Strategies for articular cartilage lesion repair and functional restoration

Progenitor cells from cartilage--no osteoarthritis-grade-specific differences in stem cell marker expression

Tissue engineering efforts for the fabrication of cartilage substitutes head toward applicability in osteoarthritis (OA). Progenitor cells can be harvested from the osteoarthritic joint itself, resembling multipotent mesenchymal stromal cells (MSC). Our objective was to analyze MSC characteristics of those cells in respect to the OA-related damage of their harvest site. OA cartilage was obtained from six patients during alloarthroplastic knee surgery, sample grading was done according to Outerbridge's classification. Upon enzymatic dissociation, primary chondrocytes were expanded in two-dimensional monolayer culture. At distinct cell passages, the process of dedifferentiation was phenotypically monitored; cell surface expression of classical MSC markers was analyzed by flow cytometry. Cells were subjected to chondrogenesis and osteogenesis after their fourth passage. At third passage, 95% of cells became positive for cluster of differentiation (CD)105 and further subclassification revealed that the majority of them were positive for both CD73 and CD90. CD105(+) CD73(+) CD90(+) phenotype meets thus the minimal surface antigen criteria for MSC definition. More than one-third of dedifferentiated chondrocytes displayed a coexpression of CD9(+) CD166(+) CD90(+) and to a lesser extent CD105(+) CD73(+) CD44(+) , irrespective of the stage of the original cartilage degradation. Finally, we could successfully demonstrate a redifferentiation of these progenitors into sulfated glycosaminoglycan producing cells. The basic level of alkaline phosphatase activity could not be enhanced upon osteogenic differentiation. In conclusion, chondrogenic progenitors derived from OA cartilages with low or high Outerbridge's grade can be seen as a potential cellular source for cartilage replacement.

One-step osteochondral repair with cartilage fragments in a composite scaffold

PURPOSE

This study proposes a single-step therapeutic approach for osteochondral defects using autologous cartilage fragments loaded onto a scaffold composed of a hyaluronic acid (HA) derivative, human fibrin glue (FG) and autologous platelet-rich-plasma (PRP), in a rabbit model. The aim is to demonstrate the in vitro outgrowth of chondrocytes from cartilage fragments and the in vivo formation of a functional repair tissue.

METHODS

In vitro: minced articular cartilage was loaded onto two different types of scaffold (paste or membrane) according to two different HA preparations (injectable HA-derivative or HA-derivative felt). In vivo: trochlear osteochondral defects were created in 50 adult rabbits, which were then assigned to 5 different treatment groups: cartilage fragments loaded onto membrane scaffolds with FG (Group 1) or without FG (Group 2); membrane scaffolds alone with FG (Group 3) or without FG (Group 4); empty defects (Group 5). Membrane scaffolds were used "in vivo" for simpler preparation and better adhesive properties. Repair processes were evaluated histologically and by immunohistochemistry at 1, 3, and 6 months.

RESULTS

An in vitro time-dependent cell outgrowth from cartilage fragments was observed with both types of scaffolds. At 6 months, in vivo, cartilage fragment-loaded scaffolds induced significantly better repair tissue than the scaffold alone using histological scoring. Repair in Group 2 was superior to that in any of the control groups (p < 0.05).

CONCLUSION

Autologous cartilage fragments loaded onto an HA felt/FG/PRP-scaffold provided an efficient cell source, and allowed for an improvement of the repair process of ostechondral defects in a rabbit model. Human FG, however, hampered the rabbit healing process. These results may have clinical relevance as they show the potential of a novel one-stage repair technique for osteochondral defects.

Modified autologous matrix-induced chondrogenesis (AMIC) for the treatment of a large osteochondral defect in a varus knee: a case report

This paper presents a case report of a 27-year-old male patient affected by a large osteochondral defect of the medial femoral condyle (6 cm(2)) in a varus knee. He was treated with a combined approach consisting of high tibial osteotomy and autologous matrix-induced chondrogenesis technique enhanced by a bone marrow-enriched bone graft. Twelve months after surgery, the patient reported considerable reduction in pain and significant increase in his quality of life. A hyaline-like cartilage completely covered the defect and was congruent with the surrounding condyle cartilage as revealed by MRI and by a second-look arthroscopy. Level of evidence IV.

