Passaged adult chondrocytes can form engineered cartilage with functional mechanical properties: a canine model

In vitro inflammatory multi-cellular model of osteoarthritis

Objective

Osteoarthritis (OA) is a chronic joint disease, with limited treatment options, characterized by inflammation and matrix degradation, and resulting in severe pain or disability. Progressive inflammatory interaction among key cell types, including chondrocytes and macrophages, leads to a cascade of intra- and inter-cellular events which culminate in OA induction. In order to investigate these interactions, we developed a multi-cellular in vitro OA model, to characterize OA progression, and identify and evaluate potential OA therapeutics in response to mediators representing graded levels of inflammatory severity.

Methods

We compared macrophages, chondrocytes and their co-culture responses to "low" Interleukin-1 (IL-1) or "high" IL-1/tumor necrosis factor (IL-1/TNF) levels of inflammation. We also investigated response changes following the administration of dexamethasone (DEX) or mesenchymal stromal cell (MSC) treatment via a combination of gene expression and secretory changes, reflecting not only inflammation, but also chondrocyte function.

Results

Inflamed chondrocytes presented an osteoarthritic-like phenotype characterized by high gene expression of pro-inflammatory cytokines and chemokines, up-regulation of ECM degrading proteases, and down-regulation of chondrogenic genes. Our results indicate that while MSC treatment attenuates macrophage inflammation directly, it does not reduce chondrocyte inflammatory responses, unless macrophages are present as well. DEX however, can directly attenuate chondrocyte inflammation.

Conclusions

Our results highlight the importance of considering multi-cellular interactions when studying complex systems such as the articular joint. In addition, our approach, using a panel of both inflammatory and chondrocyte functional genes, provides a more comprehensive approach to investigate disease biomarkers, and responses to treatment.

Modernizing Storage Conditions for Fresh Osteochondral Allografts by Optimizing Viability at Physiologic Temperatures and Conditions

Objective. Osteochondral allograft (OCA) transplantation has demonstrated good long-term outcomes in treatment of cartilage defects. Viability, a key factor in clinical success, decreases with peri-implantation storage at 4°C during pathogen testing, matching logistics, and transportation. Modern, physiologic storage conditions may improve viability and enhance outcomes. Design. Osteochondral specimens from total knee arthroplasty patients (6 males, 5 females, age 56.4 ± 2.2 years) were stored in media and incubated at normoxia (21% O₂) at 22°C or 37°C, and hypoxia (2% O₂) at 37°C. Histology, live-dead staining, and quantitative polymerase chain reaction (qPCR) was performed 24 hours after harvest and following 7 days of incubation. Tissue architecture, cell viability, and gene expression were analyzed. Results. No significant viability or gene expression deterioration of cartilage was observed 1-week postincubation at 37°C, with or without hypoxia. Baseline viable cell density (VCD) was 94.0% ± 2.7% at day 1. At day 7, VCD was 95.1% (37°C) with normoxic storage and 92.2% (37°C) with hypoxic storage (P ≥ 0.27). Day 7 VCD (22°C) incubation was significantly lower than both the baseline and 37°C storage values (65.6%; P < 0.01). COL1A1, COL1A2, and ACAN qPCR expression was unchanged from baseline (P < 0.05) for all storage conditions at day 7, while CD163 expression, indicative of inflammatory macrophages and monocytes, was significantly lower in the 37°C groups (P < 0.01). Conclusion. Physiologic storage at 37°C demonstrates improved chondrocyte viability and metabolism, and maintained collagen expression compared with storage at 22°C. These novel findings guide development of a method to optimize short-term fresh OCA storage, which may lead to improved clinical results.

Marine origin materials on biomaterials and advanced therapies to cartilage tissue engineering and regenerative medicine

The body's self-repair capacity is limited, including injuries on articular cartilage zones. Over the past few decades, tissue engineering and regenerative medicine (TERM) has focused its studies on the development of natural biomaterials for clinical applications aiming to overcome this self-therapeutic bottleneck. This review focuses on the development of these biomaterials using compounds and materials from marine sources that are able to be produced in a sustainable way, as an alternative to mammal sources (e.g., collagens) and benefiting from their biological properties, such as biocompatibility, low antigenicity, biodegradability, among others. The structure and composition of the new biomaterials require mimicking the native extracellular matrix (ECM) of articular cartilage tissue. To design an ideal temporary tissue-scaffold, it needs to provide a suitable environment for cell growth (cell attachment, proliferation, and differentiation), towards the regeneration of the damaged tissues. Overall, the purpose of this review is to summarize various marine sources to be used in the development of different tissue-scaffolds with the capability to sustain cells envisaging cartilage tissue engineering, analysing the systems displaying more promising performance, while pointing out current limitations and steps to be given in the near future.

A synthetic mechanogenetic gene circuit for autonomous drug delivery in engineered tissues

Mechanobiologic signals regulate cellular responses under physiologic and pathologic conditions. Using synthetic biology and tissue engineering, we developed a mechanically responsive bioartificial tissue that responds to mechanical loading to produce a preprogrammed therapeutic biologic drug. By deconstructing the signaling networks induced by activation of the mechanically sensitive ion channel transient receptor potential vanilloid 4 (TRPV4), we created synthetic TRPV4-responsive genetic circuits in chondrocytes. We engineered these cells into living tissues that respond to mechanical loading by producing the anti-inflammatory biologic drug interleukin-1 receptor antagonist. Chondrocyte TRPV4 is activated by osmotic loading and not by direct cellular deformation, suggesting that tissue loading is transduced into an osmotic signal that activates TRPV4. Either osmotic or mechanical loading of tissues transduced with TRPV4-responsive circuits protected constructs from inflammatory degradation by interleukin-1α. This synthetic mechanobiology approach was used to develop a mechanogenetic system to enable long-term, autonomously regulated drug delivery driven by physiologically relevant loading.

Pulsed electromagnetic fields promote repair of focal articular cartilage defects with engineered osteochondral constructs

Articular cartilage injuries are a common source of joint pain and dysfunction. We hypothesized that pulsed electromagnetic fields (PEMFs) would improve growth and healing of tissue-engineered cartilage grafts in a direction-dependent manner. PEMF stimulation of engineered cartilage constructs was first evaluated in vitro using passaged adult canine chondrocytes embedded in an agarose hydrogel scaffold. PEMF coils oriented parallel to the articular surface induced superior repair stiffness compared to both perpendicular PEMF (p = .026) and control (p = .012). This was correlated with increased glycosaminoglycan deposition in both parallel and perpendicular PEMF orientations compared to control (p = .010 and .028, respectively). Following in vitro optimization, the potential clinical translation of PEMF was evaluated in a preliminary in vivo preclinical adult canine model. Engineered osteochondral constructs (∅ 6 mm × 6 mm thick, devitalized bone base) were cultured to maturity and implanted into focal defects created in the stifle (knee) joint. To assess expedited early repair, animals were assessed after a 3-month recovery period, with microfracture repairs serving as an additional clinical control. In vivo, PEMF led to a greater likelihood of normal chondrocyte (odds ratio [OR]: 2.5, p = .051) and proteoglycan (OR: 5.0, p = .013) histological scores in engineered constructs. Interestingly, engineered constructs outperformed microfracture in clinical scoring, regardless of PEMF treatment (p < .05). Overall, the studies provided evidence that PEMF stimulation enhanced engineered cartilage growth and repair, demonstrating a potential low-cost, low-risk, noninvasive treatment modality for expediting early cartilage repair.

