Composition of cartilagenous tissue with mineralized and non-mineralized zones formed in vitro

Cellular therapy and tissue engineering for cartilage repair

Articular cartilage (AC) has limited capacity for repair. The first attempt to repair cartilage using tissue engineering was reported in 1977. Since then, cell-based interventions have entered clinical practice in orthopaedics, and several tissue engineering approaches to repair cartilage are in the translational pipeline towards clinical application. Classically, these involve a scaffold, substrate or matrix to provide structure, and cells such as chondrocytes or mesenchymal stromal cells to generate the tissue. We discuss the advantages and drawbacks of the use of various cell types, natural and synthetic scaffolds, multiphasic or gradient-based scaffolds, and self-organizing or self-assembling scaffold-free systems, for the engineering of cartilage constructs. Several challenges persist including achieving zonal tissue organization and integration with the surrounding tissue upon implantation. Approaches to improve cartilage thickness, organization and mechanical properties include mechanical stimulation, culture under hypoxic conditions, and stimulation with growth factors or other macromolecules. In addition, advanced technologies such as bioreactors, biosensors and 3D bioprinting are actively being explored. Understanding the underlying mechanisms of action of cell therapy and tissue engineering approaches will help improve and refine therapy development. Finally, we discuss recent studies of the intrinsic cellular and molecular mechanisms of cartilage repair that have identified novel signals and targets and are inspiring the development of molecular therapies to enhance the recruitment and cartilage reparative activity of joint-resident stem and progenitor cells. A one-fits-all solution is unrealistic, and identifying patients who will respond to a specific targeted treatment will be critical.

Biofabrication Strategies for Musculoskeletal Disorders: Evolution towards Clinical Applications

Biofabrication has emerged as an attractive strategy to personalise medical care and provide new treatments for common organ damage or diseases. While it has made impactful headway in e.g., skin grafting, drug testing and cancer research purposes, its application to treat musculoskeletal tissue disorders in a clinical setting remains scarce. Albeit with several in vitro breakthroughs over the past decade, standard musculoskeletal treatments are still limited to palliative care or surgical interventions with limited long-term effects and biological functionality. To better understand this lack of translation, it is important to study connections between basic science challenges and developments with translational hurdles and evolving frameworks for this fully disruptive technology that is biofabrication. This review paper thus looks closely at the processing stage of biofabrication, specifically at the bioinks suitable for musculoskeletal tissue fabrication and their trends of usage. This includes underlying composite bioink strategies to address the shortfalls of sole biomaterials. We also review recent advances made to overcome long-standing challenges in the field of biofabrication, namely bioprinting of low-viscosity bioinks, controlled delivery of growth factors, and the fabrication of spatially graded biological and structural scaffolds to help biofabricate more clinically relevant constructs. We further explore the clinical application of biofabricated musculoskeletal structures, regulatory pathways, and challenges for clinical translation, while identifying the opportunities that currently lie closest to clinical translation. In this article, we consider the next era of biofabrication and the overarching challenges that need to be addressed to reach clinical relevance.

Polymeric mesh and insulin-like growth factor 1 delivery enhance cell homing and graft-cartilage integration

Cartilage injury, such as full-thickness lesions, predisposes patients to the premature development of osteoarthritis, a degenerative joint disease. While surgical management of cartilage lesions has improved, long-term clinical efficacy has stagnated, owing to the lack of hyaline cartilage regeneration and inadequate graft-host integration. This study tests the hypothesis that integration of cartilage grafts with native cartilage can be improved by enhancing the migration of chondrocytes across the graft-host interface via the release of chemotactic factor from a degradable polymeric mesh. To this end, a polylactide-co-glycolide/poly-ε-caprolactone mesh was designed to localize the delivery of insulin-like growth factor 1 (IGF-1), a well-established chondrocyte attractant. The release of IGF-1 (100 ng/mg) enhanced cell migration from cartilage explants, and the mesh served as critical structural support for cell adhesion, growth, and production of a cartilaginous matrix in vitro, which resulted in increased integration strength compared with mesh-free repair. Further, this neocartilage matrix was structurally contiguous with native and grafted cartilage when tested in an osteochondral explant model in vivo. These results demonstrate that this combined approach of a cell homing factor and supportive matrix will promote cell-mediated integrative cartilage repair and improve clinical outcomes of cartilage grafts in the treatment of osteoarthritis.

Current strategies for integrative cartilage repair

Osteoarthritis (OA) is a degenerative joint condition characterized by painful cartilage lesions that impair joint mobility. Current treatments such as lavage, microfracture, and osteochondral implantation fail to integrate newly formed tissue with host tissues and establish a stable transition to subchondral bone. Similarly, tissue-engineered grafts that facilitate cartilage and bone regeneration are challenged by how to integrate the graft seamlessly with surrounding host cartilage and/or bone. This review centers on current approaches to promote cartilage graft integration. It begins with an overview of articular cartilage structure and function, as well as degenerative changes to this relationship attributed to aging, disease, and trauma. A discussion of the current progress in integrative cartilage repair follows, focusing on graft or scaffold design strategies targeting cartilage-cartilage and/or cartilage-bone integration. It is emphasized that integrative repair is required to ensure long-term success of the cartilage graft and preserve the integrity of the newly engineered articular cartilage. Studies involving the use of enzymes, choice of cell source, biomaterial selection, growth factor incorporation, and stratified versus gradient scaffolds are therefore highlighted. Moreover, models that accurately evaluate the ability of cartilage grafts to enhance tissue integrity and prevent ectopic calcification are also discussed. A summary and future directions section concludes the review.

Extracellular matrix components and culture regimen selectively regulate cartilage formation by self-assembling human mesenchymal stem cells in vitro and in vivo

BACKGROUND

Cartilage formation from self-assembling mesenchymal stem cells (MSCs) in vitro recapitulate important cellular events during mesenchymal condensation that precedes native cartilage development. The goal of this study was to investigate the effects of cartilaginous extracellular matrix (ECM) components and culture regimen on cartilage formation by self-assembling human MSCs in vitro and in vivo.