Computational Investigation of Fibrin Mechanical and Damage Properties at the Interface Between Native Cartilage and Implant

Scaffold-based tissue-engineered constructs as well as cell-free implants offer promising solutions to focal cartilage lesions. However, adequate mechanical stability of these implants in the lesion is required for successful repair. Fibrin is the most common clinically available adhesive for cartilage implant fixation, but fixation quality using fibrin is not well understood. The objectives of this study were to investigate the conditions leading to damage in the fibrin adhesive and to determine which adhesive properties are important in preventing delamination at the interface. An idealized finite element model of the medial compartment of the knee was created, including a circular defect and an osteochondral implant. Damage and failure of fibrin at the interface was represented by a cohesive zone model with coefficients determined from an inverse finite element method and previously published experimental data. Our results demonstrated that fibrin glue alone may not be strong enough to withstand physiologic loads in vivo while fibrin glue combined with chondrocytes more effectively prevents damage at the interface. The results of this study suggest that fibrin fails mainly in shear during off-axis loading and that adhesive materials that are stronger or more compliant than fibrin may be good alternatives due to decreased failure at the interface. The present model may be used to improve design and testing protocols of bioadhesives and give insight into the failure mechanisms of cartilage implant fixation in the knee joint.

Lentiviral-mediated RNAi knockdown of Cbfa1 gene inhibits endochondral ossification of antler stem cells in micromass culture

Articular cartilage (AC) lacks ability to repair defects due to its avascular nature as healing process relies on cells being brought in by blood vessels. Multiple approaches have been taken to facilitate cartilage repair in clinics, to date there is no effective treatment available that can restores the AC lesion to a normally functioning level over extended periods. In this regard, antler cartilage is unique in being richly vascularised and hence can effectively repair and regenerate. Interestingly, antler stem cells, from which the vascularised cartilage is derived, can form avascular cartilage when taken away from their original niche, suggesting that the vascular or avascular state of antler cartilage is controlled by extrinsic factors. Understanding the mechanisms underlying this phenotype switch may help us to devise a way to trigger the effective intrinsic repair of AC. However, adoption of antler cartilage model for AC repair requires the demonstration that the cartilage specific signalling pathways also prevail in antler chondrogenesis. To achieve this, in the present study we silenced expression of Cbfa1, a key factor regulatingendochondral ossification, using RNAi, and showed that expression of the downstream genes type I collagen and osteocalcin were suppressed which, in turn, inhibited endochondral ossification process taking place in the antler stem cell-formed nodules. Therefore, we provided further evidence at molecular level that antler could be developed as novel model for the study of AC repair. The eventual identification of the extrinsic factors dictating the phenotype switch between the vascular and avascular state of antler cartilage will open up a new avenue for the cure of osteoarthritis.

Building bridges: leveraging interdisciplinary collaborations in the development of biomaterials to meet clinical needs

Our laboratory at Rice University has forged numerous collaborations with clinicians and basic scientists over the years to advance the development of novel biomaterials and the modification of existing materials to meet clinical needs. This review highlights collaborative advances in biomaterials research from our laboratory in the areas of scaffold development, drug delivery, and gene therapy, especially as related to applications in bone and cartilage tissue engineering.

Biomaterial scaffolds in cartilage–subchondral bone defects influencing the repair of autologous articular cartilage transplants

The repair of articular cartilage typically involves the repair of cartilage–subchondral bone tissue defects. Although various bioactive materials have been used to repair bone defects, how these bioactive materials in subchondral bone defects influence the repair of autologous cartilage transplant remains unclear. The aim of this study was to investigate the effects of different subchondral biomaterial scaffolds on the repair of autologous cartilage transplant in a sheep model. Cylindrical cartilage–subchondral bone defects were created in the right femoral knee joint of each sheep. The subchondral bone defects were implanted with hydroxyapatite–β-tricalcium phosphate (HA–TCP), poly lactic-glycolic acid (PLGA)-HA–TCP dual-layered composite scaffolds (PLGA/HA–TCP scaffolds), or autologous bone chips. The autologous cartilage layer was placed on top of the subchondral materials. After 3 months, the effect of different subchondral scaffolds on the repair of autologous cartilage transplant was systematically studied by investigating the mechanical strength, structural integration, and histological responses. The results showed that the transplanted cartilage layer supported by HA–TCP scaffolds had better structural integration and higher mechanical strength than that supported by PLGA/HA–TCP scaffolds. Furthermore, HA–TCP-supported cartilage showed higher expression of acid mucosubstances and glycol-amino-glycan contents than that supported by PLGA/HA–TCP scaffolds. Our results suggested that the physicochemical properties, including the inherent mechanical strength and material chemistry of the scaffolds, play important roles in influencing the repair of autologous cartilage transplants. The study may provide useful information for the design and selection of proper subchondral biomaterials to support the repair of both subchondral bone and cartilage defects.