Sustained low-dose dexamethasone delivery via a PLGA microsphere-embedded agarose implant for enhanced osteochondral repair

Articular cartilage defects are a common source of joint pain and dysfunction. We hypothesized that sustained low-dose dexamethasone (DEX) delivery via an acellular osteochondral implant would have a dual pro-anabolic and anti-catabolic effect, both supporting the functional integrity of adjacent graft and host tissue while also attenuating inflammation caused by iatrogenic injury. An acellular agarose hydrogel carrier with embedded DEX-loaded poly(lactic-co-glycolic) acid (PLGA) microspheres (DLMS) was developed to provide sustained release for at least 99 days. The DLMS implant was first evaluated in an in vitro pro-inflammatory model of cartilage degradation. The implant was chondroprotective, as indicated by maintenance of Young's modulus (EY) (p = 0.92) and GAG content (p = 1.0) in the presence of interleukin-1β insult. In a subsequent preliminary in vivo experiment, an osteochondral autograft transfer was performed using a pre-clinical canine model. DLMS implants were press-fit into the autograft donor site and compared to intra-articular DEX injection (INJ) or no DEX (CTL). Functional scores for DLMS animals returned to baseline (p = 0.39), whereas CTL and INJ remained significantly worse at 6 months (p < 0.05). DLMS knees were significantly more likely to have improved OARSI scores for proteoglycan, chondrocyte, and collagen pathology (p < 0.05). However, no significant improvements in synovial fluid cytokine content were observed. In conclusion, utilizing a targeted DLMS implant, we observed in vitro chondroprotection in the presence of IL-1-induced degradation and improved in vivo functional outcomes. These improved outcomes were correlated with superior histological scores but not necessarily a dampened inflammatory response, suggesting a primarily pro-anabolic effect. STATEMENT OF SIGNIFICANCE: Articular cartilage defects are a common source of joint pain and dysfunction. Effective treatment of these injuries may prevent the progression of osteoarthritis and reduce the need for total joint replacement. Dexamethasone, a potent glucocorticoid with concomitant anti-catabolic and pro-anabolic effects on cartilage, may serve as an adjuvant for a variety of repair strategies. Utilizing a dexamethasone-loaded osteochondral implant with controlled release characteristics, we demonstrated in vitro chondroprotection in the presence of IL-1-induced degradation and improved in vivo functional outcomes following osteochondral repair. These improved outcomes were correlated with superior histological cartilage scores and minimal-to-no comorbidity, which is a risk with high dose dexamethasone injections. Using this model of cartilage restoration, we have for the first time shown the application of targeted, low-dose dexamethasone for improved healing in a preclinical model of focal defect repair.

Large Animal Models for Osteochondral Regeneration

Namely, in the last two decades, large animal models - small ruminants (sheep and goats), pigs, dogs and horses - have been used to study the physiopathology and to develop new therapeutic procedures to treat human clinical osteoarthritis. For that purpose, cartilage and/or osteochondral defects are generally performed in the stifle joint of selected large animal models at the condylar and trochlear femoral areas where spontaneous regeneration should be excluded. Experimental animal care and protection legislation and guideline documents of the US Food and Drug Administration, the American Society for Testing and Materials and the International Cartilage Repair Society should be followed, and also the specificities of the animal species used for these studies must be taken into account, such as the cartilage thickness of the selected defect localization, the defined cartilage critical size defect and the joint anatomy in view of the post-operative techniques to be performed to evaluate the chondral/osteochondral repair. In particular, in the articular cartilage regeneration and repair studies with animal models, the subchondral bone plate should always be taken into consideration. Pilot studies for chondral and osteochondral bone tissue engineering could apply short observational periods for evaluation of the cartilage regeneration up to 12 weeks post-operatively, but generally a 6- to 12-month follow-up period is used for these types of studies.

Biologic Joint Repair Strategies: The Mizzou BioJoint Story

Because articular cartilage has very limited healing potential, most symptomatic cartilage injuries eventually result in end-stage osteoarthritis and are treated with artificial joint replacement. Our interdisciplinary, comparative orthopedic research performed by a team of DVMs, MDs, engineers, and basic scientists has yielded marked progress toward effective biologic joint restoration strategies by bringing bench-side ideas to fruition in bedside applications in both canine and human patients. This mini-review summarizes the progress of biologic joint restoration strategies at our center.

Transient expression of the diseased phenotype of osteoarthritic chondrocytes in engineered cartilage

Due to the degradation of osteoarthritic (OA) cartilage in post-traumatic OA (PTOA), these tissues are challenging to study and manipulate in vitro. In this study, chondrocytes isolated from either PTOA (meniscal-release (MR) model) or normal (contralateral limb) cartilage of canine knee joints were used to form micropellets to assess the maintenance of the OA chondrocyte phenotype in vitro. Media samples from the micropellet cultures were used to measure matrix metalloproteinase (MMP), chemokine, and cytokine concentrations. Significant differences in matrix synthesis were observed as a function of disease with OA chondrocytes generally synthesizing more extracellular matrix with increasing time in culture. No donor dependent differences were detected. Luminex multiplex analysis of pellet culture media showed disease and time-dependent differences in interleukin (IL)-8, keratinocyte chemoattractant (KC)-like protein, MMP-1, MMP-2, and MMP-3, which are differentially expressed in OA. This memory of their diseased phenotype persists for the first 2 weeks of culture. These results demonstrate the potential to use chondrocytes from an animal model of OA to study phenotype alterations during the progression and treatment of OA. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:829-836, 2017.

Concise Review: Mesenchymal Stem Cells for Functional Cartilage Tissue Engineering: Taking Cues from Chondrocyte-Based Constructs

Osteoarthritis, the most prevalent form of joint disease, afflicts 9% of the U.S. population over the age of 30 and costs the economy nearly $100 billion annually in healthcare and socioeconomic costs. It is characterized by joint pain and dysfunction, though the pathophysiology remains largely unknown. Due to its avascular nature and limited cellularity, articular cartilage exhibits a poor intrinsic healing response following injury. As such, significant research efforts are aimed at producing engineered cartilage as a cell-based approach for articular cartilage repair. However, the knee joint is mechanically demanding, and during injury, also a milieu of harsh inflammatory agents. The unforgiving mechano-chemical environment requires tissue replacements that are capable of bearing such burdens. The use of mesenchymal stem cells (MSCs) for cartilage tissue engineering has emerged as a promising cell source due to their ease of isolation, capacity to readily expand in culture, and ability to undergo lineage-specific differentiation into chondrocytes. However, to date, very few studies utilizing MSCs have successfully recapitulated the structural and functional properties of native cartilage, exposing the difficult process of uniformly differentiating stem cells into desired cell fates and maintaining the phenotype during in vitro culture and after in vivo implantation. To address these shortcomings, here, we present a concise review on modulating stem cell behavior, tissue development and function using well-developed techniques from chondrocyte-based cartilage tissue engineering. Stem Cells Translational Medicine 2017;6:1295-1303.

