Tissue engineering heart valves: valve leaflet replacement study in a lamb model

Prenatally fabricated autologous human living heart valves based on amniotic fluid derived progenitor cells as single cell source

BACKGROUND

A novel concept providing prenatally tissue engineered human autologous heart valves based on routinely obtained fetal amniotic fluid progenitors as single cell source is introduced.

METHODS AND RESULTS

Fetal human amniotic progenitors were isolated from routinely sampled amniotic fluid and sorted using CD133 magnetic beads. After expansion and differentiation, cell phenotypes of CD133- and CD133+ cells were analyzed by immunohistochemistry and flowcytometry. After characterization, CD133- derived cells were seeded onto heart valve leaflet scaffolds (n=18) fabricated from rapidly biodegradable polymers, conditioned in a pulse duplicator system, and subsequently coated with CD133+ derived cells. After in vitro maturation, opening and closing behavior of leaflets was investigated. Neo-tissues were analyzed by histology, immunohistochemistry, and scanning electron microscopy (SEM). Extracellular matrix (ECM) elements and cell numbers were quantified biochemically. Mechanical properties were assessed by tensile testing. CD133- derived cells demonstrated characteristics of mesenchymal progenitors expressing CD44 and CD105. Differentiated CD133+ cells showed features of functional endothelial cells by eNOS and CD141 expression. Engineered heart valve leaflets demonstrated endothelialized tissue formation with production of ECM elements (GAG 80%, HYP 5%, cell number 100% of native values). SEM showed intact endothelial surfaces. Opening and closing behavior was sufficient under half of systemic conditions.

CONCLUSIONS

The use of amniotic fluid as single cell source is a promising low-risk approach enabling the prenatal fabrication of heart valves ready to use at birth. These living replacements with the potential of growth, remodeling, and regeneration may realize the early repair of congenital malformations.

Tissue engineering of heart valves using decellularized xenogeneic or polymeric starter matrices

Heart valve replacement represents the most common surgical therapy for end-stage valvular heart diseases. A major drawback that all contemporary heart valve replacements have in common is the lack of growth, repair and remodelling capability. In order to overcome these limitations, the emerging new field of tissue engineering is focusing on the in vitro generation of functional, living heart valve replacements. The basic approach uses starter matrices either of decellularized xenogeneic or polymeric materials configured in the shape of the heart valve and subsequent cell seeding. This manuscript will give a detailed overview of these two concepts without giving favour to one or the other. The concluding discussion will focus on current limitations and studies as well as future challenges prior to safe clinical application.

In vivo autologous recellularization of a tissue-engineered heart valve: are bone marrow mesenchymal stem cells the best candidates?

OBJECTIVE

Bone marrow stem cells, especially the mesenchymal stem cell subpopulation, have been used to create in vitro tissue-engineered heart valves. We hypothesized that autologous bone marrow cells, injected in a decellularized porcine scaffold before surgical implantation, could promote in vivo recolonization and limit valve deterioration. We thus analyzed the effects of in situ injection of autologous bone marrow mononuclear cells and of mesenchymal stem cells on the outcome of xenogenic decellularized scaffolds in a lamb model.

METHODS

Decellularized porcine pulmonary valves were implanted in the pulmonary artery under cardiopulmonary bypass in 14 lambs after injection in the scaffold of autologous bone marrow mononuclear cells (BMMC) group (n = 7) or of mesenchymal stem cells (MSC) group (n = 7). At 4 months, valve function was evaluated by echocardiography, and valves were explanted for macroscopic and histologic analysis.

RESULTS

Mean transvalvular and distal gradients (millimeters of mercury) were lower in the MSC than those in the BMMC group (1.3 +/- 0.39 vs 4.24 +/- 0.91 and 4.05 +/- 1.89 vs 12.02 +/- 6.95, respectively; P < .02). Histologic examination showed significant recolonization and re-endothelialization in both groups. However, significant valve thickening and inflammatory cell infiltration were observed in the BMMC group. By contrast, valves from the MSC group displayed extracellular matrix and cell disposition close to those of native pulmonary valves.

CONCLUSIONS

Tissue-engineered heart valves created from mesenchymal stem cells, injected directly in a decellularized xenograft scaffold, exhibited satisfactory hemodynamic and histologic aspects after 4 months. Further long-term studies are needed to demonstrate the potential of mesenchymal stem cells for clinical application in heart valve surgery.

Endothelial progenitor and mesenchymal stem cell-derived cells persist in tissue-engineered patch in vivo: application of green and red fluorescent protein-expressing retroviral vector

An unresolved question regarding tissue-engineered (TE) cardiac valves and vessels is the fate of the transplanted cells in vivo. We have developed a strategy to track the anatomic location of seeded cells within TE constructs and neighboring tissues using a retroviral vector system encoding green and red fluorescent proteins (GFPs and RFPs, respectively) in ovine circulating endothelial progenitor cells (EPCs) and bone marrow-derived mesenchymal stem cells (BMSCs). We demonstrate that stable transduction ex vivo with high-titer Moloney murine leukemia virus-based retroviral vector yields transduction efficiency of greater than 97% GFP(+) EPC- and RFP(+) mesenchymal stem cell (MSC)-derived cells. Cellular phenotype and transgene expression were also maintained through 25 subsequent passages. Using a retroviral vector system to distinguish our pre-seeded cells from tissue-resident progenitor cells and circulating endothelial and marrow-derived precursors, we simultaneously co-seeded 2 x 10(6) GFP(+) EPCs and 2 x 10(5) RFP(+) MSCs onto the TE patches. In a series of ovine pulmonary artery patch augmentation studies, transplanted GFP(+) EPC- and RFP(+) MSC-derived cells persisted within the TE patch 7 to 14 days after implantation, as identified using immunofluorescence. Analysis showed 81% luminal coverage of the TE patches before implantation with transduced cells, increasing to 96% at day 7 and decreasing to 67% at day 14 post-implantation. This suggests a temporal association between retroviral expression of progenitor cells and mediating effects of these cells on the physiological remodeling and maturation of the TE constructs. To our knowledge, this is the first cardiovascular tissue-engineering in vivo study using a double-labeling method to demonstrate a direct evidence of the source, persistence, and incorporation into a TE vascular patch of co-cultured and simultaneously pre-seeded adult progenitor cells.

