Approaches to heart valve tissue engineering scaffold design

Chondroitin sulfate-based composites: a tour d'horizon of their biomedical applications

Chondroitin sulfate (CS), a natural anionic mucopolysaccharide, belonging to the glycosaminoglycan family, acts as the primary element of the extracellular matrix (ECM) of diverse organisms. It comprises repeating units of disaccharides possessing β-1,3-linked N-acetyl galactosamine (GalNAc), and β-1,4-linked D-glucuronic acid (GlcA), and exhibits antitumor, anti-inflammatory, anti-coagulant, anti-oxidant, and anti-thrombogenic activities. It is a naturally acquired bio-macromolecule with beneficial properties, such as biocompatibility, biodegradability, and immensely low toxicity, making it the center of attention in developing biomaterials for various biomedical applications. The authors have discussed the structure, unique properties, and extraction source of CS in the initial section of this review. Further, the current investigations on applications of CS-based composites in various biomedical fields, focusing on delivering active pharmaceutical compounds, tissue engineering, and wound healing, are discussed critically. In addition, the manuscript throws light on preclinical and clinical studies associated with CS composites. A short section on Chondroitinase ABC has also been canvassed. Finally, this review emphasizes the current challenges and prospects of CS in various biomedical fields.

3D Tissue-Engineered Vascular Drug Screening Platforms: Promise and Considerations

Despite the efforts devoted to drug discovery and development, the number of new drug approvals have been decreasing. Specifically, cardiovascular developments have been showing amongst the lowest levels of approvals. In addition, concerns over the adverse effects of drugs to the cardiovascular system have been increasing and resulting in failure at the preclinical level as well as withdrawal of drugs post-marketing. Besides factors such as the increased cost of clinical trials and increases in the requirements and the complexity of the regulatory processes, there is also a gap between the currently existing pre-clinical screening methods and the clinical studies in humans. This gap is mainly caused by the lack of complexity in the currently used 2D cell culture-based screening systems, which do not accurately reflect human physiological conditions. Cell-based drug screening is widely accepted and extensively used and can provide an initial indication of the drugs' therapeutic efficacy and potential cytotoxicity. However, in vitro cell-based evaluation could in many instances provide contradictory findings to the in vivo testing in animal models and clinical trials. This drawback is related to the failure of these 2D cell culture systems to recapitulate the human physiological microenvironment in which the cells reside. In the body, cells reside within a complex physiological setting, where they interact with and respond to neighboring cells, extracellular matrix, mechanical stress, blood shear stress, and many other factors. These factors in sum affect the cellular response and the specific pathways that regulate variable vital functions such as proliferation, apoptosis, and differentiation. Although pre-clinical in vivo animal models provide this level of complexity, cross species differences can also cause contradictory results from that seen when the drug enters clinical trials. Thus, there is a need to better mimic human physiological conditions in pre-clinical studies to improve the efficiency of drug screening. A novel approach is to develop 3D tissue engineered miniaturized constructs in vitro that are based on human cells. In this review, we discuss the factors that should be considered to produce a successful vascular construct that is derived from human cells and is both reliable and reproducible.

Leaflet Tissue Generation from Microfibrous Heart Valve Leaflet Scaffolds with Native Characteristics

Mechanical and bioprosthetic valves that are currently applied for replacing diseased heart valves are not fully efficient. Heart valve tissue engineering may solve the issues faced by the prosthetic valves in heart valve replacement. The leaflets of native heart valves have a trilayered structure with layer-specific orientations; thus, it is imperative to develop functional leaflet tissue constructs with a native trilayered, oriented structure. Its key solution is to develop leaflet scaffolds with a native morphology and structure. In this study, microfibrous leaflet scaffolds with a native trilayered and oriented structure were developed in an electrospinning system. The scaffolds were implanted for 3 months in rats subcutaneously to study the scaffold efficiencies in generating functional tissue-engineered leaflet constructs. These in vivo tissue-engineered leaflet constructs had a trilayered, oriented structure similar to native leaflets. The tensile properties of constructs indicated that they were able to endure the hydrodynamic load of the native heart valve. Collagen, glycosaminoglycans, and elastin─the predominant extracellular matrix components of native leaflets─were found sufficiently in the leaflet tissue constructs. The residing cells in the leaflet tissue constructs showed vimentin and α-smooth muscle actin expression, i.e., the constructs were in a growing state. Thus, the trilayered, oriented fibrous leaflet scaffolds produced in this study could be useful to develop heart valve scaffolds for successful heart valve replacements.

Biomaterials in Valvular Heart Diseases

Valvular heart disease (VHD) occurs as the result of valvular malfunction, which can greatly reduce patient's quality of life and if left untreated may lead to death. Different treatment regiments are available for management of this defect, which can be helpful in reducing the symptoms. The global commitment to reduce VHD-related mortality rates has enhanced the need for new therapeutic approaches. During the past decade, development of innovative pharmacological and surgical approaches have dramatically improved the quality of life for VHD patients, yet the search for low cost, more effective, and less invasive approaches is ongoing. The gold standard approach for VHD management is to replace or repair the injured valvular tissue with natural or synthetic biomaterials. Application of these biomaterials for cardiac valve regeneration and repair holds a great promise for treatment of this type of heart disease. The focus of the present review is the current use of different types of biomaterials in treatment of valvular heart diseases.

