A simplified technique for the cryopreservation of vein allografts

Engineered Test Tissues: A Model for Quantifying the Effects of Cryopreservation Parameters

Engineered tissues are showing promise as implants to repair or replace damaged tissues in vivo or as in vitro tools to discover new therapies. A major challenge of the tissue engineering field is the sample preservation and storage until their transport and desired use. To successfully cryopreserve tissue, its viability, structure, and function must be retained post-thaw. The outcome of cryopreservation is impacted by several parameters, including the cryopreserving agent (CPA) utilized, the cooling rate, and the storage temperature. Although a number of CPAs are commercially available for cell cryopreservation, there are few CPAs designed specifically for tissue cryostorage and recovery. In this study, we present a flexible, relatively high-throughput method that utilizes engineered tissue rings as test tissues for screening the commercially available CPAs and cryopreservation parameters. Engineered test tissues can be fabricated with low batch-to-batch variability and characteristic morphology due to their endogenous extracellular matrix, and they have mechanical properties and a ring format suitable for testing with standard methods. The tissues were grown for 7 days in standard 48-well plates and cryopreserved in standard cryovials. The method allowed for the quantification of metabolic recovery, tissue apoptosis/necrosis, morphology, and mechanical properties. In addition to establishing the method, we tested different CPA formulations, freezing rates, and freezing points. Our proposed method enables timely preliminary screening of CPA formulations and cryopreservation parameters that may improve the storage of engineered tissues.

Inhibiting connexin channels protects against cryopreservation-induced cell death in human blood vessels

OBJECTIVES

Cryopreserved blood vessels are being increasingly employed in vascular reconstruction procedures but freezing/thawing is associated with significant cell death that may lead to graft failure. Vascular cells express connexin proteins that form gap junction channels and hemichannels. Gap junction channels directly connect the cytoplasm of adjacent cells and may facilitate the passage of cell death messengers leading to bystander cell death. Two hemichannels form a gap junction channel but these channels are also present as free non-connected hemichannels. Hemichannels are normally closed but may open under stressful conditions and thereby promote cell death. We here investigated whether blocking gap junctions and hemichannels could prevent cell death after cryopreservation.

MATERIALS AND METHODS

Inclusion of Gap27, a connexin channel inhibitory peptide, during cryopreservation and thawing of human saphenous veins and femoral arteries was evaluated by terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) assays and histological examination.

RESULTS

We report that Gap27 significantly reduces cell death in human femoral arteries and saphenous veins when present during cryopreservation/thawing. In particular, smooth muscle cell death was reduced by 73% in arteries and 71% in veins, while endothelial cell death was reduced by 32% in arteries and 51% in veins.

CONCLUSIONS

We conclude that inhibiting connexin channels during cryopreservation strongly promotes vascular cell viability.

Activated Rho/Rho kinase and modified calcium sensitivity in cryopreserved human saphenous veins

BACKGROUND

We have shown previously that cryopreservation of human internal mammary arteries activates protein kinase C and enhances intracellular Ca(2+) [Ca(2+)](i). We now present evidence that in human saphenous veins (HSV) cryoinjury is associated with activation of the Rho/Rho kinase signaling pathways and enhanced [Ca(2+)](i).

METHODS

HSV were investigated in vitro either unfrozen within 12h after removal or after storage at -196 degrees C in a cryomedium containing 1.8M dimethyl sulfoxide and 0.1M sucrose as cryoprotectant additives.

RESULTS

Cryostorage diminished responses to receptor-mediated contractile agonists such as noradrenaline, 5-HT and endothelin-1 by up to 30% whereas responses to KCl were attenuated by about 50%. Concentration-response curves for CaCl(2) on unfrozen and cryopreserved HSV revealed similar inhibitory activities of both blocking 1,4-dihydropyridine derivatives nifedipine and the (-)-(R) enantiomer of SDZ 202-791 whereas the Ca(2+) channel activating (+)-(S) enantiomer of SDZ 202-791 was 10 times less effective at enhancing contractions to CaCl(2) when tested after cryostorage. These functional effects were reflected by changes in [Ca(2+)](i) as demonstrated by fluorescence of Fluo-3AM loaded veins. The diminished activity of (+)-(S) SDZ 202-791 in cryopreserved HSV was reversed partially when the potassium channel opener pinacidil (1 microM) was present during the freezing/thawing process. Blockade of Rho kinase by HA-1077 proved to be significantly more effective at attenuating contractile responses to both endothelin-1 and KCl after cryostorage.