Engineering the heart piece by piece: state of the art in cardiac tissue engineering

According to the National Transplant Society, more than 7000 Americans in need of organs die every year owing to a lack of lifesaving organs. Bioengineering 3D organs in vitro for subsequent implantation may provide a solution to this problem. The field of tissue engineering in its most rudimentary form is focused on the developed of transplantable organ substitutes in the laboratory. The objective of this article is to introduce important technological hurdles in the field of cardiac tissue engineering. This review starts with an overview of tissue engineering, followed by an introduction to the field of cardiovascular tissue engineering and finally summarizes some of the key advances in cardiac tissue engineering; specific topics discussed in this article include cell sourcing and biomaterials, in vitro models of cardiac muscle and bioreactors. The article concludes with thoughts on the utility of tissue-engineering models in basic research as well as critical technological hurdles that need to be addressed in the future.

The cardiovascular tissue-reactor: a novel device for the engineering of heart valves

The shortage of human donor valves and disadvantages of current mechanical and xenogenic valve grafts cause a growing demand for biologically engineered valves. We developed a bioreactor for in vitro transformation of porcine semilunar heart valves into human valves. The reactor design was optimized in order to achieve a complete removal of porcine cells by trypsinization and cellularization of the remaining matrix with human vascular cells. The physical parameters of the reactor were characterized. Based on these data, decellularization and cellularization protocols were developed and the successful application of protocols was investigated by immunohistochemistry. The resulting reactor consisted of two connectable and perfuseable modules. Heart valve structures of varying sizes could be mounted into the reactor and could be completely depopulated within 4 days by a sequence of enzymatic and mechanical treatments applied simultaneously to the valves. Complete cellularization with human cells could be achieved after optimizing the seeding design, density, and time.

Heart valve tissue engineering: concepts, approaches, progress, and challenges

Potential applications of tissue engineering in regenerative medicine range from structural tissues to organs with complex function. This review focuses on the engineering of heart valve tissue, a goal which involves a unique combination of biological, engineering, and technological hurdles. We emphasize basic concepts, approaches and methods, progress made, and remaining challenges. To provide a framework for understanding the enabling scientific principles, we first examine the elements and features of normal heart valve functional structure, biomechanics, development, maturation, remodeling, and response to injury. Following a discussion of the fundamental principles of tissue engineering applicable to heart valves, we examine three approaches to achieving the goal of an engineered tissue heart valve: (1) cell seeding of biodegradable synthetic scaffolds, (2) cell seeding of processed tissue scaffolds, and (3) in-vivo repopulation by circulating endogenous cells of implanted substrates without prior in-vitro cell seeding. Lastly, we analyze challenges to the field and suggest future directions for both preclinical and translational (clinical) studies that will be needed to address key regulatory issues for safety and efficacy of the application of tissue engineering and regenerative approaches to heart valves. Although modest progress has been made toward the goal of a clinically useful tissue engineered heart valve, further success and ultimate human benefit will be dependent upon advances in biodegradable polymers and other scaffolds, cellular manipulation, strategies for rebuilding the extracellular matrix, and techniques to characterize and potentially non-invasively assess the speed and quality of tissue healing and remodeling.

Biocompatibility and biodegradation of novel PHB porous substrates with controlled multi-pore size by emulsion templates method

PHB porous substrates were prepared based on the mono-membrane fabricated by emulsion templates method. The key factors of the method affecting the pore size and porosity of the PHB porous substrates were studied. The surface of PHB porous substrates were investigated by scanning electron microscope (SEM), which showed the even pore size and regular arranged pore. The transect of the PHB porous substrates prepared using the templates method was good. Moreover, the effects of variation of surfactant content (P%) and water content (R) on the pore size and porosity of PHB films were discussed. Preliminary studies showed that when P% is less than 20%, the pore size made by emulsion templates ranged from 5 microm to 30 microm with the value of P increasing. As P% is up to 20%. It was interesting to see that the porous substrates had muti-pore size distribution, i.e., median pore sizes were about 5 microm and inside the wall of pore, there existed numerous micro-pores size can be controlled from 100 nm to 500 nm only by adjusting the parameter R of the microemulsion. The cell-compatibility was evaluated via Chinese Hamster Lung (CHL) fibroblast cultivation in vitro. The Cells were cultured on both the mono-pore size membrane prepared by emulsion templates and the multi-pore size membrane prepared by microemulsion templates. It can be seen that the cells cultured on multi-pore size membrane stretched their morphology and proliferated better than that of mono-pore size membrane. These results indicated that the multi-pore size membrane had better cell-compatibility and was more suitable for tissue engineering. The degradation experiment indicated that the degradation of PHB porous substrates were accelerated by enzyme in vitro and the porous configuration was favorable to its degradation.

Engineering of Biologically Active Living Heart Valve Leaflets Using Human Umbilical Cord–Derived Progenitor Cells

This study demonstrates the engineering of biologically active heart valve leaflets using prenatally available human umbilical cord-derived progenitor cells as the only cell source. Wharton's Jelly-derived cells and umbilical cord blood-derived endothelial progenitor cells were subsequently seeded on biodegradable scaffolds and cultured in a biomimetic system under biochemical or mechanical stimulation or both. Depending on the stimulation, leaflets showed mature layered tissue formation with functional endothelia and extracellular matrix production comparable with that of native tissues. This demonstrates the feasibility of heart valve leaflet fabrication from prenatal umbilical cord-derived progenitor cells as a further step in overcoming the lack of living autologous replacements with growth and regeneration potential for the repair of congenital malformation.

Engineered Living Blood Vessels: Functional Endothelia Generated From Human Umbilical Cord-Derived Progenitors

BACKGROUND

Tissue-engineered living blood vessels (TEBV) with growth capacity represent a promising new option for the repair of congenital malformations. We investigate the functionality of TEBV with endothelia generated from human umbilical cord blood-derived endothelial progenitor cells.

METHODS

Tissue-engineered living blood vessels were generated from human umbilical cord-derived myofibroblasts seeded on biodegradable vascular scaffolds, followed by endothelialization with differentiated cord blood-derived endothelial progenitor cells. During in vitro maturation the TEBV were exposed to physiologic conditioning in a flow bioreactor. For functional assessment, a subgroup of TEBV was stimulated with tumor necrosis factor-alpha. Control vessels endothelialized with standard vascular endothelial cells were treated in parallel. Analysis of the TEBV included histology, immunohistochemistry, biochemistry (extracellular matrix analysis, DNA), and biomechanical testing. Endothelia were analyzed by flow cytometry and immunohistochemistry (CD31, von Willebrand factor, thrombomodulin, tissue factor, endothelial nitric oxide synthase).

