Transformation of nonvascular acellular tissue matrices into durable vascular conduits

Mechanical characterization of porcine ureter for the evaluation of tissue-engineering applications

Introduction: Clinics increasingly require readily deployable tubular substitutes to restore the functionality of structures like ureters and blood vessels. Despite extensive exploration of various materials, both synthetic and biological, the optimal solution remains elusive. Drawing on abundant literature experiences, there is a pressing demand for a substitute that not only emulates native tissue by providing requisite signals and growth factors but also exhibits appropriate mechanical resilience and behaviour. Methods: This study aims to assess the potential of porcine ureters by characterizing their biomechanical properties in their native configuration through ring and membrane flexion tests. In order to assess the tissue morphology before and after mechanical tests and the eventual alteration of tissue microstructure that would be inserted in material constitutive description, histological staining was performed on samples. Corresponding computational analyses were performed to mimic the experimental campaign to identify the constitutive material parameters. Results: The absence of any damages to muscle and collagen fibres, which only compacted after mechanical tests, was demonstrated. The experimental tests (ring and membrane flexion tests) showed non-linearity for material and geometry and the viscoelastic behaviour of the native porcine ureter. Computational models were descriptive of the mechanical behaviour ureteral tissue, and the material model feasible. Discussion: This analysis will be useful for future comparison with decellularized tissue for the evaluation of the aggression of cell removal and its effect on microstructure. The computational model could lay the basis for a reliable tool for the prediction of solicitation in the case of tubular substitutions in subsequent simulations.

Cardiac aorta-derived extracellular matrix scaffold enhances critical mediators of angiogenesis in isoproterenol-induced myocardial infarction mice

An incapability to improve lost cardiac muscle caused by acute ischemic injury remains the most important deficiency of current treatments to prevent heart failure. We investigated whether cardiomyocytes culturing on cardiac aorta-derived extracellular matrix scaffold has advantageous effects on cardiomyocytes survival and angiogenesis biomarkers' expression. Ten male NMRI mice were randomly divided into two groups: (1) control (healthy mice) and (2) myocardial infarction (MI)-induced model group (Isoproterenol/subcutaneously injection/single dose of 85 mg/kg). Two days after isoproterenol injection, all animals were sacrificed to isolate cardiomyocytes from myocardium tissues. The fresh thoracic aorta was obtained from male NMRI mice and decellularized using 4% sodium deoxycholate and 2000 kU DNase-I treatments. Control and MI-derived cardiomyocytes were seeded on decellularized cardiac aorta (DCA) considered three-dimensional (3D) cultures. To compare, the isolated cardiomyocytes from control and MI groups were also cultured as a two-dimensional (2D) culture system for 14 days. The cell viability was examined by MTT assay. The expression levels of Hif-1α and VEGF genes and VEGFR1 protein were tested by real-time PCR and western blotting, respectively. Moreover, the amount of VEGF protein was evaluated in the conditional media of the 2D and 3D systems. The oxidative stress was assessed via MDA assay. Hif-1α and VEGF genes were downregulated in MI groups compared to controls. However, the resulting data showed that decellularized cardiac aorta matrices positively affect the expression of Hif-1α and VEGF genes. The expression level of VEGFR1 protein was significantly (p ≤ 0.01) upregulated in both MI and healthy cell groups cultured on decellularized cardiac aorta matrices as a 3D system compared to the MI cell group cultured in the 2D systems. Furthermore, MDA concentration significantly decreased in 3D-cultured cells (MI and healthy cell groups) rather than the 2D-cultured MI group (p ≤ 0.015). The findings suggest that cardiac aorta-derived extracellular scaffold by preserving VEGF, improving the cell viability, and stimulating angiogenesis via upregulating Hif-1α, VEGF, and VEGFR1 in cardiomyocytes could be considered as a potential approach along with another therapeutic method to reduce the complications of myocardial infarction and control the progressive pathological conditions related to MI.

The performance of heparin modified poly(ε-caprolactone) small diameter tissue engineering vascular graft in canine-A long-term pilot experiment in vivo

Long-term in vivo observation in large animal model is critical for evaluating the potential of small diameter tissue engineering vascular graft (SDTEVG) in clinical application, but is rarely reported. In this study, a SDTEVG is fabricated by the electrospinning of poly(ε-caprolactone) and subsequent heparin modification. SDTEVG is implanted into canine's abdominal aorta for 511 days in order to investigate its clinical feasibility. An active and robust remodeling process was characterized by a confluent endothelium, macrophage infiltrate, extracellular matrix deposition and remodeling on the explanted graft. The immunohistochemical and immunofluorescence analysis further exhibit the regeneration of endothelium and smooth muscle layer on tunica intima and tunica media, respectively. Thus, long-term follow-up reveals viable neovessel formation beyond graft degradation. Furthermore, the von Kossa staining exhibits no occurrence of calcification. However, although no TEVG failure or rupture happens during the follow-up, the aneurysm is found by both Doppler ultrasonic and gross observation. Consequently, as-prepared TEVG shows promising potential in vascular tissue engineering if it can be appropriately strengthened to prevent the occurrence of aneurysm.