* Constrained Cage Culture Improves Engineered Cartilage Functional Properties by Enhancing Collagen Network Stability

When cultured with sufficient nutrient supply, engineered cartilage synthesizes proteoglycans rapidly, producing an osmotic swelling pressure that destabilizes immature collagen and prevents the development of a robust collagen framework, a hallmark of native cartilage. We hypothesized that mechanically constraining the proteoglycan-induced tissue swelling would enhance construct functional properties through the development of a more stable collagen framework. To test this hypothesis, we developed a novel "cage" growth system to mechanically prevent tissue constructs from swelling while ensuring adequate nutrient supply to the growing construct. The effectiveness of constrained culture was examined by testing constructs embedded within two different scaffolds: agarose and cartilage-derived matrix hydrogel (CDMH). Constructs were seeded with immature bovine chondrocytes and cultured under free swelling (FS) conditions for 14 days with transforming growth factor-β before being placed into a constraining cage for the remainder of culture. Controls were cultured under FS conditions throughout. Agarose constructs cultured in cages did not expand after the day 14 caging while FS constructs expanded to 8 × their day 0 weight after 112 days of culture. In addition to the physical differences in growth, by day 56, caged constructs had higher equilibrium (agarose: 639 ± 179 kPa and CDMH: 608 ± 257 kPa) and dynamic compressive moduli (agarose: 3.4 ± 1.0 MPa and CDMH 2.8 ± 1.0 MPa) than FS constructs (agarose: 193 ± 74 kPa and 1.1 ± 0.5 MPa and CDMH: 317 ± 93 kPa and 1.8 ± 1.0 MPa for equilibrium and dynamic properties, respectively). Interestingly, when normalized to final day wet weight, cage and FS constructs did not exhibit differences in proteoglycan or collagen content. However, caged culture enhanced collagen maturation through the increased formation of pyridinoline crosslinks and improved collagen matrix stability as measured by α-chymotrypsin solubility. These findings demonstrate that physically constrained culture of engineered cartilage constructs improves functional properties through improved collagen network maturity and stability. We anticipate that constrained culture may benefit other reported engineered cartilage systems that exhibit a mismatch in proteoglycan and collagen synthesis.

Systematic Study of Functionalizable, Non-Biofouling Agarose Films with Protein and Cellular Patterns on Glass Slides

Herein we demonstrate a systematic investigation of chemically functionalizable, non-biofouling agarose films over large-area glass surfaces. Agarose films, prepared with various concentrations of aqueous agarose, were activated by using periodate oxidation to generate aldehyde groups at the termini of the agarose chains. The non-biofouling efficacy and binding capabilities of the activated films were evaluated by using protein and cellular patterning, performed by using a microarrayer, microcontact printing, and micromolding in capillaries. Characterization by using a fluorescence slide scanner and a scanning-probe microscope revealed that the pore sizes of the agarose films played an important role in achieving desirable film performance; the 0.2 wt % agarose film exhibited the optimum efficacy in this work.

Nutrient Channels Aid the Growth of Articular Surface-Sized Engineered Cartilage Constructs

Symptomatic osteoarthritic lesions span large regions of joint surfaces and the ability to engineer cartilage constructs at clinically relevant sizes would be highly desirable. We previously demonstrated that nutrient transport limitations can be mitigated by the introduction of channels in 10 mm diameter cartilage constructs. In this study, we scaled up our previous system to cast and cultivate 40 mm diameter constructs (2.3 mm overall thickness); 4 mm diameter and channeled 10 mm diameter constructs were studied for comparison. Furthermore, to assess whether prior results using primary bovine cells are applicable for passaged cells-a more clinically realistic scenario-we cast constructs of each size with primary or twice-passaged cells. Constructs were assessed mechanically for equilibrium compressive Young's modulus (EY), dynamic modulus at 0.01 Hz (G*), and friction coefficient (μ); they were also assessed biochemically, histologically, and immunohistochemically for glycosaminoglycan (GAG) and collagen contents. By maintaining open channels, we successfully cultured robust constructs the size of entire human articular cartilage layers (growing to ∼52 mm in diameter, 4 mm thick, mass of 8 g by day 56), representing a 100-fold increase in scale over our 4 mm diameter constructs, without compromising their functional properties. Large constructs reached EY of up to 623 kPa and GAG contents up to 8.9%/ww (% of wet weight), both within native cartilage ranges, had G* >2 MPa, and up to 3.5%/ww collagen. Constructs also exhibited some of the lowest μ reported for engineered cartilage (0.06-0.11). Passaged cells produced tissue of lower quality, but still exhibited native EY and GAG content, similar to their smaller controls. The constructs produced in this study are, to our knowledge, the largest engineered cartilage constructs to date which possess native EY and GAG, and are a testament to the effectiveness of nutrient channels in overcoming transport limitations in cartilage tissue engineering.

Cell-based tissue engineering strategies used in the clinical repair of articular cartilage

One of the most important issues facing cartilage tissue engineering is the inability to move technologies into the clinic. Despite the multitude of current research in the field, it is known that 90% of new drugs that advance past animal studies fail clinical trials. The objective of this review is to provide readers with an understanding of the scientific details of tissue engineered cartilage products that have demonstrated a certain level of efficacy in humans, so that newer technologies may be developed upon this foundation. Compared to existing treatments, such as microfracture or autologous chondrocyte implantation, a tissue engineered product can potentially provide more consistent clinical results in forming hyaline repair tissue and in filling the entirety of the defect. The various tissue engineering strategies (e.g., cell expansion, scaffold material, media formulations, biomimetic stimuli, etc.) used in forming these products, as collected from published literature, company websites, and relevant patents, are critically discussed. The authors note that many details about these products remain proprietary, not all information is made public, and that advancements to the products are continuously made. Nevertheless, by understanding the design and production processes of these emerging technologies, one can gain tremendous insight into how to best use them and also how to design the next generation of tissue engineered cartilage products.

High seeding density of human chondrocytes in agarose produces tissue-engineered cartilage approaching native mechanical and biochemical properties

Animal cells have served as highly controllable model systems for furthering cartilage tissue engineering practices in pursuit of treating osteoarthritis. Although successful strategies for animal cells must ultimately be adapted to human cells to be clinically relevant, human chondrocytes are rarely employed in such studies. In this study, we evaluated the applicability of culture techniques established for juvenile bovine and adult canine chondrocytes to human chondrocytes obtained from fresh or expired osteochondral allografts. Human chondrocytes were expanded and encapsulated in 2% agarose scaffolds measuring ∅3-4mm×2.3mm, with cell seeding densities ranging from 15 to 90×10(6)cells/mL. Subsets of constructs were subjected to transient or sustained TGF-β treatment, or provided channels to enhance nutrient transport. Human cartilaginous constructs physically resembled native human cartilage, and reached compressive Young's moduli of up to ~250kPa (corresponding to the low end of ranges reported for native knee cartilage), dynamic moduli of ~950kPa (0.01Hz), and contained 5.7% wet weight (%/ww) of glycosaminoglycans (≥ native levels) and 1.5%/ww collagen. We found that the initial seeding density had pronounced effects on tissue outcomes, with high cell seeding densities significantly increasing nearly all measured properties. Transient TGF-β treatment was ineffective for adult human cells, and tissue construct properties plateaued or declined beyond 28 days of culture. Finally, nutrient channels improved construct mechanical properties, presumably due to enhanced rates of mass transport. These results demonstrate that our previously established culture system can be successfully translated to human chondrocytes.