Stem cells and new intervention measures as emerging therapy in cardiac surgery

Cardiovascular disease (CVD) presents a great burden for elderly patients, their caregivers, and health systems. Structural and functional alterations of vessels accumulate throughout life, culminating in increased risk of developing CVD. Several inflammatory pathway are involved in vascular ageing. The growing elderly population worldwide highlights the need to understand how aging promotes CVD in order to develop new strategies to confront this challenge. In this review we analyzed the role of stem cells and new intervention measures as emerging drugs for vascular aging.

Trilayer scaffold with cardiosphere-derived cells for heart valve tissue engineering

Natural polymers collagen, glycosaminoglycans, and elastin are promising candidate materials for heart valve tissue engineering scaffolds. This work produced trilayer scaffolds that resembled the layered structures of the extracellular matrices of native heart valves. The scaffolds showed anisotropic bending moduli (in both dry and hydrated statuses) depending on the loading directions (lower in the With Curvature direction than in the Against Curvature direction), which mimicked the characteristic behavior of the native heart valves. The interactions between cardiosphere-derived cells and the scaffolds were characterized by multiphoton microscopy, and relatively similar cell distributions were observed on different layers (a cell density of 3,000-4,000 mm^-3 and a migration depth of 0.3-0.4 mm). The trilayer scaffold has represented a forwarding step from the previous studies, in attempting to better replicate a native heart valve structurally, mechanically, and biologically.

Heart valve tissue-derived hydrogels: Preparation and characterization of mitral valve chordae, aortic valve, and mitral valve gels

Heart valve (HV) diseases are among the leading causes of death and continue to threaten public health worldwide. The current clinical options for HV replacement include mechanical and biological prostheses. However, an ongoing problem with current HV prostheses is their failure to integrate with the host tissue and their inability grow and remodel within the body. Tissue engineered heart valves (TEHVs) are a promising solution to these problems, as they are able to grow and remodel somatically with the rest of the body. Recently, decellularized HVs have demonstrated great potential as valve replacements because they are tissue specific, but recellularization is still a challenge due to the dense HV extracellular matrix (ECM) network. In this proof-of-concept work, we decellularized porcine mitral valve chordae, aortic valve leaflets, and mitral valve leaflets and processed them into injectable hydrogels that could accommodate any geometry. While the three valvular ECMs contained various amounts of collagen, they displayed similar glycosaminoglycan contents. The hydrogels had similar nanofibrous structures and gelation kinetics with various compressive strengths. When encapsulated with NIH 3 T3 fibroblasts, all the hydrogels supported cell survivals up to 7 days. Decellularized HV ECM hydrogels may show promising potential HV tissue engineering applications. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1732-1740, 2019.

Quantitative Characterization of Aortic Valve Endothelial Cell Viability and Morphology In Situ Under Cyclic Stretch

Current protocols for mechanical preconditioning of tissue engineered heart valves have focused on application of pressure, flexure and fluid flow to stimulate collagen production, ECM remodeling and improving mechanical performance. The aim of this study was to determine if mechanical preconditioning with cyclic stretch could promote an intact endothelium that resembled the viability and morphology of a native valve. Confocal laser scanning microscopy was used to image endothelial cells on aortic valve strips subjected to static incubation or physiological strain regimens. An automated image analysis program was designed and implemented to detect and analyze live and dead cells in images captured of a live aortic valve endothelium. The images were preprocessed, segmented, and quantitatively analyzed for live/dead cell ratio, minimum neighbor distance and circularity. Significant differences in live/dead cellular ratio and the minimum distance between cells were observed between static and strained endothelia, indicating that cyclic strain is an important stimulus for maintaining a healthy endothelium. In conclusion, in vitro application of physiological levels of cyclic strain to tissue engineered heart valves seeded with autologous endothelial cells would be advantageous.

Heart valve tissue engineering: an overview of heart valve decellularization processes

Despite recent advances in medicine and surgery, many people still suffer from cardiovascular diseases, which affect their life span and morbidity. Regenerative medicine and tissue engineering are novel approaches based on restoring or replacing injured tissues and organs with scaffolds, cells and growth factors. Scaffolds are acquired from two major sources, synthetic materials and naturally derived scaffolds. Biological scaffolds derived from native tissues and cell-derived matrix offer many advantages. They are more biocompatible with a higher affinity to cells, which facilitate tissue reconstruction. Interestingly, xenogeneic recipients generally tolerate their components. Therefore, heart valve tissue engineering is increasingly benefiting from naturally derived scaffolds. In this review, we investigated the different protocols and methods that have been used for heart valve decellularization.