METHODS

Human bone marrow-derived MSCs (hMSCs) were seeded and compacted in 6.5-mm-diameter transwell inserts with coated (type I, type II collagen) or uncoated (vehicle) membranes, at different densities (0.5 × 10⁶, 1.0 × 10⁶, 1.5 × 10⁶ per insert). Pellets were formed by aggregating hMSCs (0.25 × 10⁶) in round-bottomed wells. All tissues were cultured for up to 6 weeks for in vitro analyses. Discs (cultured for 6, 8 or 10 weeks) and pellets (cultured for 10 weeks) were implanted subcutaneously in immunocompromised mice to evaluate the cartilage stability in vivo.

RESULTS

Type I and type II collagen coatings enabled cartilage disc formation from self-assembling hMSCs. Without ECM coating, hMSCs formed dome-shaped tissues resembling the pellets. Type I collagen, expressed in the prechondrogenic mesenchyme, improved early chondrogenesis versus type II collagen. High seeding density improved cartilage tissue properties but resulted in a lower yield of disc formation. Discs and pellets exhibited compositional and organizational differences in vitro and in vivo. Prolonged chondrogenic induction of the discs in vitro expedited endochondral ossification in vivo.

CONCLUSIONS

The outcomes of cartilage tissues formed from self-assembling MSCs in vitro and in vivo can be modulated by the control of culture parameters. These insights could motivate new directions for engineering cartilage and bone via a cartilage template from self-assembling MSCs.

Applications of Chondrocyte-Based Cartilage Engineering: An Overview

Chondrocytes are the exclusive cells residing in cartilage and maintain the functionality of cartilage tissue. Series of biocomponents such as different growth factors, cytokines, and transcriptional factors regulate the mesenchymal stem cells (MSCs) differentiation to chondrocytes. The number of chondrocytes and dedifferentiation are the key limitations in subsequent clinical application of the chondrocytes. Different culture methods are being developed to overcome such issues. Using tissue engineering and cell based approaches, chondrocytes offer prominent therapeutic option specifically in orthopedics for cartilage repair and to treat ailments such as tracheal defects, facial reconstruction, and urinary incontinence. Matrix-assisted autologous chondrocyte transplantation/implantation is an improved version of traditional autologous chondrocyte transplantation (ACT) method. An increasing number of studies show the clinical significance of this technique for the chondral lesions treatment. Literature survey was carried out to address clinical and functional findings by using various ACT procedures. The current study was conducted to study the pharmacological significance and biomedical application of chondrocytes. Furthermore, it is inferred from the present study that long term follow-up studies are required to evaluate the potential of these methods and specific positive outcomes.

Mesenchymal Stem Cells for Osteochondral Tissue Engineering

Mesenchymal stem cells (MSC) are of major interest in regenerative medicine, as they are easily harvested from a variety of sources (including bone marrow and fat aspirates) and they are able to form a range of mesenchymal tissues, in vitro and in vivo. We focus here on the use of MSCs for engineering of cartilage, bone, and complex osteochondral tissue constructs, using protocols that replicate some aspects of natural mesodermal development. For engineering of human bone, we discuss some of the current advances, and highlight the use of perfusion bioreactors for supporting anatomically exact human bone grafts. For engineering of human cartilage, we discuss the limitations of current approaches, and highlight engineering of stratified, mechanically functional human cartilage interfaced with bone by mesenchymal condensation of MSCs. Taken together, current advances enable engineering of physiologically relevant bone, cartilage and osteochondral composites, and physiologically relevant studies of osteochondral development and disease.

Building an anisotropic meniscus with zonal variations

Toward addressing the difficult problems of knee meniscus regeneration, a self-assembling process has been used to re-create the native morphology and matrix properties. A significant problem in such attempts is the recapitulation of the distinct zones of the meniscus, the inner, more cartilaginous and the outer, more fibrocartilaginous zones. In this study, an anisotropic and zonally variant meniscus was produced by self-assembly of the inner meniscus (100% chondrocytes) followed by cell seeding the outer meniscus (coculture of chondrocytes and meniscus cells). After 4 weeks in culture, the engineered, inner meniscus exhibited a 42% increase in both instantaneous and relaxation moduli and a 62% increase in GAG/DW, as compared to the outer meniscus. In contrast, the circumferential tensile modulus and collagen/DW of the outer zone was 101% and 129% higher, respectively, than the values measured for the inner zone. Furthermore, there was no difference in the radial tensile modulus between the control and zonal engineered menisci, suggesting that the inner and outer zones of the engineered zonal menisci successfully integrated. These data demonstrate that not only can biomechanical and biochemical properties be engineered to differ by the zone, but they can also recapitulate the anisotropic behavior of the knee meniscus.

A comparison of self-assembly and hydrogel encapsulation as a means to engineer functional cartilaginous grafts using culture expanded chondrocytes

Despite an increased interest in the use of hydrogel encapsulation and cellular self-assembly (often termed "self-aggregating" or "scaffold-free" approaches) for tissue-engineering applications, to the best of our knowledge, no study to date has been undertaken to directly compare both approaches for generating functional cartilaginous grafts. The objective of this study was to directly compare self-assembly (SA) and agarose hydrogel encapsulation (AE) as a means to engineer such grafts using passaged chondrocytes. Agarose hydrogels (5 mm diameter × 1.5 mm thick) were seeded with chondrocytes at two cell seeding densities (900,000 cells or 4 million cells in total per hydrogel), while SA constructs were generated by adding the same number of cells to custom-made molds. Constructs were either supplemented with transforming growth factor (TGF)-β3 for 6 weeks, or only supplemented with TGF-β3 for the first 2 weeks of the 6 week culture period. The SA method was only capable of generating geometrically uniform cartilaginous tissues at high seeding densities (4 million cells). At these high seeding densities, we observed that total sulphated glycosaminoglycan (sGAG) and collagen synthesis was greater with AE than SA, with higher sGAG retention also observed in AE constructs. When normalized to wet weight, however, SA constructs exhibited significantly higher levels of collagen accumulation compared with agarose hydrogels. Furthermore, it was possible to engineer such functionality into these tissues in a shorter timeframe using the SA approach compared with AE. Therefore, while large numbers of chondrocytes are required to engineer cartilaginous grafts using the SA approach, it would appear to lead to the faster generation of a more hyaline-like tissue, with a tissue architecture and a ratio of collagen to sGAG content more closely resembling native articular cartilage.