RESULTS

Histologically, a three-layered tissue organization of the TEBV analogous to native vessels was observed, and biochemistry revealed the major matrix constituents (collagen, proteoglycans) of blood vessels. Biomechanical properties (Young's modulus, 2.03 +/- 0.65 MPa) showed profiles resembling those of native tissue. Endothelial progenitor cells expressed typical endothelial cell markers CD31, von Willebrand factor, and endothelial nitric oxide synthase comparable to standard vascular endothelial cells. Stimulation with tumor necrosis factor-alpha resulted in physiologic upregulation of tissue factor and downregulation of thrombomodulin expression.

CONCLUSIONS

These results indicate that TEBV with tissue architecture and functional endothelia similar to native blood vessels can be successfully generated from human umbilical cord progenitor cells. Thus, blood-derived progenitor cells obtained before or at birth may enable the clinical realization of tissue engineering constructs for pediatric applications.

Reference Models for Mitral Valve Tissue Engineering Based on Valve Cell Phenotype and Extracellular Matrix Analysis

The advance of mitral valve repair techniques through tissue engineering is impeded by the lack of information regarding the cellular and extracellular components of the mitral valve. The present study aims to expand our understanding of the mitral valve structure by analysing the synthesis of extracellular matrix (ECM) proteins and the expression of nitric oxide synthase (NOS). Valvular endothelial cells (VECs) and valvular interstitial cells (VICs) were isolated from porcine mitral valves. Immunochemical staining of ECM components, including type I, II, III, IV and V collagen, laminin, fibronectin, elastin and chondroitin sulphate (CS), was performed on both mitral valve tissue and cell cultures. Reverse transcription polymerase chain reaction and immunochemistry were used to analyse NOS expression in native valve and in culture. Both VECs and VICs synthesised the basement membrane components, laminin and type IV collagen both in vivo and in vitro, amongst other fibrous ECM proteins. Synthesis of type I collagen and CS was absent in VEC cultures. Each cell type had a characteristic profile of NOS expression. VECs synthesised endothelial NOS both in vivo and in vitro, with a minority of VICs expressing neuronal NOS in vitro. The present study reports newly recognised aspects of the mitral valve structure and the in vitro behaviour of mitral valve cell populations based on ECM synthesis and NOS expression. The presented profiles can be used as base tools for the generation of data necessary for the selection of ideal cell sources and for the design of appropriate scaffolds for the development of effective tissue-engineered mitral valves.

Potential for Synthesis and Degradation of Extracellular Matrix Proteins by Valve Interstitial Cells Seeded onto Collagen Scaffolds

Matrix remodeling, which involves proteolytic enzymes, such as the matrix metalloproteinases (MMPs), is of significant importance with respect to tissue engineering a heart valve construct. The ability of valve interstitial cells (ICs) to release these enzymes in biological scaffolds and to synthesize their own matrix has not been adequately studied, and this has important implications for tissue engineering. Cultured human aortic valve ICs were seeded onto a 3-dimensional type I collagen matrix for 28 days, whereby the presence of the remodeling enzymes, MMPs, were determined using immunohistochemistry, and detection of extracellular matrix (ECM) gene expression was performed using in situ hybridization. The collagenases, stromelysins, and membrane-type MMPs were expressed in 1%, 2%, and 5% collagen scaffolds after 28 days, whereas gelatinase expression was not observed. In situ hybridization revealed the presence of the ECM messenger ribonucleic acid (mRNA) in cells cultured in collagen scaffolds however, an increase in all three mRNAs was only detected in the 1% collagen scaffolds. The presence of collagenases, stromelysins, and membrane-type MMPs indicate that human valve ICs have the capacity to remodel type I collagen scaffold and that the genes necessary for synthesizing matrix have been turned on within the cells themselves. Scaffold composition also demonstrated differential effects onMMPexpression. These observations are of relevance with respect to the development of tissue-engineered heart valves.

Living autologous heart valves engineered from human prenatally harvested progenitors

BACKGROUND

Heart valve tissue engineering is a promising strategy to overcome the lack of autologous growing replacements, particularly for the repair of congenital malformations. Here, we present a novel concept using human prenatal progenitor cells as new and exclusive cell source to generate autologous implants ready for use at birth.

METHODS AND RESULTS

Human fetal mesenchymal progenitors were isolated from routinely sampled prenatal chorionic villus specimens and expanded in vitro. A portion was cryopreserved. After phenotyping and genotyping, cells were seeded onto synthetic biodegradable leaflet scaffolds (n=12) and conditioned in a bioreactor. After 21 days, leaflets were endothelialized with umbilical cord blood-derived endothelial progenitor cells and conditioned for additional 7 days. Resulting tissues were analyzed by histology, immunohistochemistry, biochemistry (amounts of extracellular matrix, DNA), mechanical testing, and scanning electron microscopy (SEM) and were compared with native neonatal heart valve leaflets. Fresh and cryopreserved cells showed comparable myofibroblast-like phenotypes. Genotyping confirmed their fetal origin. Neo-tissues exhibited organization, cell phenotypes, extracellular matrix production, and DNA content comparable to their native counterparts. Leaflet surfaces were covered with functional endothelia. SEM showed cellular distribution throughout the polymer and smooth surfaces. Mechanical profiles approximated those of native heart valves.

CONCLUSIONS

Prenatal fetal progenitors obtained from routine chorionic villus sampling were successfully used as an exclusive, new cell source for the engineering of living heart valve leaflets. This concept may enable autologous replacements with growth potential ready for use at birth. Combined with the use of cell banking technology, this approach may be applied also for postnatal applications.

Autologous human tissue-engineered heart valves: prospects for systemic application

BACKGROUND

Tissue engineering represents a promising approach for the development of living heart valve replacements. In vivo animal studies of tissue-engineered autologous heart valves have focused on pulmonary valve replacements, leaving the challenge to tissue engineer heart valves suitable for systemic application using human cells.

METHODS AND RESULTS

Tissue-engineered human heart valves were analyzed up to 4 weeks and conditioning using bioreactors was compared with static culturing. Tissue formation and mechanical properties increased with time and when using conditioning. Organization of the tissue, in terms of anisotropic properties, increased when conditioning was dynamic in nature. Exposure of the valves to physiological aortic valve flow demonstrated proper opening motion. Closure dynamics were suboptimal, most likely caused by the lower degree of anisotropy when compared with native aortic valve leaflets.

