Potential of Human Embryonic Stem Cells in Cartilage Tissue Engineering and Regenerative Medicine

The current surgical intervention of using autologous chondrocyte implantation (ACI) for cartilage repair is associated with several problems such as donor site morbidity, de-differentiation upon expansion and fibrocartilage repair following transplantation. This has led to exploration of the use of stem cells as a model for chondrogenic differentiation as well as a potential source of chondrogenic cells for cartilage tissue engineering and repair. Embryonic stem cells (ESCs) are advantageous, due to their unlimited self-renewal and pluripotency, thus representing an immortal cell source that could potentially provide an unlimited supply of chondrogenic cells for both cell and tissue-based therapies and replacements. This review aims to present an overview of emerging trends of using ESCs in cartilage tissue engineering and regenerative medicine. In particular, we will be focusing on ESCs as a promising cell source for cartilage regeneration, the various strategies and approaches employed in chondrogenic differentiation and tissue engineering, the associated outcomes from animal studies, and the challenges that need to be overcome before clinical application is possible.     

Functional biomaterials for cartilage regeneration

The injury and degeneration of articular cartilage and associated arthritis are leading causes of disability worldwide. Cartilage tissue engineering as a treatment modality for cartilage defects has been investigated for over 20 years. Various scaffold materials have been developed for this purpose, but has yet to achieve feasibility and effectiveness for widespread clinical use. Currently, the regeneration of articular cartilage remains a formidable challenge, due to the complex physiology of cartilage tissue and its poor healing capacity. Although intensive research has been focused on the developmental biology and regeneration of cartilage tissue and a diverse plethora of biomaterials have been developed for this purpose, cartilage regeneration is still suboptimal, such as lacking a layered structure, mechanical mismatch with native cartilage and inadequate integration between native tissue and implanted scaffold. The ideal scaffold material should have versatile properties that actively contribute to cartilage regeneration. Functional scaffold materials may overcome the various challenges faced in cartilage tissue engineering by providing essential biological, mechanical, and physical/chemical signaling cues through innovative design. This review thus focuses on the complex structure of native articular cartilage, the critical properties of scaffolds required for cartilage regeneration, present strategies for scaffold design, and future directions for cartilage regeneration with functional scaffold materials.     

The tissue-engineered auricle: past, present, and future

The reconstruction, repair, and regeneration of the external auricular framework continue to be one of the greatest challenges in the field of tissue engineering. To replace like with like, we should emulate the native structure and composition of auricular cartilage by combining a suitable chondrogenic cell source with an appropriate scaffold under optimal in vitro and in vivo conditions. Due to the fact that a suitable and reliable substitute for auricular cartilage has yet to be engineered, hand-carved autologous costal cartilage grafts and ear-shaped porous polyethylene implants are the current treatment modalities for auricular reconstruction. However, over the last decade, significant advances have been made in the field of regenerative medicine and tissue engineering. A variety of scaffolds and innovative approaches have been investigated as alternatives to using autologous carved costal cartilage or porous polyethylene implants. A review of recent developments and the current state of the art and science is presented, focusing on scaffolds, cell sources, seeding densities, and mechanical characteristics of tissue-engineered auricular cartilage.     

细胞外基质材料在骨组织工程中的研究进展

由疾病、外部创伤等原因引起的大骨骼缺损的治疗需要通过骨移植手术,寻找安全易得的替代骨已经成为临床上的重要课题,近年来快速发展的组织工程骨为解决这一难题提供了一种新的途径。支架材料作为组织工程的核心要素,其表面性状、结构,机械性能和生物学性能均能调控细胞的各种生命活动和体内组织的修复再生。细胞外基质由于其天然性、低免疫排斥性和优异的生物相容性等特点,已被广泛用作再生医学的支架材料。通过回顾近些年来细胞外基质材料在骨组织工程中的应用,阐述多种细胞外基质材料的构建修饰方法及其体外、体内的生物学效应,并对其在骨再生领域的应用前景进行展望。     

Founder's award to Antonios G. Mikos, Ph.D., 2011 Society for Biomaterials annual meeting and exposition, Orlando, Florida, April 13-16, 2011: Bones to biomaterials and back again--20 years of taking cues from nature to engineer synthetic polymer scaffolds