Strategic design and fabrication of engineered scaffolds for articular cartilage repair

Damage to articular cartilage can eventually lead to osteoarthritis (OA), a debilitating, degenerative joint disease that affects millions of people around the world. The limited natural healing ability of cartilage and the limitations of currently available therapies make treatment of cartilage defects a challenging clinical issue. Hopes have been raised for the repair of articular cartilage with the help of supportive structures, called scaffolds, created through tissue engineering (TE). Over the past two decades, different designs and fabrication techniques have been investigated for developing TE scaffolds suitable for the construction of transplantable artificial cartilage tissue substitutes. Advances in fabrication technologies now enable the strategic design of scaffolds with complex, biomimetic structures and properties. In particular, scaffolds with hybrid and/or biomimetic zonal designs have recently been developed for cartilage tissue engineering applications. This paper reviews critical aspects of the design of engineered scaffolds for articular cartilage repair as well as the available advanced fabrication techniques. In addition, recent studies on the design of hybrid and zonal scaffolds for use in cartilage tissue repair are highlighted.

Culture of primary bovine chondrocytes on a continuously expanding surface inhibits dedifferentiation

Expansion of autologous chondrocytes in vitro is used to generate adequate populations for cell-based therapies. However, standard (SD) culture methods cause loss of chondrocyte phenotype and dedifferentiation to fibroblast-like cells. Here, we use a novel surface expansion culture system in an effort to inhibit chondrocyte dedifferentiation. A highly elastic silicone rubber culture surface was continuously stretched over a 13-day period to 600% of its initial surface area. This maintained cells at a high density while limiting contact inhibition and reducing the need for passaging. Gene expression analysis, biochemical assays, and immunofluorescence microscopy of follow-on pellet cultures were used to characterize the results of continuous expansion (CE) culture versus SD cultures on rigid polystyrene. CE culture yielded cells with a more chondrocyte-like morphology and higher RNA-level expression of the chondrogenic markers collagen type II, aggrecan, and cartilage oligomeric matrix protein. Furthermore, the expression of collagen type I RNA and α-smooth muscle actin protein were significantly reduced, indicating suppression of fibroblastic features. Pellet cultures from CE chondrocytes contained more sulphated glycosaminoglycan and collagen type II than pellets from SD culture. Additional control cultures on static (unexpanded) silicone (SS culture) indicated that benefits of CE culture were partially due to features of the culture surface itself and partially due to the reduced passaging which that surface enabled through CE. Chondrocytes grown in CE culture may, therefore, be a superior source for cell-based therapies.

Inorganic polyphosphate stimulates cartilage tissue formation

Clinical utilization of tissue-engineered cartilage constructs has been limited by their inferior mechanical properties compared to native articular cartilage. A number of strategies have been investigated to increase the accumulation of major extracellular matrix components within in vitro-formed cartilage, including the administration of growth factors and mechanical stimulation. In this study, the anabolic effect of inorganic polyphosphates, a linear polymer of orthophosphate residues linked by phosphoanhydride bonds, was demonstrated in both chondrocyte cultures and native articular cartilage cultured ex vivo. Compared to untreated controls, polyphosphate treatment of three-dimensional primary chondrocyte cultures induced increased glycosaminoglycan and collagen accumulation in a concentration- and chain length-dependent manner. This effect was transient, because chondrocytes express exopolyphosphatases that hydrolyze polyphosphate. The anabolic effect of polyphosphates was accompanied by a lower rate of DNA increase within the chondrocyte cultures treated with inorganic polyphosphate. Inorganic polyphosphate enhances cartilage matrix accumulation and is a promising approach to improve the quality of tissue-engineered cartilage constructs.

The incorporation of a zone of calcified cartilage improves the interfacial shear strength between in vitro-formed cartilage and the underlying substrate

A major challenge for cartilage tissue engineering remains the proper integration of constructs with surrounding tissues in the joint. Biphasic osteochondral constructs that can be anchored in a joint through bone ingrowth partially address this requirement. In this study, a methodology was devised to generate a cell-mediated zone of calcified cartilage (ZCC) between the in vitro-formed cartilage and a porous calcium polyphosphate (CPP) bone substitute in an attempt to improve the mechanical integrity of that interface. To do so, a calcium phosphate (CaP) film was deposited on CPP by a sol-gel process to prevent the accumulation of polyphosphates and associated inhibition of mineralization as the substrate degrades. Cartilage formed in vitro on the top surface of CaP-coated CPP by deep-zone chondrocytes was histologically and biochemically comparable to that formed on uncoated CPP. Furthermore, the mineral in the ZCC was similar in crystal structure, morphology and length to that formed on uncoated CPP and native articular cartilage. The generation of a ZCC at the cartilage-CPP interface led to a 3.3-fold increase in the interfacial shear strength of biphasic constructs. Improved interfacial strength of these constructs may be critical to their clinical success for the repair of large cartilage defects.

