Comparative Analysis of Frozen and Acellularized Vascular Xenografts

Background

We implanted frozen and acellularized porcine xenograft vessels as small-diameter arterial grafts in goats and comparatively analyzed the explanted grafts by gross observation and by light microscopy at predetermined periods.

Materials and Methods

Porcine carotid arteries were harvested and immediately stored within a tissue preservation solution at −70°C in a freezer designated for frozen xenograft vessels. The acellularized xenograft vessels were prepared with NaCl-SDS solution and stored frozen until use. One pair of porcine xenograft vessels were used to compare the frozen and acellularized grafts in the bilateral carotid arteries in one goat. The grafts were implanted for one, 3, and 6 months in three animals. Periodic ultrasonographic examinations were performed during the observation period. Explanted grafts were analyzed by gross observation, and by light microscopy.

Results

All animals survived the experimental procedure without specific problems. Ultrasonographic examinations showed excellent patency in all grafts during the observation period. Gross observations revealed nonthrombotic patent smooth lumens. Microscopic examinations of the explanted grafts showed satisfactory cellular reconstruction to the 6-month stage. Although more inflammatory responses were observed in the early phase of implantation of frozen xenografts than of acellularized xenografts, there was no evidence of significant rejection of the frozen xenografts.

Conclusion

These findings suggest that porcine vessel xenografts, regardless of them being acellularized or simply frozen xenografts, can be acceptably implanted in goats as a form of small-diameter vascular graft.

Artificial Aortic Valves: An Overview

This review discusses strategies that may address some of the limitations associated with replacing diseased or dysfunctional aortic valves with mechanical or tissue valves. These limitations range from structural failure and thromboembolic complications associated with mechanical valves to a limited durability and calcification with tissue valves. In pediatric patients there is an issue with the inability of substitutes to grow with the recipient. The emerging science of tissue engineering potentially provides an attractive alternative by creating viable tissue structures based on a resorbable scaffold. Morphometrically precise, biodegradable polymer scaffolds may be fabricated from data obtained from scans of natural valves by rapid prototyping technologies such as fused deposition modelling. The scaffold provides a mechanical profile until seeded cells produce their own extra cellular matrix. The microstructure of the forming tissue may be aligned into predetermined spatial orientations via fluid transduction in a bioreactor. Although there are many technical obstacles that must be overcome before tissue engineered heart valves are introduced into routine surgical practice these valves have the potential to overcome many of the shortcomings of current heart valve substitutes.

Tissue-Engineered Acellularized Valve Xenografts: A Comparative Animal Study between Plain Acellularized Xenografts and Autologous Endothelial Cell Seeded Acellularized Xenografts

Background

Acellularized valve xenografts are considered a promising way of overcoming the inherent limitations of current prosthetic valves. The aim of this study was to compare the biological responses of an autologous endothelial cell seeded acellularized xenograft (AAX) and a plain acellularized xenograft (PAX) implanted in the pulmonary valve leaflet in the same animal.

Methods

Endothelial cells were isolated and cultured from the jugular vein of goats. Porcine valve leaflets were acellularized with Nacl-SDS, and for AAX, leaflets were then seeded with autologous endothelial cells. A PAX and an AAX were implanted as double pulmonary valve leaflet replacement in the same animal in a goat model (n=6). After sacrifice, the implanted valve leaflet tissues were retrieved and analyzed visually and under a light microscope.

Results and Conclusions

Six animals were sacrificed as scheduled during the short-term (6 and 24 hours), mid-term (1 week and 1 month) and long-term (3 and 6 months). Gross and ultrasonographic examinations revealed good valve function with no thrombosis but with slight thickening. Microscopic analysis of the leaflets showed abundant cellular ingrowth into the acellularized leaflets over time. The role of endothelial cell seeding remains controversial. This animal experiment demonstrates the practical feasibility of using acellularized valve xenografts.

Off-the-shelf tissue engineered heart valves for in situ regeneration: current state, challenges and future directions

INTRODUCTION

Transcatheter aortic valve replacement (TAVR) is continuously evolving and is expected to surpass surgical valve implantation in the near future. Combining durable valve substitutes with minimally invasive implantation techniques might increase the clinical relevance of this therapeutic option for younger patient populations. Tissue engineering offers the possibility to create tissue engineered heart valves (TEHVs) with regenerative and self-repair capacities which may overcome the pitfalls of current TAVR prostheses.