For biomaterials scientists focusing on tissue engineering applications, the gold standard material is healthy, autologous tissue. Ideal material properties and construct design parameters are thus both obvious and often times unachievable; additional considerations such as construct delivery and the underlying pathology necessitating new tissue yield additional design challenges with solutions that are not evident in nature. For the past nearly two decades, our laboratory and collaborators have aimed to develop both new biomaterials and a better understanding of the complex interplay between material and host tissue to facilitate bone and cartilage regeneration. Various approaches have ranged from mimicking native tissue material properties and architecture to developing systems for bioactive molecule delivery as soluble factors or bound directly to the biomaterial substrate. Such technologies have allowed others and us to design synthetic biomaterials incorporating increasing levels of complexity found in native tissues with promising advances made toward translational success. Recent work focuses on translation of these technologies in specific clinical situations through the use of adjunctive biomaterials designed to address existing pathologies or guide host-material integration.     

Application of chitosan-based polysaccharide biomaterials in cartilage tissue engineering: a review

Once damaged, articular cartilage has very little capacity for spontaneous healing because of the avascular nature of the tissue. Although many repair techniques have been proposed over the past four decades, none has sucessfully regenerated long-lasting hyaline cartilage tissue to replace damaged cartilage. Tissue engineering approaches, such as transplantation of isolated chondrocytes, have recently demonstrated tremendous clinical potential for regeneration of hyaline-like cartilage tissue and treatment of chondral lesions. As such a new approach emerges, new important questions arise. One of such questions is: what kinds of biomaterials can be used with chondrocytes to tissue-engineer articular cartilage? The success of chondrocyte transplantation and/or the quality of neocartilage formation strongly depend on the specific cell-carrier material. The present article reviews some of those biomaterials, which have been suggested to promote chondrogenesis and to have potentials for tissue engineering of articular cartilage. A new biomaterial, a chitosan-based polysaccharide hydrogel, is also introduced and discussed in terms of the biocompatibility with chondrocytes.     

Tissue Engineering Strategies for the Regeneration of Orthopedic Interfaces

A major focus in the field of orthopedic tissue engineering is the development of tissue engineered bone and soft tissue grafts with biomimetic functionality to allow for their translation to the clinical setting. One of the most significant challenges of this endeavor is promoting the biological fixation of these grafts with each other as well as the implant site. Such fixation requires strategic biomimicry to be incorporated into the scaffold design in order to re-establish the critical structure–function relationship of the native soft tissue-to-bone interface. The integration of distinct tissue types (e.g. bone and soft tissues such as cartilage, ligaments, or tendons), necessitates a multi-phased or stratified scaffold with distinct yet continuous tissue regions accompanied by a gradient of mechanical properties. This review discusses tissue engineering strategies for regenerating common tissue-to-tissue interfaces (ligament-to-bone, tendon-to-bone, or cartilage-to-bone), and the strategic biomimicry implemented in stratified scaffold design for multi-tissue regeneration. Potential challenges and future directions in this emerging field will also be presented. It is anticipated that interface tissue engineering will enable integrative soft tissue repair, and will be instrumental for the development of complex musculoskeletal tissue systems with biomimetic complexity and functionality.     

Gene-activated matrices for cartilage defect reparation

A cartilage defect has a very limited ability to repair itself spontaneously due to the shortage of blood. Many attempts have been made to restore the integrity of cartilage in clinical and experimental studies. Recently, tissue engineering has emerged as a new protocol for lost tissue regeneration. Meanwhile, the defect-repairing environment can be improved by gene therapy methods. Gene-activated matrices (GAM) fabricated with biomaterials and plasmids fill the cartilage defects to restore the integrity of joint surface, facilitating repair cell adhesion and proliferation as well as the synthesis of extracelluar matrix. And they also serve as a local gene delivery system, inducing therapeutic agent expression at the repair site. In the present study, we fabricated two- and three-dimensional matrices from chitosan and gelatin, then added a plasmid DNA encoding transforming growth factors-ss1 (TGF-ss1) for cartilage defect regeneration. First, we demonstrated primary chondrocytes could maintain their biological characteristics and secrete therapeutic proteins when they were cultured onto GAM in vitro. Subsequently we inserted three-dimensional GAM into cartilage defects of rabbit knee joints. With the results of the new cartilage tissue formation, we came to the conclusion that GAM was helpful for new tissue production and this therapeutic protocol provided a cheap, simple, and effective method for cartilage defect reparation.     

气管组织工程的研究策略及其展望

气管组织工程是近10年来国内外研究的热点和难点问题,取得了一些进展.但如何更好地遴选基质材料、实现种子细胞共培养、上皮化以及再血管化来构建组织工程化气管仍需要不断寻求新的研究思路和策略.本文从气管重建和理想化气管替代物的角度,回顾了近年来在组织工程气管方面的研究进展,并对未来的研究作出展望.     