Tissue engineering of articular cartilage with biomimetic zones

Articular cartilage damage is a persistent and increasing problem with the aging population, and treatments to achieve biological repair or restoration remain a challenge. Cartilage tissue engineering approaches have been investigated for over 20 years, but have yet to achieve the consistency and effectiveness for widespread clinical use. One of the potential reasons for this is that the engineered tissues do not have or establish the normal zonal organization of cells and extracellular matrix that appears critical for normal tissue function. A number of approaches are being taken currently to engineer tissue that more closely mimics the organization of native articular cartilage. This review focuses on the zonal organization of native articular cartilage, strategies being used to develop such organization, the reorganization that occurs after culture or implantation, and future prospects for the tissue engineering of articular cartilage with biomimetic zones.

Calcification of cartilage formed in vitro on calcium polyphosphate bone substitutes is regulated by inorganic polyphosphate

A major challenge to the successful clinical application of bioengineered cartilage remains its integration to surrounding tissues upon implantation. One way to address this consists of generating biphasic constructs composed of articular cartilage formed in vitro on the top surface and integrated with the porous sub-surface of a bone substitute material - in the case of this study, calcium polyphosphate (CPP). To improve the mechanical integrity of the cartilage-bone substitute interface, attempts have been made to generate a zone of calcified cartilage (ZCC) within the CPP-cartilage interface, thereby mimicking the native joint architecture. The purpose of this work was to establish the effects of the degradation products of CPP on cartilage calcification in order to explain the observed positioning of a ZCC away from the interface junction. It was determined that polyphosphate released from the CPP accumulates within in vitro-grown cartilage and inhibits cartilage calcification in a concentration and chain length (i.e. molecular weight) dependent manner. It was found that this effect is transient as chondrocytes express exopolyphosphatases which hydrolyze polyphosphate to release orthophosphate. Hence, the generation of biphasic constructs with a properly located ZCC will require tailoring of CPP substrates with lower degradation rates or the upregulation of exopolyphosphatases by 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.

The role of tissue engineering in articular cartilage repair and regeneration

Articular cartilage repair and regeneration continue to be largely intractable because of the poor regenerative properties of this tissue. The field of articular cartilage tissue engineering, which aims to repair, regenerate, and/or improve injured or diseased articular cartilage functionality, has evoked intense interest and holds great potential for improving articular cartilage therapy. This review provides an overall description of the current state of and progress in articular cartilage repair and regeneration. Traditional therapies and related problems are introduced. More importantly, a variety of promising cell sources, biocompatible tissue engineered scaffolds, scaffoldless techniques, growth factors, and mechanical stimuli used in current articular cartilage tissue engineering are reviewed. Finally, the technical and regulatory challenges of articular cartilage tissue engineering and possible future directions are also discussed.

In vivo evaluation of a multiphased scaffold designed for orthopaedic interface tissue engineering and soft tissue-to-bone integration

Achieving functional graft integration with subchondral bone poses a significant challenge for orthopaedic soft tissue repair and reconstruction. Soft tissues such as the anterior cruciate ligament (ACL) integrate with bone through a fibrocartilage interface, which minimizes stress concentrations and mediates load transfer between soft and hard tissues. We propose that biological fixation can be achieved by regenerating this fibrocartilage interface on biological or synthetic ACL grafts. This study focuses on the in vivo evaluation of a stratified scaffold predesigned to mimic the multitissue transition found at the ACL-to-bone interface. Specifically, the scaffold consists of three distinct yet continuous phases: Phase A for ligament formation, Phase B for the interface, and Phase C for the bone region. Interface-relevant cell types, specifically fibroblasts, chondrocytes, and osteoblasts, will be tri-cultured on this scaffold, and the formation of cell type- and phase-specific matrix heterogeneity as well as fibrocartilage formation will be evaluated over 8 weeks in a subcutaneous athymic rat model. Acellular scaffolds as well as scaffolds co-cultured with fibroblasts and osteoblasts will serve as controls. It was found that the triphasic scaffold supported multilineage cellular interactions as well as tissue infiltration and abundant matrix production in vivo. In addition, controlled phase-specific matrix heterogeneity was induced on the scaffold, with distinct mineral and fibrocartilage-like tissue regions formed in the tri-cultured group. Cell seeding had a positive effect on both host infiltration and matrix elaboration, which also translated into increased mechanical properties in the seeded groups compared to the acellular controls. In summary, the biomimetic and multiphasic design coupled with spatial control of cell distribution enables multitissue regeneration on the stratified scaffold, and demonstrates the potential for regenerating the interface between soft tissue grafts and bone.

Self-assembly of fibrochondrocytes and chondrocytes for tissue engineering of the knee meniscus

Chondrocyte self-assembly in high-density scaffoldless culture has shown success in producing articular cartilage constructs, and a similar process could be applied to fibrocartilage tissue engineering. Three cell combinations were compared in self-assembly culture-100% chondrocytes, 100% meniscal fibrochondrocytes, and 50:50 co-cultures of fibrochondrocytes and chondrocytes with the goal of creating a proteoglycan, collagen I, and collagen II matrix similar to native meniscus. Two culture surfaces were also compared for self-assembly: agarose-coated wells and tissue culture plastic. After 4 weeks, the resulting self-assembled chondrocyte constructs were 10.24+/-0.63 mm in diameter and 0.96+/-0.14 mm thick, weighing 84.5+/-7.2 mg. Co-culture constructs were smaller and weighed 22.5+/-1.0 mg. In contrast, the fibrochondrocyte constructs contracted into spheres weighing 1.3+/-0.3 mg. Immunostaining showed collagen II in the chondrocyte constructs, both collagen I and collagen II in the co-cultures, and only collagen I in the fibrochondrocyte constructs. Collagen densities for chondrocyte, co-culture, and fibrochondrocyte constructs were 41+/-3, 38+/-3, and 20+/-2 microg/mg dry weight, and glycosaminoglycan densities were 230+/-2, 80+/-6, and 10+/-1 microg/mg dry weight, respectively. Self-assembled co-cultures, with their mixed collagen I and II matrix and robust gross characteristics, appear promising for tissue engineering of the knee meniscus.