Cell-based tissue engineering strategies used in the clinical repair of articular cartilage

One of the most important issues facing cartilage tissue engineering is the inability to move technologies into the clinic. Despite the multitude of current research in the field, it is known that 90% of new drugs that advance past animal studies fail clinical trials. The objective of this review is to provide readers with an understanding of the scientific details of tissue engineered cartilage products that have demonstrated a certain level of efficacy in humans, so that newer technologies may be developed upon this foundation. Compared to existing treatments, such as microfracture or autologous chondrocyte implantation, a tissue engineered product can potentially provide more consistent clinical results in forming hyaline repair tissue and in filling the entirety of the defect. The various tissue engineering strategies (e.g., cell expansion, scaffold material, media formulations, biomimetic stimuli, etc.) used in forming these products, as collected from published literature, company websites, and relevant patents, are critically discussed. The authors note that many details about these products remain proprietary, not all information is made public, and that advancements to the products are continuously made. Nevertheless, by understanding the design and production processes of these emerging technologies, one can gain tremendous insight into how to best use them and also how to design the next generation of tissue engineered cartilage products.

Dexamethasone Release from Within Engineered Cartilage as a Chondroprotective Strategy Against Interleukin-1α

While significant progress has been made toward engineering functional cartilage constructs with mechanical properties suitable for in vivo loading, the impact on these grafts of inflammatory cytokines, chemical factors that are elevated with trauma or osteoarthritis, is poorly understood. Previous work has shown dexamethasone to be a critical compound for cultivating cartilage with functional properties, while also providing chondroprotection from proinflammatory cytokines. This study tested the hypothesis that the incorporation of poly(lactic-co-glycolic acid) (PLGA) (75:25) microspheres that release dexamethasone from within chondrocyte-seeded agarose hydrogel constructs would promote development of constructs with functional properties and protect constructs from the deleterious effects of interleukin-1α (IL-1α). After 28 days of growth culture, experimental groups were treated with IL-1α (10 ng/mL) for 7 days. Reaching native equilibrium moduli and proteoglycan levels, dexamethasone-loaded microsphere constructs exhibited tissue properties similar to microsphere-free control constructs cultured in dexamethasone-supplemented culture media and were insensitive to IL-1α exposure. These findings are in stark contrast to constructs containing dexamethasone-free microspheres or no microspheres, cultured without dexamethasone, where IL-1α exposure led to significant tissue degradation. These results support the use of dexamethasone delivery from within engineered cartilage, through biodegradable microspheres, as a strategy to produce mechanically functional tissues that can also combat the deleterious effects of local proinflammatory cytokine exposure.

A puzzle assembly strategy for fabrication of large engineered cartilage tissue constructs

Engineering of large articular cartilage tissue constructs remains a challenge as tissue growth is limited by nutrient diffusion. Here, a novel strategy is investigated, generating large constructs through the assembly of individually cultured, interlocking, smaller puzzle-shaped subunits. These constructs can be engineered consistently with more desirable mechanical and biochemical properties than larger constructs (~4-fold greater Young׳s modulus). A failure testing technique was developed to evaluate the physiologic functionality of constructs, which were cultured as individual subunits for 28 days, then assembled and cultured for an additional 21-35 days. Assembled puzzle constructs withstood large deformations (40-50% compressive strain) prior to failure. Their ability to withstand physiologic loads may be enhanced by increases in subunit strength and assembled culture time. A nude mouse model was utilized to show biocompatibility and fusion of assembled puzzle pieces in vivo. Overall, the technique offers a novel, effective approach to scaling up engineered tissues and may be combined with other techniques and/or applied to the engineering of other tissues. Future studies will aim to optimize this system in an effort to engineer and integrate robust subunits to fill large defects.

Long-term storage and preservation of tissue engineered articular cartilage

With limited availability of osteochondral allografts, tissue engineered cartilage grafts may provide an alternative treatment for large cartilage defects. An effective storage protocol will be critical for translating this technology to clinical use. The purpose of this study was to evaluate the efficacy of the Missouri Osteochondral Allograft Preservation System (MOPS) for room temperature storage of mature tissue engineered grafts, focusing on tissue property maintenance during the current allograft storage window (28 days). Additional research compares MOPS to continued culture, investigates temperature influence, and examines longer-term storage. Articular cartilage constructs were cultured to maturity using adult canine chondrocytes, then preserved with MOPS at room temperature, in refrigeration, or kept in culture for an additional 56 days. MOPS storage maintained desired chondrocyte viability for 28 days of room temperature storage, retaining 75% of the maturity point Young's modulus without significant decline in biochemical content. Properties dropped past this time point. Refrigeration maintained properties similar to room temperature at 28 days, but proved better at 56 days. For engineered grafts, MOPS maintained the majority of tissue properties for the 28-day window without clearly extending that period as it had for native grafts. These results are the first evaluating engineered cartilage storage.

Cytokine preconditioning of engineered cartilage provides protection against interleukin-1 insult

BACKGROUND

During osteoarthritis and following surgical procedures, the environment of the knee is rich in proinflammatory cytokines such as IL-1. Introduction of tissue-engineered cartilage constructs to a chemically harsh milieu may limit the functionality of the implanted tissue over long periods. A chemical preconditioning scheme (application of low doses of IL-1) was tested as a method to prepare developing engineered tissue to withstand exposure to a higher concentration of the cytokine, known to elicit proteolysis as well as rapid degeneration of cartilage.

METHODS

Using an established juvenile bovine model system, engineered cartilage was preconditioned with low doses of IL-1α (0.1 ng/mL, 0.5 ng/mL, and 1.0 ng/mL) for 7 days before exposure to an insult dose (10 ng/mL). The time frame over which this protection is afforded was investigated by altering the amount of time between preconditioning and insult as well as the time following insult. To explore a potential mechanism for this protection, one set of constructs was preconditioned with CoCl2, a chemical inducer of hypoxia, before exposure to the IL-1α insult. Finally, we examined the translation of this preconditioning method to extend to clinically relevant adult, passaged chondrocytes from a preclinical canine model.

RESULTS

Low doses of IL-1α (0.1 ng/mL and 0.5 ng/mL) protected against subsequent catabolic degradation by cytokine insult, preserving mechanical stiffness and biochemical composition. Regardless of amount of time between preconditioning scheme and insult, protection was afforded. In a similar manner, preconditioning with CoCl2 similarly allowed for mediation of catabolic damage by IL-1α. The effects of preconditioning on clinically relevant adult, passaged chondrocytes from a preclinical canine model followed the same trends with low-dose IL-1β offering variable protection against insult.

CONCLUSIONS

Chemical preconditioning schemes have the ability to protect engineered cartilage constructs from IL-1-induced catabolic degradation, offering potential modalities for therapeutic treatments.

Porous titanium bases for osteochondral tissue engineering

Tissue engineering of osteochondral grafts may offer a cell-based alternative to native allografts, which are in short supply. Previous studies promote the fabrication of grafts consisting of a viable cell-seeded hydrogel integrated atop a porous, bone-like metal. Advantages of the manufacturing process have led to the evaluation of porous titanium as the bone-like base material. Here, porous titanium was shown to support the growth of cartilage to produce native levels of Young's modulus, using a clinically relevant cell source. Mechanical and biochemical properties were similar or higher for the osteochondral constructs compared to chondral-only controls. Further investigation into the mechanical influence of the base on the composite material suggests that underlying pores may decrease interstitial fluid pressurization and applied strains, which may be overcome by alterations to the base structure. Future studies aim to optimize titanium-based tissue engineered osteochondral constructs to best match the structural architecture and strength of native grafts.