Concise Review: Patency of Small-Diameter Tissue-Engineered Vascular Grafts: A Meta-Analysis of Preclinical Trials

Several patient groups undergoing small‐diameter (<6 mm) vessel bypass surgery have limited autologous vessels for use as grafts. Tissue‐engineered vascular grafts (TEVG) have been suggested as an alternative, but the ideal TEVG remains to be generated, and a systematic overview and meta‐analysis of clinically relevant studies is lacking. We systematically searched PubMed and Embase databases for (pre)clinical trials and identified three clinical and 68 preclinical trials ([>rabbit]; 873 TEVGs) meeting the inclusion criteria. Preclinical trials represented low to medium risk of bias, and binary logistic regression revealed that patency was significantly affected by recellularization, TEVG length, TEVG diameter, surface modification, and preconditioning. In contrast, scaffold types were less important. The patency was 63.5%, 89%, and 100% for TEVGs with a median diameter of 3 mm, 4 mm, and 5 mm, respectively. In the group of recellularized TEVGs, patency was not improved by using smooth muscle cells in addition to endothelial cells nor affected by the endothelial origin, but seems to benefit from a long‐term (46–240 hours) recellularization time. Finally, data showed that median TEVG length (5 cm) and median follow‐up (56 days) used in preclinical settings are relatively inadequate for direct clinical translation. In conclusion, our data imply that future studies should consider a TEVG design that at least includes endothelial recellularization and bioreactor preconditioning, and we suggest that more standard guidelines for testing and reporting TEVGs in large animals should be considered to enable interstudy comparisons and favor a robust and reproducible outcome as well as clinical translation.

In Vivo Performance of Decellularized Vascular Grafts: A Review Article

Due to poor vessel quality in patients with cardiovascular diseases, there has been an increased demand for small-diameter tissue-engineered blood vessels that can be used as replacement grafts in bypass surgery. Decellularization techniques to minimize cellular inflammation have been applied in tissue engineering research for the development of small-diameter vascular grafts. The biocompatibility of allogenic or xenogenic decellularized matrices has been evaluated in vitro and in vivo. Both short-term and long-term preclinical studies are crucial for evaluation of the in vivo performance of decellularized vascular grafts. This review offers insight into the various preclinical studies that have been performed using decellularized vascular grafts. Different strategies, such as surface-modified, recellularized, or hybrid vascular grafts, used to improve neoendothelialization and vascular wall remodeling, are also highlighted. This review provides information on the current status and the future development of decellularized vascular grafts.

Nucleotide Catabolism on the Surface of Aortic Valve Xenografts; Effects of Different Decellularization Strategies

Extracellular nucleotide metabolism controls thrombosis and inflammation and may affect degeneration and calcification of aortic valve prostheses. We evaluated the effect of different decellularization strategies on enzyme activities involved in extracellular nucleotide metabolism. Porcine valves were tested intact or decellularized either by detergent treatment or hypotonic lysis and nuclease digestion. The rates of ATP hydrolysis, AMP hydrolysis, and adenosine deamination were estimated by incubation of aorta or valve leaflet sections with substrates followed by HPLC analysis. We demonstrated relatively high activities of ecto-enzymes on porcine valve as compared to the aortic wall. Hypotonic lysis/nuclease digestion preserved >80 % of ATP and AMP hydrolytic activity but reduced adenosine deamination to <10 %. Detergent decellularization completely removed (<5 %) all these activities. These results demonstrate high intensity of extracellular nucleotide metabolism on valve surface and indicate that various valve decellularization techniques differently affect ecto-enzyme activities that could be important in the development of improved valve prostheses.

MicroRNAs in vascular tissue engineering and post-ischemic neovascularization

Increasing numbers of paediatric patients with congenital heart defects are surviving to adulthood, albeit with continuing clinical needs. Hence, there is still scope for revolutionary new strategies to correct vascular anatomical defects. Adult patients are also surviving longer with the adverse consequences of ischemic vascular disease, especially after acute coronary syndromes brought on by plaque erosion and rupture. Vascular tissue engineering and therapeutic angiogenesis provide new hope for these patients. Both approaches have shown promise in laboratory studies, but have not yet been able to deliver clear evidence of clinical success. More research into biomaterials, molecular medicine and cell and molecular therapies is necessary. This review article focuses on the new opportunities offered by targeting microRNAs for the improved production and greater empowerment of vascular cells for use in vascular tissue engineering or for increasing blood perfusion of ischemic tissues by amplifying the resident microvascular network.