Development of controlled matrix heterogeneity on a triphasic scaffold for orthopedic interface tissue engineering

Biological fixation of orthopedic soft tissue grafts to bone poses a significant clinical challenge. The clinical success of soft tissue-based grafts for anterior cruciate ligament (ACL) reconstruction is limited by the lack of functional graft integration with subchondral bone. Soft tissues such as the ACL connect to subchondral bone via a complex interface whereby three distinct tissue regions (ligament, fibrocartilage, and bone) work in concert to facilitate load transfer from soft to hard tissue while minimizing stress concentration at the interface. Although a fibrovascular tissue forms at the graft-to-bone interface following surgery, this tissue is nonphysiologic and represents a weak link between the graft and bone. We propose that the re-establishment of the native multi-tissue interface is essential for biological graft fixation. In vivo observations and our in vitro monolayer co-culture results suggest that osteoblast-fibroblast interaction is important for interface regeneration. This study focuses on the design of a triphasic scaffold system mimicking the multi-tissue organization of the native ACL-to-bone interface and the evaluation of osteoblast-fibroblast interactions during three-dimensional co-culture on the triphasic scaffold. We found that the triphasic scaffold supported cell proliferation, migration and phenotypic matrix production while maintaining distinct cellular regions and phase-specific extracellular matrix deposition over time. This triphasic scaffold is designed to guide the eventual reestablishment of an anatomically oriented and mechanically functional fibrocartilage interfacial region directly on biological and synthetic soft tissue grafts. The results of this study demonstrate the feasibility of multi-tissue regeneration on a single scaffold, and the potential of interface tissue engineering to enable the biological fixation of soft tissue grafts to bone.

Engineering complex tissues

This article summarizes the views expressed at the third session of the workshop "Tissue Engineering--The Next Generation," which was devoted to the engineering of complex tissue structures. Antonios Mikos described the engineering of complex oral and craniofacial tissues as a "guided interplay" between biomaterial scaffolds, growth factors, and local cell populations toward the restoration of the original architecture and function of complex tissues. Susan Herring, reviewing osteogenesis and vasculogenesis, explained that the vascular arrangement precedes and dictates the architecture of the new bone, and proposed that engineering of osseous tissues might benefit from preconstruction of an appropriate vasculature. Jennifer Elisseeff explored the formation of complex tissue structures based on the example of stratified cartilage engineered using stem cells and hydrogels. Helen Lu discussed engineering of tissue interfaces, a problem critical for biological fixation of tendons and ligaments, and the development of a new generation of fixation devices. Rita Kandel discussed the challenges related to the re-creation of the cartilage-bone interface, in the context of tissue engineered joint repair. Frederick Schoen emphasized, in the context of heart valve engineering, the need for including the requirements derived from "adult biology" of tissue remodeling and establishing reliable early predictors of success or failure of tissue engineered implants. Mehmet Toner presented a review of biopreservation techniques and stressed that a new breakthrough in this field may be necessary to meet all the needs of tissue engineering. David Mooney described systems providing temporal and spatial regulation of growth factor availability, which may find utility in virtually all tissue engineering and regeneration applications, including directed in vitro and in vivo vascularization of tissues. Anthony Atala offered a clinician's perspective for functional tissue regeneration, and discussed new biomaterials that can be used to develop new regenerative technologies.

A self-assembling process in articular cartilage tissue engineering

Current therapies for articular cartilage defects often result in fibrocartilaginous tissue. To achieve regeneration with hyaline articular cartilage, tissue-engineering approaches employing cell-seeded scaffolds have been investigated. However, limitations of scaffolds include phenotypic alteration of cells, stress-shielding, hindrance of neotissue organization, and degradation product toxicity. This study employs a self-assembling process to produce tissue-engineered constructs over agarose in vitro without using a scaffold. Compared to past studies using various meshes and gels as scaffolding materials, the self-assembly method yielded constructs with comparable GAG and collagen content. By 12 weeks, the self-assembling process resulted in tissue-engineered constructs that were hyaline- like in appearance with histological, biochemical, and biomechanical properties approaching those of native articular cartilage. Overall, constructs contained two thirds more GAG per dry weight than calf articular cartilage. Collagen per dry weight reached more than one third the level of native tissue. IHC and gel electrophoresis showed collagen type II production and absence of collagen type I. More importantly, self-assembled constructs reached well over one third the stiffness of native tissue.

Osteogenic protein-1 promotes the formation of tissue-engineered cartilage using the alginate-recovered-chondrocyte method

OBJECTIVE

This study examined the effects of a growth factor, recombinant human osteogenic protein-1 (rhOP-1), on the formation of tissue-engineered cartilaginous tissue by adult bovine articular chondrocytes using the alginate-recovered-chondrocyte (ARC) method.

DESIGN

To ascertain if rhOP-1 enhances the formation of the cell-associated matrix (CM) and the characteristics of CM formation, bovine articular chondrocytes were first cultured for up to 14 days in alginate beads in medium supplemented with serum, with or without rhOP-1. Then, the recovered chondrocytes and their associated CM were resuspended in medium, with or without OP-1, seeded onto culture inserts, and incubated for an additional 14 days. The fabricated ARC tissues were subjected to biochemical and histological analyses.

RESULTS

The addition of rhOP-1 to the medium in the alginate bead culture step resulted in an increased accumulation of both proteoglycan (PG) and collagen, with a ratio of PG to collagen that was higher than that found in native adult cartilage. The addition of rhOP-1 in the second step had a similar stimulatory effect during 14 days of culture. Histological examination of the tissue formed under all conditions revealed a cartilage-like matrix, stained strongly by toluidine blue. The thickness of the tissues obtained from culture conditions that included the addition of rhOP-1 was four times greater than that of the tissues cultured without rhOP-1.

CONCLUSIONS

Using the ARC method, rhOP-1 enhanced the formation of matrix and generated a voluminous tissue-engineered cartilaginous construct. These characteristics may be beneficial in generating constructs that can cover large defects.

