In Vivo Behavior of Decellularized Vein Allograft1,2

The journey of decellularized vessel: from laboratory to operating room

Over the past few decades, there has been a remarkable advancement in the field of transplantation. But the shortage of donors is still an urgent problem that requires immediate attention. As with xenotransplantation, bioengineered organs are promising solutions to the current shortage situation. And decellularization is a unique technology in organ-bioengineering. However, at present, there is no unified decellularization method for different tissues, and there is no gold-standard for evaluating decellularization efficiency. Meanwhile, recellularization, re-endothelialization and modification are needed to form transplantable organs. With this mind, we can start with decellularization and re-endothelialization or modification of small blood vessels, which would serve to address the shortage of small-diameter vessels while simultaneously gathering the requisite data and inspiration for further recellularization of the whole organ-scale vascular network. In this review, we collect the related experiments of decellularization and post-decellularization approaches of small vessels in recent years. Subsequently, we summarize the experience in relation to the decellularization and post-decellularization combinations, and put forward obstacle we face and possible solutions.

Coatings in Decellularized Vascular Scaffolds for the Establishment of a Functional Endothelium: A Scoping Review of Vascular Graft Refinement

Developments in tissue engineering techniques have allowed for the creation of biocompatible, non-immunogenic alternative vascular grafts through the decellularization of existing tissues. With an ever-growing number of patients requiring life-saving vascular bypass grafting surgeries, the production of functional small diameter decellularized vascular scaffolds has never been more important. However, current implementations of small diameter decellularized vascular grafts face numerous clinical challenges attributed to premature graft failure as a consequence of common failure mechanisms such as acute thrombogenesis and intimal hyperplasia resulting from insufficient endothelial coverage on the graft lumen. This review summarizes some of the surface modifying coating agents currently used to improve the re-endothelialization efficiency and endothelial cell persistence in decellularized vascular scaffolds that could be applied in producing a better patency small diameter vascular graft. A comprehensive search yielding 192 publications was conducted in the PubMed, Scopus, Web of Science, and Ovid electronic databases. Careful screening and removal of unrelated publications and duplicate entries resulted in a total of 16 publications, which were discussed in this review. Selected publications demonstrate that the utilization of surface coating agents can induce endothelial cell adhesion, migration, and proliferation therefore leads to increased re-endothelialization efficiency. Unfortunately, the large variance in methodologies complicates comparison of coating effects between studies. Thus far, coating decellularized tissue gave encouraging results. These developments in re-endothelialization could be incorporated in the fabrication of functional, off-the-shelf alternative small diameter vascular scaffolds.

Individualized tissue-engineered veins as vascular grafts: A proof of concept study in pig

Personalized tissue engineered vascular grafts are a promising advanced therapy medicinal product alternative to autologous or synthetic vascular grafts utilized in blood vessel bypass or replacement surgery. We hypothesized that an individualized tissue engineered vein (P-TEV) would make the body recognize the transplanted blood vessel as autologous, decrease the risk of rejection and thereby avoid lifelong treatment with immune suppressant medication as is standard with allogenic organ transplantation. To individualize blood vessels, we decellularized vena cava from six deceased donor pigs and tested them for cellular removal and histological integrity. A solution with peripheral blood from the recipient pigs was used for individualized reconditioning in a perfusion bioreactor for seven days prior to transplantation. To evaluate safety and functionality of the individualized vascular graft in vivo, we transplanted reconditioned porcine vena cava into six pigs and analyzed histology and patency of the graft at different time points, with three pigs at the final endpoint 4-5 weeks after surgery. Our results showed that the P-TEV was fully patent in all animals, did not induce any occlusion or stenosis formation and we did not find any signs of rejection. The P-TEV showed rapid recellularization in vivo with the luminal surface covered with endothelial cells. In summary, the results indicate that P-TEV is functional and have potential for use as clinical transplant grafts.

Sonication-Assisted Method for Decellularization of Human Umbilical Artery for Small-Caliber Vascular Tissue Engineering

Decellularized vascular grafts are useful for the construction of biological small-diameter tissue-engineered vascular grafts (≤6 mm). Traditional chemical decellularization requires a long treatment time, which may damage the structure and alter the mechanical properties. Decellularization using sonication is expected to solve this problem. The aim of this study was to develop an effective decellularization method using ultrasound followed by washing. Different power values of sonication at 40 kHz were tested for 2, 4, and 8 h followed by a washing procedure. The efficacy of sonication of decellularized human umbilical artery (sDHUA) was evaluated via DNA content, histological staining, mechanical properties, and biocompatibility. The sDHUAs were further implanted into rats for up to 90 days and magnetic resonance angiography (MRA) was performed for the implanted grafts. The results demonstrated that treatment of human umbilical artery (HUA) by sonication at ultrasonic power of 204 W for 4 h followed by washing for 24 h in 2% SDS buffer could eliminate more than 90% of cells and retain similar mechanical properties of the HUA. Recellularization was assessed by scanning electron microscopy (SEM), which indicated that sDHUA provided niches for human umbilical vein endothelial cells (HUVECs) to reside, indicating in vitro cytocompatibility. Further implantation tests also indicated the fitness of the sonication-treated HUA as a scaffold for small-caliber tissue engineering vascular grafts.

In vitro evaluation of surface biological properties of decellularized aorta for cardiovascular use

The aim of this study was to determine an in vitro evaluation method that could directly predict in vivo performance of decellularized tissue for cardiovascular use. We hypothesized that key factors for in vitro evaluation would be found by in vitro assessment of decellularized aortas that previously showed good performance in vivo, such as high patency. We chose porcine aortas, decellularized using three different decellularization methods: sodium dodecyl-sulfate (SDS), freeze-thawing, and high-hydrostatic pressurization (HHP). Immunohistological staining, a blood clotting test, scanning electron microscopy (SEM) analysis, and recellularization of endothelial cells were used for the in vitro evaluation. There was a significant difference in the remaining extracellular matrix (ECM) components, ECM structure, and the luminal surface structure between the three decellularized aortas, respectively, resulting in differences in the recellularization of endothelial cells. On the other hand, there was no difference observed in the blood clotting test. These results suggested that the blood clotting test could be a key evaluation method for the prediction of in vivo performance. In addition, evaluation of the luminal surface structure and the recellularization experiment should be packaged as an in vitro evaluation because the long-term patency was probably affected. The evaluation approach in this study may be useful to establish regulations and a quality management system for a cardiovascular prosthesis.