Recellularization of decellularized heart valves: Progress toward the tissue-engineered heart valve

The tissue-engineered heart valve portends a new era in the field of valve replacement. Decellularized heart valves are of great interest as a scaffold for the tissue-engineered heart valve due to their naturally bioactive composition, clinical relevance as a stand-alone implant, and partial recellularization in vivo. However, a significant challenge remains in realizing the tissue-engineered heart valve: assuring consistent recellularization of the entire valve leaflets by phenotypically appropriate cells. Many creative strategies have pursued complete biological valve recellularization; however, identifying the optimal recellularization method, including in situ or in vitro recellularization and chemical and/or mechanical conditioning, has proven difficult. Furthermore, while many studies have focused on individual parameters for increasing valve interstitial recellularization, a general understanding of the interacting dynamics is likely necessary to achieve success. Therefore, the purpose of this review is to explore and compare the various processing strategies used for the decellularization and subsequent recellularization of tissue-engineered heart valves.

Elastomeric Fibrous Hybrid Scaffold Supports In Vitro and In Vivo Tissue Formation

Biomimetic materials with biomechanical properties resembling those of native tissues while providing an environment for cell growth and tissue formation, are vital for tissue engineering (TE). Mechanical anisotropy is an important property of native cardiovascular tissues and directly influences tissue function. This study reports fabrication of anisotropic cell-seeded constructs while retaining control over the construct's architecture and distribution of cells. Newly synthesized poly-4-hydroxybutyrate (P4HB) is fabricated with a dry spinning technique to create anelastomeric fibrous scaffold that allows control of fiber diameter, porosity, and rate ofdegradation. To allow cell and tissue ingrowth, hybrid scaffolds with mesenchymalstem cells (MSCs) encapsulated in a photocrosslinkable hydrogel were developed. Culturing the cellularized scaffolds in a cyclic stretch/flexure bioreactor resulted in tissue formation and confirmed the scaffold's performance under mechanical stimulation. In vivo experiments showed that the hybrid scaffold is capable of withstanding physiological pressures when implanted as a patch in the pulmonary artery. Aligned tissue formation occurred on the scaffold luminal surface without macroscopic thrombus formation. This combination of a novel, anisotropic fibrous scaffold and a tunable native-like hydrogel for cellular encapsulation promoted formation of 3D tissue and provides a biologically functional composite scaffold for soft-tissue engineering applications.

Living nano-micro fibrous woven fabric/hydrogel composite scaffolds for heart valve engineering

Regeneration and repair of injured or diseased heart valves remains a clinical challenge. Tissue engineering provides a promising treatment approach to facilitate living heart valve repair and regeneration. Three-dimensional (3D) biomimetic scaffolds that possess heterogeneous and anisotropic features that approximate those of native heart valve tissue are beneficial to the successful in vitro development of tissue engineered heart valves (TEHV). Here we report the development and characterization of a novel composite scaffold consisting of nano- and micro-scale fibrous woven fabrics and 3D hydrogels by using textile techniques combined with bioactive hydrogel formation. Embedded nano-micro fibrous scaffolds within hydrogel enhanced mechanical strength and physical structural anisotropy of the composite scaffold (similar to native aortic valve leaflets) and also reduced its compaction. We determined that the composite scaffolds supported the growth of human aortic valve interstitial cells (HAVIC), balanced the remodeling of heart valve ECM against shrinkage, and maintained better physiological fibroblastic phenotype in both normal and diseased HAVIC over single materials. These fabricated composite scaffolds enable the engineering of a living heart valve graft with improved anisotropic structure and tissue biomechanics important for maintaining valve cell phenotypes.

STATEMENT OF SIGNIFICANCE

Heart valve-related disease is an important clinical problem, with over 300,000 surgical repairs performed annually. Tissue engineering offers a promising strategy for heart valve repair and regeneration. In this study, we developed and tissue engineered living nano-micro fibrous woven fabric/hydrogel composite scaffolds by using textile technique combined with bioactive hydrogel formation. The novelty of our technique is that the composite scaffolds can mimic physical structure anisotropy and the mechanical strength of natural aortic valve leaflet. Moreover, the composite scaffolds prevented the matrix shrinkage, which is major problem that causes the failure of TEHV, and better maintained physiological fibroblastic phenotype in both normal and diseased HAVIC. This work marks the first report of a combination composite scaffold using 3D hydrogel enhanced by nano-micro fibrous woven fabric, and represents a promising tissue engineering strategy to treat heart valve injury.