Long-term intermittent compressive stimulation improves the composition and mechanical properties of tissue-engineered cartilage

Tissue engineering of articular cartilage is a promising alternative for cartilage repair. However, it has been difficult to develop tissue in vitro that mimicks native cartilage. Cartilaginous tissue formed in vitro does not accumulate enough extracellular matrix, is deficient in collagen, and possesses only a fraction of the mechanical properties of native cartilage. In this study, we investigated whether long-term intermittent compressive stimulation would improve the quality of the generated tissue. Chondrocyte cultures were established on the surface of porous calcium polyphosphate substrates and allowed to form cartilaginous tissue. In vitro-formed tissues were subjected to different stimulation protocols for 1 week. The optimal mechanical stimulation parameters identified in this short-term study were then applied to the cultures for up to 4 weeks. Mechanical stimulation applied at a 5% compressive amplitude at a frequency of 1 Hz for 400 cycles every second day resulted in the greatest increase in collagen synthesis (37 +/- 9% over control) while not significantly affecting proteoglycan synthesis (2 +/- 8% over control). This condition, applied to the chondrocyte cultures for 4 weeks, resulted in a significant increase in the amount of tissue that formed (stimulated, 2.4 +/- 0.2 mg dry wt; unstimulated, 1.61 +/- 0.08 mg dry wt). Stimulated tissues contained approximately 40% more collagen (stimulated, 590 +/- 58 microg; unstimulated, 420 +/- 42 microg), and 30% more proteoglycans (stimulated, 393 +/- 34 microg; unstimulated, 302 +/- 32 microg) as well as displaying a 2- to 3-fold increase in compressive mechanical properties (maximal equilibrium stress: stimulated, 10 +/- 1 kPa; unstimulated, 5 +/- 1 kPa; maximal equilibrium modulus: stimulated, 80 +/- 23 kPa; unstimulated, 24 +/- 6 kPa). The results of this study demonstrate that intermittent mechanical stimulation can increase collagen synthesis and, when applied over a 4-week period, can accelerate extracellular matrix accumulation as well as improve the material properties of the developed tissue. Interestingly, only short periods of mechanical stimulation (6 min every second day) were needed to affect the quality of cartilaginous tissue formed in vitro.

The effect of detachment of the articular cartilage from its calcified zone on the cartilage microstructure, assessed by 2H-spectroscopic double quantum filtered MRI

Most studies on articular cartilage properties have been conducted after detachment of the cartilage from the bone. In the present work we investigated the effect of detachment on collagen fiber architecture. We used one-dimensional (2)H double quantum filtered MRI on cartilage bone plugs equilibrated in deuterated saline. The quadrupolar splittings observed in the different zones were related to the degree of order and the density of the collagen fibers. The method is non-destructive, allowing for measurements on the same plug without the need for fixation, dehydration, sectioning and decalcification. Detachment of the radial from the calcified zone resulted in swelling of the cartilage plug in physiological saline and a concomitant decrease in the quadrupolar splitting. The effect of mechanical pressure on the (2)H quadrupolar splittings for the detached cartilage and for the calcified zone-bone plugs were compared with those of the same zones in the intact cartilage-bone plug. The splitting in the radial zone of the detached cartilage collapsed at much smaller loads compared to the intact cartilage-bone plug. The effect of the load on the size of the cartilage was also greater for the detached plug. These results indicate that anchoring of the cartilage to the bone through the calcified zone plays an important role in retaining the order of the collagen fibers. The water (2)H quadrupolar splitting in intact and proteoglycan-depleted cartilage was the same, indicating that the proteoglycans do not contribute to the ordering of the collagen fibers.

Tissue engineering of stratified articular cartilage from chondrocyte subpopulations

OBJECTIVE

To test if subpopulations of chondrocytes from different cartilage zones could be used to engineer cartilage constructs with features of normal stratification. ESIGN: Chondrocytes from the superficial and middle zones of immature bovine cartilage were cultured in alginate, released, and seeded either separately or sequentially to form cartilage constructs. Constructs were cultured for 1 or 2 weeks and were assessed for growth, compressive properties, and deposition, and localization of matrix molecules and superficial zone protein (SZP).

RESULTS

The cartilaginous constructs formed from superficial zone chondrocytes exhibited less matrix growth and lower compressive properties than constructs from middle zone chondrocytes, with the stratified superficial-middle constructs exhibiting intermediate properties. Expression of SZP was highest at the construct surfaces, with the localization of SZP in superficial-middle constructs being concentrated at the superficial surface.

CONCLUSIONS

Manipulation of subpopulations of chondrocytes can be useful in engineering cartilage tissue with a biomimetic approach, and in fabricating constructs that exhibit stratified features of normal articular cartilage.

A novel two-step method for the formation of tissue-engineered cartilage by mature bovine chondrocytes: the alginate-recovered-chondrocyte (ARC) method

Most attempts to tissue-engineer cartilage have involved seeding of cultured cells into a biological or synthetic scaffold. We have developed a novel two-step culture approach that makes possible the in vitro formation of cartilaginous-like tissue by mature adult bovine chondrocytes without the aid of a synthetic matrix. The first step consists of culturing chondrocytes under conditions that maintain their rounded shape and their molecular phenotype as assessed by type II collagen and aggrecan production. This step was accomplished by culturing the isolated chondrocytes in alginate beads until the cells have reestablished a proteoglycan-rich cell-associated matrix (CM). The second step consists of culturing the cells with their CM, after recovery from the beads, on a tissue culture insert with a porous membrane. In this study, young adult bovine articular chondrocytes were cultured in alginate beads in the presence of 10% or 20% fetal bovine serum (FBS). After 7 days of culture, the alginate beads were dissolved by incubating the beads for 20 min in sodium citrate buffer, a calcium chelator. Following a brief centrifugation, the cells with their CM were recovered, resuspended in medium containing 10% or 20% FBS and seeded onto a tissue culture insert. After 1 week of culture on the insert, the individual cells with their CM progressively became incorporated into a mass of cartilaginous tissue. Culture with 20% FBS resulted in the best formation of tissues. These tissues, easily recovered from the insert, were then subjected to biochemical and histological analyses. The biochemical results showed that the chondrocytes remain phenotypically stable in the tissues. The de novo tissue has a relatively high ratio of PG/collagen. Histological examination of the tissue revealed it contained a cartilage-like matrix strongly stained with toluidine blue. This scaffold-free system appears ideal to study, in vitro, the development of transplantable cartilaginous tissue.