Potential of mesenchymal stem cells for bioengineered blood vessels in comparison with other eligible cell sources

Application of stem cells in tissue engineering has proved to be effective in many cases due to great proliferation and differentiation potentials as well as possible paracrine effects of these cells. Human mesenchymal stem cells (MSCs) are recognized as a valuable source for vascular tissue engineering, which requires endothelial and perivascular cells. The goal of this review is to survey the potential of MSCs for engineering functional blood vessels in comparison with other cell types including bone marrow mononuclear cells, endothelial precursor cells, differentiated adult autologous smooth muscle cells, autologous endothelial cells, embryonic stem cells, and induced pluripotent stem cells. In conclusion, MSCs represent a preference in making autologous tissue-engineered vascular grafts (TEVGs) as well as off-the-shelf TEVGs for emergency vascular surgery cases.

Small Diameter Xenogeneic Extracellular Matrix Scaffolds for Vascular Applications

Currently, despite the success of percutaneous coronary intervention (PCI), coronary artery bypass graft (CABG) remains among the most commonly performed cardiac surgical procedures in the United States. Unfortunately, the use of autologous grafts in CABG presents a major clinical challenge as complications due to autologous vessel harvest and limited vessel availability pose a significant setback in the success rate of CABG surgeries. Acellular extracellular matrix (ECM) scaffolds derived from xenogeneic vascular tissues have the potential to overcome these challenges, as they offer unlimited availability and sufficient length to serve as "off-the-shelf" CABGs. Unfortunately, regardless of numerous efforts to produce a fully functional small diameter xenogeneic ECM scaffold, the combination of factors required to overcome all failure mechanisms in a single graft remains elusive. This article covers the major failure mechanisms of current xenogeneic small diameter vessel ECM scaffolds, and reviews the recent advances in the field to overcome these failure mechanisms and ultimately develop a small diameter ECM xenogeneic scaffold for CABG. Impact Statement Currently, the use of autologous vessel in coronary artery bypass graft (CABG) is common practice. However, the use of autologous tissue poses significant complications due to tissue harvest and limited availability. Developing an alternative vessel for use in CABG can potentially increase the success rate of CABG surgery by eliminating complications related to the use of autologous vessel. However, this development has been hindered by an array of failure mechanisms that currently have not been overcome. This article describes the currently identified failure mechanisms of small diameter vascular xenogeneic extracellular matrix scaffolds and reviews current research targeted to overcoming these failure mechanisms toward ensuring long-term graft patency.

Renal Tubular- and Vascular Basement Membranes and their Mimicry in Engineering Vascularized Kidney Tubules

The high prevalence of chronic kidney disease leads to an increased need for renal replacement therapies. While there are simply not enough donor organs available for transplantation, there is a need to seek other therapeutic avenues as current dialysis modalities are insufficient. The field of regenerative medicine and whole organ engineering is emerging, and researchers are looking for innovative ways to create (part of) a functional new organ. To biofabricate a kidney or its functional units, it is necessary to understand and learn from physiology to be able to mimic the specific tissue properties. Herein is provided an overview of the knowledge on tubular and vascular basement membranes' biochemical components and biophysical properties, and the major differences between the two basement membranes are highlighted. Furthermore, an overview of current trends in membrane technology for developing renal replacement therapies and to stimulate kidney regeneration is provided.

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.

Construction of tissue-engineered lymphatic vessel using human adipose derived stem cells differentiated lymphatic endothelial like cells and decellularized arterial scaffold: A preliminary study

We have previously demonstrated that human adipose-derived stem cells (hADSCs) can be differentiated into lymphatic endothelial like cells. The purpose of this study was to investigate the feasibility of utilizing the induced lymphatic endothelial like cells and decellularized arterial scaffold to construct the tissue-engineered lymphatic vessel. The hADSCs were isolated from adipose tissue in healthy adults and were characterized the multilineage differentiation potential. Decellularized arterial scaffold was prepared using the Triton x-100 method. ADSCs were differentiated into lymphatic-like endothelial cells, and the induced cells were then seeded onto the decellularized arterial scaffold to engineer the lymphatic vessel. The histological analyses were performed to examine the endothelialized construct. The decellularized arterial scaffold was successfully obtained and was able to maintain its vessel morphology. The isolated ADSCs can be differentiated into osteocytes and adipocytes. After seeding onto the scaffold, the seeded cells attached and grew well on the decellularized arterial scaffold. Our preliminary results demonstrated that the induced lymphatic endothelial like cells combined with decellularized arterial scaffold could be utilized to successfully engineer the lymphatic vessel. Our findings may be helpful for the development of tissue-engineering of the lymphatic graft.

* A Rat Model for the In Vivo Assessment of Biological and Tissue-Engineered Valvular and Vascular Grafts

The demand for an improvement of the biocompatibility and durability of vascular and valvular implants requires translational animal models to study the in vivo fate of cardiovascular grafts. In the present article, a review on the development and application of a microsurgical rat model of infrarenal implantation of aortic grafts and aortic valved conduits is provided. By refinement of surgical techniques and inclusion of hemodynamic considerations, a functional model has been created, which provides a modular platform for the in vivo assessment of biological and tissue-engineered grafts. Through optional addition of procalcific diets, disease-inducing agents, and genetic modifications, complex multimorbidity scenarios mimicking the clinical reality in cardiovascular patients can be simulated. Applying this model, crucial aspects of the biocompatibility, biofunctionality and degeneration of vascular and valvular implants in dependency on graft preparation, and modification as well as systemic antidegenerative treatment of the recipient have been and will be addressed.