Epiphyseal Chondroprogenitors Provide a Stable Cell Source for Cartilage Cell Therapy

Articular cartilage regeneration poses particularly tough challenges for implementing cell-based therapies. Many cell types have been investigated looking for a balanced combination of responsiveness and stability, yet techniques are still far from defining a gold standard. The work presented focuses on the reliable expansion and characterization of a clinical grade human epiphyseal chondroprogenitor (ECP) cell bank from a single tissue donation. A parental human ECP cell bank was established, which provides the seed material for master and working cell banks. ECPs were investigated at both low and high cumulative population doublings looking at morphology, monolayer expansion kinetics, resistance to cryogenic shock, colony-forming efficiency, and cell surface markers. Three-dimensional micropellet assays were used to determine spontaneous extracellular matrix deposition at varying population doublings and monolayer 2D differentiation studies were undertaken to assess the propensity for commitment into other lineages and their stability. ECPs exhibited remarkable homogeneity in expansion with a steady proliferative potential averaging three population doublings over 8 days. Surface marker analysis revealed no detectable contaminating subpopulations or population enrichment during prolonged culture periods. Despite a slight reduction in Sox9 expression levels at higher population doublings in monolayer, nuclear localization was equivalent both in monolayer and in micropellet format. Equally, ECPs were capable of depositing glycosaminoglycans and producing aggrecan, collagen I, and collagen II in 3D pellets both at low and high population doublings indicating a stable spontaneous chondrogenic potential. Osteogenic induction was differentially restricted in low and high population doublings as observed by Von Kossa staining of calcified matrix, with a notable collagen X, MMP13, and ADAMTS5 downregulation. Rare adipogenic induction was seen as evidenced by cytoplasmic lipid accumulation detectable by Oil Red O staining. These findings highlight the reliability, stability, and responsiveness of ECPs over prolonged culture, making them ideal candidates in defining novel strategies for cartilage regeneration.

Regenerative medicine as applied to general surgery

The present review illustrates the state of the art of regenerative medicine (RM) as applied to surgical diseases and demonstrates that this field has the potential to address some of the unmet needs in surgery. RM is a multidisciplinary field whose purpose is to regenerate in vivo or ex vivo human cells, tissues, or organs to restore or establish normal function through exploitation of the potential to regenerate, which is intrinsic to human cells, tissues, and organs. RM uses cells and/or specially designed biomaterials to reach its goals and RM-based therapies are already in use in several clinical trials in most fields of surgery. The main challenges for investigators are threefold: Creation of an appropriate microenvironment ex vivo that is able to sustain cell physiology and function in order to generate the desired cells or body parts; identification and appropriate manipulation of cells that have the potential to generate parenchymal, stromal and vascular components on demand, both in vivo and ex vivo; and production of smart materials that are able to drive cell fate.

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.

Clinical relevance of scaffolds for cartilage engineering

The repair of articular cartilage defects in patients' knees presents a particular challenge to the orthopedic surgeon because cartilage lacks the ability to repair or regenerate itself. Various cartilage repair techniques have not produced a superior or uniform outcome, which has led to a new generation of cartilage repair based on tissue-engineering strategies and the use of biological scaffolds. Clinical advances have been made regarding the regeneration of articular cartilage, and continue to be made toward the achievement of a suitable treatment method for resurfacing osteochondral defects, through cartilage tissue engineering and the use of pluripotent cells seeded on bio-scaffolds.