Characterisation of reconstituted equine cartilage formed in vitro

Lesions in cartilage of equine weightbearing joints commonly result in lameness. Cell-based resurfacing techniques are currently being developed for human and veterinary applications. Biopsies of stifle joint cartilage (1 g) were harvested aseptically and chondrocytes were isolated by sequential enzyme digestion. The cells were grown in vitro on filter inserts. Analysis of cultures 8 weeks later showed that the cells had accumulated extracellular matrix and formed a continuous layer of cartilagenous tissue as determined histologically. The cells maintained their phenotype as they synthesised type II collagen and proteoglycans similar in size to those synthesised by chondrocytes in native cartilage, but this reconstituted tissue had more sulphated glycosaminoglycan and lower collagen content than native cartilage. This experiment tests the feasibility of growing equine cartilagenous tissue in vitro. This tissue may be useful in the management of chondral injuries in the horse in a scenario where the patient donates cells, the cells are propagated under laboratory conditions and the resulting tissue becomes the therapeutic agent.

Tumor necrosis factor alpha-dependent proinflammatory gene induction is inhibited by cyclic tensile strain in articular chondrocytes in vitro

OBJECTIVE

To understand the intracellular mechanisms of the action of mechanical strain on articular chondrocytes during inflammation.

METHODS

One of the major mediators responsible for cartilage destruction in inflamed articular joints is tumor necrosis factor alpha (TNFalpha). Therefore, in this study we examined the intracellular mechanisms of actions of cyclic tensile strain (CTS) on the recombinant human TNFalpha (rHuTNFalpha)-induced proinflammatory pathways in primary cultures of chondrocytes. The expression of messenger RNA (mRNA) for TNFalpha-dependent proinflammatory proteins was examined by semiquantitative reverse transcriptase-polymerase chain reaction. The synthesis of proinflammatory proteins was examined by Western blot analysis in cell extracts, followed by semiquantitative measurement of bands using densitometric analysis. Nitric oxide production was measured by Griess reaction, and prostaglandin E2 production was assessed by radioimmunoassays. The proteoglycan synthesis in chondrocytes was assessed by incorporation of Na2(35)SO4 in chondroitin sulfate proteoglycans.

RESULTS

By exposing chondrocytes to CTS in the presence of TNFalpha in vitro, we showed that CTS is an effective antagonist of TNFalpha actions and acts as both an antiinflammatory signal and a reparative signal. CTS of low magnitude suppresses TNFalpha-induced mRNA expression of multiple proinflammatory proteins involved in catabolic responses, such as inducible nitric oxide synthase, cyclooxygenase 2, and collagenase. CTS also counteracts cartilage degradation by augmenting induction of tissue inhibitor of metalloproteinase 2. Additionally, CTS augments the reparative process via abrogation of TNFalpha-induced suppression of proteoglycan synthesis. Nonetheless, CTS acts on chondrocytes in a TNFalpha-dependent manner, since exposure of chondrocytes to CTS alone had no effect on these parameters.

CONCLUSION

CTS of low magnitude acts as an effective antagonist of TNFalpha not only by inhibiting the TNFalpha-dependent induction of proinflammatory proteins upstream of mRNA transcription, but also by augmenting the proteoglycan synthesis that is inhibited by TNFalpha.

Cyclic tensile strain suppresses catabolic effects of interleukin-1beta in fibrochondrocytes from the temporomandibular joint

OBJECTIVE

To discern the effects of continuous passive motion on inflamed temporomandibular joints (TMJ).

METHODS

The effects of continuous passive motion on TMJ were simulated by exposing primary cultures of rabbit TMJ fibrochondrocyte monolayers to cyclic tensile strain (CTS) in the presence of recombinant human interleukin-1beta (rHuIL-1beta) in vitro. The messenger RNA (mRNA) induction of rHuIL-1beta response elements was examined by semiquantitative reverse transcriptase-polymerase chain reaction. The synthesis of nitric oxide was examined by Griess reaction, and the synthesis of prostaglandin E2 (PGE2) was examined by radioimmunoassay. The synthesis of proteins was examined by Western blot analysis of the cell extracts, and synthesis of proteoglycans via incorporation of 35S-sodium sulfate in the culture medium.

RESULTS

Exposure of TMJ fibrochondrocytes to rHuIL-1beta resulted in the induction of inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX-2), which were paralleled by NO and PGE2 production. Additionally, IL-1beta induced significant levels of collagenase (matrix metalloproteinase 1 [MMP-1]) within 4 hours, and this was sustained over a period of 48 hours. Concomitant application of CTS abrogated the catabolic effects of IL-1beta on TMJ chondrocytes by inhibiting iNOS, COX-2, and MMP-1 mRNA production and NO, PGE2, and MMP-1 synthesis. CTS also counteracted cartilage degradation by augmenting expression of mRNA for tissue inhibitor of metalloproteinases 2 that is inhibited by rHuIL-1beta. In parallel, CTS also counteracted rHuIL-1beta-induced suppression of proteoglycan synthesis. Nevertheless, the presence of an inflammatory signal was a prerequisite for the observed CTS actions, because fibrochondrocytes, when exposed to CTS alone, did not exhibit any of the effects described above.

