethyl sulphoxide permeability (chi-squared = 0.43, df = 20) of fresh human oocytes. Oocytes that had failed to fertilize had a slightly lower hydraulic conductivity and dimethyl sulphoxide permeability and, after exposure to 1.5 mol dimethyl sulphoxide l-1, these cells appeared to become permeable to normally impermeable solutes. These permeability properties have been used to design a protocol for the addition and removal of dimethyl sulphoxide to control the magnitude of volumetric changes.

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.

Sensitivity of kidney perfusion protocol design to physical and physiological parameters

The introduction and removal of cryoprotective agents (CPA) to a kidney via vascular perfusion may induce changes in cell volume that are destructive to the tubular epithelial or capillary endothelial cells as well as causing significant increases in vascular resistance that compromise the perfusion process. A network thermodynamic model of the coupled osmotic, hydrodynamic and elastic properties of the kidney was applied to evaluate the sensitivity of these critical outputs to a set of physiological and perfusion variables. Simulation results suggest that in the design of perfusion protocols for CPAs such as glycerol, it may be advantageous to: (a) select a CPA with as high a cell membrane permeability as possible; (b) increase the concentration of mannitol in the perfusate to about 200 mos/kg, beyond which there is no discernible benefit; (c) when glycerol is the CPA, limit the rate of reduction in the perfusate during removal to 30 mM/min or less; (d) limit the perfusion pressure to 20-30 mm Hg, within the practical constraints of the perfusion system; (e) increase the concentration of impermeant in the perfusate to as high as 400 mos/kg, although it is recognized that this departure from plasma-like composition might impose other problems that are not considered in this model. Further, it was observed that the vascular membrane permeability plays a relatively minor role in controlling cellular osmotic injury and vascular perfusion resistance and is therefore not a critical parameter in the perfusion design process.

Permeation of human ovarian tissue with cryoprotective agents in preparation for cryopreservation

The recent improvements in the treatment of cancer by chemo- and radiotherapy have led to a significant increase in the survival rates of patients with malignant disease, but at the expense of distressing side effects. One major problem, especially for younger patients, is that aggressive therapy destroys a significant proportion of the follicular population, which can result in either temporary or permanent infertility. Freeze-banking pieces of ovarian cortex prior to treatment is one strategy for preserving fecundity. When the patient is in remission, fertility could, theoretically, be restored by autografting the thawed tissue at the orthotopic site or by growing isolated follicles to maturity in vitro. Recent studies have found good follicular survival in frozen-thawed human ovarian tissue but to optimize the process an effective cryopreservation method needs to be developed. An essential part of such a technique is to permeate the tissue with a cryoprotectant to minimize ice formation and the extent of this equilibration is an important determinant of post-thaw cellular survival. In the current study, we have investigated the diffusion of four cryoprotective agents into human tissue at both 4 degrees C and 37 degrees C. We have also studied the effect of adding different concentrations of the non penetrating cryoprotective agent, sucrose, to the freezing media using the release of lactate dehydrogenase as a measure of its protective effect. At 4 degrees C propylene glycol and glycerol penetrated the tissue significantly slower than either ethylene glycol or dimethyl sulphoxide. At the higher temperature of 37 degrees C all four cryoprotectants penetrated at a faster rate, however concern about enhanced toxicity prevents the use of these conditions in practice. Thus, the results suggest that the best method of preparing tissue for freezing is exposure for 30 min to 1.5 M solutions of ethylene glycol or dimethyl sulphoxide at 4 degrees C; this achieved a mean tissue concentration that was almost 80% that of the bathing solution. We also report that the addition of low concentrations of sucrose to the freezing medium does not have a significant protective effect against freezing injury.

Cryopreservation of rabbit corneas in dimethyl sulfoxide

PURPOSE

To minimize the injury to endothelial cells during cryopreservation of rabbit corneas with dimethyl sulfoxide.

METHODS

Rabbit corneas were cryopreserved using 20% wt/wt dimethyl sulfoxide (Me2SO), added and removed in stages to maintain the osmotically induced excursions in cell volume to within +/-40% of their isotonic volume. The vehicle solution, cooling rate, and conditions of storage used were those already reported to be optimal for endothelial cell survival after exposure to low temperatures. Survival was assessed by confocal microscopy with vital staining and by the ability of the endothelium to control stromal hydration during 3 hours of normothermic perfusion. The effect of temperature of addition and removal of Me2SO (room temperature [RT] or 2 degrees C) on endothelial viability also was measured.