Biodegradable and biomimetic elastomeric scaffolds for tissue-engineered heart valves

Valvular heart diseases are the third leading cause of cardiovascular disease, resulting in more than 25,000 deaths annually in the United States. Heart valve tissue engineering (HVTE) has emerged as a putative treatment strategy such that the designed construct would ideally withstand native dynamic mechanical environment, guide regeneration of the diseased tissue and more importantly, have the ability to grow with the patient. These desired functions could be achieved by biomimetic design of tissue-engineered constructs that recapitulate in vivo heart valve microenvironment with biomimetic architecture, optimal mechanical properties and possess suitable biodegradability and biocompatibility. Synthetic biodegradable elastomers have gained interest in HVTE due to their excellent mechanical compliance, controllable chemical structure and tunable degradability. This review focuses on the state-of-art strategies to engineer biomimetic elastomeric scaffolds for HVTE. We first discuss the various types of biodegradable synthetic elastomers and their key properties. We then highlight tissue engineering approaches to recreate some of the features in the heart valve microenvironment such as anisotropic and hierarchical tri-layered architecture, mechanical anisotropy and biocompatibility.

STATEMENT OF SIGNIFICANCE

Heart valve tissue engineering (HVTE) is of special significance to overcome the drawbacks of current valve replacements. Although biodegradable synthetic elastomers have emerged as promising materials for HVTE, a mature HVTE construct made from synthetic elastomers for clinical use remains to be developed. Hence, this review summarized various types of biodegradable synthetic elastomers and their key properties. The major focus that distinguishes this review from the current literature is the thorough discussion on the key features of native valve microenvironments and various up-and-coming approaches to engineer synthetic elastomers to recreate these features such as anisotropic tri-layered architecture, mechanical anisotropy, biodegradability and biocompatibility. This review is envisioned to inspire and instruct the design of functional HVTE constructs and facilitate their clinical translation.

Generation and Assessment of Functional Biomaterial Scaffolds for Applications in Cardiovascular Tissue Engineering and Regenerative Medicine

Current clinically applicable tissue and organ replacement therapies are limited in the field of cardiovascular regenerative medicine. The available options do not regenerate damaged tissues and organs, and, in the majority of the cases, show insufficient restoration of tissue function. To date, anticoagulant drug-free heart valve replacements or growing valves for pediatric patients, hemocompatible and thrombus-free vascular substitutes that are smaller than 6 mm, and stem cell-recruiting delivery systems that induce myocardial regeneration are still only visions of researchers and medical professionals worldwide and far from being the standard of clinical treatment. The design of functional off-the-shelf biomaterials as well as automatable and up-scalable biomaterial processing methods are the focus of current research endeavors and of great interest for fields of tissue engineering and regenerative medicine. Here, various approaches that aim to overcome the current limitations are reviewed, focusing on biomaterials design and generation methods for myocardium, heart valves, and blood vessels. Furthermore, novel contact- and marker-free biomaterial and extracellular matrix assessment methods are highlighted.

Expression of COLLAGEN 1 and ELASTIN Genes in Mitral Valvular Interstitial Cells within Microfiber Reinforced Hydrogel

OBJECTIVE

The incidence of heart valve disease is increasing worldwide and the number of heart valve replacements is expected to increase in the future. By mimicking the main tissue structures and properties of heart valve, tissue engineering offers new options for the replacements. Applying an appropriate scaffold in fabricating tissue-engineered heart valves (TEHVs) is of importance since it affects the secretion of the main extracellular matrix (ECM) components, collagen 1 and elastin, which are crucial in providing the proper mechanical properties of TEHVs.

MATERIALS AND METHODS

Using real-time polymerase chain reaction (PCR) in this experi- mental study, the relative expression levels of COLLAGEN 1 and ELASTIN were obtained for three samples of each examined sheep mitral valvular interstitial cells (MVICs)-seeded onto electrospun poly (glycerol sebacate) (PGS)-poly (ε-caprolactone) (PCL) microfibrous, gelatin and hyaluronic acid based hydrogel-only and composite (PGS-PCL/hydrogel) scaffolds. This composite has been shown to create a synthetic three-dimensional (3D) microenvironment with appropriate mechanical and biological properties for MVICs.

RESULTS

Cell viability and metabolic activity were similar among all scaffold types. Our results showed that the level of relative expression of COLLAGEN 1 and ELASTIN genes was higher in the encapsulated composite scaffolds compared to PGS-PCL-only and hydrogel-only scaffolds with the difference being statistically significant (P<0.05).

CONCLUSION

The encapsulated composite scaffolds are more conducive to ECM secretion over the PGS-PCL-only and hydrogel-only scaffolds. This composite scaffold can serve as a model scaffold for heart valve tissue engineering.

Bioengineering Strategies for Polymeric Scaffold for Tissue Engineering an Aortic Heart Valve: An Update

The occurrence of dysfunctional aortic valves is increasing every year, and current replacement heart valves, although having been shown to be clinically successful, are only short-term solutions and suffer from many agonizing long-term drawbacks. The tissue engineering of heart valves is recognized as one of the most promising answers for aortic valve disease therapy, but overcoming current shortcomings will require multidisciplinary efforts. The use of a polymeric scaffold to guide the growth of the tissue is the most common approach to generate a new tissue for an aortic heart valve. However, optimizing the design of the scaffold, in terms of biocompatibility, surface morphology for cell attachments and the correct rate of degradation is critical in creating a viable tissue-engineered aortic heart valve. This paper highlights the bioengineering strategies that need to be followed to construct a polymeric scaffold of sufficient mechanical integrity, with superior surface morphologies, that is capable of mimicking the valve dynamics in vivo. The current challenges and future directions of research for creating tissue-engineered aortic heart valves are also discussed.