Blood vessel-derived acellular matrix for vascular graft application

To overcome the issues connected to the use of autologous vascular grafts and artificial materials for reconstruction of small diameter (<6 mm) blood vessels, this study aimed to develop acellular matrix- (AM-) based vascular grafts. Rat iliac arteries were decellularized by a detergent-enzymatic treatment, whereas endothelial cells (ECs) were obtained through enzymatic digestion of rat skin followed by immunomagnetic separation of CD31-positive cells. Sixteen female Lewis rats (8 weeks old) received only AM or previously in vitro reendothelialized AM as abdominal aorta interposition grafts (about 1 cm). The detergent-enzymatic treatment completely removed the cellular part of vessels and both MHC class I and class II antigens. One month after surgery, the luminal surface of implanted AMs was partially covered by ECs and several platelets adhered in the areas lacking cell coverage. Intimal hyperplasia, already detected after 1 month, increased at 3 months. On the contrary, all grafts composed by AM and ECs were completely covered at 1 month and their structure was similar to that of native vessels at 3 months. Taken together, our findings show that prostheses composed of AM preseeded with ECs could be a promising approach for the replacement of blood vessels.

Vascular engineering for bypass surgery

Since the introduction of synthetic vascular grafts in the 1960s, only two-stage endothelial cell seeding has demonstrated any significant improvement over conventional vascular grafts, and its benefits have yet to be demonstrated on a large scale. Tissue engineering is a rapidly expanding field with great potential, but efforts to construct tissue-engineered arterial grafts have, to date, yielded little clinical success. This review explores the latest approaches to the construction of a superior vascular graft, along with its potential for use in the clinic in the future.

Tissue engineered vascular grafts--preclinical aspects

Tissue engineering enables the development of fully biological vascular substitutes that restore, maintain and improve tissue function in a manner identical to natural host tissue. However the development of the appropriate preclinical evaluation techniques for the generation of fully functional tissue-engineered vascular graft (TEVG) is required to establish their safety for use in clinical trials and to test clinical effectiveness. This review gives an insight on the various preclinical studies performed in the area of tissue engineered vascular grafts highlighting the different strategies used with respect to cells and scaffolds, typical animal models used and the major in vivo evaluation studies that have been carried out. The review emphasizes the combined effort of engineers, biologists and clinicians which can take this clinical research to new heights of regenerative therapy.

Development of endothelium-denuded human umbilical veins as living scaffolds for tissue-engineered small-calibre vascular grafts

Tissue-engineered small-calibre vessel grafts may help to alleviate the lack of graft material for coronary and peripheral bypass grafting in an increasing number of patients. This study explored the use of endothelium-denuded human umbilical veins (HUVs) as scaffolds for vascular tissue engineering in a perfusion bioreactor. Vessel diameter (1.2 ± 0.4 mm), wall thickness (0.38 ± 0.09 mm), uniaxial ultimate failure stress (8029 ± 1714 kPa) and burst pressure (48.4 ± 20.2 kPa, range 28.4-83.9 kPa) were determined in native samples. The effects of endothelium removal from HUVs by enzymatic digestion, hypotonic lysis and dehydration were assessed. Dehydration did not significantly affect contractile function, tetrazolium dye reduction, mechanical strength and vessel structure, whereas the other methods failed in at least one of these parameters. Denudation by dehydration retained laminin, fibronectin, collagen and elastic fibres. Denuded HUVs were seeded in a perfusion bioreactor with either allogeneic HUVs endothelial cells or with saphenous vein endothelial cells harvested from patients with coronary artery disease. Seeding in a perfusion bioreactor resulted in a confluent monolayer of endothelial cells from both sources, as judged by histology and scanning electron microscopy. Seeded cells contained von Willebrand factor and CD31. In conclusion, denuded HUVs should be considered an alternative to decellularized blood vessels, as the process keeps the smooth muscle layer intact and functional, retains proteins relevant for biomechanic properties and for cell attachment and provides a suitable scaffold for seeding an autologous and flow-resistant endothelium.