STATEMENT OF SIGNIFICANCE

The studies described in this manuscript follow up on previous studies from our lab pertaining to the fabrication of osteochondral grafts that consist of a bone-like porous metal and a chondrocyte-seeded hydrogel. Here, tissue engineered osteochondral grafts were cultured to native stiffness using adult chondrocytes, a clinically relevant cell source, and a porous titanium base, a material currently used in clinical implants. This porous titanium is manufactured via selective laser melting, offering the advantages of precise control over shape, pore size, and orientation. Additionally, this manuscript describes the mechanical influence of the porous base, which may have applicability to porous bases derived from other materials.

Fabrication of tissue engineered osteochondral grafts for restoring the articular surface of diarthrodial joints

Osteochondral allograft implantation is an effective cartilage restoration technique for large defects (>10 cm(2)), though the demand far exceeds the supply of available quality donor tissue. Large bilayered engineered cartilage tissue constructs with accurate anatomical features (i.e. contours, thickness, architecture) could be beneficial in replacing damaged tissue. When creating these osteochondral constructs, however, it is pertinent to maintain biofidelity to restore functionality. Here, we describe a step-by-step framework for the fabrication of a large osteochondral construct with correct anatomical architecture and topology through a combination of high-resolution imaging, rapid prototyping, impression molding, and injection molding.

Functional consequences of glucose and oxygen deprivation on engineered mesenchymal stem cell-based cartilage constructs

OBJECTIVE

Tissue engineering approaches for cartilage repair have focused on the use of mesenchymal stem cells (MSCs). For clinical success, MSCs must survive and produce extracellular matrix in the physiological context of the synovial joint, where low nutrient conditions engendered by avascularity, nutrient utilization, and waste production prevail. This study sought to delineate the role of microenvironmental stressors on MSC viability and functional capacity in three dimensional (3D) culture.

DESIGN

We evaluated the impact of glucose and oxygen deprivation on the functional maturation of 3D MSC-laden agarose constructs. Since MSC isolation procedures result in a heterogeneous cell population, we also utilized micro-pellet culture to investigate whether clonal subpopulations respond to these microenvironmental stressors in a distinct fashion.

RESULTS

MSC health and the functional maturation of 3D constructs were compromised by both glucose and oxygen deprivation. Importantly, glucose deprivation severely limited viability, and so compromised the functional maturation of 3D constructs to the greatest extent. The observation that not all cells died suggested there exists heterogeneity in the response of MSC populations to metabolic stressors. Population heterogeneity was confirmed through a series of studies utilizing clonally derived subpopulations, with a spectrum of matrix production and cell survival observed under conditions of metabolic stress.

CONCLUSIONS

Our findings show that glucose deprivation has a significant impact on functional maturation, and that some MSC subpopulations are more resilient to metabolic challenge than others. These findings suggest that pre-selection of subpopulations that are resilient to metabolic challenge may improve in vivo outcomes.

Animal models of cartilage repair

Cartilage repair in terms of replacement, or regeneration of damaged or diseased articular cartilage with functional tissue, is the ‘holy grail’ of joint surgery. A wide spectrum of strategies for cartilage repair currently exists and several of these techniques have been reported to be associated with successful clinical outcomes for appropriately selected indications. However, based on respective advantages, disadvantages, and limitations, no single strategy, or even combination of strategies, provides surgeons with viable options for attaining successful long-term outcomes in the majority of patients. As such, development of novel techniques and optimisation of current techniques need to be, and are, the focus of a great deal of research from the basic science level to clinical trials. Translational research that bridges scientific discoveries to clinical application involves the use of animal models in order to assess safety and efficacy for regulatory approval for human use. This review article provides an overview of animal models for cartilage repair.

Cite this article: Bone Joint Res 2014;4:89–94.

Phenotype and polarization of autologous T cells by biomaterial-treated dendritic cells

Given the central role of dendritic cells (DCs) in directing T-cell phenotypes, the ability of biomaterial-treated DCs to dictate autologous T-cell phenotype was investigated. In this study, we demonstrate that differentially biomaterial-treated DCs differentially directed autologous T-cell phenotype and polarization, depending on the biomaterial used to pretreat the DCs. Immature DCs (iDCs) were derived from human peripheral blood monocytes and treated with biomaterial films of alginate, agarose, chitosan, hyaluronic acid, or 75:25 poly(lactic-co-glycolic acid) (PLGA), followed by co-culture of these biomaterial-treated DCs and autologous T cells. When autologous T cells were co-cultured with DCs treated with biomaterial film/antigen (ovalbumin, OVA) combinations, different biomaterial films induced differential levels of T-cell marker (CD4, CD8, CD25, CD69) expression, as well as differential cytokine profiles [interferon (IFN)-γ, interleukin (IL)-12p70, IL-10, IL-4] in the polarization of T helper (Th) types. Dendritic cells treated with agarose films/OVA induced CD4+CD25+FoxP3+ (T regulatory cells) expression, comparable to untreated iDCs, on autologous T cells in the DC-T co-culture system. Furthermore, in this co-culture, agarose treatment induced release of IL-12p70 and IL-10 at higher levels as compared with DC treatment with other biomaterial films/OVA, suggesting Th1 and Th2 polarization, respectively. Dendritic cells treated with PLGA film/OVA treatment induced release of IFN-γ at higher levels compared with that observed for co-cultures with iDCs or DCs treated with all other biomaterial films. These results indicate that DC treatment with different biomaterial films has potential as a tool for immunomodulation by directing autologous T-cell responses.

Growth factor priming differentially modulates components of the extracellular matrix proteome in chondrocytes and synovium-derived stem cells

To make progress in cartilage repair it is essential to optimize protocols for two-dimensional cell expansion. Chondrocytes and SDSCs are promising cell sources for cartilage repair. We previously observed that priming with a specific growth factor cocktail (1 ng/mL transforming growth factor-β1, 5 ng/mL basic fibroblast growth factor, and 10 ng/mL platelet-derived growth factor-BB) in two-dimensional culture, led to significant improvement in mechanical and biochemical properties of synovium-derived stem cell (SDSC)-seeded constructs. The current study assessed the effect of growth factor priming on the proteome of canine chondrocytes and SDSCs. In particular, growth factor priming modulated the proteins associated with the extracellular matrix in two-dimensional cultures of chondrocytes and SDSCs, inducing a partial dedifferentiation of chondrocytes (most proteins associated with cartilage were down-regulated in primed chondrocytes) and a partial differentiation of SDSCs (some collagen-related proteins were up-regulated in primed SDSCs). However, when chondrocytes and SDSCs were grown in pellet culture, growth factor-primed cells maintained their chondrogenic potential with respect to glycosaminoglycan and collagen production. In conclusion, the strength of the label-free proteomics technique is that it allows for the determination of changes in components of the extracellular matrix proteome in chondrocytes and SDSCs in response to growth factor priming, which could help in future tissue engineering strategies.