CONCLUSIONS

Data suggested that cryopreservation modified [Ca(2+)](i) of venous smooth muscle cells (1) through depolarization-induced changes in Ca(2+) influx and (2) through activation of Rho kinase signaling pathways.

Impact of freezing/thawing procedures on the post-thaw viability of cryopreserved human saphenous vein conduits

BACKGROUND

Cryopreserved human blood vessels are important tools in reconstructive surgery. However, patency of frozen/thawed conduits depends largely on the freezing/thawing procedures employed.

METHODS

Changes in tone were recorded on rings from human saphenous vein (SV) and used to quantify the degree of cryoinjury after different periods of exposure at room temperature to the cryomedium (Krebs-Henseleit solution containing 1.8M dimethyl sulfoxide and 0.1M sucrose) and after different cooling speeds and thawing rates following storage at -196 degrees C.

RESULTS

Without freezing, exposure of SV to the cryomedium for up to 240 min did not modify contractile responses to noradrenaline (NA). Pre-freezing exposure to the cryomedium for 10-120 min attenuated significantly post-thaw maximal contractile responses to NA, endothelin-1 (ET-1) and potassium chloride (KCl) by 30-44%. Exposure for 240 min attenuated post-thaw contractile responses to all tested agents markedly by 62-67%. Optimal post-thaw contractile activity was obtained with SV frozen at about -1.2 degrees C/min and thawed slowly at about 15 degrees C/min. In these SV maximal contractile responses to NA, ET-1 and KCl amounted to 66%, 70% and 60% of that produced by unfrozen controls. Following cryostorage of veins for up to 10 years the responsiveness of vascular smooth muscle to NA was well maintained.

CONCLUSION

Cryopreservation allows long-term banking of viable human SV with only minor loss in contractility.

Quantitative reduction of methyl tetrazolium by fresh vein homograft biopsies in vitro is an index of viability

OBJECTIVE

The color density of the methyl tetrazolium (MTT) test is proportional to mitochondrial enzyme activity thus reflecting cellular viability. The aim was to evaluate the MTT test as a viability assay for vein homograft studies.

MATERIALS AND METHODS

Fresh intact vein samples were harvested during multi-organ procurement. The reliability of the MTT assay was tested by a fluorescent dye combination (1 microg/ml propidium iodide PI and 4 microM/ml SYTO-16 stains). The enzyme kinetics of the reaction was also investigated. The optimal reagent concentration, biopsy size and incubation period were established.

RESULTS

There was a linear relationship between the vein homograft's weight and the pigment production activity. A nonspecific reaction (8.6%) was observed in negative controls. The MTT cleavage up to 0.1% (w/v) follows the Michaelis kinetics. The Michaelis constant (2,805 +/- 130 microM), the maximal velocity (196 +/- 2.2 x 10(-5 )microM s(-1)) and the velocity constant (6.98 +/- 0.2 x 10(-7) s(-1)) was calculated. The viability assessed by fluorescent dyes simultaneously visualized the live/dead cell ratio, which can be calculated by image analysis software.

CONCLUSION

The use of MTT in colorimetric assays offers high sensitivity. The assay is simple, inexpensive, and reproducible in vein homograft studies.

Preservation of Vein Allograft Viability during Long-Term Storage

BACKGROUND

The aim of this study was to compare the vein allograft viability following cryopreservation with that remaining after prolonged refrigerated storage.

MATERIALS AND METHODS

Great saphenous vein biopsies had been cryopreserved, and the samples were divided into two matched groups and stored in tissue culture medium for 42 days at +4 degrees C, either with or without regular medium replacement. Each vein allograft was biopsied and assayed for viability every third day by the methyltetrazolium reduction assay. Viability indexes of vein allografts harvested from brain-dead multi-organ donors and from cadavers whose warm ischemic periods were maximally 24 h were also compared.