Tissue Engineering of Blood Vessels: Functional Requirements, Progress, and Future Challenges

Vascular disease results in the decreased utility and decreased availability of autologus vascular tissue for small diameter (< 6 mm) vessel replacements. While synthetic polymer alternatives to date have failed to meet the performance of autogenous conduits, tissue-engineered replacement vessels represent an ideal solution to this clinical problem. Ongoing progress requires combined approaches from biomaterials science, cell biology, and translational medicine to develop feasible solutions with the requisite mechanical support, a non-fouling surface for blood flow, and tissue regeneration. Over the past two decades interest in blood vessel tissue engineering has soared on a global scale, resulting in the first clinical implants of multiple technologies, steady progress with several other systems, and critical lessons-learned. This review will highlight the current inadequacies of autologus and synthetic grafts, the engineering requirements for implantation of tissue-engineered grafts, and the current status of tissue-engineered blood vessel research.

Nonthermal Irreversible Electroporation for Tissue Decellularization

Tissue scaffolding is a key component for tissue engineering, and the extracellular matrix (ECM) is nature’s ideal scaffold material. A conceptually different method is reported here for producing tissue scaffolds by decellularization of living tissues using nonthermal irreversible electroporation (NTIRE) pulsed electrical fields to cause nanoscale irreversible damage to the cell membrane in the targeted tissue while sparing the ECM and utilizing the body’s host response for decellularization. This study demonstrates that the method preserves the native tissue ECM and produces a scaffold that is functional and facilitates recellularization. A two-dimensional transient finite element solution of the Laplace and heat conduction equations was used to ensure that the electrical parameters used would not cause any thermal damage to the tissue scaffold. By performing NTIRE in vivo on the carotid artery, it is shown that in 3 days post NTIRE the immune system decellularizes the irreversible electroporated tissue and leaves behind a functional scaffold. In 7 days, there is evidence of endothelial regrowth, indicating that the artery scaffold maintained its function throughout the procedure and normal recellularization is taking place.

Animal models for the assessment of novel vascular conduits

The development of an ideal small-diameter conduit for use in vascular bypass surgery has yet to be achieved. The ongoing innovation in biomaterial design generates novel conduits that require preclinical assessment in vivo, and a number of animal models have been used for this purpose. This article examines the rationale behind animal models used in the assessment of small-diameter vascular conduits encompassing the commonly used species: baboons, sheep, pigs, dogs, rabbits, and rodents. Studies on the comparative hematology for these species relative to humans are summarized, and the hydrodynamic values for common implant locations are also compared. The large- and small-animal models are then explored, highlighting the characteristics of each that determine their relative utility in the assessment of vascular conduits. Where possible, the performance of expanded polytetrafluoroethylene is given in each animal and in each location to allow direct comparisons between species. New challenges in animal modeling are outlined for the assessment of tissue-engineered graft designs. Finally, recommendations are given for the selection of animal models for the assessment of future vascular conduits.

The pathology of depopulated bovine ureter xenografts utilized for vascular access in haemodialysis patients

AIMS

Vascular access for long-term haemodialysis is obtained through the surgical fashioning of arteriovenous fistulae, utilizing the patients' native blood vessels, or by insertion of synthetic grafts or non-synthetic gluteraldehyde cross-linked biological xenografts. These non-native grafts have high complication rates and a depopulated bovine ureter xenograft has recently been developed as an alternative. The aim was to undertake the first systematic review of the histopathology of bovine ureter xenografts (n = 25) utilized for haemodialysis vascular access in humans.

METHODS AND RESULTS

Pre-insertion specimens (n = 7) showed preservation of some cellular architecture and histological antigenicity. Uncomplicated segments of post-insertion specimens (n = 18) showed myofibroblastic in-growth but no luminal endothelialization and no vascularization of the wall, other than at sites of needle puncture. Post-insertion, 50% showed a severe adventitial host inflammatory response with a dominant granulomatous and eosinophil-rich infiltrate. Inflammation was present in grafts with various complications (stenosis, thrombosis, aneurysm), but there was no clear pathogenic link.

CONCLUSIONS

We conclude that repopulation of bovine ureter xenografts by host cells is limited and that, in specimens removed for complications, an inflammatory reaction to the xenograft is common. This could reflect retention of some antigenicity following pre-insertion 'depopulation' of the grafts.

The use of three-dimensional nanostructures to instruct cells to produce extracellular matrix for regenerative medicine strategies

Synthetic polymers or naturally-derived extracellular matrix (ECM) proteins have been used to create tissue engineering scaffolds; however, the need for surface modification in order to achieve polymer biocompatibility and the lack of biomechanical strength of constructs built using proteins alone remain major limitations. To overcome these obstacles, we developed novel hybrid constructs composed of both strong biosynthetic materials and natural human ECM proteins. Taking advantage of the ability of cells to produce their own ECM, human foreskin fibroblasts were grown on silicon-based nanostructures exhibiting various surface topographies that significantly enhanced ECM protein production. After 4 weeks, cell-derived sheets were harvested and histology, immunochemistry, biochemistry and multiphoton imaging revealed the presence of collagens, tropoelastin, fibronectin and glycosaminoglycans. Following decellularization, purified sheet-derived ECM proteins were mixed with poly(epsilon-caprolactone) to create fibrous scaffolds using electrospinning. These hybrid scaffolds exhibited excellent biomechanical properties with fiber and pore sizes that allowed attachment and migration of adipose tissue-derived stem cells. Our study represents an innovative approach to generate strong, non-cytotoxic scaffolds that could have broad applications in tissue regeneration strategies.