RESULTS

After thawing, all the cryopreserved corneas appeared structurally intact when assessed by vital staining and could limit stromal swelling during subsequent normothermic perfusion. Analysis of the rate of stromal swelling during the first 1.5 hours of normothermic perfusion indicated a substantial benefit when the Me2SO was removed at RT. Adding and removing the Me2SO at RT, which allowed a briefer exposure to Me2SO before cooling, resulted in better structural integrity of the endothelial layer than when the addition of cryoprotectant took place on ice.

CONCLUSIONS

These results demonstrate the importance of osmotic stresses in the generation of injury to corneal endothelium during cryopreservation and the possibility of eventual successful cryopreservation of this tissue.

Repopulation of freeze-dried porcine valves with human fibroblasts and endothelial cells

BACKGROUND AND AIMS OF THE STUDY

There is a need for a replacement cardiac valve constructed from non-immunogenic materials but incorporating living, and preferably autologous, cells. The object of this study was to colonize freeze-dried porcine valve leaflets with human fibroblasts and vascular endothelial cells.

METHODS

Porcine pulmonary valve leaflets were freeze-dried to produce a porous matrix having communicating cavities of appropriate dimensions for fibroblast repopulation. Cultured human fibroblasts and vascular endothelial cells that had been cryopreserved by standard methods were added to freeze-dried leaflets. Following culture at 37 degrees C, the leaflets were examined by confocal scanning microscopy and transmission electron microscopy.

RESULTS

Mechanical perforation of the leaflet surface permitted colonization of the freeze-dried matrix by fibroblasts; under the conditions we studied, the cell density did not reach physiologic levels but those cells that were present were well attached and metabolically active. Gentle cotton abrasion of the surface of the freeze-dried leaflets provided a suitable substrate for endothelial cell attachment and confluence was achieved in 10 days. Leaflets were perforated, cultured with human fibroblasts for 10 days, then gently rubbed with a cotton bud and cultured for a further 10 days with human endothelial cells. The endothelial cells formed a confluent layer on the surface and viable fibroblasts were present within the substance of the leaflet.

CONCLUSION

Although these results are preliminary, they demonstrate the basic feasibility of this approach to the production of xenogeneic valves that contain the patient's own cells.

Fractures in cryopreserved elastic arteries

The aim of this study was to define the conditions under which macroscopic fractures occur in vascular tissue during cryopreservation and to develop a practical cryopreservation method that prevents fracturing. The common carotid artery of the rabbit was subjected to a cryopreservation process that has been optimised for retention of in vitro function and cytological structure. This involves the stepwise addition and subsequent removal of dimethyl sulfoxide using a calculated protocol that avoids osmotic injury and minimises toxic action, controlled cooling, storage at -180 degrees C, and rapid warming. Seventy-five percent of such arteries were grossly fractured. The cooling and warming conditions were systematically varied to determine when in the cooling/storage/warming process the fractures occurred. Differential scanning calorimetry was then used to identify any corresponding thermal events. It was found that the fractures occurred as the temperature range -150 to -100 degrees C was traversed during the warming phase of the process. The glass transition temperature of a maximally freeze-concentrated solution of the cryoprotectant used was found to be -123 degrees C. Reducing the warming rate between the storage temperature (-180 degrees C) and -100 degrees C to < 50 degrees C/min prevented the fractures. Subsequent thawing could then be carried out rapidly in a 37 degrees C water bath without risk of fracture. We suggest that the fractures probably result from the thermal stresses created by rapid warming of the vitreous material that is produced by freeze-concentration of the aqueous phase. Relatively slow warming to -100 degrees C, at which temperature the vitreous material has softened, reduces these stresses and avoids the fractures.