Articular chondrocyte redifferentiation in 3D co-cultures with mesenchymal stem cells

In this work, we evaluated the ability of 3D co-cultures with mesenchymal stem cells (MSCs) to redifferentiate monolayer expanded articular chondrocytes (ACs) and produce cartilaginous extracellular matrix at varying stages of the dedifferentiation process and further examined the dependency of this effect on the culture medium composition. Primary bovine ACs were expanded in monolayers for up to nine population doublings to obtain seven cell stocks with gradually increasing levels of dedifferentiation. Culture expanded ACs were then seeded as monocultures and co-cultures with rabbit bone marrow-derived MSCs (30:70 ratio of ACs-to-MSCs) on porous scaffolds. Parallel cultures were established for each cell population in serum-containing growth medium and serum-free induction medium supplemented with dexamethasone and TGF-β3. After 3 weeks, all groups were analyzed for DNA content, glycosaminoglycan (GAG) and hydroxyproline (HYP) production, and chondrogenic gene expression. Significant enhancements in cellularity, GAG content and GAG/HYP ratio, and chondrogenic phenotype were observed in the induction medium compared to growth medium at all levels of AC expansion. Furthermore, primary co-cultures showed similarly enhanced chondrogenesis compared to monocultures in both culture media, whereas passaged ACs benefitted from co-culturing only in the induction medium. We conclude that co-cultures of ACs and MSCs can produce superior in vitro engineered cartilage in comparison to pure AC cultures, due to both heterotypic cellular interactions and decreased need for monolayer expansion of biopsied chondrocytes. While the initial level of AC dedifferentiation affected the quality of the engineered constructs, co-culture benefits were realized at all stages of AC expansion when suitable chondroinductive culture medium was used.

Articular Cartilage Repair: Where We Have Been, Where We Are Now, and Where We Are Headed

This review traces the genealogy of the field of articular cartilage repair from its earliest attempts to its present day vast proliferation of research advances. Prior to the 1980s there was only sporadic efforts to regenerate articular cartilage as it was considered to be incapable of regeneration based on historical dogma. The first flurry of reports documented the use of various cell types ultimately leading to the first successful demonstration of autologous chondrocyte transplantation which was later translated to clinical use and has resulted in the revised axiom that cartilage regeneration is possible. The current field of cartilage repair is multifaceted and some of the 1980s' vintage concepts have been revisited with state of the art technology now available. The future of the field is now poised to undertake the repair of whole cartilage surfaces beyond focal defects and an appreciation for integrated whole joint health to restore cartilage homeostasis.

Platelet-rich plasma combined with agarose as a bioactive scaffold to enhance cartilage repair: an in vitro study

OBJECTIVE

The purpose of this study was to determine whether the platelet-rich plasma-agarose gel scaffold could be a bioactive scaffold capable of growth factors release for cartilage repair.

METHODS

Porcine chondrocytes were seeded in agarose gel and platelet-rich plasma-agarose gel. During the 28-days culture, microstructure of hydrogels and morphologies of chondrocytes seeded in the hydrogels were observed using scanning electron microscope; viability of chondrocytes in gels was examined by live/dead assay; qualitative and quantitative analysis of glycosaminoglycan, collagen and DNA were assessed by histological, immunohistochemical staining and biochemical assay; gene expression was measured by real-time polymerase chain reaction. In vitro cartilage ring models were used to evaluate the integration of the scaffolds, and the integration strength was analyzed by mechanical push-out tests.

RESULTS

Scanning electron microscope revealed both scaffolds had highly uniform porous structure. Live/dead scaffolds showed 100% cells alive in both groups. After 28-days culture, glycosaminoglycan, collagen, DNA content and chondrocyte-related genes expression in platelet-rich plasma-agarose gel were significantly higher than pure agarose gel. Integration strength in platelet-rich plasma-agarose gel was also higher compared to pure agarose gel.

CONCLUSION

Platelet-rich plasma showed a positive effect on chondrocytes proliferation, differentiation and integration between native cartilage and engineered tissue when combined with agarose gel. Our findings suggest that platelet-rich plasma-agarose gel scaffold is a promising bioactive scaffold for future cartilage tissue engineering and future clinical works.

Insulin, ascorbate, and glucose have a much greater influence than transferrin and selenous acid on the in vitro growth of engineered cartilage in chondrogenic media

The primary goal of this study was to characterize the response of chondrocyte-seeded agarose constructs to varying concentrations of several key nutrients in a chondrogenic medium, within the overall context of optimizing the key nutrients and the placement of nutrient channels for successful growth of cartilage tissue constructs large enough to be clinically relevant in the treatment of osteoarthritis (OA). To this end, chondrocyte-agarose constructs (ø4×2.34 mm, 30×10(6) cells/mL) were subjected to varying supplementation levels of insulin (0× to 30× relative to standard supplementation), transferrin (0× to 30×), selenous acid (0× to 10×), ascorbate (0× to 30×), and glucose (0× to 3×). The quality of resulting engineered tissue constructs was evaluated by their compressive modulus (E(-Y)), tensile modulus (E(+Y)), hydraulic permeability (k), and content of sulfated glycosaminoglycans (sGAG) and collagen (COL); DNA content was also quantified. Three control groups from two separate castings of constructs (1× concentrations of all medium constituents) were used. After 42 days of culture, values in each of these controls were, respectively, E(-Y)=518±78, 401±113, 236±67 kPa; E(+Y)=1420±430, 1140±490, 1240±280 kPa; k=2.3±0.8×10(-3), 5.4±7.0×10(-3), 3.3±1.3×10(-3) mm(4)/N·s; sGAG=7.8±0.3, 6.3±0.4, 4.1±0.5%/ww; COL=1.3±0.2, 1.1±0.3, 1.4±0.4%/ww; and DNA=11.5±2.2, 12.1±0.6, 5.2±2.8 μg/disk. The presence of insulin and ascorbate was essential, but their concentrations may drop as low as 0.3× without detrimental effects on any of the measured properties; excessive supplementation of ascorbate (up to 30×) was detrimental to E(-Y), and 30× insulin was detrimental to both E(+Y) and E(-Y). The presence of glucose was similarly essential, and matrix elaboration was significantly dependent on its concentration (p<10(-6)), with loss of functional properties, composition, and cellularity observed at ≤0.3×; excessive glucose supplementation (up to 3×) showed no detrimental effects. In contrast, transferrin and selenous acid had no influence on matrix elaboration. These findings suggest that adequate distributions of insulin, ascorbate, and glucose, but not necessarily of transferrin and selenous acid, must be ensured within large engineered cartilage constructs to produce a viable substitute for joint tissue lost due to OA.