CONCLUSIONS

This study presents autologous tissue-engineered heart valves based on human saphenous vein cells and a rapid degrading synthetic scaffold. Tissue properties and mechanical behavior might allow for use as living aortic valve replacements.

Valvular endothelial cells regulate the phenotype of interstitial cells in co-culture: effects of steady shear stress

Valvular endothelial cells interact with interstitial cells in a complex hemodynamic and mechanical environment to maintain leaflet tissue integrity. The precise roles of each cell type are difficult to ascertain in a controlled manner in vivo. The objective of this study was to develop a three-dimensional aortic valve leaflet model, comprised of valvular endothelium and interstitial cells, and determine the cellular responses to imposed lumenal fluid flow. Two leaflet models were created using type I collagen hydrogels. Model 1 contained 1 million/mL porcine aortic valve interstitial cells (PAVICs). Model 2 added a seeding of the lumenal surface of Model 1 with approximately 50,000/cm(2) porcine aortic valve endothelial cells (PAVECs). Both leaflet models were exposed to 20 dynes/cm(2) steady shear for up to 96 h, with static constructs serving as controls. Endothelial cell alignment, matrix production, and cell phenotype were monitored. The results indicate that PAVECs align perpendicularly to flow similar to 2D culture. We report that PAVICs in model 1 express vimentin strongly and alpha-smooth-muscle actin (SMA) to a lesser extent, but SMA expression is increased by shear stress, particularly near the lumenal surface. Model 1 constructs increase in cell number, maintain protein levels, but lose glycosaminoglycans in response to shear. Co-culture with PAVECs (Model 2) modulates these responses in both static and flow environments, resulting in PAVIC phenotype that is more similar to the native condition. PAVECs stimulated a decrease in PAVIC proliferation, an increase in protein synthesis with shear stress, and reduced the loss of glycosaminoglycans with flow. Additionally, PAVECs stimulated PAVIC differentiation to a more quiescent phenotype, defined by reduced expression of SMA. These results suggest that valvular endothelial cells are necessary to properly regulate interstitial cell phenotype and matrix synthesis. Additionally, we show that tissue-engineered models can be used to discover and understand complex biomechanical relationships between cells that interact in vivo.

Allograft tissue for use in valve replacement

Homograft or allograft tissue has been available for use as replacement for diseased valves or reconstruction of major vessels for decades. However, with respect to replacement of diseased valvular tissue the search for the ideal valve still continues. In this review we will discuss the clinical indications, surgical techniques, and outcome of aortic homografts.

Transforming Growth Factor-β1 Modulates Extracellular Matrix Production, Proliferation, and Apoptosis of Endothelial Progenitor Cells in Tissue-Engineering Scaffolds

Background— Valvular endothelial cells and circulating endothelial progenitor cells (EPCs) can undergo apparent phenotypic change from endothelial to mesenchymal cell type. Here we investigated whether EPCs can promote extracellular matrix formation in tissue engineering scaffolds in response to transforming growth factor (TGF)-β1.

Method and Results— Characterized ovine peripheral blood EPCs were seeded onto poly (glycolic acid)/poly (4-hydroxybutyrate) scaffolds for 5 days. After seeding at 2×10 6 cells/cm 2 , scaffolds were incubated for 5 days in a roller bottle, with or without the addition of TGF-β1. After seeding at 15×10 6 cells/cm 2 , scaffolds were incubated for 10 days in a roller bottle with or without the addition of TGF-β1 for the first 5 days. Using immunofluorescence and Western blotting, we demonstrated that EPCs initially exhibit an endothelial phenotype (ie, CD31 + , von Willebrand factor + , and α–smooth muscle actin (SMA) − ) and can undergo a phenotypic change toward mesenchymal transformation (ie, CD31 + and α-SMA + ) in response to TGF-β1. Scanning electron microscopy and histology revealed enhanced tissue formation in EPC-TGF-β1 scaffolds. In both the 10- and 15-day experiments, EPC-TGF-β1 scaffolds exhibited a trend of increased DNA content compared with unstimulated EPC scaffolds. TGF-β1–mediated endothelial to mesenchymal transformation correlated with enhanced expression of laminin and fibronectin within scaffolds evidenced by Western blotting. Strong expression of tropoelastin was observed in response to TGF-β1 equal to that in the unstimulated EPC. In the 15-day experiments, TGF-β1–stimulated scaffolds revealed dramatically enhanced collagen production (types I and III) and incorporated more 5-bromodeoxyuridine and TUNEL staining compared with unstimulated controls.

Conclusions— Stimulation of EPC-seeded tissue engineering scaffolds with TGF-β1 in vitro resulted in a more organized cellular architecture with glycoprotein, collagen, and elastin synthesis, and thus noninvasively isolated EPCs coupled with the pleiotropic actions of TGF-β1 could offer new strategies to guide tissue formation in engineered cardiac valves.

Tissue engineering of viable pulmonary arteries for surgical correction of congenital heart defects

BACKGROUND

Tissue-engineered pulmonary arteries could overcome the drawbacks of homografts or prosthetic conduits used in the repair of many congenital cardiac defects. However, the ideal scaffold material for tissue-engineered conduits is still subject of intensive debate. In this study, we evaluated an acellularized allogeneic matrix scaffold for pulmonary artery tissue engineering with and without in-vitro reseeding with autologous endothelial cells in the pulmonary circulation in a growing sheep model.

METHODS

Ovine pulmonary arteries (n = 10) were acellularized by trypsin/ethylenediamine tetraacetic acid incubation. Autologous endothelial cells were harvested from carotid arteries, and the pulmonary conduits were seeded with endothelial cells. We implanted in-vitro, autologous, reendothelialized (group A, n = 5) and acellularized pulmonary conduits (group B, n = 5) in the pulmonary circulation. The animals were sacrificed 6 months after the operation. Explanted valves were examined histologically and by immunohistochemistry.

RESULTS

The conduit diameter increased in both groups (group A, 44% +/- 11%; group B, 87% +/- 18%; p < 0.05). In group A, however, a proportional increase in diameter was present, whereas in group B, a disproportionate increase resulting in aneurysm formation was observed. Histologically, the conduit wall integrity was destroyed in group B and preserved in group A. In group B, the extracellularmatrix degenerated with a reduced amount of collagens and proteoglycanes. Furthermore, no elastic fibers were detectable. In contrast, the extracellularmatrix in group A was close to native ovine tissue.