Tissue-engineered blood vessel graft produced by self-derived cells and allogenic acellular matrix: a functional performance and histologic study

INTRODUCTION

Current prosthetic alternatives to autologous vascular grafts remain poor in terms of patency. Growing biologic blood vessels has been proposed as an alternative. In this work, the authors demonstrate a method for producing a tissue-engineered vascular graft (TEVG) with self-derived endothelial cells, smooth muscle cells, and allogenic acellular matrix in vitro. The aim of this study was to find out if the graft is suitable as the carotid artery substitute.

METHODS

A canine model was developed for this study. Endothelial and smooth muscle cells were used as seeding cells, and allogenic acellular matrix was used as scaffold to produce the TEVG. Endothelial and smooth muscle cells from the saphenous vein were harvested by trypsin and collagenase digestion respectively. These isolated cells were cultured and expanded by routine cell culture technique. The common carotid artery was harvested from other fresh dog cadavers and processed by a multistep decellularizing technique to remove original cells and preserve elastic and collagen fibers. Then, the inner surface of the acellular matrix as a scaffold was sequentially seeded with cells. Smooth muscle cells were seeded onto the scaffold. It was placed in bioreactors filled Dulbecco modified Eagle medium supplemented with growth factors. After 4 weeks, the vessels were harvested from the bioreactors and seeded with endothelial cells at the lumen for 7 days. Finally, the cell-seeded graft was transplanted to the cell-donated dog to substitute part of the native common carotid artery (2 cm in length). All animals were followed up for 6 months. Twenty-four dogs were divided into 3 groups randomly: group A (native artery graft), group B (allogenic acellular matrix graft only), and group C (acellular allogenic matrix coated with endothelial and smooth muscle cells). These grafts were subjected to regular echocardiography at 1, 3, and 6 months postoperatively. Then, the TEVG were harvested for histologic evaluation at 6 months after transplantation. The vascular luminal surfaces were observed by electronic scanning microscopy.

RESULTS

The TEVG showed good functional performance demonstrated by regular Doppler ultrasonography at 1, 3, and 6 months postoperatively, compared with that of native arteries. All vascular grafts in group A and C provided patent rates of 100%; however, the patency rate of group B was 50% at 3 and 6 months postoperatively. The TEVG had a similar macroscopic appearance to that of native vessels. Histologic and immunohistochemical analyses indicated the presence of high cell density and development of a highly organized structure of ECM.

CONCLUSION

Cultured self-derived endothelial and smooth muscle cells could be used as seeding cells and allogenic acellularized matrix could be used as scaffold in producing the TEVG. The TEVG had histologic and functional properties consistent with native arteries.

Massive aneurysmal dilatation of a depopulated ureteric hemodialysis xenograft

Use of depopulated bovine ureteric xenografts for hemodialysis vascular access has recently been described. Cellular components have been removed, giving a connective tissue matrix which can be neocellularized, retaining native biomechanics. A 24-year-old male with end-stage renal disease from focal segmental glomerulosclerosis presented with particularly difficult vascular access. A depopulated bovine ureteric xenograft was implanted from the left subclavian artery to innominate vein. It became massively aneurysmal, requiring emergency embolization. Biopsy of the graft stained positive for alpha-gal. We believe this is the first reported case of massive aneurysmal dilatation of a depopulated bovine ureteric xenograft.

Randomized clinical trial comparing decellularized bovine ureter with expanded polytetrafluoroethylene for vascular access

BACKGROUND

The SynerGraft model 100 (SG 100) is a decellularized bovine uereter graft developed to improve on prosthetic conduits for vascular access. Its clinical performance was compared with polytetrafluoroethylene (ePTFE) in a prospective, pilot randomized study.

METHODS

Patients requiring haemodialysis with no native vein options were included. Between June 2004 and June 2007, 29 patients received SG 100 and 27 ePTFE grafts. Forty-five patients had undergone previous access surgery. All grafts were between the brachial artery and the axillary vein.