Methods for producing scaffold-free engineered cartilage sheets from auricular and articular chondrocyte cell sources and attachment to porous tantalum

Scaffold-free cartilage engineering techniques may provide a simple alternative to traditional methods employing scaffolds. We previously reported auricular chondrocyte-derived constructs for use in an engineered trachea model; however, the construct generation methods were not reported in detail. In this study, methods for cartilage construct generation from auricular and articular cell sources are described in detail, and the resulting constructs are compared for use in a joint resurfacing model. Attachment of cartilage sheets to porous tantalum is also investigated as a potential vehicle for future attachment to subchondral bone. Large scaffold-free cartilage constructs were produced from culture-expanded chondrocytes from skeletally mature rabbits, and redifferentiated in a chemically-defined culture medium. Auricular constructs contained more glycosaminoglycan (39.6±12.7 vs. 9.7±1.9 μg/mg wet weight, mean and standard deviation) and collagen (2.7±0.45 vs. 1.1±0.2 μg/mg wet weight, mean and standard deviation) than articular constructs. Aggregate modulus was also higher for auricular constructs vs. articular constructs (0.23±0.07 vs. 0.12±0.03 MPa, mean and standard deviation). Attachment of constructs to porous tantalum was achieved by neocartilage ingrowth into tantalum pores. These results demonstrate that large scaffold-free neocartilage constructs can be produced from mature culture-expanded chondrocytes in a chemically-defined medium, and that these constructs can be attached to porous tantalum.

Mechanical properties of native and tissue-engineered cartilage depend on carrier permeability: a bioreactor study

The implantation of osteochondral constructs-tissue-engineered (TE) cartilage on a bone substitute carrier-is a promising method to treat defects in articular cartilage. Currently, however, the TE cartilage's mechanical properties are clearly inferior to those of native cartilage. Their improvement has been the subject of various studies, mainly focusing on growth factors and physical loading during cultivation. With the approach of osteochondral constructs another aspect arises: the permeability of the carrier materials. The purpose of this study was to investigate whether and how the permeability of the subchondral bone influences the properties of native cartilage and whether the bone substitute carrier's permeability influences the TE cartilage of osteochondral constructs accordingly. Consequently, the influence of the subchondral bone's permeability on native cartilage was determined: Native porcine cartilage-bone cylinders were cultivated for 2 weeks in a bioreactor under mechanical loading with and without restricted permeability of the bone. For the TE cartilage these two permeability conditions were investigated using permeable and impermeable tricalciumphosphate carriers under equivalent cultivation conditions. All specimens were evaluated mechanically, biochemically, and histologically. The restriction of the bone's permeability significantly decreased the Young's modulus of native cartilage in vitro. No biochemical differences were found. This finding was confirmed for TE cartilage: While the biochemical parameters were not affected, a permeable carrier improved the cell morphology and mechanical properties in comparison to an impermeable one. In conclusion, the carrier permeability was identified as a determining factor for the mechanical properties of TE cartilage of osteochondral constructs.

Improved osteochondral allograft preservation using serum-free media at body temperature

BACKGROUND

Osteochondral allografts (OCAs) are currently preserved at 4°C and used within 28 days of donor harvest. The window of opportunity for implantation is limited to 14 days due to a 2-week disease testing protocol.

HYPOTHESIS

Osteochondral allograft tissues stored at 37°C will have significantly higher chondrocyte viability, as well as superior biochemical and biomechanical properties, than those stored at 4°C.

STUDY DESIGN

Controlled laboratory study.

METHODS

Osteochondral allografts from 15 adult canine cadavers were aseptically harvested within 4 hours of death. Medial and lateral femoral condyles were stored in Media 1, similar to the current standard, or Media 2, an anti-inflammatory and chondrogenic media containing dexamethasone and transforming growth factor-β3, at 4°C or 37°C for up to 56 days. Chondrocyte viability, glycosaminoglycan (GAG) and collagen (hydroxyproline [HP]) content, biomechanical properties, and collagen II and aggrecan content were assessed at days 28 and 56. Five femoral condyles were stored overnight and assessed the next day to serve as controls.

RESULTS

Storage in Media 1 at 37°C maintained chondrocyte viability at significantly higher levels than in any other media-temperature combination and at levels not significantly different from controls. Osteochondral allografts stored in either media at 4°C showed a significant decrease in chondrocyte viability throughout storage. Glycosaminoglycan and HP content were maintained through 56 days of storage in OCAs in Media 1 at 37°C. There were no significant differences in elastic or dynamic moduli among groups at day 56. Qualitative immunohistochemistry demonstrated the presence of collagen II and aggrecan throughout all layers of cartilage.

CONCLUSION

Osteochondral allograft viability, matrix content and composition, and biomechanical properties were maintained at "fresh" levels through 56 days of storage in Media 1 at 37°C. Osteochondral allografts stored at 4°C were unable to maintain viability or matrix integrity through 28 days of storage. These findings suggest that storage of OCAs in a defined media at 37°C is superior to current protocols (4°C) for tissue preservation prior to transplantation.

CLINICAL RELEVANCE

Storage of OCAs in serum-free chemically defined media at 37°C can increase the "window of opportunity" for implantation of optimal tissue from 14 days to 42 days after disease testing clearance.

Recapitulating aspects of the oxygen and substrate environment of the damaged joint milieu for stem cell-based cartilage tissue engineering

Human infrapatellar fat pad contains a source of mesenchymal stem cells (FPSCs) that potentially offer a novel population for the treatment of damaged or diseased articular cartilage. Existing cartilage repair strategies such as microfracture harness the presence of a low-oxygen microenvironment, fibrin clot formation at sites of microfracture, and elevations in growth factors in the damaged joint milieu. Bearing this in mind, the objective of this study was to determine the chondrogenic potential of diseased human FPSCs in a model system that recapitulates some of these features. In the first phase of the study, the role of transforming growth factor beta-3 (TGF-β3) and fibroblast growth factor-2 (FGF-2), in addition to an altered oxygen-tension environment, on the colony-forming unit-fibroblast (CFU-F) capacity and growth kinetics of human FPSCs during monolayer expansion was evaluated. The subsequent chondrogenic capacity of these cells was quantified in both normoxic (20%) and low- (5%) oxygen conditions. Expansion in FGF-2 was shown to reduce CFU-F numbers, but simultaneously increase both the colony size and the cell yield compared to standard expansion conditions. Supplementation with both FGF-2 and TGF-β3 significantly reduced cell-doubling time. Expansion in FGF-2, followed by differentiation at 5% oxygen tension, was observed to synergistically enhance subsequent sulfated glycosaminoglycan (sGAG) accumulation after chondrogenic induction. FPSCs expanded in FGF-2 were then encapsulated in either agarose or fibrin hydrogels in an attempt to engineer cartilaginous grafts. sGAG synthesis was higher in fibrin constructs, and was further enhanced by differentiation at 5% oxygen tension, accumulating 2.7% (ww) sGAG after 42 days in culture. These results indicate that FPSCs, a readily accessible cell population, form cartilage in an in vitro environment that recapitulates several key biological features of cartilage repair during microfracture and also point toward the potential utility of such cells when combined with fibrin hydrogel scaffolds.