Basic science and surgical treatment options for articular cartilage injuries of the knee

The complex structure of articular cartilage allows for diverse knee function throughout range of motion and weight bearing. However, disruption to the structural integrity of the articular surface can cause significant morbidity. Due to an inherently poor regenerative capacity, articular cartilage defects present a treatment challenge for physicians and therapists. For many patients, a trial of nonsurgical treatment options is paramount prior to surgical intervention. In instances of failed conservative treatment, patients can undergo an array of palliative, restorative, or reparative surgical procedures to treat these lesions. Palliative methods include debridement and lavage, while restorative techniques include marrow stimulation. For larger lesions involving subchondral bone, reparative procedures such as osteochondral grafting or autologous chondrocyte implantation are considered. Clinical success not only depends on the surgical techniques but also requires strict adherence to rehabilitation guidelines. The purpose of this article is to review the basic science of articular cartilage and to provide an overview of the procedures currently performed at our institution for patients presenting with symptomatic cartilage lesions.

New insights into cartilage repair - the role of migratory progenitor cells in osteoarthritis

Osteoarthritis is one of the most common musculo-skeletal diseases with a complex patholoy and a strong impact on cell biology, differentiation and migration behavior of mesenchymal stem cell-derived progenitor cells. In this review, we elucidate the influence of the pathologically altered extracellular matrix on progenitor cell behavior. Moreover, we discuss the modulation of progenitor cells especially of previously characterized chondrogenic progenitor cells (Koelling et al., 2009) in situ to enhance their regeneration potential. These options comprise the application of growth factors like fibroblast growth factor-2, a Runx-2 knock down and a contemporary anti-inflammatory therapy. This supports endogenous regeneration on behalf of the diseased osteoarthritic cartilage, which otherwise results mainly in an insufficient fibro-cartilaginous repair tissue. Furthermore, new results indicate a role of pericytes in osteoarthritis for these repair attempts. We discuss the biological mechanisms potentially leading to new therapeutic options in osteoarthritis to enhance regeneration in situ.

New Frontiers for Cartilage Repair and Protection

OBJECTIVE

Articular cartilage injury is common after athletic injury and remains a difficult treatment conundrum both for the surgeon and athlete. Although recent treatments for damage to articular cartilage have been successful in alleviating symptoms, more durable and complete, long-term articular surface restoration remains the unattained goal. In this article, we look at both new ways to prevent damage to articular surfaces as well as new techniques to recreate biomechanically sound and biochemically true articular surfaces once an athlete injures this surface. This goal should include reproducing hyaline cartilage with a well-integrated and flexible subchondral base and the normal zonal variability in the articular matrix.

RESULTS

A number of nonoperative interventions have shown early promise in mitigating cartilage symptoms and in preclinical studies have shown evidence of chondroprotection. These include the use of glucosamine, chondroitin, and other neutraceuticals, viscosupplementation with hyaluronic acid, platelet-rich plasma, and pulsed electromagnetic fields. Newer surgical techniques, some already in clinical study, and others on the horizon offer opportunities to improve the surgical restoration of the hyaline matrix often disrupted in athletic injury. These include new scaffolds, single-stage cell techniques, the use of mesenchymal stem cells, and gene therapy.

CONCLUSION

Although many of these treatments are in the preclinical and early clinical study phase, they offer the promise of better options to mitigate the sequelae of athletically induced cartilage.

Bioengineered human vascular networks transplanted into secondary mice reconnect with the host vasculature and re-establish perfusion

The ability to form anastomoses with the host circulation is essential for vascular networks incorporated within cell-seeded bioengineered tissues. Here, we tested whether and how rapidly human endothelial colony forming cell (ECFC)/mesenchymal progenitor cell (MPC)-derived bioengineered vessels, originally perfused in one mouse, could become reperfused in a secondary mouse. Using in vivo labeling with a systemically injected mixture of human- and murine-specific lectins, we demonstrate that ECFC/MPC blood vessels reconnect and are perfused at day 3 after transplantation. Furthermore, we quantified the longitudinal change in perfusion volume in the same implants before and after transplantation using contrast-enhanced micro-ultrasonic imaging. Perfusion was restored at day 3 after transplantation and increased with time, suggesting an important new feature of ECFC/MPC blood vessels: the bioengineered vessels can reconnect with the vasculature when transplanted to a new site. This feature extends the potential applications of this postnatal progenitor cell-based technology for transplantable large tissue-engineered constructs.