CONCLUSION

CTS acts as an effective antagonist of rHuIL-1beta by potentially diminishing its catabolic actions on TMJ fibrochondrocytes. Furthermore, CTS actions appear to involve disruption/regulation of signal transduction cascade of rHuIL-1beta upstream of mRNA transcription.

Cyclic tensile strain acts as an antagonist of IL-1 beta actions in chondrocytes

Inflammatory cytokines play a major role in cartilage destruction in diseases such as osteoarthritis and rheumatoid arthritis. Because physical therapies such as continuous passive motion yield beneficial effects on inflamed joints, we examined the intracellular mechanisms of mechanical strain-mediated actions in chondrocytes. By simulating the effects of continuous passive motion with cyclic tensile strain (CTS) on chondrocytes in vitro, we show that CTS is a potent antagonist of IL-1 beta actions and acts as both an anti-inflammatory and a reparative signal. Low magnitude CTS suppresses IL-1 beta-induced mRNA expression of multiple proteins involved in catabolic responses, such as inducible NO synthase, cyclo-oxygenase II, and collagenase. CTS also counteracts cartilage degradation by augmenting mRNA expression for tissue inhibitor of metalloproteases and collagen type II that are inhibited by IL-1 beta. Additionally, CTS augments the reparative process via hyperinduction of aggrecan mRNA expression and abrogation of IL-1 beta-induced suppression of proteoglycan synthesis. Nonetheless, the presence of an inflammatory signal is a prerequisite for the observed CTS actions, as exposure of chondrocytes to CTS alone has little effect on these parameters. Functional analysis suggests that CTS-mediated anti-inflammatory actions are not mediated by IL-1R down-regulation. Moreover, as an effective antagonist of IL-1 beta, the actions of CTS may involve disruption/regulation of signal transduction cascade of IL-1 beta upstream of mRNA transcription. These observations are the first to show that CTS directly acts as an anti-inflammatory signal on chondrocytes and provide a molecular basis for its actions.

Deep zone articular chondrocytes in vitro express genes that show specific changes with mineralization

We have developed a method to form reconstituted mineralized articular cartilagenous tissue in vitro from isolated deep zone chondrocytes. The aim of this study was to characterize further these cultures prior to and during mineralization. Histologic examination of the cells up to 8 days in culture showed that the chondrocytes had formed cartilagenous tissue. Similar to the in vivo cartilage, the chondrocytes expressed aggrecan, types II, I, and X collagens, osteopontin, and alkaline phosphatase (ALP). No osteocalcin mRNA expression was detected in either the in vivo cartilage or in vitro-generated tissue. Addition of beta-glycerophosphate (beta-GP) to the medium on day 5 induced mineralization and changes in gene expression. Expression of type X collagen, type II collagen, aggrecan core protein, and ALP were inhibited significantly between 2 h and 24 h after the addition of beta-GP. At 72 h, expression of these genes were still significantly depressed. These changes correlated with a decrease in collagen and proteoglycan synthesis, and ALP activity. Osteopontin expression increased within 8 h but returned to constitutive levels by 72 h. No change in type I collagen expression was detected. The changes in gene expression were not due to a direct effect of beta-GP itself, because similar gene changes occurred in the presence of phosphoethanolamine, another agent which induces mineralization. No changes in gene expression were seen in nonmineralizing cultures. In summary, articular chondrocytes grown on filter culture show expression of similar genes to the chondrocytes in the deep zone of articular cartilage and that changes in expression of specific genes were observed during tissue mineralization, suggesting that it is a suitable model to use to study the mechanism(s) regulating the localized mineralization of articular cartilage.

Cyclic Tensile Stress Exerts Antiinflammatory Actions on Chondrocytes by Inhibiting Inducible Nitric Oxide Synthase

Continuous passive motion manifests therapeutic effects on inflamed articular joints by an as-yet-unknown mechanism. Here, we show that application of cyclic tensile stress (CTS) in vitro abrogates the catabolic effects of IL-1β on chondrocytes. The effects of CTS are mediated by down-regulation of IL-1β-dependent inducible NO production, and are directly attributed to the inhibition of inducible NO synthase (iNOS) mRNA expression and protein synthesis. The inhibition of iNOS induction by CTS is paralleled by abrogation of IL-1β-induced down-regulation of proteoglycan synthesis. Furthermore, CTS inhibits iNOS expression and up-regulates proteoglycan synthesis at concentrations of IL-1β frequently observed in inflamed arthritic joints, suggesting that the actions of CTS may be clinically relevant in suppressing the sustained effects of pathological levels of IL-1β in vivo. These results are the first to demonstrate that mechanisms of the intracellular actions of CTS in IL-1β-activated chondrocytes are mediated through inhibition of a key molecule in the signal transduction pathway that leads to iNOS expression.

Characterization of the mineral in calcified articular cartilagenous tissue formed in vitro

Cartilagenous tissue with mineralized and nonmineralized layers was generated in vitro using bovine chondrocytes isolated from the deep zone of articular cartilage. Mineralization was induced by addition of either beta-glycerophosphate (beta-GP), phosphoethanolamine (PEA), or adenosine triphosphate (ATP). As this tissue might be suitable for use in joint resurfacing, the mineral of the calcified layer was characterized and compared to that present in the in vivo mineralized zone of bovine articular cartilage. Von Kossa staining demonstrated the presence of mineralization in the lower half of the tissue. The calcium content in the tissue varied from 4.9% to 7.8% of dry weight. Electron diffraction demonstrated a pattern consistent with hydroxyapatite. Brightfield transmission electron microscopy showed that the crystals were acicular and when measured under electron diffraction dark field imaging were 16.6 +/- 3.8 (beta-GP), 16.4 +/- 3.8 nm (ATP), and 17.0 +/- 6.3 nm (PEA) in length. The crystals were similar in size (16.0 +/- 5.5 nm) and appearance to the crystals in the in vivo calcified cartilage. This data suggests that the mineralization that occurs in vitro is similar to the in vivo cartilage.