Tissue Engineering of Vein Valves Based on Decellularized Natural Matrices

Valvular repair or transplantation, designed to restore the venous valve function of the legs, has been proposed as treatment in chronic venous insufficiency. Available grafts or surgeries have provided limited durability so far. Generating venous valve substitutes by means of tissue engineering could be a solution. We generated decellularized jugular ovine vein conduits containing valves (oVVC) after reseeding with ovine endothelial cells differentiated from peripheral blood-derived endothelial cells (oPBEC), cultivated in vitro corresponding to the circulatory situation in the lower leg at rest and under exertion. oVVC were decellularized by detergent treatment. GFP-labeled oPBEC were seeded onto the luminal side of the decellularized oVVC and cultivated under static-rotational conditions for 6 h (group I) and 12 h (group II), respectively. Reseeded matrices of group I were exposed to continuous low flow conditions (“leg at rest”). The tissues of group II were exposed to a gradually increasing flow (“leg under effort”). After 5 days, the grafts of group I revealed a uniform luminal endothelial cell coverage of the examined areas of the venous walls and adjacent venous valve leaflets. In group II, the cell coverage on luminal areas of the venous wall parts was found to be nearly complete. The endothelial cell coverage of adjacent venous valve leaflets was revealed to be less dense and confluent. Endothelial cells cultured on acellular vein tissues of both groups were distinctly orientated uniformly in the flow direction, clearly creating a stable and flow-orientated layer. Thus, an endothelium could successfully be reestablished on the luminal surface of a decellularized venous valve by seeding peripheral blood endothelial cells and culturing under different conditions.

Acute Limb Shortening for Major Near and Complete Upper Extremity Amputations with Associated Neurovascular Injury: A Review of the Literature

In the setting a near or complete upper extremity amputations with significant soft tissue loss and neurovascular compromise, upper extremity surgeons are faced with the challenge of limb salvage. There are a multitude of treatment options for managing skeletal and soft tissue injuries including provisional fixation, staged reconstruction, and an acute shortening osteotomy with primary rigid internal fixation. However, many complications are associated with these techniques. Complications of provisional fixation include pin tract infection and loosening, tethering of musculotendinous units, nonunion, and additional surgeries. Staged reconstruction includes a variety of techniques: distraction osteogenesis, bone transport, or vascularized and non-vascularized structural autograft or allograft, but the risks often outweigh the benefits. Risks include nonunion, postoperative vascular complications necessitating reoperation, and the inability to return to the previous level of function at an average of 24 months. Acute shortening osteotomy with internal fixation offers the advantage of a single-stage procedure that provides for decreasing the soft tissue loss, provides a rigid platform to protect the delicate neurovascular repair, and alleviates unwanted tension at the repair sites. This review discusses the literature on the surgical treatment of severe upper extremity trauma with associated neurovascular injury over the past 75 years, and aims to evaluate the indications, surgical techniques, clinical and functional outcomes, and complications associated with acute shortening osteotomy with rigid internal fixation. Although this technique is not without risks, it is well-tolerated in the acute setting with a complication profile comparable to other techniques of fixation while remaining a single procedure.

Development of a Tissue-Engineered Bypass Graft Seeded with Stem Cells

The gold standard conduit for bypass of diseased small-diameter arteries remains autologous vascular tissue. In the absence of such tissue, patients are offered bypass with prosthetic material, with far less durable results. Vascular tissue engineering, the creation of a vascular conduit by seeding a tubular scaffold with various cells, may offer an alternative approach to this difficult situation. Herein we review some of the significant challenges that remain in designing an ideal vascular conduit and outline potential solutions offered by a graft created by seeding natural vascular tissue (decellularized vein allograft) with readily available autologous cells (adipose-derived stem cells).

Decellularized biological matrices: an interesting approach for cardiovascular tissue repair and regeneration

The repair and replacement of blood vessels is one of the most challenging topics for biomedical research. Autologous vessels are preferred as graft materials, but they still have many issues to overcome: for instance, they need multiple surgical procedures and often patients may not have healthy and surgically valuable arteries useful as an autograft. A tissue-engineering approach is widely desirable to generate biological vascular prostheses. Recently, decellularization of native tissue has gained significant attention in the biomedical research field. This method is used to obtain biological scaffolds that are expected to maintain the complex three-dimensional structure of the extracellular matrix, preserving the biomechanical properties of the native tissues. The decellularizing methods and the biomechanical characteristics of these products are presented in this review. Decellularization of biological matrices induces the loss of major histocompatibility complex (MHC), which is expected to promote an immunological response by the host. All the studies showed that decellularized biomaterials possess adequate properties for xenografting. Concerning their mechanical properties, several studies have demonstrated that, although chemical decellularization methods do not affect the scaffolds' mechanical properties, these materials can be modified through different treatments in order to provide the desired mechanical characteristics, depending on the specific application. A short overview of legislative issues concerning the use of decellularized substitutes and future perspectives in surgical applications is also presented. Copyright © 2015 John Wiley & Sons, Ltd.

CD133 antibody conjugation to decellularized human heart valves intended for circulating cell capture

The long term efficacy of tissue based heart valve grafts may be limited by progressive degeneration characterized by immune mediated inflammation and calcification. To avoid this degeneration, decellularized heart valves with functionalized surfaces capable of rapid in vivo endothelialization have been developed. The aim of this study is to examine the capacity of CD133 antibody-conjugated valve tissue to capture circulating endothelial progenitor cells (EPCs). Decellularized human pulmonary valve tissue was conjugated with CD133 antibody at varying concentrations and exposed to CD133 expressing NTERA-2 cl.D1 (NT2) cells in a microflow chamber. The amount of CD133 antibody conjugated on the valve tissue surface and the number of NT2 cells captured in the presence of shear stress was measured. Both the amount of CD133 antibody conjugated to the valve leaflet surface and the number of adherent NT2 cells increased as the concentration of CD133 antibody present in the surface immobilization procedure increased. The data presented in this study support the hypothesis that the rate of CD133(+) cell adhesion in the presence of shear stress to decellularized heart valve tissue functionalized by CD133 antibody conjugation increases as the quantity of CD133 antibody conjugated to the tissue surface increases.

c-Kit+ progenitors generate vascular cells for tissue-engineered grafts through modulation of the Wnt/Klf4 pathway

The development of decellularised scaffolds for small diameter vascular grafts is hampered by their limited patency, due to the lack of luminal cell coverage by endothelial cells (EC) and to the low tone of the vessel due to absence of a contractile smooth muscle cells (SMC). In this study, we identify a population of vascular progenitor c-Kit+/Sca-1- cells available in large numbers and derived from immuno-privileged embryonic stem cells (ESCs). We also define an efficient and controlled differentiation protocol yielding fully to differentiated ECs and SMCs in sufficient numbers to allow the repopulation of a tissue engineered vascular graft. When seeded ex vivo on a decellularised vessel, c-Kit+/Sca-1-derived cells recapitulated the native vessel structure and upon in vivo implantation in the mouse, markedly reduced neointima formation and mortality, restoring functional vascularisation. We showed that Krüppel-like transcription factor 4 (Klf4) regulates the choice of differentiation pathway of these cells through β-catenin activation and was itself regulated by the canonical Wnt pathway activator lithium chloride. Our data show that ESC-derived c-Kit+/Sca-1-cells can be differentiated through a Klf4/β-catenin dependent pathway and are a suitable source of vascular progenitors for the creation of superior tissue-engineered vessels from decellularised scaffolds.