Tissue engineering of functional articular cartilage: the current status

Osteoarthritis is a degenerative joint disease characterized by pain and disability. It involves all ages and 70% of people aged >65 have some degree of osteoarthritis. Natural cartilage repair is limited because chondrocyte density and metabolism are low and cartilage has no blood supply. The results of joint-preserving treatment protocols such as debridement, mosaicplasty, perichondrium transplantation and autologous chondrocyte implantation vary largely and the average long-term result is unsatisfactory. One reason for limited clinical success is that most treatments require new cartilage to be formed at the site of a defect. However, the mechanical conditions at such sites are unfavorable for repair of the original damaged cartilage. Therefore, it is unlikely that healthy cartilage would form at these locations. The most promising method to circumvent this problem is to engineer mechanically stable cartilage ex vivo and to implant that into the damaged tissue area. This review outlines the issues related to the composition and functionality of tissue-engineered cartilage. In particular, the focus will be on the parameters cell source, signaling molecules, scaffolds and mechanical stimulation. In addition, the current status of tissue engineering of cartilage will be discussed, with the focus on extracellular matrix content, structure and its functionality.

Chondrogenesis of hMSC in affinity-bound TGF-beta scaffolds

Herein we describe a bio-inspired, affinity binding alginate-sulfate scaffold, designed for the presentation and sustained release of transforming growth factor beta 1 (TGF-β1), and examine its effects on the chondrogenesis of human mesenchymal stem cells (hMSCs). When attached to matrix via affinity interactions with alginate sulfate, TGF-β1 loading was significantly greater and its initial release from the scaffold was attenuated compared to its burst release (>90%) from scaffolds lacking alginate-sulfate. The sustained TGF-β1 release was further supported by the prolonged activation (14 d) of Smad-dependent (Smad2) and Smad-independent (ERK1/2) signaling pathways in the seeded hMSCs. Such presentation of TGF-β1 led to hMSC chondrogenic differentiation; differentiated chondrocytes with deposited collagen type II were seen within three weeks of in vitro hMSC seeding. By contrast, in scaffolds lacking alginate-sulfate, the effect of TGF-β1 was short-term and hMSCs could not reach a similar differentiation degree. When hMSC constructs were subcutaneously implanted in nude mice, chondrocytes with deposited type II collagen and aggrecan typical of the articular cartilage were found in the TGF-β1 affinity-bound constructs. Our results highlight the fundamental importance of appropriate factor presentation to its biological activity, namely - inducing efficient stem cell differentiation.

The maturity of tissue-engineered cartilage in vitro affects the repairability for osteochondral defect

Cartilage tissue engineering using cells and biocompatible scaffolds has emerged as a promising approach to repair of cartilage damage. To date, however, no engineered cartilage has proven to be equivalent to native cartilage in terms of biochemical and compression properties, as well as histological features. An alternative strategy for cartilage engineering is to focus on the in vivo regeneration potential of immature engineered cartilage. Here, we used a rabbit model to evaluate the extent to which the maturity of engineered cartilage influenced the remodeling and integration of implanted extracellular matrix scaffolds containing allogenous chondrocytes. Full-thickness osteochondral defects were created in the trochlear groove of New Zealand white rabbits. Left knee defects were left untreated as a control (group 1), and right knee defects were implanted with tissue-engineered cartilage cultured in vitro for 2 days (group 2), 2 weeks (group 3), or 4 weeks (group 4). Histological, chemical, and compression assays of engineered cartilage in vitro showed that biochemical composition became more cartilagenous, and biomechanical property for compression gradually increased with culture time. In an in vivo study, gross imaging and histological observation at 1 and 3 months after implanting in vitro-cultured engineered cartilage showed that defects in groups 3 and 4 were repaired with hyaline cartilage-like tissue, whereas defects were only partially filled with fibrocartilage after 1 month in groups 1 and 2. At 3 months, group 4 showed striking features of hyaline cartilage tissue, with a mature matrix and a columnar arrangement of chondrocytes. Zonal distribution of type II collagen was most prominent, and the International Cartilage Repair Society score was also highest at this time. In addition, the subchondral bone was well ossified. In conclusion, in vivo engineered cartilage was remodeled when implanted; however, its extent to maturity varied with cultivation period. Our results showed that the more matured the engineered cartilage was, the better repaired the osteochondral defect was, highlighting the importance of the in vitro cultivation period.