RESULTS

Clinical details were similar between the groups; overall mean(s.d.) follow-up was 469(398) days. After 1 year, there were no significant differences in primary patency (28 per cent for SG 100 versus 48 per cent for ePTFE; P = 0.290), assisted primary patency (52 versus 64 per cent; P = 0.430) or secondary patency (57 versus 68 per cent; P = 0.370). Freedom from infection at 1 year was 96 per cent for SG 100 and 91 per cent for ePTFE (P = 0.410). Fifty-seven further procedures (18 endovascular and 39 surgical) were needed to maintain patency in 50 grafts (23 SG 100 and 27 ePTFE).

CONCLUSION

Both grafts were adequate conduits for haemodialysis and were amenable to repair. Anticipated advantages for SG 100 were not seen in either patency or stability.

Decellularized ureter for tissue-engineered small-caliber vascular graft

Previous attempts to create small-caliber vascular prostheses have been limited. The aim of this study was to generate tissue-engineered small-diameter vascular grafts using decellularized ureters (DUs). Canine ureters were decellularized using one of four different chemical agents [Triton-X 100 (Tx), deoxycholate (DCA), trypsin, or sodium dodecyl sulfate (SDS)] and the histology, residual DNA contents, and immunogenicity of the resulting DUs were compared. The mechanical properties of the DUs were evaluated in terms of water permeability, burst strength, tensile strength, and compliance. Cultured canine endothelial cells (ECs) and myofibroblasts were seeded onto DUs and evaluated histologically. Canine carotid arteries were replaced with the EC-seeded DUs (n = 4). As controls, nonseeded DUs (n = 5) and PTFE prostheses (n = 4) were also used to replace carotid arteries. The degree of decellularization and the maintenance of the matrix were best in the Tx-treated DUs. Tx-treated and DCA-treated DUs had lower remnant DNA contents and immunogenicity than the others. The burst strength of the DUs was more than 500 mmHg and the maximum tensile strength of the DUs was not different to that of native ureters. DU compliance was similar to that of native carotid artery. The cell seeding test resulted in monolayered ECs and multilayered alpha-smooth muscle actin-positive cells on the DUs. The animal implantation model showed that the EC-seeded DUs were patent for at least 6 months after the operation, whereas the nonseeded DUs and PTFE grafts become occluded within a week. These results suggest that tissue-engineered DUs may be a potential alternative conduit for bypass surgery.

Tissue engineering of vascular conduits

BACKGROUND

Autologous conduits are not available in up to 40 per cent of patients with arteriopathy who require coronary or lower limb revascularization, and access sites for renal dialysis may eventually become exhausted. Synthetic prostheses achieve a poor patency rate in small-calibre anastomoses. This review examines how vascular tissue engineering may be used to address these issues.

METHODS

A Medline search was performed, using the keywords "vascular tissue engineering", "small diameter vascular conduit", "vascular cell biology", "biomechanics", "cell seeding" and "graft endothelialization". Key references were hand-searched for relevant papers.

RESULTS AND CONCLUSION

In vitro and in vivo approaches are currently being used for guided cell repopulation of both biological and synthetic scaffolds. The major clinical problem has been extended culture time (approximately 6 weeks), which precludes their use in the acute setting. However, recent advances have led not only to improved patency rates for prostheses, but also to a potential reduction in culture time. In addition, increased mobilization of endothelial progenitor cells in the presence of ischaemic tissue may increase the autologous cell yield for scaffold reseeding with further reduction in culture time.

Evaluation of Compliance and Stiffness of Decellularized Tissues as Scaffolds for Tissue-Engineered Small Caliber Vascular Grafts Using Intravascular Ultrasound

This study evaluated the compliance and stiffness of decellularized canine common carotid artery as well as decellularized canine ureter and compared it with that of polytetrafluoroethylene, elastin gel combined with polylactic acid tube, and canine common carotid artery. To calculate the compliance and stiffness, internal diameters and cross-sectional areas were measured according to changes in the intraluminal pressures using intravascular ultrasound in a closed circuit system equipped with a syringe pump. The pressure-area curve, stiffness parameter beta, and diameter compliance were evaluated. Canine common carotid artery and decellularized canine common carotid artery, as well as decellularized canine ureter, showed a compliant response, a J-shaped curve. However, the latter evidenced different characteristics in the low pressure range. Although the cross-sectional area of the elastin gel combined with polylactic acid tube showed some changes, it did not present a J-shape curve. Polytetrafluoroethylene exhibited a noncompliant response.The results in this study have shown that the compliance in the decellularized matrices was maintained after cell extraction, which demonstrated the importance of the remaining matrix structure in the mechanical properties of decellularized tissue. A clear difference between the decellularized matrices and synthetic materials was noted in terms of the compliance, even in materials composed of relatively elastic materials.