Tissue engineered scaffolds for an effective healing and regeneration: reviewing orthotopic studies

It is commonly stated that tissue engineering is the most promising approach to treat or replace failing tissues/organs. For this aim, a specific strategy should be planned including proper selection of biomaterials, fabrication techniques, cell lines, and signaling cues. A great effort has been pursued to develop suitable scaffolds for the restoration of a variety of tissues and a huge number of protocols ranging from in vitro to in vivo studies, the latter further differentiating into several procedures depending on the type of implantation (i.e., subcutaneous or orthotopic) and the model adopted (i.e., animal or human), have been developed. All together, the published reports demonstrate that the proposed tissue engineering approaches spread toward multiple directions. The critical review of this scenario might suggest, at the same time, that a limited number of studies gave a real improvement to the field, especially referring to in vivo investigations. In this regard, the present paper aims to review the results of in vivo tissue engineering experimentations, focusing on the role of the scaffold and its specificity with respect to the tissue to be regenerated, in order to verify whether an extracellular matrix-like device, as usually stated, could promote an expected positive outcome.

Stem cells accumulate on a decellularized arterial xenograft in vivo

BACKGROUND

Surgical interest in complete arterial revascularization is expanding, but some patients lack suitable conduits for this goal. The field of stem cell biology is rapidly expanding and, together with the concepts of tissue engineering, offers the promise of growing autologous grafts in the laboratory. We aim to develop a model using human arteries as vascular grafts in a murine model and to assess the cellular accumulation on these grafts.

METHODS

Human arterial samples were collected and decellularized using an ionic detergent. These vessel scaffolds were then used as grafts in an in vivo mouse model, and the cellular accumulation on them was examined histologically and by cell culture with assessment of their physiologic properties.

RESULTS

Left internal mammary artery branches were fully decellularized and successfully implanted into a murine model. Grafts were repopulated by cells expressing stem cell markers cluster of differentiation 34 and stage-specific embryonic antigen, and subsequently expressed markers of mature endothelial and smooth muscle cells (cluster of differentiation 31, calponin, and myosin heavy chain). The migratory capacity of the cultured cells was significantly higher than that of mouse smooth muscle cells (p < 0.001).

CONCLUSIONS

We describe the successful use of human arteries in a murine graft model, allowing the study of repopulation. Decellularized grafts are repopulated by cells expressing stem cell markers and subsequently express smooth muscle and endothelial cell markers. This model has the potential to be used for further development of laboratory-grown vascular grafts.

Vascular tissue engineering: from in vitro to in situ

Blood vessels transport blood to deliver oxygen and nutrients. Vascular diseases such as atherosclerosis may result in obstruction of blood vessels and tissue ischemia. These conditions require blood vessel replacement to restore blood flow at the macrocirculatory level, and angiogenesis is critical for tissue regeneration and remodeling at the microcirculatory level. Vascular tissue engineering has focused on addressing these two major challenges. We provide a systematic review on various approaches for vascular graft tissue engineering. To create blood vessel substitutes, bioengineers and clinicians have explored technologies in cell engineering, materials science, stem cell biology, and medicine. The scaffolds for vascular grafts can be made from native matrix, synthetic polymers, or other biological materials. Besides endothelial cells, smooth muscle cells, and fibroblasts, expandable cells types such as adult stem cells, pluripotent stem cells, and reprogrammed cells have also been used for vascular tissue engineering. Cell-seeded functional tissue-engineered vascular grafts can be constructed in bioreactors in vitro. Alternatively, an autologous vascular graft can be generated in vivo by harvesting the capsule layer formed around a rod implanted in soft tissues. To overcome the scalability issue and make the grafts available off-the-shelf, nonthrombogenic vascular grafts have been engineered that rely on the host cells to regenerate blood vessels in situ. The rapid progress in the field of vascular tissue engineering has led to exciting preclinical and clinical trials. The advancement of micro-/nanotechnology and stem cell engineering, together with in-depth understanding of vascular regeneration mechanisms, will enable the development of new strategies for innovative therapies.

Granulocyte colony-stimulating factor minimizes negative remodeling of decellularized small diameter vascular graft conduits but not medial degeneration

BACKGROUND

Poor endothelialization and intimal hyperplasia are major causes of small diameter vascular conduit (SDVC) failure. The present study was aimed to investigate the influence of granulocyte colony-stimulating factor (G-CSF) on inhibiting adverse remodeling of decellularized SDVCs.

METHODS

Sprague-Dawley rats implanted with allograft infra renal abdominal aortic conduits were divided into 2 groups according to whether they were treated with G-CSF (+G-CSF group; n=6) or without (Decell group; n=6). The conduits were harvested at 8 weeks after surgery and examined for intimal hyperplasia, collagen deposition, and -actin-staining cells. The medial layer was also examined for signs of cellular repopulation and changes in the elastic fiber morphology.

RESULTS

Intergroup comparison of the intimal composition showed relatively sparse collagen content and predominance of -actin-staining cells in the +G-CSF group. The medial layer in the 2 groups showed similar degrees of elastic fiber degeneration and wall thinning relative to the normal aortic wall. However, the enhanced staining for von Willebrand factor and CD31, along with transmission electron microscopy findings of superior cellular and ultrastructural preservation, suggested that the remodeling and endothelialization in the +G-CSF conduits were superior to those in the Decell conduits.