RESULTS

Vein allografts stored for 42 days at +4 degrees C showed a similar viability (58.9 +/- 1.2%) to that of cryopreserved veins (59.7 +/- 2.3%). This was true even when cryopreserved and thawed allografts were subjected to 3 days of post-thaw incubation under presumably favorable conditions (58.7 +/- 1.6%). There was no viability index difference between the samples with medium replaced and not replaced and the specimens harvested from two different donor groups.

CONCLUSIONS

Long-term storage of vein allografts at +4 degrees C is a valuable option for regular banking practice. Sufficient amounts can be procured from cadavers similar to tissue donors.

Effects of green tea polyphenol on preservation of human saphenous vein

The potential role of green tea polyphenol (GtPP) in preserving the human saphenous vein was investigated under physiological conditions. The vein segments were incubated for 1, 3, 5, 7 and 14 days, either after 4h of treatment with 1.0mg/ml GtPP or in the presence of GtPP at the same concentration. After incubation, the endothelial cell viability, endothelial nitric oxide synthase (eNOS) expression and the vein histology were evaluated. When the veins were not treated with GtPP, the viability of the endothelial cells was significantly reduced with the progress in the culture time, and none of the cells expressed eNOS after 5 days. Furthermore, severe histological changes and structural damage were observed in the non-treated veins. In contrast, incubating the veins after 4h of GtPP treatment significantly prevented these phenomena. The cellular viability of the GtPP-treated vein was approximately 64% after 7 days, and eNOS expression was maintained up to 40%, compared to that of the fresh vein. The histological observations showed that the vasculature was quite similar to that of the fresh vein. When incubated with GtPP, the vein could also be preserved for 1 week under physiological conditions retaining both its cellular viability (61%) and eNOS expression level (45%) and maintaining its venous structure without any morphological changes. These results demonstrate that GtPP treatment may be a useful method for preserving the HSV.

Effect of the thawing process on cryopreserved arteries

This study was designed to explore the changes that occur in cryopreserved pig arteries following different thawing procedures, before and after being placed in an in vitro flow circuit. Segments of minipig iliac artery were cryopreserved in complete minimal essential medium plus 10% dimethylsulphoxide and stored in liquid nitrogen at -196 degrees C for 30 days. Three study groups were established according to whether the arterial specimens were fresh (control, n = 20), cryopreserved and rapidly thawed (RT) at 37 degrees C (n = 22) or cryopreserved and subjected to controlled, automated slow thawing (ST) (n = 22). Half of the specimens of each group were subsequently placed in the flow circuit for 72 hr. Evaluation was made of morphological and ultrastructural changes. Cell damage was established using the TUNEL method. All cryopreserved specimens showed endothelial denudation that was most extensive in those subjected to rapid thawing. Slowly thawed specimens showed improved cell viability and organization of the vessel wall, compared to those thawed rapidly. Under conditions of flow, the damage induced by the freezing/thawing process was enhanced. These findings suggest that (a) slow thawing of cryopreserved arteries results in improved preservation of the structure and viability of vessels, and (b) the damage induced by freezing/thawing is enhanced when vessels are subjected to flow in an in vitro circuit.

Gradual thawing improves the preservation of cryopreserved arteries

This study was designed to test a slow, controlled, automated process for the thawing of cryopreserved arteries, whereby specimen warming is synchronized with the warming of its environment. Segments of minipig iliac artery, 4-5 cm in length, were subjected to controlled, automated cryopreservation in a biological freezer at a cooling rate of 1 degrees C/min to -120 degrees C, followed by storage in liquid nitrogen at -196 degrees C for 30 days. Following storage, the arterial segments were subjected to rapid (warming rate of approximately 100 degrees C/min) or gradual (1 degrees C/min) thawing. Thawed specimens were processed for light microscopy and for scanning and transmission electron microscopy, Cell death was determined by the TUNEL method. Metalloproteinase (MMP) expression was estimated by immunohistochemical analysis. Most of the cryopreserved vessels subjected to rapid thawing showed spontaneous fractures, mainly microfractures, whereas these were absent in slowly thawed specimens. In rapidly thawed vessels, the proportion of damaged cells was double that observed in those thawed more gradually. Increased intensity and extent of MMP-2 expression was shown by rapidly thawed specimens. The slow-thawing protocol tested avoids the formation of spontaneous fractures and microfractures and the accumulation of fluid within the arterial wall tissue. This results in improved tissue preservation.