Evaluation of cyropreserved internal thoracic artery as an alternative coronary graft: evidence for preserved functional, metabolic and structural integrity

Effects of cryo-preservation on skeletal muscle tissues mechanical behavior under tensile and peeling tests until rupture

The most common method to study the mechanical behavior of soft tissue is to test animal specimens, which should be prepared as soon as possible after the death to avoid biological deterioration effects such as rigor mortis. Freezing and cryo-preservation could allow extending the time between procurement and implantation. From a mechanical perspective, tissue preservation could influence mechanical testing results. Therefore, this study focuses on the influence of cryo-preserved samples on their mechanical behavior, especially at the rupture. In order to analyze this aspect, two tests were performed on the porcine abdominal wall. A tensile test to study the elastic behavior of samples and the tensile strength until rupture. A peeling test to more finely investigate the cohesion between muscle fibers. No statistical difference could be observed following tensile test. However, peeling tests between cryo-preserved and control samples showed a clear statistical difference with a p-value of 0.0097 for G_p. Indeed, energy release rate was higher for the Cryo-preserve group than the Control group with G_p = 0.36 ± 0.07 N/mm vs 0.26 ± 0.10 N/mm. This difference suggests that the characterization of rupture energies for muscular tissue should be done without having frozen the samples, even with a cryopreservative agent. These results could also indicate that even if the rupture mode is the same between mechanical tests, a different rupture direction could imply different mechanical preservations for soft tissues. This study could help to understand the difficult mechanical preservation of soft tissues, especially on the rupture behavior. Future studies on skeletal muscles will be necessary to compare our results, especially in peeling.

Patency and structural changes in cryopreserved arterial grafts used as vessel substitutes in the rat

OBJECTIVE

To evaluate the patency and structural changes that occur in the short- and mid-term when cryopreserved syngenic arterial grafts are implanted in an experimental animal model.

MATERIAL AND METHODS

Segments of iliac artery from the Spraque-Dawley rat were cryopreserved in a biological freezer according a controlled, computerized freezing protocol whereby the specimens are cooled at a rate of 1 degrees C/min. After storage at -145 degrees C in liquid N2 vapor for 30 days, the cryografts were slowly thawed. These vessels were grafted to the common iliac artery in syngenic animals. The following study groups were established: group I (GI), non-implanted cryografts; group II (GII), autografts; and group III (GIII), cryoisografts. The control group (CG) was comprised of fresh iliac arteries. The animals were sacrificed 14, 30, or 90 days post-surgery. At each of these follow-up times, graft specimens were morphologically evaluated by light and scanning and transmission electron microscopy and immunolabeling of endothelial cells (vWf). Cell damage attributed to the cryopreservation or grafting process was also determined.

RESULTS

At the time of sacrifice, graft patency was 100% for the autografts, while 26.6% of the cryoisografts showed fully occlusive thrombosis. Among other complications, two pseudoaneurysms were detected. After cryopreservation, the grafts (GI) showed patches of endothelial denudation and good cellularity of the medial layer. The intimal hyperplasia observed in autografts implanted for 14 days (GII) was significantly delayed until day 30 when the graft was cryopreserved (GIII). Cryoisografts showed general thinning of the arterial wall and degeneration accompanied by medial layer cell loss. These grafts showed most cell damage at 90 days post-implant. Expression of the vWf in all specimens showing intimal hyperplasia was confined to the outermost graft layer.

CONCLUSIONS

Cryopreservation modified the reparative response of the grafts. Owing to faster degeneration of the medial layer and a delay in the appearance of intimal hyperplasia, arterial wall thickness was reduced relative to that of the non-cryopreserved autografts. This thinning, at least in the short-term (90 days), does not seem to give rise to aneurysms owing to the generation of a neointima that stabilizes the vessel wall.

Long-term Behaviour of Cryopreserved Arterial Grafts Versus Prosthetic Micrografts

INTRODUCTION

When a patient has no suitable vessels for use as grafts in bypass or reconstruction procedures, two of the options available are the use of a cryopreserved vessel or an expanded polytetrafluoroethylene (ePTFE) prosthesis. This study was designed to compare the long-term behaviour of these vascular substitutes.

MATERIAL AND METHODS

We established three study groups by grafting the following vessel substitutes to the iliac artery in Spraque-Dawley rats: arterial autografts (GI, n=12), cryopreserved syngenic arterial grafts (cryoisografts) (GII, n=12), and ePTFE micrografts (GIII, n=12). The animals were sacrificed 180 days after surgery, at which time the graft specimens were morphologically evaluated by light and electron microscopy, immunolabelling (ED1/alpha-actin) and morphometric analysis of the neointima.