CONCLUSIONS

This study suggests that G-CSF exerts a positive influence on inhibiting adverse vascular remodeling of decellularized vascular conduit implants. However, whether G-CSF administration may also effectuate an improved ability to preserve the medial structural integrity is unclear.

eNOS transfection of adipose-derived stem cells yields bioactive nitric oxide production and improved results in vascular tissue engineering

This study evaluates the durability of a novel tissue engineered blood vessel (TEBV) created by seeding a natural vascular tissue scaffold (decellularized human saphenous vein allograft) with autologous adipose-derived stem cells (ASC) differentiated into endothelial-like cells. Previous work with this model revealed the graft to be thrombogenic, likely due to inadequate endothelial differentiation as evidenced by minimal production of nitric oxide (NO). To evaluate the importance of NO expression by the seeded cells, we created TEBV using autologous ASC transfected with the endothelial nitric oxide synthase (eNOS) gene to produce NO. We found that transfected ASC produced NO at levels similar to endothelial cell (EC) controls in vitro which was capable of causing vasorelaxation of aortic specimens ex vivo. TEBV (n = 5) created with NO-producing ASC and implanted as interposition grafts within the aorta of rabbits remained patent for two months and demonstrated a non-thrombogenic surface compared to unseeded controls (n = 5). Despite the xenograft nature of the scaffold, the TEBV structure remained well preserved in seeded grafts. In sum, this study demonstrates that upregulation of NO expression within adult stem cells differentiated towards an endothelial-like lineage imparts a non-thrombogenic phenotype and highlights the importance of NO production by cells to be used as endothelial cell substitutes in vascular tissue engineering applications.

Advances in biomimetic regeneration of elastic matrix structures

Elastin is a vital component of the extracellular matrix, providing soft connective tissues with the property of elastic recoil following deformation and regulating the cellular response via biomechanical transduction to maintain tissue homeostasis. The limited ability of most adult cells to synthesize elastin precursors and assemble them into mature crosslinked structures has hindered the development of functional tissue-engineered constructs that exhibit the structure and biomechanics of normal native elastic tissues in the body. In diseased tissues, the chronic overexpression of proteolytic enzymes can cause significant matrix degradation, to further limit the accumulation and quality (e.g., fiber formation) of newly deposited elastic matrix. This review provides an overview of the role and importance of elastin and elastic matrix in soft tissues, the challenges to elastic matrix generation in vitro and to regenerative elastic matrix repair in vivo, current biomolecular strategies to enhance elastin deposition and matrix assembly, and the need to concurrently inhibit proteolytic matrix disruption for improving the quantity and quality of elastogenesis. The review further presents biomaterial-based options using scaffolds and nanocarriers for spatio-temporal control over the presentation and release of these biomolecules, to enable biomimetic assembly of clinically relevant native elastic matrix-like superstructures. Finally, this review provides an overview of recent advances and prospects for the application of these strategies to regenerating tissue-type specific elastic matrix structures and superstructures.

Development of a growing rat model for the in vivo assessment of engineered aortic conduits

BACKGROUND

Numerous limitations of aortic valve grafts currently used in pediatric patients cause the need for alternative prostheses. For the purpose of in vivo evaluation of novel engineered aortic conduit grafts, we aimed at downsizing a previously described model to create a growing rodent model.

MATERIALS AND METHODS

U-shaped aortic conduits were sutured to the infrarenal aorta of young Wistar rats (70-80 g, n = 10) in an end-to-side manner. Functional assessment was performed by Doppler sonography and high resolution rodent MRI. Histology and immunohistochemistry followed after 8 wk.

RESULTS

Postoperative recovery rate was 80%. Conforming to clinical observations, postoperative MRI (d 5) and Doppler sonography (wk 8) revealed unimpaired conduit perfusion. Explanted implants were luminally completely covered by an endothelial cell layer with local hyperplasia and accumulation of α-smooth muscle actin (+) cells. Moreover microcalcification of the decellularized scaffolds was observed.

CONCLUSIONS

Our downsized model of aortic conduit transplantation enables overall characterization with detailed analysis of maturation of engineered aortic grafts in a growing organism.

Morphological changes in the ovine carotid artery wall induced by cold storage

Blood vessels obtained from cadavers and amputated limbs stored at 4°C (i.e., cold stored) potentially represent an economical and readily sourced alternative to autologous vessels and synthetic prostheses for vascular reconstructive surgery. However, cold-stored vessels would need to have a reduced antigenicity and an antithrombogenic autologous endothelial cell (EC) lining before they could function as patent vascular allografts. The aim of this study was to determine the effect of cold storage for 1-16 weeks on the morphology of the ovine carotid artery wall. Ovine carotid arteries (n = 6) were rinsed and flushed with 0.9% saline, cut into segments, wrapped in 0.9% saline-soaked gauze, and stored at 4°C for 1, 2, 4, 8, or 16 weeks. Following storage, the segments were sampled and the samples fixed and sectioned for light microscopic, immunohistochemical, or transmission electron microscopic examination. After 1 and 2 weeks the ECs were karyolitic or contained pyknotic nuclei. After 4 weeks the EC layer was depleted, the subendothelial matrix exposed, and the number of smooth muscle cells (SMCs) and fibroblasts reduced. The 8- and 16-week samples demonstrated complete loss of the EC lining and only occasional remnants of SMCs or fibroblasts. The subendothelial basement membrane appeared to undergo degradative changes as early as 1 week following cold storage. At each time point examined, the subendothelial connective tissue stroma, the internal elastic lamina (IEL), and the collagenous and elastic extracellular framework were retained. These results demonstrate that the ovine carotid artery wall progressively loses its cells but retains its extracellular components during cold storage for up to 16 weeks. They suggest that cold-stored vessels may function as allografts with a reduced antigenicity for vascular reconstructive surgery. It is conceivable that seeded autologous ECs may be used to restore the antithrombogenic EC lining prior to graft implantation.

The decellularized vascular allograft as an experimental platform for developing a biocompatible small-diameter graft conduit in a rat surgical model

PURPOSE

The present study was aimed to assess the feasibility of using decellularized aortic allograft in a rat small animal surgical model for conducting small diameter vascular tissue engineering research.

MATERIALS AND METHODS

Decellularized aortic allografts were infra-renally implanted in 12 Sprague-Dawley (SD) adult rats. The conduits were harvested at 2 (n = 6) and 8 weeks (n = 6), and assessed by hematoxylin and eosin (H&E), van Gieson, Masson Trichrome staining, and immunohistochemistry for von Willebrand factor, CD 31(+), and actin.