Cell sources for the regeneration of articular cartilage: the past,the horizon and the future

Avascular, aneural articular cartilage has a low capacity for self‐repair and as a consequence is highly susceptible to degradative diseases such as osteoarthritis. Thus the development of cell‐based therapies that repair focal defects in otherwise healthy articular cartilage is an important research target, aiming both to delay the onset of degradative diseases and to decrease the need for joint replacement surgery. This review will discuss the cell sources which are currently being investigated for the generation of chondrogenic cells. Autologous chondrocyte implantation using chondrocytes expanded ex vivo was the first chondrogenic cellular therapy to be used clinically. However, limitations in expansion potential have led to the investigation of adult mesenchymal stem cells as an alternative cell source and these therapies are beginning to enter clinical trials. The chondrogenic potential of human embryonic stem cells will also be discussed as a developmentally relevant cell source, which has the potential to generate chondrocytes with phenotype closer to that of articular cartilage. The clinical application of these chondrogenic cells is much further away as protocols and tissue engineering strategies require additional optimization. The efficacy of these cell types in the regeneration of articular cartilage tissue that is capable of withstanding biomechanical loading will be evaluated according to the developing regulatory framework to determine the most appropriate cellular therapy for adoption across an expanding patient population.     

Scaffolds for tissue engineering of cartilage

Articular cartilage lesions resulting from trauma or degenerative diseases are commonly encountered clinical problems. It is well-established that adult articular cartilage has limited regenerative capacity, and, although numerous treatment protocols are currently employed clinically, few approaches exist that are capable of consistently restoring long-term function to damaged articular cartilage. Tissue engineering strategies that focus on the use of three-dimensional scaffolds for repairing articular cartilage lesions offer many advantages over current treatment strategies. Appropriate design of biodegradable scaffold conduits (either preformed or injectable) allow for the delivery of reparative cells bioactive factors, or gene factors to the defect site in an organized manner. This review seeks to highlight pertinent design considerations and limitations related to the development, material selection, and processing of scaffolds for articular cartilage tissue engineering, evidenced over the last decade. In particular, considerations for novel repair strategies that use scaffolds in combination with controlled release of bioactive factors or gene therapy are discussed, as are scaffold criteria related to mechanical stimulation of cell-seeded constructs. Furthermore, the subsequent impact of current and future aspects of these multidisciplinary scaffold-based approaches related to in vitro and in vivo cartilage tissue engineering are reported herein.     

Bilayered scaffolds for osteochondral tissue engineering

Osteoarthritis (OA) is a prevalent degenerative joint disease that places a significant burden on the socioeconomic efficacy of communities around the world. Tissue engineering repair of articular cartilage in synovial joints represents a potential OA treatment strategy superior to current surgical techniques. In particular, osteochondral tissue engineering, which promotes the simultaneous regeneration of articular cartilage and underlining subchondral bone, may be a clinically relevant approach toward impeding OA progression. The unique and complex functional demands of the two contrasting tissues that comprise osteochondral tissue require the use of bilayered scaffolds to promote individual growth of both on a single integrated implant. This paper reviews the three current bilayered scaffold strategies applied to solve this challenging problem, with a focus on the need for an innovative approach to design and fabrication of new optimized scaffold combinations to reinforce materials science as an important element of osteochondral tissue engineering.     

纳米支架与关节软骨重建

背景：纳米羟基磷灰石因具有与骨组织中天然羟磷灰石晶体尺寸相关的特性以及具有良好的生物相容性和骨传导性,被广泛用于骨组织工程。 目的：对纳米支架材料的性能进行阐述,探讨仿生多层纳米支架构建和纳米支架体内试验研究的新进展。 方法：由作者应用计算机检索PubMed数据库及CNKI数据库1979/2011,在英文标题和摘要中以“cartilage, nano”和“cartilage, nanofiber or nanofibrous”检索,中文文献检索以“软骨,纳米,支架”为关键词,选择内容与纳米支架、软骨损伤修复与软骨组织工程相关的文章,同一领域文献则选择近期发表或发表在权威杂志文章,共纳入50篇文献。 结果与结论：软骨自我修复能力有限,软骨组织工程作为一种新治疗手段为其治愈提供了可能,纳米支架则以其优越的性能成为软骨组织修复的重要生物材料,纳米支架的仿生分层设计与制备及其生物性能的最优化,以及大宗样本临床试验的缺乏成为制约纳米支架材料应用于临床软骨缺损修复的关键问题。 关键词：纳米;支架;软骨重建;组织工程;综述文献 doi:10.3969/j.issn.1673-8225.2012.12.041     