RESULTS

At the time of sacrifice, graft patency was 100% for the autografts and cryoisografts, while 10% of the ePTFE micrografts showed fully-occlusive thrombosis. Intimal hyperplasia was observed in grafts in GI and GII; the neointima being thinner in the cryoisografts (54.36 +/- 2.26 microm) than the autografts (161.30 +/- 3.91 microm). The endothelium formed over the prosthetic micrografts was unstable, with areas of subendothelial thickening (9.37 +/- 3.18 microm). Cell loss and medial layer degeneration were observed in both GI and GII specimens, while the GIII grafts were colonised by cells on their luminal surface.

CONCLUSIONS

All three grafts show good long-term tolerance when used in an arterial setting. Following long-term implant, autografts and cryoisografts show similar alterations that give rise to the complete loss of the muscle component of the tunica media along with the formation of a stable neointima. This new layer takes on the role of the tunica media.

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.

Rapid thawing increases the fragility of the cryopreserved arterial wall

OBJECTIVE

To extend present knowledge of the biomechanical and structural changes which occur in the cryopreserved, rapidly thawed arterial wall.

MATERIALS AND METHODS

Minipig iliac arterial segments were cryopreserved at -196 degrees C in either minimum essential medium or Wisconsin solution. Fresh segments served as the control group. After 1 month, the specimens were rapidly thawed (37 degrees C) and processed for biomechanical, ultrastructural, morphological and immunohistochemical (MMP-1, MMP-2, MMP-3 and MMP-9) analysis. Visualisation of apoptotic cells was performed by TUNEL method. For the mechanical distension analysis, an in vitro circuit was designed.

RESULTS

The cryopreserved segments showed a 42% incidence of spontaneous fracture and the appearance of microfractures which affected the endoluminal third of the vessel. An accumulation of liquid in the subelastica was observed. An increased expression of wall-degradative enzymes (mainly MMP-2) was also observed following cryopreservation. No significant differences were detected in the proportional elasticity module or tensile strength of the specimen groups. No differences in mechanical distension were observed between groups after the vessel segments were subjected to the pulsatile circuit flow for 72 h. Cell damage was most intense in the specimens cryopreserved in Wisconsin solution.

CONCLUSIONS

Cryopreservation in both the solutions employed, followed by rapid thawing, induce changes in the permeability which increase the fragility of the cryopreserved arterial wall. Both increased expression of wall-degradative enzymes and accumulation of liquid may contribute to graft failure after implantation.

Changes in the cooling rate and medium improve the vascular function in cryopreserved porcine femoral arteries

PURPOSE

The purpose of this study was to design an adequate technique with which to cryopreserve pig femoral arteries and to assess the influence of storage times in vascular function.

METHODS

Fifty-two femoral arteries were distributed in seven groups. In group A (control), 10 arteries were studied after harvest; in groups B1 and B2, 19 arteries were suspended in RPMI 1640 plus fetal calf serum plus dimethylsulfoxide and were cryopreserved at 1 degrees C per minute or 0.3 degrees C per minute, respectively. In groups C1 to C4, 23 arteries were suspended in modified Krebs-Henseleit plus dimethylsulfoxide plus sucrose, cryopreserved at 0.7 degrees C per minute, and kept frozen for 1, 15, 60, or 180 days, respectively. After being thawed, arteries were examined for contraction and endothelial-dependent vasodilation (organ bath studies), antithrombotic properties of the endothelial layer(perfusion studies), and vessel structure (electron microscopy).

RESULTS

Endothelial cells were present in both cryopreserved and control arteries. The control vessels showed a mean contraction to norepinephrine (10(-7) mol/L) of 13010 +/- 3181 mg. Arteries in groups B1 and B2 did not respond to norepinephrine. Contraction in groups C1 to C4 was as follows: C1, 5354 +/- 1222 mg; C2, 5187 +/- 2672 mg; C3, 6867 +/- 2292 mg; C4, 7000 +/- 2858 mg, which represent 50% of the control values (P <.001). Vasodilation was similar in control (99% +/- 3%) and cryopreserved arteries (C1, 90% +/- 13%; C2, 93% +/- 12%; C3, 89% +/- 15%; C4, 88% +/- 22%). Storage time did not influence vascular function. Platelet interaction was almost absent and similar in all groups.

CONCLUSION

A modified cryopreservation technique preserves endothelial function independently of the storage time up to 6 months.