RESULTS

Consistent, predictable, and reproducible results were produced by means of a standardized surgical procedure. All animals survived without major complications. Inflammatory immune reaction was minimal, and there was no evidence of aneurysmal degeneration or rupture of the decellularized vascular implants. However, the aortic wall appeared thinner and the elastic fibers in the medial layer showed decreased undulation compared to the normal aorta. There was also minimal cellular repopulation of the vascular media. The remodeling appeared progressive from 2 to 8 weeks with increased intimal thickening and accumulation of both collagen and cells staining for actin. Although the endothelial like cells appeared largely confluent at 8 weeks, they were not as concentrated in appearance as in the normal aorta.

CONCLUSION

The results showed the present rat animal model using decellularized vascular allograft implants to be a potentially durable and effective experimental platform for conducting further research on small diameter vascular tissue engineering.

Lineage restriction of human hepatic stem cells to mature fates is made efficient by tissue-specific biomatrix scaffolds

Current protocols for differentiation of stem cells make use of multiple treatments of soluble signals and/or matrix factors and result typically in partial differentiation to mature cells with under- or overexpression of adult tissue-specific genes. We developed a strategy for rapid and efficient differentiation of stem cells using substrata of biomatrix scaffolds, tissue-specific extracts enriched in extracellular matrix, and associated growth factors and cytokines, in combination with a serum-free, hormonally defined medium (HDM) tailored for the adult cell type of interest. Biomatrix scaffolds were prepared by a novel, four-step perfusion decellularization protocol using conditions designed to keep all collagen types insoluble. The scaffolds maintained native histology, patent vasculatures, and ≈1% of the tissue's proteins but >95% of its collagens, most of the tissue's collagen-associated matrix components, and physiological levels of matrix-bound growth factors and cytokines. Collagens increased from almost undetectable levels to >15% of the scaffold's proteins with the remainder including laminins, fibronectins, elastin, nidogen/entactin, proteoglycans, and matrix-bound cytokines and growth factors in patterns that correlate with histology. Human hepatic stem cells (hHpSCs), seeded onto liver biomatrix scaffolds and in an HDM tailored for adult liver cells, lost stem cell markers and differentiated to mature, functional parenchymal cells in ≈1 week, remaining viable and with stable mature cell phenotypes for more than 8 weeks.

CONCLUSION

Biomatrix scaffolds can be used for biological and pharmaceutical studies of lineage-restricted stem cells, for maintenance of mature cells, and, in the future, for implantable, vascularized engineered tissues or organs.

The isolation of cell derived extracellular matrix constructs using sacrificial open-cell foams

Extracellular matrix derived from human and animal tissues is being used to repair and reconstruct a variety of tissues clinically. The utility of such constructs is limited by the geometry, composition and constitutive properties of the tissue or organ from which the ECM is harvested. To address this limitation, we have developed an approach to isolate extracellular matrix in bulk from populations of living cells grown in culture on three-dimensional substrates. Human biopsy derived fibroblasts were seeded within open-cell foams and cultured in-vitro for periods up to three weeks, after which the synthetic component was removed by incubation in a water miscible solvent. After several wash steps and lyophilization, a white, lacy, multi-molecular construct was isolated. Tandem mass spectroscopy showed that it contained 22 extracellular matrix constituents, including such proteins and proteoglycans as collagen type I and type III, fibronectin, transforming growth factor beta, decorin and biglycan among others. On average 47 mg of construct was isolated for each gram of synthetic substrate initially seeded with cells. The biomaterial harvested from human tracheal fibroblasts had an elastic modulus (250 kPa) and a composition similar to that of human vocal fold tissue, and supported reseeding with human tracheal derived fibroblasts. An important finding was that the approach was useful in isolating ECM from a variety of cell lineages and developmental stages including skin fibroblasts, brain derived astrocytes and mesenchymal stem cells. The results, together with the archival literature, suggest that the approach can be used to produce a range of cell derived constructs with unique physical and chemical attributes for a variety of research and medical applications.

Novel utilization of serum in tissue decellularization

Decellularization of native tissues is a promising technique with numerous applications in tissue engineering and regenerative medicine. However, there are various limitations of currently available decellularization methods, such as alteration of extracellular matrix mechanics and restricted use on certain tissues. This study was conducted to explore the effect of serum on the decellularization of various types of tissues. Fetal bovine serum-containing cell culture medium endothelial growth media-2 removed DNA but not cellular beta-actin from human umbilical artery after detergent treatment, without compromising the tissue mechanical strength assessed by burst pressure. In addition, the effect of serum-containing endothelial growth media-2 on DNA removal was replicated in other types of tissues such as tissue-engineered vessels and myocardium. Other types of serum, including human serum, were also shown to remove DNA from detergent-pretreated tissues. In conclusion, we describe a novel utilization of serum that may have broad applications in tissue decellularization.

Linear shear conditioning improves vascular graft retention of adipose-derived stem cells by upregulation of the alpha5beta1 integrin

Use of adult adipose-derived stem cells (ASCs) as endothelial cell substitutes in vascular tissue engineering is attractive because of their availability. However, when seeded onto decellularized vascular scaffolding and exposed to physiological fluid shear force, ASCs are physically separated from the graft lumen. Herein we have investigated methods of increasing initial ASC attachment using luminal precoats and a novel protocol for the gradual introduction of shear stress to optimize ASC retention. Fibronectin coating of the graft lumen increased ASC attachment by nearly sixfold compared with negative controls. Gradual introduction of near physiological fluid shear stress using a novel bioreactor whereby flow rate was increased every second at a rate of 1.5 dynes/cm(2) per day resulted in complete luminal coverage compared with near complete cell loss following conventional daily abrupt increases. An upregulation of the alpha(5)beta(1) integrin was evinced following exposure to shear stress, which accounts for the observed increase in ASC retention on the graft lumen. These results indicated a novel method for seeding, conditioning, and retaining of adult stem cells on a decellularized vein scaffold within a high-shear stress microenvironment.

Development of decellularized human umbilical arteries as small-diameter vascular grafts

OBJECTIVE

Developing a tissue-engineered small-diameter (<6mm) vascular graft for reconstructive surgery has remained a challenge for the past several decades. This study was conducted to develop a decellularized umbilical artery and to evaluate its composition, endothelial cell compatibility, mechanical properties, and in vivo stability for potential use as a small-diameter vascular graft.