CONCLUSIONS

Tissue-engineered pulmonary conduits (autologous endothelial cells and allogeneic matrix scaffolds) functioned well in the pulmonary circulation. They demonstrated an increase in diameter and an extracellular matrix comparable to that of native ovine tissue.

Cell transplantation for heart failure, and tissue engineering of cardiovascular structures

The emergence of selective cell transplantation and tissue engineering has opened up new alternatives for the treatment of heart failure. These involve the use of stem and progenitor cells, as well as new biodegradable polymer scaffolds. The last two decades have seen a dramatic increase in our knowledge of how to fabricate a wide array of tissues, including cardiovascular structures. This article reviews current trends in selective cell transplantation and tissue engineering, and summarises recent achievements and remaining questions in constructing functional cardiac tissue.

Cell Sourcing and Culture Conditions for Fibrin-Based Valve Constructs

Cell sourcing for tissue-engineered heart valves remains a critical issue. In this work, human dermal fibroblasts (HDF) or porcine valve interstitial cells (PVIC) were entrapped in adherent fibrin disk constructs and harvested at 3 and 5 weeks. We compared the fibrin remodeling abilities of each cell type in Dulbecco's Modified Eagle's Medium (DMEM) and DMEM/F12 supplemented with transforming growth factor beta (TGF), and the response of PVIC to DMEM/F12 supplemented with fibroblast growth factor (FGF), a combination of FGF and TGF, and TGF with varying ascorbic acid (AA) concentrations. Culture media were supplemented with serum, insulin, AA, a fibrinolysis inhibitor, and antibiotics. DMEM maximized collagen and elastin deposition by HDF, while DMEM/F12 with FGF yielded the highest fibrin remodeling response by PVIC. HDF degraded fibrin slower than PVIC, and PVIC constructs had higher cellularity than HDF constructs in DMEM and DMEM/F12 at 3 weeks. FGF addition increased collagen content, collagen deposited per cell, and collagen as percentage of total protein compared to medium supplemented with TGF or TGF and FGF. AA addition increased collagen deposition by PVIC, but there was no dose dependence between 50 and 150 microg/mL AA. These results collectively show that PVIC are able to remodel fibrin faster and exhibit greater mechanical stiffening compared to HDF. Conditions for increased collagen deposition are also identified toward the engineering of valve constructs.

Decellularized heart valve as a scaffold for in vivo recellularization: Deleterious effects of granulocyte colony-stimulating factor

BACKGROUND

Autologous recellularization of decellularized heart valve scaffolds is a promising challenge in the field of tissue-engineered heart valves and could be boosted by bone marrow progenitor cell mobilization. The aim of this study was to examine the spontaneous in vivo recolonization potential of xenogeneic decellularized heart valves in a lamb model and the effects of granulocyte colony-stimulating factor mobilization of bone marrow cells on this process.

METHODS

Decellularized porcine aortic valves were implanted in 12 lambs. Six lambs received granulocyte colony-stimulating factor (10 microg x kg(-1) x d(-1) for 7 days, granulocyte colony-stimulating factor group), and 6 received no granulocyte colony-stimulating factor (control group). Additionally, nondecellularized porcine valves were implanted in 5 lambs (xenograft group). Angiographic and histologic evaluation was performed at 3, 6, 8, and 16 weeks.

RESULTS

Few macroscopic modifications of leaflets and the aortic wall were observed in the control group, whereas progressive shrinkage and thickening of the leaflets appeared in the granulocyte colony-stimulating factor and xenograft groups. In the 3 groups progressive ovine cell infiltration (fluorescence in situ hybridization) was observed in the leaflets and in the adventitia and the intima of the aortic wall but not in the media. Neointimal proliferation of alpha-actin-positive cells, inflammatory infiltration, adventitial neovascularization, and calcifications were more important in the xenograft and the granulocyte colony-stimulating factor groups than in the control group. Continuous re-endothelialization appeared only in the control group.

CONCLUSION

Decellularized xenogeneic heart valve scaffolds allowed partial autologous recellularization. Granulocyte colony-stimulating factor led to accelerated heart valve deterioration similar to that observed in nondecellularized xenogeneic cardiac bioprostheses.

A novel composite scaffold for cardiac tissue engineering

One approach to the engineering of functional cardiac tissue for basic studies and potential clinical use involves bioreactor cultivation of dissociated cells on a biomaterial scaffold. Our objective was to develop a scaffold that is (1) highly porous with large interconnected pores (to facilitate mass transport), (2) hydrophilic (to enhance cell attachment), (3) structurally stable (to withstand the shearing forces during bioreactor cultivation), (4) degradable (to provide ultimate biocompatibility of the tissue graft), and (5) elastic (to enable transmission of contractile forces). The scaffold of choice was made as a composite of poly(dl-lactide-co-caprolactone), poly(dl-lactide-co-glycolide) (PLGA), and type I collagen, with open interconnected pores and the average void volume of 80 +/- 5%. Neonatal rat heart cells suspended in Matrigel were seeded into the scaffold at a physiologically high density (1.35 x 10(8) cells/cm(3)) and cultivated for 8 d in cartridges perfused with culture medium or in orbitally mixed dishes (25 rpm); collagen sponge (Ultrafoam) and PLGA sponge served as controls. Construct cellularity, presence of cardiac markers, and contractile properties were markedly improved in composite scaffolds as compared with both controls.

Stem cells, tissue engineering and organogenesis in transplantation

Tissue engineering is an attempt to generate living tissues for surgical transplantation. In vitro and in vivo approaches have led to the production of vascular and cardiovascular components, bones, cartilages and gastrointestinal tissues. Organogenesis has a different aim, which is to create transplantable organs from embryonic tissue implanted into the recipient's omentum. This approach has been successful in creating kidneys and pancreases in animals. The use of stem cells in organogenesis and in tissue engineering has vastly enlarged the potential for clinical applications. The technique of nuclear transfer offers the possibility of creating cells, which are genetically identical to the host. Tissue engineering and organogenesis represent the future of transplantation in medicine. The progress in this field is of tremendous importance because it can produce a new generation of morphologically complex tissues and organs. In this review, the most relevant experiences in this area are summarized, including its perspectives for therapeutical applications.