Freeze drying of cardiac valves in preparation for cellular repopulation

When freeze-dried cardiac valves have been implanted they remained acellular. This study is the initial step in the development of a method designed to repopulate the substance of the freeze-dried valve with fibroblasts and the lumenal surface with endothelial cells. In this scheme, the freeze-drying process performs three functions; it provides a porous matrix, it kills the donor cells, and it preserves the collagen structure and hence the mechanical strength of the valve. This paper describes the production of appropriate porosity in freeze-dried porcine pulmonary valve leaflets. We found that Tg' for this material is -83 degrees C, which made it impracticable to freeze-dry exclusively from the glassy state. Uncontrolled freeze-drying produced a variable structure with most of the pores considerably smaller than the desired size and a dense layer, apparently devoid of perforations, on the surface. Compacted layers also occurred within the substance of the leaflets. These appearances suggested that extensive collapse had occurred during the drying process. Variation of the cooling rate, the primary drying temperature, and the warming rate during secondary drying enabled us to identify the following conditions that provided satisfactory internal porosity: cooling at 5 degrees C/min, vacuum drying for 6 h at -20 degrees C, and secondary drying for 10 h during rewarming at 0.06-0.08 degrees C/min. The internal cavities measured 100-350 microns2 by ca. 400 microns2, which is adequate to provide access for the fibroblasts (cross-sectional area ca. 150-200 microns2 when rounded but fusiform when attached. However, the internal porous structure rarely communicated with the surface and mechanical perforation was required to provide continuity between the surface and the internal sponge. The resulting method provides a basis for studies of cell colonization.

Effects of ionizing radiation on the mechanical properties of human bone

Allogeneic bone grafts are frequently sterilized by means of ionizing radiation. We investigated the effects of ionizing radiation on both quasistatic and impact mechanical properties of human bone. Specimens from four paired femora of four donors received doses of 29.5 kGy ("standard," frequently used by tissue banks), 94.7 kGy ("high"), or 17 kGy ("low") of ionizing radiation. Young's modulus was unchanged by any level of radiation. Radiation significantly reduced bending strength, work to fracture, and impact energy absorption; in each case, the severity of the effect increased from low to standard to high doses of radiation. Work to fracture was particularly severely degraded; specimens irradiated with the high dose absorbed only 5% of the energy of the controls. Radiation, even at relatively low doses, makes the bone more brittle and thereby reduces its energy-absorbing capacity. We suggest that because the level of radiation required to produce an acceptable level of viral inactivation (90 kGy) produces an unacceptable reduction in the mechanical integrity of the bone, low levels of radiation, sufficient to produce bacterial safety, should be used in conjunction with biological tests to ensure viral safety.

The effect of cooling rate and temperature on the toxicity of ethylene glycol in the rabbit internal carotid artery

The smooth muscle and vascular endothelium of small elastic arteries (the rabbit common carotid artery) are injured by exposure to 40% ethylene glycol (EG) at 4 degrees C, and additional damage occurs when the arteries are cooled without freezing to -20 degrees C. This paper reports attempts to reduce this injury by altering the cooling rate and temperature of exposure to the cryoprotectant. Very slow cooling (0.1 degree C/min) removed all residual smooth muscle and endothelial function when assessed in vitro after rewarming and removal of the cryoprotectant. Very rapid cooling to -20 degrees C also increased the injury, both to the endothelium and to the smooth muscle. Reducing the temperature of exposure to 40% EG from +4 degrees C to -20 degrees C had no beneficial effect on the smooth muscle but enabled the vascular endothelium to retain some functional activity. These data suggest that the mechanism responsible may be related to the physical properties of ethylene glycol rather than to a biochemical interaction with metabolic processes, and that it is a mechanism which is highly specific for the cell types involved. It also underlines the difficulties involved in the successful cryopreservation of complex tissues and organs.

Cryopreservation of the common carotid artery of the rabbit: optimization of dimethyl sulfoxide concentration and cooling rate

This paper describes the continuation of studies that demonstrated the suitability of CP-Tes solution as a medium for the introduction and removal of dimethyl sulfoxide in rabbit common carotid arteries and established the kinetics of cryoprotectant permeation in that tissue. In this paper we report the tolerance of rabbit common carotid artery to dimethyl sulfoxide, in concentrations up to 30% (w/w), using a technique of exposure that was designed to control osmotic stress. The maximum concentration achieved without damage was 15% (w/w). Vessels were then equilibrated with 15% dimethyl sulfoxide and cooled to -80 degrees C at 0.22, 0.69, 2.15, or 9.63 degrees C/min: they were then transferred to the gas phase of a liquid nitrogen refrigerator (temperature below -160 degrees C) for storage. Thawing was carried out in a 37 degrees C water bath. The optimum rate of cooling for these conditions was found to be 0.69 degrees C/min. The maximal recovery of contractile force in response to 10(-6) M norepinephrine was 30-40%; relaxation to acetylcholine (an endothelium-mediated function) was 80% of control, and an estimated 71% of endothelial cells survived with minimal ultrastructural change.