METHODS AND RESULTS

Human umbilical arteries were isolated and decellularized by incubation in CHAPS and sodium dodecyl sulfate buffers followed by incubation in endothelial growth media-2. Decellularized umbilical arteries were completely devoid of cellular and nuclear material while retaining the integrity of extracellular collagenous matrix. The mechanical strength of the decellularized umbilical artery as assessed by its burst pressure in vitro showed no significant change from its native form. Decellularized umbilical arteries supported endothelial adherence as indicated by the re-endotheliazation with a monolayer of human umbilical vein endothelial cells. Furthermore, decellularized vessels that were implanted into nude rats as abdominal aorta interposition grafts remained mechanically intact and patent for up to 8 weeks.

CONCLUSION

Decellularized human umbilical arteries preserved the extracellular matrix, supported endothelialization, and retained function in vivo for up to 8 weeks. These properties suggest the potential use of decellularized umbilical arteries as small-diameter vascular grafts.

Heparin coating of small-caliber decellularized xenografts reduces macrophage infiltration and intimal hyperplasia

Small-caliber decellularized xenografts with surface heparin coating are known to reduce in vivo thrombogenicity. This study was performed to examine whether heparin coating on the small-caliber decellularized xenografts would reduce macrophage infiltration and intimal hyperplasia. In a rabbit model of bilateral carotid implantation, each of the animals (n = 18) received a heparin-coated decellularized xenograft from a canine carotid artery on one side and a nonheparin-coated one on the other side. These experiments were terminated respectively at 1 week (n = 6), 3 weeks (n = 6), and 12 weeks (n = 6). Results showed that, compared with the nonheparin-coated grafts, the heparin-coated grafts had significantly less macrophage infiltration 1 week after implantation, identified by the mouse antirabbit macrophage antibody (RAM11)-positive cells on the vascular wall, covering all the proximal, middle, and distal parts of the grafts (P < 0.01). Moreover, the heparin-coated grafts also showed less deposition of proliferation cell nuclear antigen (PCNA)-positive cells on the vascular wall, indicating less cell proliferation, which was significant not only at 1 week (P < 0.01) but also at 12 weeks (P < 0.01). Intimal hyperplasia, measured by the intimal : media (I : M) ratio, was found similar in both groups at 1 and 3 weeks. However, the I : M ratio was significantly lower in the heparin-coated group than in the nonheparin-coated group at 12 weeks, especially in the proximal anastomosis area (0.76 +/- 0.12 vs. 0.345 +/- 0.06, P < 0.01). Heparin coating of small-caliber decellularized xenografts is associated with an early reduction of macrophage infiltration and intimal hyperplasia in a rabbit model of bilateral carotid artery implantation for 12 weeks. Thus, heparin coating appears to deliver not only the antithrombogeneity but also the antiproliferative property for small-caliber decellularized xenografts.

Evidence for in vivo growth potential and vascular remodeling of tissue-engineered artery

Nondegradable synthetic polymer vascular grafts currently used in cardiovascular surgery have no growth potential. Tissue-engineered vascular grafts (TEVGs) may solve this problem. In this study, we developed a TEVG using autologous bone marrow-derived cells (BMCs) and decellularized tissue matrices, and tested whether the TEVGs exhibit growth potential and vascular remodeling in vivo. Vascular smooth muscle-like cells and endothelial-like cells were differentiated from bone marrow mononuclear cells in vitro. TEVGs were fabricated by seeding these cells onto decellularized porcine abdominal aortas and implanted into the abdominal aortas of 4-month-old, bone marrow donor pigs (n = 4). Eighteen weeks after implantation, the dimensions of TEVGs were measured and compared with those of native abdominal aortas. Expression of molecules associated with vascular remodeling was examined with reverse transcription-polymerase chain reaction assay and immunohistochemistry. Eighteen weeks after implantation, all TEVGs were patent with no sign of thrombus formation, dilatation, or stenosis. Histological and immunohistochemical analyses of the retrieved TEVGs revealed regeneration of endothelium and smooth muscle and the presence of collagen and elastin. The outer diameter of three of the four TEVGs increased in proportion to increases in body weight and outer native aorta diameter. Considerable extents of expression of molecules associated with extracellular matrix (ECM) degradation (i.e., matrix metalloproteinase and tissue inhibitor of matrix metalloproteinase) and ECM precursors (i.e., procollagen I, procollagen III, and tropoelastin) occurred in the TEVGs, indicating vascular remodeling associated with degradation of exogenous ECMs (implanted decellularized matrices) and synthesis of autologous ECMs. This study demonstrates that the TEVGs with autologous BMCs and decellularized tissue matrices exhibit growth potential and vascular remodeling in vivo of tissue-engineered artery.

A small diameter, fibrous vascular conduit generated from a poly(ester urethane)urea and phospholipid polymer blend

The thrombotic and hyperplastic limitations associated with synthetic small diameter vascular grafts have generated sustained interest in finding a tissue engineering solution for autologous vascular segment generation in situ. One approach is to place a biodegradable scaffold at the site that would provide acute mechanical support while vascular tissue develops. To generate a scaffold that possessed both non-thrombogenic character and mechanical properties appropriate for vascular tissue, a biodegradable poly(ester urethane)urea (PEUU) and non-thrombogenic bioinspired phospholipid polymer, poly(2-methacryloyloxyethyl phosphorylcholine-co-methacryloyloxyethyl butylurethane) (PMBU) were blended at PMBU weight fractions of 0-15% and electrospun to create fibrous scaffolds. The composite scaffolds were flexible with breaking strains exceeding 300%, tensile strengths of 7-10MPa and compliances of 2.9-4.4 x 10(-4) mmHg(-1). In vitro platelet deposition on the scaffold surfaces significantly decreased with increasing PMBU content. Rat smooth muscle cell proliferation was also inhibited on PEUU/PMBU blended scaffolds with greater inhibition at higher PMBU content. Fibrous vascular conduits (1.3mm inner diameter) implanted in the rat abdominal aorta for 8 weeks showed greater patency for grafts with 15% PMBU blending versus PEUU without PMBU (67% versus 40%). A thin neo-intimal layer with endothelial coverage and good anastomotic tissue integration was seen for the PEUU/PMBU vascular grafts. These results are encouraging for further evaluation of this technique in larger diameter applications for longer implant periods.