AREAS COVERED

This review focuses on off-the-shelf TEHVs which rely on a clinically-relevant in situ tissue engineering approach and which have already advanced into preclinical or first-in-human investigation.

EXPERT COMMENTARY

Among the off-the-shelf in situ TEHVs reported in literature, the vast majority covers pulmonary valve substitutes, and only few are combined with transcatheter implantation technologies. Hence, further innovations should include the development of transcatheter tissue engineered aortic valve substitutes, which would considerably increase the clinical relevance of such prostheses.

'Printability' of Candidate Biomaterials for Extrusion Based 3D Printing: State-of-the-Art

Regenerative medicine has been highlighted as one of the UK's 8 'Great Technologies' with the potential to revolutionize patient care in the 21st Century. Over the last decade, the concept of '3D bioprinting' has emerged, which allows the precise deposition of cell laden bioinks with the aim of engineering complex, functional tissues. For 3D printing to be used clinically, there is the need to produce advanced functional biomaterials, a new generation of bioinks with suitable cell culture and high shape/print fidelity, to match or exceed the physical, chemical and biological properties of human tissue. With the rapid increase in knowledge associated with biomaterials, cell-scaffold interactions and the ability to biofunctionalize/decorate bioinks with cell recognition sequences, it is important to keep in mind the 'printability' of these novel materials. In this illustrated review, we define and refine the concept of 'printability' and review seminal and contemporary studies to highlight the current 'state of play' in the field with a focus on bioink composition and concentration, manipulation of nozzle parameters and rheological properties.

Is there an alternative to systemic anticoagulation, as related to interventional biomedical devices?

To reduce the toxic effects, related clinical problems and complications such as bleeding disorders associated with systemic anticoagulation, it has been hypothesized that by coating the surfaces of medical devices, such as stents, bypass grafts, extracorporeal circuits, guide wires and catheters, there will be a significant reduction in the requirement for systemic anticoagulation or, ideally, it will no longer be necessary. However, current coating processes, even covalent ones, still result in leaching followed by reduced functionality. Alternative anticoagulants and related antiplatelet agents have been used for improvement in terms of reduced restenosis, intimal hyperphasia and device failure. This review focuses on existing heparinization processes, their application in clinical devices and the updated list of alternatives to heparinization in order to obtain a broad overview, it then highlights, in particular, the future possibilities of using heparin and related moieties to tissue engineer scaffolds.

Gene regulation of extracellular matrix remodeling in human bone marrow stem cell-seeded tissue-engineered grafts

Tissue-engineered heart valves are prone to early structural deterioration. We hypothesize that cell-scaffold interaction and mechanical deformation results in upregulation of genes related to osteogenic/chondrogenic differentiation and thus changes extracellular matrix (ECM) composition in human bone marrow mesenchymal stem cell (hBMSC)-derived tissue-engineered grafts. hBMSC were expanded and seeded onto poly-glycolic acid/poly-lactic acid scaffold for 14 days. Seeded tissue-engineered constructs (TEC) were subjected to cyclic flexure for 24 h, whereas control TEC was maintained in roller bottles for the same duration. hBMSC, TEC, and mechanically deformed TEC were subjected to gene-array and histological analysis. Expression levels of RNA and/or protein markers related to chondrogenesis (Sox9, MGP, RunX2, Col II, Col X, and Col XI) and osteogenesis (ALPL, BMP2, EDN1, RunX1, and Col I) were increased in TEC compared to unseeded hBMSC. Histological sections of TEC stained positive for Saffranin O, alkaline phosphatase activity, and calcium deposits. The expression levels of the above gene and protein markers further increased in deformed TEC compared to static TEC. Cell-scaffold interactions and mechanical stress results in gene expression suggestive of endochondral-ossification that impact upon ECM composition and may predispose them to eventual calcification.