组织工程技术修复损伤关节软骨的研究与应用

背景：软骨是一种无血管的组织,软骨损伤后自身修复能力有限。当前用于治疗关节软骨损伤的方法从保守治疗到手术治疗多种多样。随着组织工程技术的发展,关节软骨的修复又进入了新的高度。 目的：综述组织工程方法修复软骨损伤的新进展。 方法：由第一作者在2013年5月应用计算机检索2000至2013年PubMed 数据库及CNKI 数据库,英文以“cartilage tissue engineering,cartilage defect;stem cell,scaffold;growth factor”为关键词,中文以“软骨组织工程,软骨缺损,干细胞,支架,生长因子”为关键词,选择内容与软骨组织工程、软骨损伤修复相关的文章,同一领域文献则选择近期发表或发表在权威杂志文章,共纳入64篇文献。 结果与结论：软骨组织工程三大要素——种子细胞、支架和细胞因子,三者必须协调发展和互利。现阶段组织工程方法修复关节软骨损伤的研究虽已取得很大进展,但大多停留于实验探索阶段,尚未应用于临床。随着新材料的不断研发,新的组织工程软骨修复材料将兼顾材料学和生物科学的需要,使其更接近机体自身组织生物学特性。在新的技术支持下,动物实验研究也将向临床试验转变,使关节软骨损伤的治疗取得突破性进展。     

Regenerative dental medicine: stem cells and tissue engineering in dentistry.

The dawn of this century is brightened by the growing understanding and experimentation with stem cells as primary tools in the expanding regenerative medicine and tissue engineering revolution. The tradition of using prosthetic artificial implants to restore lost or damaged dental tissue will gradually be supplanted by more natural alternatives, including biological tooth replacement or induction. The practice of dentistry is likely to be revolutionized by biological therapies based on growth and differentiation factors that accelerate and/or induce a natural biological regeneration. This prospect has flourished from the gained knowledge provided by the molecular biological characterization of the genetic makeup of human cells and from a growing understanding of the effect of environmental factors. Prevention of dental diseases will also gain new ground as more insight is gained into the genetic makeup of microbial pathogens, their interactions with the host, and the host repair mechanisms. This review summarizes current knowledge, barriers, and challenges in the clinical use of stem cells with an emphasis on applications in dentistry.     

Maximizing cartilage formation and integration via a trajectory-based tissue engineering approach

Given the limitations of current surgical approaches to treat articular cartilage injuries, tissue engineering (TE) approaches have been aggressively pursued. Despite reproduction of key mechanical attributes of native tissue, the ability of TE cartilage constructs to integrate with native tissue must also be optimized for clinical success. In this paper, we propose a “trajectory-based” tissue engineering (TB-TE) approach, based on the hypothesis that time-dependent increases in construct maturation in-vitro prior to implantation (i.e. positive rates) may provide a reliable predictor of in-vivo success. As an example TE system, we utilized hyaluronic acid hydrogels laden with mesenchymal stem cells. We first modeled the maturation of these constructs in-vitro to capture time-dependent changes. We then performed a sensitivity analysis of the model to optimize the timing and amount of data collection. Finally, we showed that integration to cartilage in-vitro is not correlated to the maturation state of TE constructs, but rather their maturation rate, providing a proof-of-concept for the use of TB-TE to enhance treatment outcomes following cartilage injury. This new approach challenges the traditional TE paradigm of matching only native state parameters of maturity and emphasizes the importance of also establishing an in-vitro trajectory in constructs in order to improve the chance of in-vivo success.     