Tri-layered elastomeric scaffolds for engineering heart valve leaflets

Tissue engineered heart valves (TEHVs) that can grow and remodel have the potential to serve as permanent replacements of the current non-viable prosthetic valves particularly for pediatric patients. A major challenge in designing functional TEHVs is to mimic both structural and anisotropic mechanical characteristics of the native valve leaflets. To establish a more biomimetic model of TEHV, we fabricated tri-layered scaffolds by combining electrospinning and microfabrication techniques. These constructs were fabricated by assembling microfabricated poly(glycerol sebacate) (PGS) and fibrous PGS/poly(caprolactone) (PCL) electrospun sheets to develop elastic scaffolds with tunable anisotropic mechanical properties similar to the mechanical characteristics of the native heart valves. The engineered scaffolds supported the growth of valvular interstitial cells (VICs) and mesenchymal stem cells (MSCs) within the 3D structure and promoted the deposition of heart valve extracellular matrix (ECM). MSCs were also organized and aligned along the anisotropic axes of the engineered tri-layered scaffolds. In addition, the fabricated constructs opened and closed properly in an ex vivo model of porcine heart valve leaflet tissue replacement. The engineered tri-layered scaffolds have the potential for successful translation towards TEHV replacements.

Electrospun PGS:PCL microfibers align human valvular interstitial cells and provide tunable scaffold anisotropy

Tissue engineered heart valves (TEHV) can be useful in the repair of congenital or acquired valvular diseases due to their potential for growth and remodeling. The development of biomimetic scaffolds is a major challenge in heart valve tissue engineering. One of the most important structural characteristics of mature heart valve leaflets is their intrinsic anisotropy, which is derived from the microstructure of aligned collagen fibers in the extracellular matrix (ECM). In the present study, a directional electrospinning technique is used to fabricate fibrous poly(glycerol sebacate):poly(caprolactone) (PGS:PCL) scaffolds containing aligned fibers, which resemble native heart valve leaflet ECM networks. In addition, the anisotropic mechanical characteristics of fabricated scaffolds are tuned by changing the ratio of PGS:PCL to mimic the native heart valve's mechanical properties. Primary human valvular interstitial cells (VICs) attach and align along the anisotropic axes of all PGS:PCL scaffolds with various mechanical properties. The cells are also biochemically active in producing heart-valve-associated collagen, vimentin, and smooth muscle actin as determined by gene expression. The fibrous PGS:PCL scaffolds seeded with human VICs mimick the structure and mechanical properties of native valve leaflet tissues and would potentially be suitable for the replacement of heart valves in diverse patient populations.

Transesophageal Echocardiography in Healthy Young Adult Male Baboons (Papio hamadryas anubis): Normal Cardiac Anatomy and Function in Subhuman Primates Compared to Humans

Implantable, viable tissue engineered cardiovascular constructs are rapidly approaching clinical translation. Species typically utilized as preclinical large animal models are food stock ungulates for which cross species biological and genomic differences with humans are great. Multiple authorities have recommended developing subhuman primate models for testing regenerative surgical strategies to mitigate xenotransplant inflammation. However, there is a lack of specific quantitative cardiac imaging comparisons between humans and the genomically similar baboons (Papio hamadryas anubis). This study was undertaken to translate to baboons transesophageal echocardiographic functional and dimensional criteria defined as necessary for defining cardiac anatomy and function in the perioperative setting. Seventeen young, healthy baboons (approximately 30 kg, similar to 5 year old children) were studied to determine whether the requisite 11 views and 52 measurement parameters could be reliably acquired by transesophageal echocardiography (TEE). The obtained measurements were compared to human adult normative literature values and to a large relational database of pediatric "normal heart" echo measurements. Comparisons to humans, when normalized to BSA, revealed a trend in baboons toward larger mitral and aortic valve effective orifice areas and much larger left ventricular muscle mass and wall thickness, but similar pulmonary and tricuspid valves. By modifying probe positioning relative to human techniques, all recommended TEE views except transgastric could be replicated. To supplement, two transthoracic apical views were discovered that in baboons could reliably replace the transgastric TEE view. Thus, all requisite echo views could be obtained for a complete cardiac evaluation in Papio hamadryas anubis to noninvasively quantify cardiac structural anatomy, physiology, and dimensions. Despite similarities between the species, there are subtle and important physiologic and anatomic differences when compared to human.