Isolation of human nasoseptal chondrogenic cells: a promise for cartilage engineering

In cartilaginous tissues, perichondrium cambium layer may be the source of new cartilage. Human nasal septal perichondrium is considered to be a homogeneous structure in which some authors do not recognize the perichondrium internal zone or the cambium layer as a layer distinct from adjacent cartilage surface. In the present study, we isolated a chondrogenic cell population from human nasal septal cartilage surface zone. Nasoseptal chondrogenic cells were positive for surface markers described for mesenchymal stem cells, with exception of CD146, a perivascular cell marker, which is consistent with their avascular niche in cartilage. Although only Sox-9 was constitutively expressed, they also revealed osteogenic and chondrogenic, but not adipogenic, potentials in vitro, suggesting a more restricted lineage potential compared to mesenchymal stem cells. Interestingly, even in absence of chondrogenic growth factors in the pellet culture system, nasoseptal chondrogenic cells had a capacity to synthesize sulfated glycosaminoglycans, large amounts of collagen type II and to a lesser extent collagen type I. The spontaneous chondrogenic potential of this population of cells indicates that they may be a possible source for cartilage tissue engineering. Besides, the pellet culture system using nasoseptal chondrogenic cells may also be a model for studies of chondrogenesis.

Clinical cartilage restoration: evolution and overview

BACKGROUND

Clinical cartilage restoration is evolving, with established and emerging technologies. Randomized, prospective studies with adequate power comparing the myriad of surgical techniques used to treat chondral injuries are still lacking and it remains a challenge for the surgeon treating patients to make evidence-based decisions.

QUESTIONS/PURPOSES

We reviewed the history of the major cartilage repair/restorative procedures, indications for currently available repair/restorative procedures, and postoperative management.

METHODS

We performed searches using MEDLINE and cartilage-specific key words to identify all English-language literature. Articles were selected based on their contributions to our current understanding of the basic science and clinical treatment of articular cartilage lesions or historical importance. We then selected 77 articles, two of which are articles of historical importance.

RESULTS

Current cartilage restorative techniques include débridement, microfracture, osteochondral fragment repair, osteochondral allograft, osteochondral autograft, and autologous chondrocyte transplantation. Pending techniques include two-staged cell-based therapies integrated into a variety of scaffolds, single-stage cell-based therapy, and augmentation of marrow stimulation, each with suggested indications including lesion size, location, and activity demands of the patient. The literature demonstrates variable improvements in pain and function contingent upon multiple variables including indications and application.

CONCLUSIONS

For the patient with symptomatic chondral injury, numerous techniques are available to the surgeon to relieve pain and improve function. Until rigorous clinical trials (prospective, adequately powered, randomized control) are available, treatment decisions should be guided by expert extrapolation of the available literature based in historically sound principles.

The stem cell niche should be a key issue for cell therapy in regenerative medicine

Recent advances in stem cell research have highlighted the role played by such cells and their environment (the stem cell niche) in tissue renewal and homeostasis. The control and regulation of stem cells and their niche are remaining challenges for cell therapy and regenerative medicine on several tissues and organs. These advances are important for both, the basic knowledge of stem cell regulation, and their practical translational applications into clinical medicine. This article is primarily concerned with the mesenchymal stem cells (MSCs) and it reviews the current aspects of their own niche. We discuss on the need for a deeper understanding of the identity of this cell type and its microenvironment in order to improve the effectiveness of any cell therapy for regenerative medicine. Ex vivo reproduction of the conditions of the natural stem cell niche, when necessary, would provide success to tissue engineering. The first challenge of regenerative medicine is to find cells able to replace and/or repair the lost function of tissues and organs by disease or aging and the trophic and immunomodulatory effects recently found for MSCs open up for new opportunities. If MSCs are pericytes, as it has been proposed, perhaps it may explain the ubiquity of these cells and their possible role in miscellaneous repairs throughout the body opening for new chances for extensive tissue repair.