Tissue engineering, stem cells and cloning: current concepts and changing trends

Organ damage or loss can occur from congenital disorders, cancer, trauma, infection, inflammation, iatrogenic injuries or other conditions and often necessitates reconstruction or replacement. Replacement may take the form of organ transplant. At present, there is a severe shortage of donor organs that is worsening with the aging of the population. Tissue engineering follows the principles of cell transplantation, materials science and engineering towards the development of biological substitutes that can restore and maintain normal tissue function. Therapeutic cloning involves the introduction of a nucleus from a donor cell into an enucleated oocyte to generate embryonic stem cell lines whose genetic material is identical to that of its source. These autologous stem cells have the potential to become almost any type of cell in the adult body, and thus would be useful in tissue and organ replacement applications. This paper reviews recent advances in stem cell research and regenerative medicine, and describes the clinical applications of these technologies as novel therapies for tissue or organ loss.

Comparative study of cellular and extracellular matrix composition of native and tissue engineered heart valves

Tissue engineering of heart valves utilizes biodegradable or metabolizable scaffolds for remodeling by seeded autologous cells. The aim of this study was to determine and compare extracellular matrix (ECM) formations, cellular phenotypes and cell location of native and tissue engineered (TE) valve leaflets. Ovine carotid arteries, ovine and porcine hearts were obtained from slaughterhouses. Cells were isolated from carotid arteries and dissected ovine, porcine and TE leaflets. TE constructs were fabricated from decellularized porcine pulmonary valves, seeded ovine arterial cells and subsequent 16 days dynamic in vitro culture using a pulsatile bioreactor. Native and TE valves were studied by histology (hematoxylin-eosin, resorcin-fuchsin, Movat pentachrome), NIR femtosecond multiphoton laser scanning microscopy and scanning electron microscopy (SEM). Cells of native and TE tissues were identified and localized by immunohistochemistry. Arterial, valvular and re-isolated TE-construct cells were processed for immunocytochemistry and Western blotting. ECM analysis and SEM revealed characteristical and comparable structures in native and TE leaflets. Most cells in native leaflets stained strongly positive for vimentin. Cells positive to alpha-smooth muscle actin (alpha-SMA), myosin and calponin were only found at the ventricular (inflow) side of ovine aortic and porcine pulmonary valve leaflets. Cells from TE constructs had a strong expression of vimentin, alpha-SMA, myosin, calponin and h-caldesmon throughout the entire leaflet. Comparable ECM formation and endothelial cell lining of native and TE leaflets could be demonstrated. However, immunostaining revealed significant differences between valvular cell phenotypes of native and TE leaflets. These results may be essential for further cardiovascular tissue engineering efforts.

Heart Valve Tissue Engineering

Tissue-engineered heart valves have been proposed by physicians and scientists alike to be the ultimate solution for treating valvular heart disease. Rather than replacing a diseased or defective native valve with a mechanical or animal tissue-derived artificial valve, a tissue-engineered valve would be a living organ, able to respond to growth and physiological forces in the same way that the native aortic valve does. Two main approaches have been attempted over the past 10 to 15 years: regeneration and repopulation. Regeneration involves the implantation of a resorbable matrix that is expected to remodel in vivo and yield a functional valve composed of the cells and connective tissue proteins of the patient. Repopulation involves implanting a whole porcine aortic valve that has been previously cleaned of all pig cells, leaving an intact, mechanically sound connective tissue matrix. The cells of the patients are expected to repopulate and revitalize the acellular matrix, creating living tissue that already has the complex microstructure necessary for proper function and durability. Regrettably, neither of the 2 approaches has fared well in animal experiments, and the only clinical experience with tissue-engineered valves resulted in a number of early failures and patient death. This article reviews the technological details of the 2 main approaches, their rationale, their strengths and weaknesses, and the likely mechanisms for their failure. Alternative approaches to valvular tissue engineering, as well as the role of industry in shaping this field in the future, are also reviewed.

Cell sheet engineering: recreating tissues without biodegradable scaffolds

While tissue engineering has long been thought to possess enormous potential, conventional applications using biodegradable scaffolds have limited the field's progress, demonstrating a need for new methods. We have previously developed cell sheet engineering using temperature-responsive culture dishes in order to avoid traditional tissue engineering approaches, and their related shortcomings. Using temperature-responsive dishes, cultured cells can be harvested as intact sheets by simple temperature changes, thereby avoiding the use of proteolytic enzymes. Cell sheet engineering therefore allows for tissue regeneration by either direct transplantation of cell sheets to host tissues or the creation of three-dimensional structures via the layering of individual cell sheets. By avoiding the use of any additional materials such as carrier substrates or scaffolds, the complications associated with traditional tissue engineering approaches such as host inflammatory responses to implanted polymer materials, can be avoided. Cell sheet engineering thus presents several significant advantages and can overcome many of the problems that have previously restricted tissue engineering with biodegradable scaffolds.

Midterm clinical result of tissue-engineered vascular autografts seeded with autologous bone marrow cells

OBJECTIVE

Prosthetic and bioprosthetic materials currently in use lack growth potential and therefore must be repeatedly replaced in pediatric patients as they grow. Tissue engineering is a new discipline that offers the potential for creating replacement structures from autologous cells and biodegradable polymer scaffolds. In May 2000, we initiated clinical application of tissue-engineered vascular grafts seeded with cultured cells. However, cell culturing is time-consuming, and xenoserum must be used. To overcome these disadvantages, we began to use bone marrow cells, readily available on the day of surgery, as a cell source. The aim of the study was to assess the safety and feasibility of this technique for creating vascular tissue under low-pressure systems such as pulmonary artery or venous pressure.

METHODS

Since September 2001, tissue-engineered grafts seeded with autologous bone marrow cells have been implanted in 42 patients. The patients or their parents were fully informed and had given consent to the procedure. A 5-mL/kg specimen of bone marrow was aspirated with the patient under general anesthesia before the skin incision. The polymer tube serving as a scaffold for the cells was composed of a copolymer of l -lactide and -caprolactone (50:50). This copolymer is degraded by hydrolysis. The matrix is more than 80% porous, and the diameter of each pore is 20 to 100 microm. Polyglycolic acid woven fabric with a thickness of 0.5 mm was used for reinforcement. Twenty-three tissue-engineered conduits (grafts for extracardiac total cavopulmonary connection) and 19 tissue-engineered patches were used for the repair of congenital heart defects. The patients' ages ranged from 1 to 24 years (median 5.5 years). All patients underwent a catheterization study, computed tomographic scan, or both, for evaluation after the operation. The patients received anticoagulation therapy for 3 to 6 months after surgery.