Fractures in cryopreserved arteries

The common carotid artery of the rabbit, a typical small elastic artery, can be cryopreserved using dimethyl sulfoxide, slow cooling, storage at less than -160 degrees C, and rapid warming. This technique provides satisfactory preservation of muscle and endothelial cells, but in about 75% of cases, gross circumferential fractures occur in the vessel wall. This paper investigates the influence of vehicle solution composition, cryoprotectant concentration, cooling rate, and storage temperature on the occurrence of cracks. When cooling was halted at -80 degrees C and the arteries were stored at this temperature, fractures no longer occurred. Possible mechanisms are discussed and it is proposed that mechanical stresses develop in the vitreous material that separates the ice crystals and lead to structural failure.

Permeation of rabbit common carotid arteries with dimethyl sulfoxide

Proton nuclear magnetic resonance has been used to measure the kinetics of permeation of dimethyl sulfoxide in the common carotid artery of the rabbit. The process is described by the following exponential equations, where t = time in minutes: % Unexchanged at 2 degrees C = 70e-0.515t + 30e-0.104t and %Unexchanged at 22 degrees C = 70e-1.790t + 30e-0.146t. The times required for 95% equilibration were 13 min at 22 degrees C and 18 min at 2 degrees C. The corresponding times for 99% equilibration were 24 and 32 min, respectively.

Cryopreservation of the common carotid artery of the rabbit

We describe experiments on the cryopreservation of the rabbit common carotid artery aimed at improving upon previous results. We describe the design of a double clamp which holds the artery during transportation and storage, preventing twisting, shortening, and collapse of the vessel. The device allowed perfusion with solutions as desired and markedly reduced the extent of endothelial loss during procurement and processing. We also studied the effects of three vehicle solutions; a modified Hanks' solution, a solution originally developed for the cryopreservation of smooth muscle (K-Pipes), and a solution designed for corneal endothelium (CP-Tes). The criteria used to make the assessments were smooth muscle contractility and the structure and function of the vascular endothelium. A new staining method for vascular endothelium (combining propidium iodide with silver nitrate) is described. We found that there was significantly more endothelial cell damage in rabbit carotid arteries frozen in Hanks' solution than in the other solutions, and the recovery of smooth muscle contractility was lowest in the Hanks' group. Arteries cryopreserved using CP-Tes as the vehicle solution showed less endothelial cell damage than arteries preserved with either K-Pipes or Hanks' solution, and these arteries also exhibited the greatest relaxation response to acetylcholine. We conclude that careful handling of the vessels is important; of the solutions studied, CP-Tes is preferred for the cryopreservation of rabbit carotid artery with Me2SO.

The possibility of resuscitating livers after warm ischemic injury

The number of clinical liver transplants that can be performed is limited by the availability of suitable donor organs. If it were possible to harvest and use livers after cardiac arrest, the supply could be improved. The mechanisms of damage in warm ischemia are not yet well understood and the consequences of transplanting a liver that is unable to provide immediate life-support are unacceptable. This study aims to identify areas for more detailed study in an attempt to improve the quality of livers harvested after significant warm ischemia, and to select acceptable organs for transplantation. Porcine livers were subjected to 75 min of warm ischemia and then perfused at 37 degrees C for 3 hr, during which period biochemical monitoring was carried out. At the end of the perfusion, histological and transmission electron microscopical studies were made. Large amounts of the intracellular enzymes ALT, AST, and LDH were released into the perfusate during the first 30 min of perfusion, but this--and the further amounts released during the subsequent 2.5 hr--was influenced by the composition of the perfusate. The inclusion of the substrates fructose and oleate, plus amino acids, substantially reduced this release and also improved the ability of the livers to metabolize ammonia. Oxygen free-radical scavengers had a significant, but smaller, beneficial effect. Electron microscopy confirmed the value of perfusion in improving cell morphology, and the additional value of including metabolic substrates. This study shows that hepatocellular structure and function can be improved by appropriate perfusion methods that also provide a simple means of monitoring some important functions. Both metabolic support and neutralization of oxygen free-radical action have a role to play in this approach to rendering ischemically injured livers acceptable for clinical use.