Effect of biodegradation and de novo matrix synthesis on the mechanical properties of valvular interstitial cell-seeded polyglycerol sebacate-polycaprolactone scaffolds

The development of living heart valves that grow with the patient is a promising strategy for heart valve replacements in pediatric patients. Despite active research in the field of tissue engineered heart valves there have been limited efforts to optimize the balance between biodegradation of the scaffolds and de novo extracellular matrix (ECM) synthesis by cells and study their consequences on the mechanical properties of the cell-seeded construct. This study investigates the effect of in vitro degradation and ECM secretion on the mechanical properties of hybrid polyester scaffolds. The scaffolds were synthesized from blends of fast degrading polyglycerol sebacate (PGS) and slowly degrading polycaprolactone (PCL). PGS-PCL scaffolds were electrospun using a 2:1 ratio of PGS to PCL. Accelerated hydrolytic degradation in 0.1 mM sodium hydroxide revealed 2-fold faster degradation of PGS-PCL scaffolds compared with PCL scaffolds. Thermal analysis and scanning electron microscopy demonstrated marginal change in PCL scaffold properties, while PGS-PCL scaffolds showed preferential mass loss of PGS and thinning of the individual fibers during degradation. Consequently, the mechanical properties of PGS-PCL scaffolds decreased gradually with no significant change for PCL scaffolds during accelerated degradation. Valvular interstitial cells (VICs) seeded on PGS-PCL scaffolds showed higher ECM protein secretion compared with PCL. Thus the mechanical properties of the cell-seeded PCL scaffolds did not change significantly compared with acellular scaffolds, probably due to slower degradation and ECM deposition by VICs. In contrast, the PGS-PCL scaffolds exhibited a gradual decrease in the mechanical properties of the acellular scaffolds due to degradation, which was compensated for by new matrix secreted by VICs seeded on the scaffolds. Our study demonstrated that the faster degrading PGS component of PGS-PCL accelerated the degradation rate of the scaffolds. VICs, on the other hand, were able to remodel the synthetic scaffold, depositing new matrix proteins and maintaining the mechanical properties of the scaffolds.

Form Follows Function: Advances in Trilayered Structure Replication for Aortic Heart Valve Tissue Engineering

Tissue engineering the aortic heart valve is a challenging endeavor because of the particular hemodynamic and biologic conditions present in the native aortic heart valve. The backbone of an ideal valve substitute should be a scaffold that is strong enough to withstand billions of repetitive bending, flexing and stretching cycles, while also being slowly degradable to allow for remodeling. In this review we highlight three overlooked aspects that might influence the long term durability of tissue engineered valves: replication of the native valve trilayered histoarchitecture, duplication of the three-dimensional shape of the valve and cell integration efforts focused on getting the right number and type of cells to the right place within the valve structure and driving them towards homeostatic maintenance of the valve matrix. We propose that the trilayered structure in the native aortic valve that includes a middle spongiosa layer cushioning the motions of the two external fibrous layers should be our template for creation of novel scaffolds with improved mechanical durability. Furthermore, since cells adapt to micro-loads within the valve structure, we believe that interstitial cell remodeling of the valvular matrix will depend on the accurate replication of the structures and loads, resulting in successful regeneration of the valve tissue and extended durability.

Physiological performance of a detergent decellularized heart valve implanted for 15 months in Vietnamese pigs: surgical procedure, follow-up, and explant inspection

This study features the longest experimental follow-up for decellularized heart valves implanted in an animal model. Porcine aortic heart valves were decellularized according to a disclosed standardized method in which TRITON X-100 and sodium cholate (TRICOL) are used in succession, followed by a further treatment with the endonuclease Benzonase to completely remove the nucleic acid remnants. Experimental animals (n = 17), represented by Vietnamese pigs (VPs), received a decellularized aortic allograft as a substitute for the replacement of their right ventricular outflow tract. The surgical implantation of the TRICOL-treated aortic valve conduit was successful in 11 VPs, while perioperative or postoperative complications occurred in the remaining six animals. In the sham-operated group (n = 4), the native pulmonary root was excised and immediately reimplanted orthotopically in the same animal. Echocardiography demonstrated a satisfactory hemodynamic performance of the TRICOL-treated valves during follow-up as well as the absence of relevant leaflet alterations concerning thickness and motility or valve insufficiency. At explantation, macroscopic inspection of tissue-engineered heart valve conduits did not evidence calcifications and showed a decreased wall thickness, comparable to that of the reimplanted native pulmonary roots. Noteworthy, extended functional performance, recovery of DNA content, and active extracellular matrix precursor incorporation are apparently compatible with the properties of a living self-supporting substitute.