RESULTS

Mean follow-up after surgery was 490 +/- 276 days (1.3-31.6 months, median 16.7 months). There were no complications such as thrombosis, stenosis, or obstruction of the tissue-engineered autografts. One late death at 3 months after total cavopulmonary connection was noted in patient with hypoplastic left heart syndrome; this was unrelated to the tissue-engineered graft function. There was no evidence of aneurysm formation or calcification on cineangiography or computed tomography. All tube grafts were patent, and the diameter of the tube graft increased with time (110% +/- 7 % of the implanted size).

CONCLUSION

Biodegradable conduits or patches seeded with autologous bone marrow cells showed normal function (good patency to a maximum follow-up of 32 months). As living tissues, these vascular structures may have the potential for growth, repair, and remodeling. The tissue-engineering approach may provide an important alternative to the use of prosthetic materials in the field of pediatric cardiovascular surgery. Longer follow-up is necessary to confirm the durability of this approach.

From Stem Cells to Viable Autologous Semilunar Heart Valve

BACKGROUND

An estimated 275,000 patients undergo heart valve replacement each year. However, existing solutions for valve replacement are complicated by the morbidity associated with lifelong anticoagulation of mechanical valves and the limited durability of bioprostheses. Recent advances in tissue engineering and our understanding of stem cell biology may provide a lifelong solution to these problems.

METHODS AND RESULTS

Mesenchymal stem cells were isolated from ovine bone marrow and characterized by their morphology and antigen expression through immunocytochemistry, flow cytometry, and capacity to differentiate into multiple cell lineages. A biodegradable scaffold was developed and characterized by its tensile strength and stiffness as a function of time in cell-conditioned medium. Autologous semilunar heart valves were then created in vitro using mesenchymal stem cells and the biodegradable scaffold and were implanted into the pulmonary position of sheep on cardiopulmonary bypass. The valves were evaluated by echocardiography at implantation and after 4 months in vivo. Valves were explanted at 4 and 8 months and examined by histology and immunohistochemistry. Valves displayed a maximum instantaneous gradient of 17.2+/-1.33 mm Hg, a mean gradient of 9.7+/-1.3 mm Hg, an effective orifice area of 1.35+/-0.17 cm2, and trivial or mild regurgitation at implantation. Gradients changed little over 4 months of follow-up. Histology showed disposition of extracellular matrix and distribution of cell phenotypes in the engineered valves reminiscent of that in native pulmonary valves.

CONCLUSIONS

Stem-cell tissue-engineered heart valves can be created from mesenchymal stem cells in combination with a biodegradable scaffold and function satisfactorily in vivo for periods of >4 months. Furthermore, such valves undergo extensive remodeling in vivo to resemble native heart valves.

Application of tissue-engineering principles toward the development of a semilunar heart valve substitute

Heart valve disease is a significant medical problem worldwide. Current treatment for heart valve disease is heart valve replacement. State of the art replacement heart valves are less than ideal and are associated with significant complications. Using the basic principles of tissue engineering, promising alternatives to current replacement heart valves are being developed. Significant progress has been made in the development of a tissue-engineered semilunar heart valve substitute. Advancements include the development of different potential cell sources and cell-seeding techniques; advancements in matrix and scaffold development and in polymer chemistry fabrication; and the development of a variety of bioreactors, which are biomimetic devices used to modulate the development of tissue-engineered neotissue in vitro through the application of biochemical and biomechanical stimuli. This review addresses the need for a tissue-engineered alternative to the current heart valve replacement options. The basics of heart valve structure and function, heart valve disease, and currently available heart valve replacements are discussed. The last 10 years of investigation into a tissue-engineered heart valve as well as current developments are reviewed. Finally, the early clinical applications of cardiovascular tissue engineering are presented.

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.

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.

Biomatrix/Polymer Composite Material for Heart Valve Tissue Engineering

BACKGROUND

Decellularized extracellular matrix has been suggested as a scaffold for heart valve tissue engineering or direct implantation. However, cell removal impairs the physical properties of the valve structure and exposes bare collagen fibers that are highly thrombogenic. Matrix/polymer hybrid valves with improved biological and mechanical characteristics may be advantageous.

METHODS

Porcine aortic valves were decellularized enzymatically and impregnated with biodegradable poly(hydroxybutyrate) by a stepwise solvent exchange process. Biocompatibility was tested in vitro using cell proliferation and coagulation assays. Proinflammatory activity was assessed in vivo by implantation of matrix/polymer patches in the rabbit aorta. Biomechanic valve properties and fluid dynamics were tested in a pressure/flow-controlled pulse duplicating system. Matrix/polymer hybrid valves were implanted in pulmonary and aortic position in sheep.

RESULTS

Biocompatibility assays indicated that human blood vessel cells survive and proliferate on matrix/polymer hybrid tissue. In vitro activation of cellular and plasmatic coagulation cascades was lower than with uncoated control tissue. After implantation in the rabbit aorta, matrix/polymer hybrid patches healed well, with complete endothelialization, mild leukocyte infiltration, and less calcification than control tissue. Matrix/polymer hybrid tissue had superior tensile strength and suture retention strength, and hybrid valves showed good fluid dynamic performance. The two valves in aortic position performed well, with complete endothelialization and limited inflammatory cell invasion after 12 weeks. Of the two valves in pulmonary position, one failed.

CONCLUSIONS

Matrix/polymer hybrid tissue valves have good biological and biomechanic characteristics and may provide superior replacement valves.

Mechanical and Structural Properties of a Novel Hybrid Heart Valve Scaffold for Tissue Engineering

Hybrid heart valve scaffolds were fabricated from decellularized porcine aortic heart valve matrices and enhanced with bioresorbable polymers using different protocols: (i) dip coating of lyophilized decellularized matrices, and (ii) impregnation of wet decellularized matrices. The following polymers were evaluated: poly(4-hydroxybutyrate) and poly(3-hydroxybutyrate-co4-hydroxybutyrate). Tensile tests were conducted to assess the biomechanical behavior of valve leaflet strips. Suture retention strength was evaluated for the adjacent conduit. A pulse duplicator system was used for functional testing of the valves under physiological systemic load conditions. The properties of the hybrid structures were compared with native, decellularized, and glutaraldehyde-fixed specimens. Mechanisms of the polymer impregnation process were studied with IR spectroscopy, fluorescent microscopic imaging, and SEM. Altogether this study demonstrates the feasibility and improved biomechanical function of a novel hybrid heart valve scaffold for an application in tissue engineering.