Endothelial differentiation of adipose-derived stem cells: effects of endothelial cell growth supplement and shear force

BACKGROUND

Adipose tissue is a readily available source of multipotent adult stem cells for use in tissue engineering/regenerative medicine. Various growth factors have been used to stimulate acquisition of endothelial characteristics by adipose-derived stem cells (ASC). Herein we study the effects of endothelial cell growth supplement (ECGS) and physiological shear force on the differentiation of ASC into endothelial cells.

MATERIALS AND METHODS

Human ASC (CD13(+)29(+)90(+)31(-)45(-)) were isolated from periumbilical fat, cultured in ECGS media (for up to 3 wk), and exposed to physiological shear force (12 dynes for up to 8 d) in vitro. Endothelial phenotype was defined by cord formation on Matrigel, acetylated-low density lipoprotein (acLDL) uptake, and expression of nitric oxide synthase (eNOS), von Willebrand factor (vWF), and CD31 (platelet endothelial cell adhesion molecule, PECAM). Additionally, cell thrombogenicity was evaluated by seeding canine autologous ASC onto vascular grafts implanted within the canine arterial circulation for 2 wk.

RESULTS

We found that undifferentiated ASC did not display any of the noted endothelial characteristics. After culture in ECGS, ASC formed cords in Matrigel but failed to take up acLDL or express the molecular markers. Subsequent exposure to shear resulted in stem cell realignment, acLDL uptake, and expression of CD31; eNOS and vWF expression was still not observed. Grafts seeded with cells grown in ECGS (+/- shear) remained patent (six of seven) at 2 wk but had a thin coat of fibrin along the luminal surfaces.

CONCLUSIONS

This study suggests that (1) ECGS and shear promote the expression of several endothelial characteristics in human adipose-derived stem cells, but not eNOS or vWF; (2) their combined effects appear synergistic; and (3) stem cells differentiated in ECGS appear mildly thrombogenic in vitro, possibly related, in part, to insufficient eNOS expression. Thus, while the acquisition of several endothelial characteristics by adult stem cells derived from adipose tissue suggests these cells are a viable source of autologous cells for cardiovascular regeneration, further stimulation/modifications are necessary prior to using them as a true endothelial cell replacement.

Applying elastic fibre biology in vascular tissue engineering

For the treatment of vascular disease, the major cause of death in Western society, there is an urgent need for tissue-engineered, biocompatible, small calibre artery substitutes that restore biological function. Vascular tissue engineering of such grafts involves the development of compliant synthetic or biomaterial scaffolds that incorporate vascular cells and extracellular matrix. Elastic fibres are major structural elements of arterial walls that can enhance vascular graft design and patency. In blood vessels, they endow vessels with the critical property of elastic recoil. They also influence vascular cell behaviour through direct interactions and by regulating growth factor activation. This review addresses physiological elastic fibre assembly and contributions to vessel structure and function, and how elastic fibre biology is now being exploited in small diameter vascular graft design.

Tissue engineering applications to vascular bypass graft development: the use of adipose-derived stem cells

The burgeoning field of vascular tissue engineering holds promise for the creation of a practical and successful small-diameter arterial bypass graft. Many creative combinations of autologous cells and scaffolds exist along with an equally long list of microenvironmental cues used to create a functional arterial conduit. This review outlines our work using abdominal wall fat as a source of autologous stem cells for vascular tissue engineering, focusing specifically on this stem cell's availability and potency to differentiate into endothelial-like cells. In a series of 49 patients undergoing elective peripheral vascular surgery, an abundant quantity of adult stem cells was harvested from fat obtained by liposuction. The efficacy of the isolation did not appear influenced by advanced age, obesity, renal failure, or vascular disease, although fat from diabetic patients yielded significantly less stem cells. In addition, these adipose-derived stem cells acquired several morphologic and molecular endothelial phenotypes when exposed to growth factors (endothelial cell growth supplement and vascular endothelial growth factor) and physiologic shear stress in vitro. Taken together, these studies suggest that fat appears to be a viable source of autologous stem cells for use in vascular tissue engineering.

Systematic review of preservation methods and clinical outcome of infrainguinal vascular allografts

OBJECTIVE

We systematically reviewed clinical studies on the use of venous and arterial allografts for infrainguinal revascularization. We attempted to find evidence for the best infrainguinal vascular allograft by a systematic review of the available literature.

METHODS

An electronic search of the MEDLINE, EMBASE, and Cochrane databases was used to determine key articles from studies on the different types of vascular allografts used in infrainguinal reconstruction from 1966 to 2004. Articles were independently reviewed by using previously defined inclusion and exclusion criteria. Study results were gathered with cumulative primary patency as the primary end point. Secondary end points were major complications, graft disintegration, and major limb loss. Quantitative analysis was performed on the prospective randomized trials, and linear regression analysis was performed on cumulative primary patency. Fontaine's classification system was applied.

RESULTS

No systematic review of randomized controlled trials was found. Five randomized controlled trials, 3 prospective cohort or case series, and 15 retrospective case series with 3,837 vascular allografts were found. Methods of allograft preservation were cryopreservation (5 studies), cold storage (3 studies), and glutaraldehyde preservation of human umbilical veins (15 studies). One-year cumulative primary patency rates were 13% to 79% for cryopreservation, 63% to 80% for cold storage, and 40% to 91% for glutaraldehyde. The weighted mean 1-year cumulative primary graft patency rate was 41% for cryopreservation, 71% for cold storage, and 70% for glutaraldehyde allografts. Four randomized trials on femoropopliteal bypasses demonstrated higher patency rates of glutaraldehyde-preserved human umbilical veins than polytetrafluoroethylene grafts. Statistical heterogeneity between studies (I(2) = 91.4%) was too high to perform a formal meta-analysis. The rate of major limb loss was 20% to 58% for cryopreservation, 10% to 69% for cold storage, and 0% to 65% for glutaraldehyde, and the percentage of graft disintegration was 2% to 6% for cryopreservation, 4% to 15% for cold storage, and 0% to 11% for glutaraldehyde.

CONCLUSIONS

A firm conclusion could not be made because there were no studies available in which direct comparison was performed between different preservation methods of vascular allografts. In addition, heterogeneity of the individual studies hampered direct comparison of different types of vascular allografts. However, the overall graft performance of glutaraldehyde-preserved human umbilical vein allografts may be superior to that of other vascular allografts.