The effect of polyvinylpyrrolidone and the cooling rate during corneal cryopreservation

The effect upon endothelial cell survival of (a) PVP and (b) the cooling rate was investigated during the cryopreservation of rabbit corneas with 3 mol/liter dimethyl sulfoxide (Me2SO) dissolved in a hyperkalemic buffer vehicle solution that we designated CPTES; this solution was designed specifically to restrict deleterious ionic imbalances and cell swelling during hypothermic procedures. Polyvinylpyrrolidone (PVP) was used as the colloid and the corneas were cooled at 0.03, 0.1, 1, 25, or 125 degrees C/min, using the minimum amount of extracellular solution. Electron microscopy as well as staining with fluorescein diacetate and ethidium bromide (FDA/EB) was used to assess cellular integrity. To reduce osmotic stress, steps for the serial equilibration of the cryoprotectant additives (CPAs) were based upon calculations that predict endothelial volume during CPA exchange. A toxicity study showed that at 0 degrees C all the CPA equilibration protocols were well tolerated; for example, FDA/EB staining indicated that 97% intact cells were retained following direct transfer to 3 mol/liter Me2SO in CPTES to which 40% w/v PVP had been added as an osmotic buffer. However, less than 20% of cells were intact by FDA/EB staining with all corneas frozen at rates > 1 degree C/min regardless of which equilibration protocol was employed, nor were there any intact cells when 3 mol/liter Me2SO in CPTES was used alone at the lowest cooling rate. At intermediate cooling rates viability was improved: the highest mean survival of 81% was obtained using 3 mol/liter Me2SO in CPTES plus 40% PVP. Electron microscopy showed that detachment of the endothelial layer often occurred, but least damage was evident following exposure to 3 mol/liter Me2SO in CPTES plus 40% PVP and cooling at 1 degree C/min. No thawed cornea could maintain normal control of hydration immediately upon return to isotonic medium. The results show that, with these cryopreservation protocols, loss of cell integrity occurs at cooling rates greater than 1 degree C/min, whereas at lower rates higher survival of individual cells was achieved, but cellular adhesion to the basement membrane was impaired.

Analysis of electromagnetic heating patterns inside a cryopreserved organ

Computer analysis of the induced electromagnetic field and heating distribution inside cryopreserved organs subjected to electromagnetic illumination at frequencies of 84 MHz, 434 MHz and 2.45 GHz were carried out using a spherical model for the organ, with special reference to heating in a single-mode resonant cavity. The interaction between the frequency of the incident field and the size and dielectric properties of the sample was investigated. It is shown that uniform heating of organs is likely to be achieved at lower frequencies, as might be expected. However, the ratio between the power penetration depth for plane waves and the size of the organ is not a sufficient basis on which to judge quantitatively the uniformity of the power absorption. Hot or cold spots can occur within the organ even when this ratio is greater than unity; the wavelength in the material is also an important factor. The results from this study indicate that the use of a resonant cavity as a heating applicator has advantages over plane-wave illumination. A sharp upper limit can be set to the frequency suitable for rapid and uniform heating of a given organ.

Experimental results on the rewarming of a cryopreserved organ phantom in a UHF field

We describe a UHF rewarming system which has been used to measure warming rates, and particularly the uniformity of warming on three orthogonal axes, in a rabbit kidney phantom 36 mm in diameter. The stabilizing effect is demonstrated of using an E-field directed along the temperature gradient (or normal to any surface of dielectric discontinuity). One hot spot remains unaccounted for. The average warming rates and power dissipation are related to the volumetric heat capacity of the phantom material at -30 degrees C.

Design of a UHF applicator for rewarming of cryopreserved biomaterials

The dielectric properties of cryopreserved biological tissue are discussed in relation to the problems which arise when EM fields are used for rapid rewarming. The UHF band is favored from two aspects: the avoidance of thermal runaway and the uniformity of heating inhomogeneous material. Various resonant cavity applicators are considered for efficient and uniform rewarming. The square-aspect TE 111 cylindrical applicator is favored principally because it allows variation of the E-field orientation as required during the warming profile. An appropriate kidney phantom organ is described. It is used to obtain measured values of the overall efficiency of a TE 111 applicator. The efficiency values are found to fall steadily with increasing temperature from 85% at -40 degrees C to 45% at the phase change, mainly due to the decreasing tan delta value of the phantom material.