Mechanisms of function and disease of natural and replacement heart valves

Over the past several decades, there has been substantial progress toward understanding the mechanisms of heart valve function and dysfunction. This review summarizes an evolving conceptual framework of heart valve functional structure, developmental biology, and pathobiology and explores the implications of key insights. I emphasize: (a) valve cell and extracellular matrix biology and the impact of biomechanical factors on function, homeostasis, environmental adaptation, and key pathological processes; (b) the role of developmental processes, valvular cell behavior, and extracellular matrix remodeling in congenital and acquired valve abnormalities; and (c) the cell/matrix biology of degeneration in replacement tissue valves. I also summarize how these considerations may ultimately inform the potential for prevention and treatment of major diseases and potentially therapeutic regeneration of the cardiac valves. Recent advances and opportunities for research and clinical translation are highlighted.

Differences in Chemical Composition and Internal Structure Influence Systemic Host Response to Implants of Biomaterials

In reconstructive surgery, implantable devices are used to supply a missing function. In tissue engineering, biomaterials serve to guide and eventually deliver cells and/or molecules where a tissue regenerative response is needed. The host organism always reacts to implants of any biomaterial, in some instances even triggering a local cascade of events called the foreign body response (FBR), whose mechanisms are well defined. What has yet to be completely unraveled are the biomarkers systemically mirroring the FBR and the regeneration processes, which would be helpful for assessing the therapeutic efficacy of the bioscaffold. Our goal was to identify a biomarker fingerprint of the systemic reaction of host response to bioscaffold implants.

Different biomaterials chosen for their osteoconductive properties, including collagen, hydroxyapatite, in foam or granules, and poly-ɛ-caprolactone, were implanted in immunocompetent mice. We analyzed serum concentrations of cells and cytokines involved in the inflammatory/immune response, and the histological features of grafts.

Within two weeks after implantation, a wave of proinflammatory cytokines was flowing in the blood stream and the concentration of blood cells changed, revealing specific patterns depending on the chemistry and structure of the implanted biomaterials. Cells secreting pro-inflammatory, chemoactractant, and pro-angiogenic cytokines required for the early events in tissue repair were locally recruited because of the presence of a bioscaffold.

[Heart valve and myocardial tissue engineering]

Cardiac function, including the heart muscle and valves, can be severely altered by congenital and acquired heart diseases. Several graft materials are currently used to replace diseased cardiac tissue and valvular segments. Implantable grafts are either non-vital or can trigger an immune response which leads to graft calcification and degeneration. None of the existing grafts have the ability to remodel and grow in tandem with the physiological growth of a child and therefore require re-operation. Novel approaches such as tissue engineering have emerged as possible alternatives for cardiac reconstruction. The main concept of tissue engineering includes the use of biological and artificial scaffolds that form the shape of the organ structures for subsequent tissue replacement, which will provide absolute biocompatibility, no thrombogenicity, no teratogenicity, long-term durability and growth.Heart valve tissue engineering represents an important field especially in pediatric patients with valve pathologies. In order to create an autologous valve equivalent myofibroblasts and/or endothelial cells are seeded on specially designed scaffolds. Here we describe the different types of cell sources and different types of matrices currently used in heart valve tissue engineering. Valve manufacture is carried out in specially designed bioreactors providing physiological conditions. The number of clinical studies using tissue engineered valves is still limited; however, several promising results have already demonstrated their durability and ability to grow.Myocardial tissue engineering aims to repair, replace and regenerate damaged cardiac tissue using tissue constructs created ex vivo. Conceivable indications for clinical application of tissue engineered myocardial-implant substitutes include ischemic cardiomyopathies, as well as right ventricular outflow tract reconstruction in patients with congenital heart diseases. Therapeutic application of functional (contractile) tissue engineered heart muscle appears feasible once key issues such as identification of the suitable human cell source, large scale expansion and suitable scaffolds are solved. In addition, the present article discusses the importance of vascularization as an important prerequisite for successful bio-artificial myocardial tissue.Further experimental and clinical research on cardiovascular tissue engineering is felt to be of great importance for others as well as for us in order to create an ideal heart valve/myocardial substitute and help our patients with advanced cardiac pathologies.

Tissue engineering on matrix: future of autologous tissue replacement

Tissue engineering aims at the creation of living neo-tissues identical or close to their native human counterparts. As basis of this approach, temporary biodegradable supporter matrices are fabricated in the shape of a desired construct, which promote tissue strength and provide functionality until sufficient neo-tissue is formed. Besides fully synthetic polymer-based scaffolds, decellularized biological tissue of xenogenic or homogenic origin can be used. In a second step, these scaffolds are seeded with autologous cells attaching to the scaffold microstructure. In order to promote neo-tissue formation and maturation, the seeded scaffolds are exposed to different forms of stimulation. In cardiovascular tissue engineering, this "conditioning" can be achieved via culture media and biomimetic in vitro exposure, e.g., using flow bioreactors. This aims at adequate cellular differentiation, proliferation, and extracellular matrix production to form a living tissue called the construct. These living autologous constructs, such as heart valves or vascular grafts, are created in vitro, comprising a viable interstitium with repair and remodeling capabilities already prior to implantation. In situ further in vivo remodeling is intended to recapitulate physiological vascular architecture and function. The remodeling mechanisms were shown to be dominated by monocytic infiltration and chemotactic host-cell attraction leading into a multifaceted inflammatory process and neo-tissue formation. Key molecules of these processes can be integrated into the scaffold matrix to direct cell and tissue fate in vivo.