Tissue engineering and regenerative medicine: concepts for clinical application

Patients suffering from diseased and injured organs may be treated with transplanted organs. However, there is a severe shortage of donor organs that is worsening yearly given the aging population. Scientists in the field of regenerative medicine and tissue engineering apply the principles of cell transplantation, material science, and bioengineering to construct biological substitutes that will restore and maintain normal function in diseased and injured tissues. Therapeutic cloning, where the nucleus from a donor cell is transferred into an enucleated oocyte in order to extract pluripotent embryonic stem cells, offers a potentially limitless source of cells for tissue engineering applications. The stem cell field is also advancing rapidly, opening new options for therapy. This paper reviews recent advances that have occurred in regenerative medicine and describes applications of these new technologies that may offer novel therapies for patients with end-stage organ failure.

Three-dimensional biocompatible ascorbic acid-containing scaffold for bone tissue engineering

A biodegradable, biocompatible, ascorbic acid-containing three-dimensional polyurethane matrix was developed for bone tissue-engineering scaffolds. This matrix was synthesized with lysine-di-isocyanate (LDI), ascorbic acid (AA), glycerol, and polyethylene glycol (PEG). LDI-glycerol-PEG-AA prepolymer when reacted with water foamed with the liberation of CO(2) to provide a pliable, spongy urethane polymer with pore diameters of 100 to 500 microm. The LDI-glycerol-PEG-AA matrix degraded in aqueous solution and yielded lysine, glycerol, PEG, and ascorbic acid as breakdown products. The degradation products did not significantly affect the solution pH. The LDI-glycerol-PEG-AA matrix can be fabricated into diverse scaffold dimensions and the physicochemical properties of the polymer network supported in vitro cell growth. Green fluorescent protein-transgenic mouse bone marrow cells (GFP-MBMCs) attached to the polymer matrix and remained viable, and the cells became confluent cultures. Furthermore, ascorbic acid released from LDI-glycerol-PEG-AA matrix stimulated cell proliferation, type I collagen, and alkaline phosphatase synthesis in vitro. Cells grown on LDI-glycerol-PEG-AA matrix did not differ phenotypically from cells grown on tissue culture polystyrene plates as assessed by cell growth, expression of mRNA for collagen type I, and transforming growth factor beta(1). These observations suggest that AA-containing polyurethane may be useful in bone tissue-engineering applications.

Impact of decellularization of xenogeneic tissue on extracellular matrix integrity for tissue engineering of heart valves

The multidisciplinary research of tissue engineering utilizes biodegradable or decellularized scaffolds with autologous cell seeding. Objective of this study was to investigate the impact of different decellularization protocols on extracellular matrix integrity of xenogeneic tissue by means of multiphoton femtosecond laser scanning microscopy, biochemical and histological analysis. Pulmonary valves were dissected from porcine hearts and placed in a solution of trypsin-EDTA and incubated at 37 degrees C for either 5, 8, or 24 h, followed by a 24 h PBS washing. Native and decellularized valves were processed for histology, DNA, cell proliferation, matrix proteins and biomechanical testing. Multiphoton NIR laser microscopy has been applied for high-resolution 3D imaging of collagen and elastin. Distinct differences in several ECM components following decellularization time were observed. Total GAG contents decreased in a time-dependent manner, with o-sulfated GAGs being more susceptible to degradation than n-sulfated GAGs. Efficiency of insoluble collagen extraction increased proportionally with decellularization time, suggesting ECM-integrity may be compromised with prolonged incubation. Biomechanical testing revealed a gradual weakening of mechanical strength with increased decellularization time. The enzymatic decellularization process of heart valves revealed a time-dependent loss of cells, ECM components and biomechanical strength. In order to avoid any immune response a thorough decellularization of 24 h remains mandatory.

Potential tissue-engineering applications for neonatal surgery

Tissue engineering attempts to build neotissue from its cellular building blocks. This neotissue can then be used for reconstructive surgical applications such as replacement of a congenitally abnormal heart valve or repair of a craniofacial abnormality. Since its inception in the late 1980s, tissue engineering has sparked the interests of physicians and scientists alike because of its great potential. Significant progress has been made in this burgeoning branch of science. This article reviews some of the ongoing preclinical and clinical tissue engineering research as it applies to neonatology.

Decellularization protocols of porcine heart valves differ importantly in efficiency of cell removal and susceptibility of the matrix to recellularization with human vascular cells

OBJECTIVE

We compared 3 different decellularization protocols in porcine heart valves for efficiency of complete cell removal and potential for recellularization.

METHODS

Porcine aortic and pulmonary roots were treated with trypsin, sodium-dodecyl-sulphate, or a new method using 0.25% tert-octylphenyl-polyoxyethylen in combination with sodium-deoxycholate. After a subsequent ribonuclease digestion, specimens were seeded with in vitro expanded human saphenous vein endothelial cells and myofibroblasts.

RESULTS

After treatment with trypsin and subsequent ribonuclease digestion, endothelial attachment took place; however, xenogenic cells were still visible within the matrix. Unexpectedly, when human cells were seeded onto specimens that had been decellularized with sodium-dodecyl-sulphate, the matrices were surrounded by nonviable endothelial cell fragments, indicating a toxic influence of the ionic detergent; 0.25% tert-octylphenyl-polyoxyethylen together with sodium-deoxycholate completely removed porcine cells and enabled host recellularization.

CONCLUSION

Compared with trypsin and sodium-dodecyl-sulphate involving decellularization procedures, reported to be effective in cell removal and susceptible to recellularization with human cells, only the porcine matrix treated with a new detergent-based decellularization method using 0.25% tert-octylphenyl-polyoxyethylen/sodium-deoxycholate followed by nuclease digestion presented an excellent scaffold for recellularization with human cells.

Potential applications and prospects for cardiac xenotransplantation

Despite improvements in pharmacologic therapies, the outlook for patients with severe cardiac disease remains poor. At present, the only "cure" for end-stage heart failure is transplantation. However, fewer than 5% of those who need a cardiac transplant receive one in the United States each year. As an alternative, some propose using animals as a source of organs for transplantation (i.e., xenotransplantation). In this article we review the potential applications of xenotransplantation for the treatment of cardiac disease, and weigh xenotransplantation against other new technologies that might be used. We also consider the current status of addressing the hurdles to application of xenotransplantation.