Preservation of rabbit kidneys using a solution containing hydrolyzed starch

An organ preservation solution has been developed by combining some features of the hypertonic citrate formulation of Ross, Marshall, and Escott (RME) with some features of UW solution. Specifically the solution (HP16) contains a balance of cations similar to that in RME and the same concentration of citrate, but sulfate is replaced by chloride and mannitol by a starch hydrolysis product (SHP). A gelatin-derived polypeptide (Haemaccel) is included to provide colloid osmotic pressure. The objective was to increase the effectiveness of RME by using a higher-molecular-weight osmoticum than mannitol, but avoiding the expense of raffinose; reducing the osmolality to a more physiological level; and including a colloid to make the solution suitable for continuous perfusion. The effectiveness of the solution was tested by 48-hr hypothermic preservation of rabbit kidneys. The results were compared with those obtained using RME or UW. It was shown that simple hypothermic storage was more effective than continuous perfusion, and that HP16 was more effective than RME and as effective as UW. The improvement over RME was ascribed to the isotonic osmolality and the inclusion of a higher-molecular-weight osmoticum (the SHP), possibly supplemented by the colloid (Haemaccel). Two SHP preparations, both with dextrose-equivalent values of approximately 35, were equally effective. These materials contain a standardized mixture of dextrose, maltose, and tri- and oligosaccharides, and have the osmotic properties of a trisaccharide. The results provide a new, inexpensive preservation solution that is as effective as any so far tested with this model, and they support the importance of appropriate osmotic properties for solutions to be used in organ preservation.

Thermal property measurements on biological materials at subzero temperatures

The self-heated thermistor technique was used to measure the thermal conductivity and thermal diffusivity of biomaterials at low temperatures. Thermal standards were selected to calibrate the system at temperatures from -10 degrees C to -70 degrees C. The thermal probes were constructed with a convection barrier which eliminates convection inside liquid samples of low viscosity, without affecting the conductivity and diffusivity results. Using this technique, the thermal conductivity and diffusivity of two organ perfusates (HP5 and HP5 + 2M glycerol), one kidney phantom (a low ionic strength gel), as well as rabbit kidney cortex have been measured from -10 degrees C to -70 degrees C.

The effect of initial tonicity on freeze/thaw injury to human red cells suspended in solutions of sodium chloride

Human red blood cells, suspended in solutions of sodium chloride, have been frozen to temperatures between -2 and -14 degrees C and thawed, and the extent of hemolysis was measured. In parallel experiments, red cells were exposed to similar cycles of change in the composition of the suspending solution, but by dialysis at 21 degrees C. The tonicity of the saline in which the cells were initially suspended was varied between 0.6x isotonic and 4x isotonic; some samples from each experimental treatment were returned to isotonic saline before hemolysis was measured. It was found that the tonicity of the saline used to suspend the cells for the main body of the experiment affected the amount of hemolysis measured: raising the tonicity from 0.6x to 1x to 2x reduced hemolysis, both in the freezing and in the dialysis experiments, whereas raising the tonicity further to 4x reversed that trend. There was little difference between the freeze/thaw and the dialysis treatments for the cells suspended in 1x or 2x saline, whether or not the cells were returned to isotonic conditions. However, the cells suspended in 0.6x saline showed greater damage from freezing and thawing than from the comparable change in the composition of the solution, whether or not they were returned to isotonic conditions. Cells that were suspended in 4x saline and exposed to changes in salt concentration by dialysis showed less hemolysis when they were assayed in the 4x solution than cells that had received the comparable freezing/thaw treatment, but when the experiment included a return to isotonicity, the two treatments gave similar results. Returning the cells to isotonic saline had a negligible affect on the cells in 0.6x and 1x saline, but caused considerable hemolysis in the 2x and 4x samples, more so after dialysis than after freezing and thawing. We conclude that cells suspended in 0.6x and 4x saline behave differently from cells suspended in 1x and 2x saline and hence that cells suspended in a range of solutions of differing initial tonicity should not be treated as a homogeneous population. We argue that an effect of the unfrozen fraction of water (U) cannot be distinguished, within the framework of these freeze/thaw experiments alone, from an effect of initial tonicity, and that the biphasic nature of the correlation between haemolysis and U makes a causal connection improbable.(ABSTRACT TRUNCATED AT 400 WORDS)