Current developments in the tissue engineering of autologous heart valves: moving towards clinical use

The use of tissue-engineering methods to create autologous heart valve constructs has the potential to overcome the fundamental drawbacks of more traditional valve prostheses. Traditional mechanical valves, while durable, increase the risk for endocarditis and thrombogenesis, and require the recipient to continue lifelong anticoagulant therapy. Homograft or xenograft heart valve prostheses are associated with immune reaction and progressive deterioration with limited durability. Most importantly, neither option is capable of growth and remodeling in vivo and both options place the patient at risk for valve-related complications and reoperation. These shortcomings have prompted the application of tissue-engineering techniques to create fully autologous heart valve replacements. Future clinically efficacious tissue-engineered autologous valves should be nonthrombogenic, biocompatible, capable of growth and remodeling in vivo, implantable with current surgical techniques, hemodynamically perfect, durable for the patient’s life and most importantly, significantly improve quality of life for the patient. In order to meet these expectations, the nature of the ideal biochemical milieu for conditioning an autologous heart valve will need to be elucidated. In addition, standardized criteria by which to quantitatively evaluate a tissue-engineered heart valve, as well as noninvasive analytical techniques for use in long-term animal models, will be required. This article highlights the advances, challenges and future clinical prospects in the field of tissue engineering of autologous heart valves, focusing on progress made by studies that have investigated a fully autologous, tissue-engineered pulmonary valve replacement in vivo.

Current developments and future prospects for heart valve replacement therapy

Valve replacement is the most common surgical treatment in patients with advanced valvular heart disease. Mechanical and bio-prostheses have been the traditional heart valve replacements in these patients. However, currently the heart valves for replacement therapy are imperfect and subject patients to one or more ongoing risks, including thrombosis, limited durability, and need for re-operations due to the lack of growth in pediatric populations. Furthermore, they require an open heart surgery, which is risky for elderly and young children who are too weak or ill to undergo major surgery. This article reviews the current state of the art of heart valve replacements in light of their potential clinical applications. In recent years polymeric materials have been widely studied as potential prosthetic heart valve material being designed to overcome the clinical problems associated with both mechanical and bio-prosthetic valves. The review also addresses the advances in polymer materials, tissue engineering approaches, and the development of percutaneous valve replacement technology and discusses the future prospects in these fields.

The use of acellular matrices for the tissue engineering of cardiac valves

Tissue-engineering approaches to cardiac valve replacement have made considerable advances over recent years and it is likely that this application will realize clinical success in the near future. Research in this area has been driven by the inadequacy of the currently available cardiac valve prostheses for younger patients who require multiple reoperations as they grow and develop. Tissue engineering has the potential to provide a valve capable of the same growth, repair, and regeneration as a natural valve and could improve outcomes for patients of all ages. Owing to the function and physical environment of the cardiac valve, the development of tissue-engineered replacements is unusual in that the biomechanical properties of the construct must dominate the biological properties in order for the valve to be functional at the time of implantation. As a result of this, conventional tissue-engineering scaffolds based on biodegradable polymers or collagen may not at present be suitable in this situation because of their initial limited strength. Research into the use of acellular xenogeneic and allogeneic matrices for tissue-engineered heart valves has consequently become extremely popular since the biomechanical properties of the valve can potentially be preserved with an optimal decellularization technique that removes the cells without damaging the matrix. A number of acellular scaffolds have already been tested clinically both unseeded and preseeded with cells and these have met with variable results. This article reviews the concepts involved and the advantages and disadvantages of the different approaches to tissue engineering a living cardiac valve.

Approaches to heart valve tissue engineering scaffold design

Heart valve disease is a significant cause of mortality worldwide. However, to date, a nonthrombogenic, noncalcific prosthetic, which maintains normal valve mechanical properties and hemodynamic flow, and exhibits sufficient fatigue properties has not been designed. Current prosthetic designs have not been optimized and are unsuitable treatment for congenital heart defects. Research is therefore moving towards the development of a tissue engineered heart valve equivalent. Two approaches may be used in the creation of a tissue engineered heart valve, the traditional approach, which involves seeding a scaffold in vitro, in the presence of specific signals prior to implantation, and the guided tissue regeneration approach, which relies on autologous reseeding in vivo. Regardless of the approach taken, the design of a scaffold capable of supporting the growth of cells and extracellular matrix generation and capable of withstanding the unrelenting cardiovascular environment while forming a tight seal during closure, is critical to the success of the tissue engineered construct. This paper focuses on the quest to design, such a scaffold.