Tissue engineering of blood vessels

BACKGROUND

Tissue engineering techniques have been employed successfully in the management of wounds, burns and cartilage repair. Current prosthetic alternatives to autologous vascular bypass grafts remain poor in terms of patency and infection risk. Growing biological blood vessels has been proposed as an alternative.

METHODS

This review is based on a literature search using Medline, PubMed, ISIS and CAS of original articles and reviews, and unpublished material and abstracts.

RESULTS AND CONCLUSIONS

Complete incorporation into host tissues and the maintenance of a viable and self-renewing endothelial layer are the fundamental goals to be achieved when developing a tissue-engineered blood vessel. Sourcing of cells and modulating their interaction with extracellular matrix and supporting scaffold have been the focus of intense research. Although the use of tissue-engineered blood vessels in humans is so far limited, advances in our knowledge of stem cell precursors and the development of new biomaterials should enable this technology to reach routine clinical practice within a decade.

Effect of laser perforation on the remodeling of acellular matrix grafts

Autologous cells migrate only slightly into acellular matrix grafts. This study was carried out in small-diameter, allogeneic matrix grafts to investigate the effects on cell repopulation and remodeling caused by increased wall porosity induced by laser perforation. Allogeneic ovine carotid arteries were decellularized by dye-mediated photooxidation (Photofix). Matrix grafts (10 cm x 4 mm i.d.) were perforated with holes of 50 microm diameter at a density of 50 holes/cm(2) using a Ti-sapphire laser. The grafts were implanted in the carotid arteries of 10 sheep and were compared to nonperforated grafts implanted contralaterally. The prostheses were retrieved after 6 weeks or 3 or 6 months following implantation and were evaluated by histologic examination, immunohistochemical staining, and scanning electron microscopy. All grafts, except one of the perforated specimens, remained patent. Perforated implants, examined at 6 weeks, showed faster recellularization with endothelial cells than did the corresponding contralateral controls. Perforated grafts, examined at 6 months, showed a significantly thicker neointima and clear signs of neovascularization: endothelial cells, basal lamina, elastic fibers, circular and longitudinally orientated smooth muscle cells in comparison to nonperforated specimens. Repopulation of the decellularized matrix with host cells was higher in the perforated than in the nonperforated prostheses. These results suggest that the increased matrix porosity induced by laser perforation promotes graft remodeling and reconstitution with host cells.

Depopulated bovine ureteric xenograft for complex haemodialysis vascular access

OBJECTIVES

To assess performance of a de-cellularised bovine ureter vascular graft for haemodialysis in patients for whom conventional access was not possible.

METHODS

The Syner Graft Vascular Graft Model 100 (SGVG 100) is a bovine ureter modified by a tissue-engineering depopulation technology and uniquely it is not chemically cross-linked. SGVG 100 was implanted in patients with a failed fistula or vascular access grafts. Graft patency was the primary outcome; secondary outcomes included adverse events and associated treatments.

RESULTS

25 SGVG 100 were implanted in 23 patients; mean age was 59+/-14 years. Mean follow-up was 370 days. The mean time to occlusion (19 events) was 215+/-141 days with patency re-established in 14 of 18 surgical interventions. Thirty angioplasties were performed on 14 SGVG 100 for luminal/anastomotic stenosis. Two grafts demonstrated areas of dilation; however, both grafts continue to be usable at last reported follow-up (930 and 602 days) with no further changes in graft size. Primary patency, assisted primary patency, secondary patency, and freedom from infection were 29, 45, 81, and 95% at 1 year, respectively.

CONCLUSIONS

This report demonstrates SGVG 100 is a stable vascular access conduit, providing a suitable graft alternative when autologous vein is not available.

Tissue Engineering für Herzklappen und Gefäße

Current prosthetic substitutes for heart valves and blood vessels have numerous limitations such as limited durability (biological valves), susceptibility to infection, the necessity of lifelong anticoagulation therapy (prosthetic valves), and reduced patency in small-caliber grafts, for example. Tissue engineering using either polymers or decellularized native allogeneic or xenogenic heart valve/vascular matrices may provide the techniques to develop the ideal heart valve or vascular graft. The matrix scaffold serves as a basis on which seeded cells can organise and develop into the valve or vascular tissue prior to or following implantation. The scaffold is either degraded or metabolised during the formation and organisation of the newly generated matrix, leading to vital living tissue. This paper summarises current research and first clinical developments in the tissue engineering of heart valves and vascular grafts.