Hypothermic preservation of rabbit kidneys for 48 hours using low ionic strength solutions

Microwaves offer the prospect of rapid and uniform heating of frozen organs. This is significant in the context of cryopreservation, and particularly of vitrification, because microwave heating may help to avoid crystallization or recrystallization of ice during warming, minimize any effects of high cell density, and reduce thermal-mechanical stresses. Previous work has established a rationale for reducing the ionic strength of solutions used to prepare tissues for microwave heating, since this permits the use of lower frequencies, which makes heating more uniform, without increasing the risk of thermal runaway (T. P. Marsland, S. Evans, and D. E. Pegg, Cryobiology 24, 311-323, 1987). In this paper we report a study of two possible low ionic strength perfusates, in rabbit kidneys, using 48 hr of hypothermic storage and autotransplantation as the test system. This model was chosen because there is a great deal of basic information about it. Both a single-pass "flush" preservation solution and a solution designed for continuous perfusion gave excellent results. The continuous perfusion system, which would be the more suitable for introducing cryoprotectants, gave five of five surviving animals with peak serum creatinine levels of 353-555 mumol/liter normal histology in three cases, and only very minor damage in the other two. There would therefore seem to be no obstacle to the use of perfusates having a low ionic strength in renal cryopreservations studies.

Effects of cooling rate and glycerol concentration on the structure of the frozen kidney: assessment by cryo-scanning electron microscopy

An experimental technique, employing a directional solidification stage for controlled freezing of tissue samples and low-temperature scanning electron microscopy for observation of the structure of the frozen-hydrated samples, was used to study freezing processes in the kidney. Parametric studies in which the cooling rate during freezing and the concentration of glycerol in the tissue were varied confirmed the results of earlier freeze-substitution studies. The results suggest a mechanism for ice propagation in the kidney similar to that already proposed for the liver, in which ice originates in, and is subsequently propagated through, the peritubular vasculature. The ice front dehydrates the cells and tubular structures encountered in its path, thus preventing intraluminal freezing. At higher rates of cooling and increased concentrations of glycerol there is less dehydration of cortical structure and intraluminal freezing occurs.

Permeation of glycerol and propane-1,2-diol into human platelets

The permeability of human platelets to glycerol (GLY) and propane-1,2-diol (propylene glycol, PG) has been determined by measuring the time course of their change in volume following abrupt immersion in solutions of these solutes. A simple light-scattering method, and its calibration to measure mean platelet volume is described. The data are analyzed by means of the Kedem-Katchalsky (K-K) equations, modified to take into account the nonideal behavior of both intracellular and extracellular solutes. The values of the K-K parameters at 2, 21, and 37 degrees C, respectively, were as follows: the hydraulic conductivities (Lp) were 1 x 10(-7), 7 x 10(-7) and 3 x 10(-6) cm.sec-1.atm-1; the solute permeabilities for PG (omega RTPG) were 1.9 x 10(-6), 2.8 x 10(-5), and 1.3 x 10(-4) cm.sec-1; the solute permeabilities for GLY (omega RTGLY), at 21 and 37 degrees C only, were 2.6 x 10(-7) and 1.4 x 10(-6) cm.sec-1. The reflection coefficient (sigma) was 1 throughout. The relevant activation energies were -Lp, 16.5 kcal.mol-1; omega RTPG, 20.5 kcal.mol-1; and omega RTGLY, 17.9 kcal.mol-1. The use of these data is illustrated by computing schedules for the addition and removal of GLY and PG so that the amplitudes of changes in platelet volume are held within predetermined limits.