Strategies for enhancing the accumulation and retention of extracellular matrix in tissue-engineered cartilage cultured in bioreactors

Production of tissue-engineered cartilage involves the synthesis and accumulation of key constituents such as glycosaminoglycan (GAG) and collagen type II to form insoluble extracellular matrix (ECM). During cartilage culture, macromolecular components are released from nascent tissues into the medium, representing a significant waste of biosynthetic resources. This work was aimed at developing strategies for improving ECM retention in cartilage constructs and thus the quality of engineered tissues produced in bioreactors. Human chondrocytes seeded into polyglycolic acid (PGA) scaffolds were cultured in perfusion bioreactors for up to 5 weeks. Analysis of the size and integrity of proteoglycans in the constructs and medium showed that full-sized aggrecan was being stripped from the tissues without proteolytic degradation. Application of low (0.075 mL min(-1)) and gradually increasing (0.075-0.2 mL min(-1)) medium flow rates in the bioreactor resulted in the generation of larger constructs, a 4.0-4.4-fold increase in the percentage of GAG retained in the ECM, and a 4.8-5.2-fold increase in GAG concentration in the tissues compared with operation at 0.2 mL min(-1). GAG retention was also improved by pre-culturing seeded scaffolds in flasks for 5 days prior to bioreactor culture. In contrast, GAG retention in PGA scaffolds infused with alginate hydrogel did not vary significantly with medium flow rate or pre-culture treatment. This work demonstrates that substantial improvements in cartilage quality can be achieved using scaffold and bioreactor culture strategies that specifically target and improve ECM retention.

Hydrogels for the repair of articular cartilage defects

The repair of articular cartilage defects remains a significant challenge in orthopedic medicine. Hydrogels, three-dimensional polymer networks swollen in water, offer a unique opportunity to generate a functional cartilage substitute. Hydrogels can exhibit similar mechanical, swelling, and lubricating behavior to articular cartilage, and promote the chondrogenic phenotype by encapsulated cells. Hydrogels have been prepared from naturally derived and synthetic polymers, as cell-free implants and as tissue engineering scaffolds, and with controlled degradation profiles and release of stimulatory growth factors. Using hydrogels, cartilage tissue has been engineered in vitro that has similar mechanical properties to native cartilage. This review summarizes the advancements that have been made in determining the potential of hydrogels to replace damaged cartilage or support new tissue formation as a function of specific design parameters, such as the type of polymer, degradation profile, mechanical properties and loading regimen, source of cells, cell-seeding density, controlled release of growth factors, and strategies to cause integration with surrounding tissue. Some key challenges for clinical translation remain, including limited information on the mechanical properties of hydrogel implants or engineered tissue that are necessary to restore joint function, and the lack of emphasis on the ability of an implant to integrate in a stable way with the surrounding tissue. Future studies should address the factors that affect these issues, while using clinically relevant cell sources and rigorous models of repair.

Vascular tissue engineering: towards the next generation vascular grafts

The application of tissue engineering technology to cardiovascular surgery holds great promise for improving outcomes in patients with cardiovascular diseases. Currently used synthetic vascular grafts have several limitations including thrombogenicity, increased risk of infection, and lack of growth potential. We have completed the first clinical trial evaluating the feasibility of using tissue engineered vascular grafts (TEVG) created by seeding autologous bone marrow-derived mononuclear cells (BM-MNC) onto biodegradable tubular scaffolds. Despite an excellent safety profile, data from the clinical trial suggest that the primary graft related complication of the TEVG is stenosis, affecting approximately 16% of grafts within the first seven years after implantation. Continued investigation into the cellular and molecular mechanisms underlying vascular neotissue formation will improve our basic understanding and provide insights that will enable the rationale design of second generation TEVG.