Crosslinking effect of Nordihydroguaiaretic acid (NDGA) on decellularized heart valve scaffold for tissue engineering

Decellularized heart valve scaffolds possess many desirable properties in valvular tissue engineering. However, their current applications were limited by short durability, easily structural dysfunction and immunological competence. Although crosslinking with chemical reagents, such as glutaraldehyde (GA), will enhance the mechanical properties, the low long-term stability and cytotoxicity of the scaffolds remains potential problem. Nordihydroguaiaretic acid (NDGA) is a bioactive natural product which is able to crosslink collagen and was proven to be effective in preparation of scaffold for tendon tissue engineering. In this paper, NDGA crosslinked decellularized heart valve scaffolds demonstrated higher tensile strength, enzymatic hydrolysis resistance and store stability than the non-crosslinked ones. Its mechanical properties and cytocompability were superior to that of GA-crosslinked heart valve matrix. Below the concentration of 10 microg/ml, NDGA has no visible cytotoxic effect on both endothelial cells (EC) and valvular interstitial cells (VIC) and its cytotoxicity is much less than that of GA. The LC50 (50% lethal concentration) of NDGA on ECs and VICs are 32.6 microg/ml and 47.5 microg/ml, respectively, while those of GA are almost 30 times higher than NDGA (P < 0.05). ECs can attach to and maintain normal morphology on the surface of NDGA-crosslinked valvular scaffolds but not GA-crosslinked ones. This study demonstrated that NDGA-crosslinking of decellularized valvular matrix is a promising approach for preparation of heart valve tissue engineering scaffolds.

Bioengineering challenges for heart valve tissue engineering

Surgical replacement of diseased heart valves by mechanical and tissue valve substitutes is now commonplace and enhances survival and quality of life for many patients. However, repairs of congenital deformities require very small valve sizes not commercially available. Further, a fundamental problem inherent to the use of existing mechanical and biological prostheses in the pediatric population is their failure to grow, repair, and remodel. It is believed that a tissue engineered heart valve can accommodate many of these requirements, especially those pertaining to somatic growth. This review provides an overview of the field of heart valve tissue engineering, including recent trends, with a focus on the bioengineering challenges unique to heart valves. We believe that, currently, the key bioengineering challenge is to determine how biological, structural, and mechanical factors affect extracellular matrix (ECM) formation and in vivo functionality. These factors are fundamental to any approach toward developing a clinically viable tissue engineered heart valve (TEHV), regardless of the particular approach. Critical to the current approaches to TEHVs is scaffold design, which must simultaneously provide function (valves must function from the time of implant) as well as stress transfer to the new ECM. From a bioengineering point of view, a hierarchy of approaches will be necessary to connect the organ-tissue relationships with underpinning cell and sub-cellular events. Overall, such approaches need to be structured to address these fundamental issues to lay the basis for TEHVs that can be developed and designed according to truly sound scientific and engineering principles.

Potential cell sources for tissue engineering of heart valves in comparison with human pulmonary valve cells

Current techniques to resolve heart valve defects involve the use of prosthetic and bioprosthetic materials. These materials lack the potential to grow and are not ideal, especially not for pediatric patients. Novel techniques like tissue engineering involve the use of biodegradable polymers coated with autologous myofibroblast and endothelial cells. We inspected morphological and marker gene expression differences between cells harvested from the saphenous vein, or from veins and arteries of the umbilical cord, and the cells they are designed to replace: the interstitial and endothelial cells of the pulmonary heart valve. We assessed the extent to which the endothelial cells from the inspected sources in vitro resemble endothelial cells of human pulmonary heart valves, and we found that myofibroblast cells, respective of their source, in vitro differ from the interstitial cells from human pulmonary heart valves regarding collagen and smooth muscle alpha-actin. Therefore we conclude that the cells isolated from the saphenous veins, or from veins and arteries of the umbilical cord might be feasible cell sources for tissue engineering of heart valve for the pulmonary position.

Dynamics of proteins in Golgi membranes: comparisons between mammalian and plant cells highlighted by photobleaching techniques

In less than a decade the green fluorescent protein (GFP) has become one of the most popular tools for cell biologists for the study of dynamic processes in vivo. GFP has revolutionised the scientific approach for the study of vital organelles, such as the Golgi apparatus. As Golgi proteins can be tagged with GFP, in most cases without altering their targeting and function, it is a great substitute to conventional dyes used in the past to highlight this compartment. In this review, we cover the application of GFP and its spectral derivatives in the study of Golgi dynamics in mammalian and plant cells. In particular, we focus on the technique of selective photobleaching known as fluorescence recovery after photobleaching, which has successfully shed light on essential differences in the biology of the Golgi apparatus in mammalian and plant cells.