Engineering of bypass conduits to improve patency

For patients with severe coronary artery and distal peripheral vascular disease not amenable to angioplasty and lacking sufficient autologous vessels there is a pressing need for improvements to current surgical bypass options. It has been decades since any real progress in bypass material has reached mainstream surgical practice. This review looks at possible remedies to this situation. Options considered are methods to reduce prosthetic graft thrombogenicity, including endothelial cell seeding and developments of new prosthetic materials. The promise of tissue-engineered blood vessels is examined with a specific look at how peptides can improve cell adhesion to scaffolds.

The use of animal models in developing the discipline of cardiovascular tissue engineering: a review

Cardiovascular disease remains one of the major causes of death and disability in the Western world. Tissue engineering offers the prospect of being able to meet the demand for replacement of heart valves, vessels for coronary and lower limb bypass surgery and the generation of cardiac tissue for addition to the diseased heart. In order to test prospective tissue-engineered devices, these constructs must first be proven in animal models before receiving CE marking or FDA approval for a clinical trial. The choice of animal depends on the nature of the tissue-engineered construct being tested. Factors that need to be considered include technical requirements of implanting the construct, availability of the animal, cost and ethical considerations. In this paper, we review the history of animal studies in cardiovascular tissue engineering and the uses of animal tissue as sources for tissue engineering.

Functional neointima characterization of vascular prostheses in human

BACKGROUND

The purpose of this study was to evaluate neointimal functionality of synthetic vascular grafts repopulated by host cells after implantation.

METHODS

We obtained reseeded inflow and outflow cannulas of 2 patients undergoing orthotopic heart transplantation after left ventricular assist device implantation 9 and 10 months before. After cell isolation we examined cellular function of reseeded cells and their capability to form a functional endothelial layer applying immunohistologic markers and quantitative Western blot for endothelial nitric oxide synthase activity.

RESULTS

Neointima formation in inflow and outflow cannulas differs macroscopically and by histologic appearance. The neointima formation on the surface of the polyethylene terephthalate fiber (Dacron) grafts differs substantially from native aortic vessel wall with respect to cellular and extracellular matrix composition and cellular function.

CONCLUSIONS

The neointima of Dacron prostheses is composed of cells with rudimentary physiologic endothelial function. We conclude that synthetic matrices are not suitable scaffolds for generating functional cardiovascular implants.

A cautionary case: the SynerGraft vascular prosthesis

To date no prosthetic vascular prosthesis performs as well as autologous conduits for small diameter arterial reconstruction. We report the outcome of our first case using a new vascular prosthesis marketed by Cryolife, the SynerGraft. This Xenograft prostheses is claimed to eliminate the problems of the previous generation of gluteraldehyde treated grafts. Two 50 cm SynnerGrafts were anastomosed end to end and used as a femoral-posterior tibial bypass graft in a 68-year-old man who presented with an acutely ischaemic left leg. Our patient represented at 8 weeks with aneurysmal degeneration along the course of the graft. We urge caution in the use of these grafts until convincing data in humans is presented.

Decellularized cadaver vein allografts used for hemodialysis access do not cause allosensitization or preclude kidney transplantation

BACKGROUND

Dimethyl sulfoxide-cryopreserved (CRY) cadaver vein allografts used for hemodialysis access in patients with renal failure recently have been shown to cause broad recipient allosensitization, measured by panel reactive antibody (PRA) assay. Synergraft (SYN) processing is a novel method of treating tissue that decellularizes the graft (including mismatched major histocompatibility antigens) and potentially should prevent allosensitization.

METHODS

Twenty hemodialysis patients underwent placement of an SYN-processed cadaver vein allograft. PRA assay was used prospectively to assess allosensitization in these patients at baseline and 1-month intervals after engraftment. These results were compared with our historic series of CRY allograft recipients.

RESULTS

There was no significant difference in baseline PRA values for SYN and CRY patients (2.8% versus 2.6%, respectively). None of the SYN patients became allosensitized at 3 months postengraftment (mean PRA, 3.2%), whereas all CRY recipients became highly sensitized at a mean of 3.1 months (mean PRA, 84.1%). This result was highly significant (P < 0.0001).

CONCLUSION

SYN processing of cadaver vein allografts successfully removes antigenic material. The use of SYN allografts in patients with renal failure for hemodialysis access does not cause allosensitization and therefore should not preclude kidney transplantation.

Tissue engineering of small diameter vascular grafts

Tissue engineering, using either polymer or biological based scaffolds, represents the newest approach to overcoming limitations of small diameter prosthetic vascular grafts. Their disadvantages include thromboembolism and thrombosis, anticoagulant related haemorrhage, compliance mismatch, neointimal hyperplasia, as well as aneurysm formation. This current review represents an overview about previous and contemporary studies in the field of artificial vascular conduits development regarding arterial and venous autografts, allografts, xenografts, alloplastic prostheses, and tissue engineering.