Cryopreservation of human platelets with propane-1,2-diol

The preceding papers in this series have described techniques that permit the introduction and removal of propane-1,2-diol (propylene glycol, PG) with human platelets, in concentrations up to 2 M, without producing serious damage. These methods have now been used in attempts to cryopreserve platelets, with assessment of survival by the hypotonic stress response and ADP-induced aggregation. PG concentrations of 0.5, 1.0, 2.0, and 2.5 M and cooling rates between 0.4 and 100 degrees C/min were studied. The maximum response in the hypotonic stress test was no better than 17% and the greatest ADP-induced aggregation was only 6%; these results were obtained with 0.5 M PG, a cooling rate of 14 degrees C/min, and rapid warming (approximately 150 degrees C/min). The failure of PG concentrations greater than 0.5 M to improve survival was unexpected. When cooling was interrupted at progressively lower temperatures and function assessed, it was possible to relate the extent of damage to temperature and then, with the aid of phase diagrams, it was possible to show that, irrespective of the initial concentration of PG, the extent of damage was closely correlated with the concentration of PG produced at the minimum temperature used. It is concluded that the toxicity of PG increases so steeply with the increasing concentration produced by the separation of ice during freezing that this effect is sufficient to counteract the cryoprotective action of this solute for platelets.

Some effects of propane-1,2-diol on human platelets

The Kedem-Katchalsky equations and permeability data previously reported (F. G. Arnaud and D. E. Pegg. Permeation of glycerol and propane-1,2-diol into human platelets. Cryobiology 27, 130-136, 1990) have been used to design methods for adding and removing propane-1,2-diol (propylene glycol, PG) with human platelets. Mean platelet volume was kept within the tolerated range of 60 to 120% of normal. PG concentrations of 0.5, 1.0, 2.0, 2.5, and 3.0 M were studied at 2, 21, and 37 degrees C. PG was removed only at 21 degrees C. The effects of concentration of PG, temperature, and duration of exposure on the hypotonic stress response and ADP-induced aggregation were measured. It was found that platelets would tolerate exposure to PG concentrations up to 2 M at 21 or 2 degrees C for up to 15 min. The extent of damage increased considerably at higher temperatures and concentrations. These data provide the necessary basis for experiments to cryopreserve platelets with PG.

Cryopreservation of human platelets with 1.4 m glycerol at -75 degrees C in PVC blood packs

We have previously described a laboratory technique for the cryopreservation of human platelets with 1.4 molar glycerol. For the present study we adapted that method for use in clinical blood banks, and we describe the results of in vitro assays of platelets preserved in PVC packs, with storage at -75 degrees C for 1 month. The results suggested that substitution of the PVC pack for standard, 4 ml polypropylene freezing vials had little effect, but substitution of processing (addition and subsequent removal of glycerol) in PVC packs for processing in polystyrene containers resulted in a marked reduction in recovery. The final results, where the entire process was carried out in PVC packs was not satisfactory; only approximately 50% of the platelets were recovered and they exhibited only approximately 30% hypotonic stress response and approximately 3% aggregation response to ADP. We conclude that -75 degrees C may be too high a storage temperature, or that the specific blood packs that we used may be unsuitable for this process, or both.

The mechanism of action of retrograde oxygen persufflation in renal preservation

Vascular perfusion of gaseous oxygen has been used to prolong the in vitro survival of a number of isolated organs, and has been shown to improve the hypothermic preservation of ischemically injured kidneys that were subsequently transplanted. We have investigated the mechanism of this effect. Rabbit kidneys were subjected to 60 min of warm ischemia prior to preservation for 24 hr with Ross, Marshall, and Escott's hypertonic citrate solution, with or without retrograde oxygen persufflation (ROP) via the renal vein. It was found that adenine nucleotide levels were almost doubled in the ROP-preserved kidneys, principally due to higher adenosine triphosphate (ATP) concentrations. It was shown that cytochrome oxidase activity was unaffected by ischemia or preservation method, but studies with the metabolic inhibitors ouabain and a mixture of cyanide and iodoacetate suggested that ATP was being synthesized during the storage period but was also being utilized to power the active volume-regulating pump. Morphological examination revealed a much greater degree of cell swelling and cytological injury in the kidneys not subject to ROP, and the interstitial space appeared much reduced in the latter group. At the ultrastructural level, the ROP-treated kidneys showed generally well-preserved mitochondria, mostly in the energized "orthodox" configuration. In contrast, the mitochondria in the nonpersufflated kidneys were generally in the "condensed" deenergized state. We conclude that the provision of sufficient oxygen by ROP allows the continued production of ATP in sufficient quantities to permit improved maintenance of cellular volume and morphology under the conditions of low-temperature storage that we have studied.