A biomimetic extracellular matrix for cartilage tissue engineering centered on photocurable gelatin,hyaluronic acid and chondroitin sulfate

The development of hydrogels tailored for cartilage tissue engineering has been a research and clinical goal for over a decade. Directing cells towards a chondrogenic phenotype and promoting new matrix formation are significant challenges that must be overcome for the successful application of hydrogels in cartilage tissue therapies. Gelatin–methacrylamide (Gel-MA) hydrogels have shown promise for the repair of some tissues, but have not been extensively investigated for cartilage tissue engineering. We encapsulated human chondrocytes in Gel-MA-based hydrogels, and show that with the incorporation of small quantities of photocrosslinkable hyaluronic acid methacrylate (HA-MA), and to a lesser extent chondroitin sulfate methacrylate (CS-MA), chondrogenesis and mechanical properties can be enhanced. The addition of HA-MA to Gel-MA constructs resulted in more rounded cell morphologies, enhanced chondrogenesis as assessed by gene expression and immunofluorescence, and increased quantity and distribution of the newly synthesized extracellular matrix (ECM) throughout the construct. Consequently, while the compressive moduli of control Gel-MA constructs increased by 26 kPa after 8 weeks culture, constructs with HA-MA and CS-MA increased by 114 kPa. The enhanced chondrogenic differentiation, distribution of ECM, and improved mechanical properties make these materials potential candidates for cartilage tissue engineering applications.     

Biodegradable magnesium scaffolds: Part 1: appropriate inflammatory response

Current tissue engineering strategies focus on the replacement of pathologically altered tissues by the transplantation of cells in combination with supportive biocompatible scaffolds. Scaffolds for tissue engineering strategies in musculoskeletal research require an appropriate mechanical stability. In recent studies, considerable attention has thus been given to magnesium alloys as biodegradable implants. The aim of this study was to characterize the biocompatibility of magnesium scaffolds by the inflammatory host response. Open porous scaffolds made of the magnesium alloy AZ91D were implanted into the distal femur condyle of rabbits and were compared to autologous bone, which was transplanted into the contralateral condyle in a 3 and 6 months follow-up group. After 3 months, magnesium scaffolds were already largely degraded and most of the original magnesium alloy has disappeared. Concomitantly, a fibrous capsule enclosed the operation site. Histological analysis revealed that the magnesium scaffolds caused no significant harm to their neighboring tissues. This study shows that even fast degrading magnesium scaffolds show a good biocompatibility and react in vivo with an appropriate inflammatory host response. Magnesium alloy based implants are therefore a very promising approach in the development of mechanically suitable and open porous scaffolds for the replacement of subchondral bone in cartilage tissue engineering.     

Advances in Mesenchymal Stem Cell-based Strategies for Cartilage Repair and Regeneration

Significant research efforts have been undertaken in the last decade in the development of stem cell-based therapies for cartilage repair. Among the various stem cell sources, mesenchymal stem cells (MSCs) demonstrate great promise and clinical efficacy in cartilage regeneration. With a deeper understanding of stem cell biology, new therapeutics and new bioengineering approaches have emerged and showed potential for further developments. Of note, there has been a paradigm shift in applying MSCs for tissue regeneration from the use of stem cells for transplantation to the use of stem cell-derived matrix and secretome components as therapeutic tools and agents for cartilage regeneration. In this review, we will discuss the emerging role of MSCs in cartilage regeneration and the most recent advances in development of stem cell-based therapeutics for cartilage regeneration.     

Homing of endogenous stem/progenitor cells for in situ tissue regeneration: Promises, strategies, and translational perspectives

Stem cell-based therapy has been one of the best documented approaches in regenerative medicine, promising cures for a multitude of diseases and disorders. However, the ex vivo expansion of stem cells and their in vivo delivery are restricted by the limited availability of stem cell sources, the excessive cost of commercialization, and the anticipated difficulties of clinical translation and regulatory approval. An alternative to adoptively transferred stem cells are cell populations already present in a patient's body, including stem/progenitor cells, which can be actively attracted to sites of injury. This technique, known as endogenous cell homing, has the potential to provide new therapeutic options for in situ tissue regeneration. Such options would be less costly and complex than approaches that require substantial ex vivo cell manipulation and that use artificial vehicles for cell delivery. Tissue regeneration methods that rely on endogenous stem/progenitor cell homing, local tissue responses, and functional stimulation thus offer new insights into in vivo tissue engineering and hold great promise for the future of translational medicine. Although such methods that take advantage of the latent endogenous regenerative potential of the patient are promising for the repair of damaged tissue, they are in need of further experimental support before application in late-stage diseases or severe tissue injury. This review is not meant to be exhaustive but gives a brief outlook on the promises, strategies, and current applications of endogenous stem cell homing for in situ tissue regeneration, with particular emphasis placed upon pharmacological means based on cell-instructive scaffolds and release technology to direct cell mobilization and recruitment. In the future these exciting paradigms are likely to help reconcile the clinical and commercial pressures in regenerative medicine.