Shrinkage-free preparation of scaffold-free cartilage-like disk-shaped cell sheet using human bone marrow mesenchymal stem cells

Aiming for the clinical application of cartilage regeneration, a culture method for mesenchymal stem cells (MSCs) derived from human bone marrow to obtain scaffold-free cartilage-like disk-shaped sheet of uniform sizes without the shrinkage was investigated. A disk-shaped cell sheet having the same diameter as that of the membrane without the shrinkage was formed after the cultivation of MSCs (18.6 × 10(5)cells/well) for 3 weeks in a cell culture insert (CCI) containing a flat membrane whose porosity was 12%, while 6.2 and 31.0 × 10(5)MSCs/well, respectively, resulted in the shrinkage of the aggregate and the hole formation in the center part of the sheet. Cell aggregates shrunk also in a 96-well plate and CCIs having lower porosity. The disk-shaped cell sheet showed the comparable thickness (1.2mm) and sulfated glycosaminoglycan (sGAG) density to those of the pellet formed in a pellet culture. The gene expression levels of aggrecan and type II collagen in the disk-shaped cell sheet were not lower than those in the pellet. In conclusion, the usage of CCI having 12% porosity and 18.6 × 10(5)MSCs/well could avoid the shrinkage from the formation of the scaffold-free cartilage-like disk-shaped cell sheet.

Role of Cartilage Forming Cells in Regenerative Medicine for Cartilage Repair

Currently, cartilage repair remains a major challenge for researchers and physicians due to its limited healing capacity. Cartilage regeneration requires suitable cells; these must be easily obtained and expanded, able to produce hyaline matrix with proper mechanical properties, and demonstrate sustained integration with native tissue. At present, there is a wide variety of possible cell sources for cartilage regeneration; this review explores the diversity of sources for cartilage forming cells and the distinctive characteristics, advantages, limitations, and potential applications of each cell source. We place emphasis on cell sources used for in vitro and clinical studies.

Augmentation Strategies following the Microfracture Technique for Repair of Focal Chondral Defects

The operative management of focal chondral lesions continues to be problematic for the treating orthopedic surgeon secondary to the limited regenerative capacity of articular cartilage. Although many treatment options are currently available, none fulfills the criteria for an ideal repair solution, including a hyaline repair tissue that completely fills the defect and integrates well with the surrounding normal cartilage. The microfracture technique is an often-utilized, first-line treatment modality for chondral lesions within the knee, resulting in the formation of a fibrocartilaginous repair tissue with inferior biochemical and biomechanical properties compared to normal hyaline cartilage. Although symptomatic improvement has been shown in the short term, concerns about the durability and longevity of the fibrocartilaginous repair have been raised. In response, a number of strategies and techniques for augmentation of the first-generation microfracture procedure have been introduced in an effort to improve repair tissue characteristics and reduce long-term deterioration. Recent experimental approaches utilize modern tissue-engineering technologies including local supplementation of chondrogenic growth factors, hyaluronic acid, or cytokine modulation. Other second-generation microfracture-based techniques use different types of scaffold-guided in situ chondroinduction. The current article presents a comprehensive overview of both the experimental and early clinical results of these developing microfracture augmentation techniques.

A Review of Arthroscopic Bone Marrow Stimulation Techniques of the Talus: The Good, the Bad, and the Causes for Concern

Osteochondral lesions of the talus are common injuries following acute and chronic ankle sprains. Numerous surgical treatment strategies have been employed for treating these lesions; arthroscopic bone marrow stimulation is recognized as the first-line technique to provide fibrocartilage infill of the defect site. While the short- and medium-term outcomes of this technique are good, the long-term outcomes are not yet known. An increasing number of studies, however, show a cause for concern in employing this technique, including declining outcome scores over time. The current authors have therefore developed a treatment strategy based on previously established guidelines in addition to morphological cartilage-sensitive fast spin echo techniques and quantitative T2 mapping magnetic resonance imaging (MRI). Accordingly, the authors advocate arthroscopic bone marrow stimulation in lesion sizes up to 8 mm in diameter and osteochondral autograft transplant (OATS) in lesion sizes greater than 8 mm in diameter. In the absence of long-term studies, confining the use of arthroscopic bone marrow stimulation to smaller lesions may support prolonged joint life by decreasing the rate at which the fibrocartilage ultimately degenerates over time. Employing the OATS procedure in larger lesions has the advantage of replacing "like with like." The current review examines the role of arthroscopic bone marrow stimulation techniques of the talus.