Histopathologic changes of acellularized xenogenic carotid vascular grafts implanted in a pig-to-goat model

INTRODUCTION

The present study was designed to determine the in vivo patency and recellularization pattern of acellularized small-diameter xenogenic arterial grafts. We implanted acellularized porcine carotid arteries in bilateral carotid arteries of goats and microscopically analyzed the recellularization pattern of these grafts with the recipient's cells over time.

MATERIAL AND METHODS

Carotid arteries of pigs weighing 30-40 kg were harvested and decellularized with hypertonic saline followed by sodium dodecyl sulfate. Acellularized porcine carotid vascular xenografts (0.4-0.5 cm in diameter) were prepared into 4 cm-long segments and implanted bilaterally in the carotid arteries of 10 black-haired goats. The in vivo patency of the implanted acellularized xenogenic grafts was evaluated at regular intervals by color Doppler ultrasonography. The goats were sacrificed at predetermined intervals (1 week, 1 month, 3 months, 6 months, 12 months after implantation), two animals at each interval. Upon retrieval, visual inspections and histopathologic examinations of the grafts were performed. To identify smooth muscle cells and functioning endothelial cells, immunohistochemical staining for alpha-smooth muscle actin and von Willebrand factor were also performed.

RESULTS AND CONCLUSIONS

All experimental animals survived the observation period. Nineteen out of 20 implanted grafts showed patency with no thrombi. Microscopic analysis revealed that the grafts were completely covered with the hosts' endothelial cells, beginning from anastomotic sites. The grafts were gradually recellularized with recipients'cells including fibroblasts, myofibroblasts and smooth muscle cells. In conclusion, this study suggested that acellularized xenogenic vascular grafts can be a good alternative for the small-diameter vascular graft.

Assessment of Tissue Ingrowth Rates in Polyurethane Scaffolds for Tissue Engineering

The continuous development of new biomaterials for tissue engineering and the enhancement of tissue ingrowth into existing scaffolds, using growth factors, create the necessity for developing adequate tools to assess tissue ingrowth rates into porous biomaterials. Current histomorphometric techniques evaluating rates of tissue ingrowth tend either to measure the overall tissue content in an entire sample or to depend on the user to indicate a front of tissue ingrowth. Neither method is particularly suitable for the assessment of tissue ingrowth rates, as these methods either lack the sensitivity required or are problematic when there is a tissue ingrowth gradient rather than an obvious tissue ingrowth front. This study describes a histomorphometric method that requires little observer input, is sensitive, and renders detailed information for the assessment of tissue ingrowth rates into porous biomaterials. This is achieved by examining a number of computer-defined concentric zones, which are based on the distance of a pixel from the scaffold edge. Each zone is automatically analyzed for tissue content, eliminating the need for user definition of a tissue ingrowth front and thus reducing errors and observer dependence. Tissue ingrowth rates in two biodegradable polyurethane scaffolds (Estane and polycaprolactone-polyurethane [PCLPU]) specifically designed for tissue engineering of the knee meniscus were assessed. Samples were subcutaneously implanted in rats with follow-up until 6 months. Especially at the earlier follow-up points, PCLPU scaffolds showed significantly higher tissue ingrowth rates than Estane scaffolds, making the PCLPU scaffold a promising candidate for further studies investigating meniscus tissue engineering.

Tissue engineering a functional aortic heart valve: an appraisal

Valvular heart disease is an important cause of morbidity and mortality, and currently available substitutes for failing hearts have serious limitations. A new promising alternative that may overcome these shortcomings is provided by the relatively new field of tissue engineering (TE). TE techniques involve the growth of autologous cells on a 3D matrix that can be a biodegradable polymer scaffold, or an acellular tissue matrix. These approaches provide the potential to create living matrix valve structures with an ability to grow, repair and remodel within the recipient. This article provides an appraisal of artificial heart valves and an overview of developments in TE that includes the current limitations and challenges for creating a fully functional valve. Biomaterials and stem cell technologies are now providing the potential for new avenues of research and if combined with advances in the rapid prototyping of biomaterials, the engineering of personalized, fully functional, and autologous tissue valve replacements, may become a clinical alternative.

Autologous cardiomyocyte transplantation using a biodegradable polymer scaffold

BACKGROUND

To achieve a more reliable way of transplanting cardiomyocytes, we conducted an autologous cardiomyocyte transplantation using a biodegradable scaffold, instead of a syringe injection, as a vehicle for transporting cells in an ovine myocardial infarction model.

MATERIALS AND METHODS

A myocardial infarction was created in sheep using sequential ligation of the homonymous artery and its diagonal branch. Autologous cardiomyocytes from the right ventricular infundibulum were cultured and seeded onto a biodegradable polymer scaffold. Three months after creating myocardial infarction, the two animals were re-anesthetized and cardiomyocyte-seeded scaffolds were implanted in the infarcted area. The animals were kept alive for a further month, and then sacrificed for postmortem heart examinations. Light microscopic analysis and an immunohistochemical study for myoglobin were performed.

RESULTS

On postmortem gross examinations, the polymer scaffolds were visible in the background of well-demarcated thin-walled anteroseptal myocardial infarcts. Microscopic analysis showed abundant myoglobin-stained cells between the fiber strands of the polymer scaffolds. However, there is a possibility that some of these cells might have been giant cells reacting to foreign material.

CONCLUSION

The transplantation of cultured autologous cardiomyocytes into an infarct region using a biodegradable scaffold instead of syringe injection provides another promising option for cardiomyocyte transplantation, which warrants further study.

Role of inflammation and ischemia after implantation of xenogeneic pulmonary valve conduits: histological evaluation after 6 to 12 months in sheep

OBJECTIVE

Commercially available biological heart valve prostheses undergo degenerative changes, which finally lead to complete destruction. Here we evaluate the role of inflammation and ischemia after implantation of xenogeneic heart valve conduits (XPC) generated by novel concepts of tissue engineering.

METHODS

Acellularized (a-)XPC and autologus re-seeded (s-)XPC were implanted into sheep. Samples were taken as follows: after acellularization (n = 2), after re-seeding (n = 2), 6 months (seeded/non-seeded: n = 3/5), 9 months (n = 2/5), and 12 months (n = 3/2) post implantation. Five native porcine conduits served as control. Using histological methods, samples were evaluated for pathological changes and existence/density of microvessels.

RESULTS

Prior to implantation a-XPC were completely free of cells. Six months after implantation, leaflets and pulmonary arteries of s-XPC and a-XPC showed good endothelial surface coverage. Microvessel density within the myocardial cuffs and pulmonary vessel walls were comparable to control in all grafts. However, 6, 9 and 12 months after implantation pathological severe microvessel ingrowth, calcification and cellular infiltrations were observed on a-XPC and s-XPC valves, whereas myocardial cuffs and XPC-artery walls showed only mild degenerative alterations.

CONCLUSIONS

Inflammatory reactions play a pivotal role in the degeneration of a-XPC and s-XPC. Thus, since ischemia seems to have little or no influence on this process, inflammation inductive factors should be the center of interest.

Tissue-engineered heart valve leaflets: an effective method of obtaining acellularized valve xenografts

To determine the most effective method of producing the acellularized xenograft heart valve leaflets, we compared pathological findings of the xenograft heart valve leaflets produced by three methods; freeze-thawing, Triton and NaCl-SDS treatment and further analyzed the pattern of endothelial cells seeded onto them.

MATERIALS AND METHODS

Two pigs were sacrificed and three pulmonary valve leaflets were harvested from each animal. They were immediately stored in a tissue preservation solution and assigned in one of the three preparation methods for acellularization. Endothelial cells from the jugular vein of a goat were isolated and seeded onto the acellularized xenograft heart valve leaflets. Light and Electron microscopic analyses were performed.

RESULT AND CONCLUSION

H & E stain showed that cells were almost absent in the leaflet treated with NaCl-SDS, while cells were partly present in the leaflets treated, one with Triton and the other Freeze-thawing. Transmission microscopic analyses showed cell-free matrix with well preserved collagen architecture under the seeded endothelial cells in the leaflets treated with NaCl-SDS. In conclusion, the valve leaflets treated with NaCl-SDS among three representative methods of acellularization of tissues (freeze-thawing, Triton X-100, and NaCl-SDS) showed the better results than the others in terms of the efficacy of decellularization and response to endothelial cell seeding.