High mesenchymal stem cell seeding densities in hyaluronic acid hydrogels produce engineered cartilage with native tissue properties

Engineered cartilage based on adult mesenchymal stem cells (MSCs) is an alluring goal for the repair of articular defects. However, efforts to date have failed to generate constructs with sufficient mechanical properties to function in the demanding environment of the joint. Our findings with a novel photocrosslinked hyaluronic acid (HA) hydrogel suggest that stiff gels (high HA concentration, 5% w/v) foster chondrogenic differentiation and matrix production, but limit overall functional maturation due to the inability of the formed matrix to diffuse away from the point of production and form a contiguous network. In the current study, we hypothesized that increasing the MSC seeding density would decrease the required diffusional distance, and so expedite the development of functional properties. To test this hypothesis bovine MSCs were encapsulated at seeding densities of either 20,000,000 or 60,000,000 cells ml(-1) in 1%, 3%, and 5% (w/v) HA hydrogels. Counter to our hypothesis the higher concentration HA gels (3% and 5%) did not develop more rapidly with increased MSC seeding density. However, the biomechanical properties of the low concentration (1%) HA constructs increased markedly (nearly 3-fold with a 3-fold increase in seeding density). To ensure that optimal nutrient access was delivered, we next cultured these constructs under dynamic culture conditions (with orbital shaking) for 9 weeks. Under these conditions 1% HA seeded at 60,000,000 MSCs ml(-1) reached a compressive modulus in excess of 1 MPa (compared with 0.3-0.4 MPa for free swelling constructs). This is the highest level we have reported to date in this HA hydrogel system, and represents a significant advance towards functional stem cell-based tissue engineered cartilage.

Toward engineering a biological joint replacement

Osteoarthritis is a major cause of disability and pain for patients in the United States. Treatments for this degenerative disease represent a significant challenge considering the poor regenerative capacity of adult articular cartilage. Tissue-engineering techniques have advanced over the last two decades such that cartilage-like tissue can be cultivated in the laboratory for implantation. Even so, major challenges remain for creating fully functional tissue. This review article overviews some of these challenges, including overcoming limitations in nutrient supply to cartilage, improving in vitro collagen production, improving integration of engineered cartilage with native tissue, and exploring the potential for engineering full articular surface replacements.

A functional agarose-hydroxyapatite scaffold for osteochondral interface regeneration

Regeneration of the osteochondral interface is critical for integrative and functional cartilage repair. This study focuses on the design and optimization of a hydrogel-ceramic composite scaffold of agarose and hydroxyapatite (HA) for calcified cartilage formation. The first study objective was to compare the effects of HA on non-hypertrophic and hypertrophic chondrocytes cultured in the composite scaffold. Specifically, cell growth, biosynthesis, hypertrophy, and scaffold mechanical properties were evaluated. Next, the ceramic phase of the scaffold was optimized in terms of particle size (200 nm vs. 25 μm) and dose (0-6 w/v%). It was observed that while deep zone chondrocyte (DZC) biosynthesis and hypertrophy remained unaffected, hypertrophic chondrocytes measured higher matrix deposition and mineralization potential with the addition of HA. Most importantly, higher matrix content translated into significant increases in both compressive and shear mechanical properties. While cell hypertrophy was independent of ceramic size, matrix deposition was higher only with the addition of micron-sized ceramic particles. In addition, the highest matrix content, mechanical properties and mineralization potential were found in scaffolds with 3% micro-HA, which approximates both the mineral aggregate size and content of the native interface. These results demonstrate that the biomimetic hydrogel-ceramic composite is optimal for calcified cartilage formation and is a promising design strategy for osteochondral interface regeneration.

Cartilage matrix formation by bovine mesenchymal stem cells in three-dimensional culture is age-dependent

BACKGROUND

Cartilage degeneration is common in the aged, and aged chondrocytes are inferior to juvenile chondrocytes in producing cartilage-specific extracellular matrix. Mesenchymal stem cells (MSCs) are an alternative cell type that can differentiate toward the chondrocyte phenotype. Aging may influence MSC chondrogenesis but remains less well studied, particularly in the bovine system.

QUESTIONS/PURPOSES

The objectives of this study were (1) to confirm age-related changes in bovine articular cartilage, establish how age affects chondrogenesis in cultured pellets for (2) chondrocytes and (3) MSCs, and (4) determine age-related changes in the biochemical and biomechanical development of clinically relevant MSC-seeded hydrogels.

METHODS

Native bovine articular cartilage from fetal (n = 3 donors), juvenile (n = 3 donors), and adult (n = 3 donors) animals was analyzed for mechanical and biochemical properties (n = 3-5 per donor). Chondrocyte and MSC pellets (n = 3 donors per age) were cultured for 6 weeks before analysis of biochemical content (n = 3 per donor). Bone marrow-derived MSCs of each age were also cultured within hyaluronic acid hydrogels for 3 weeks and analyzed for matrix deposition and mechanical properties (n = 4 per age).

RESULTS

Articular cartilage mechanical properties and collagen content increased with age. We observed robust matrix accumulation in three-dimensional pellet culture by fetal chondrocytes with diminished collagen-forming capacity in adult chondrocytes. Chondrogenic induction of MSCs was greater in fetal and juvenile cell pellets. Likewise, fetal and juvenile MSCs in hydrogels imparted greater matrix and mechanical properties.

CONCLUSIONS

Donor age and biochemical microenvironment were major determinants of both bovine chondrocyte and MSC functional capacity.

CLINICAL RELEVANCE

In vitro model systems should be evaluated in the context of age-related changes and should be benchmarked against human MSC data.

Bioactive glass 13-93 as a subchondral substrate for tissue-engineered osteochondral constructs: a pilot study

BACKGROUND

Replacement of diseased areas of the joint with tissue-engineered osteochondral grafts has shown potential in the treatment of osteoarthritis. Bioactive glasses are candidates for the osseous analog of these grafts.

QUESTIONS/PURPOSES

(1) Does Bioactive Glass 13-93 (BG 13-93) as a subchondral substrate improve collagen and glycosaminoglycan production in a tissue-engineered cartilage layer? (2) Does BG 13-93 as a culture medium supplement increase the collagen and glycosaminoglycan production and improve the mechanical properties in a tissue-engineered cartilage layer?

METHODS

In Study 1, bioactive glass samples (n = 4) were attached to a chondrocyte-seeded agarose layer to form an osteochondral construct, cultured for 6 weeks, and compared to controls. In Study 2, bioactive glass samples (n = 5) were cocultured with cell-seeded agarose for 6 weeks. The cell-seeded agarose layer was exposed to BG 13-93 either continuously or for the first or last 2 weeks in culture or had no exposure.

RESULTS

Osteochondral constructs with a BG 13-93 base had improved glycosaminoglycan deposition but less collagen II content. Agarose scaffolds that had a temporal exposure to BG 13-93 within the culture medium had improved mechanical and biochemical properties compared to continuous or no exposure.

CONCLUSIONS

When used as a subchondral substrate, BG 13-93 did not improve biochemical properties compared to controls. However, as a culture medium supplement, BG 13-93 improved the biochemical and mechanical properties of a tissue-engineered cartilage layer.

CLINICAL RELEVANCE

BG 13-93 may not be suitable in osteochondral constructs but could have potential as a medium supplement for neocartilage formation.

Tissue responses against tissue-engineered cartilage consisting of chondrocytes encapsulated within non-absorbable hydrogel

To disclose the influence of foreign body responses raised against a non-absorbable hydrogel consisting of tissue-engineered cartilage, we embedded human/canine chondrocytes within agarose and transplanted them into subcutaneous pockets in nude mice and donor beagles. One month after transplantation, cartilage formation was observed in the experiments using human chondrocytes in nude mice. No significant invasion of blood cells was noted in the areas where the cartilage was newly formed. Around the tissue-engineered cartilage, agarose fragments, a dense fibrous connective tissue and many macrophages were observed. On the other hand, no cartilage tissue was detected in the autologous transplantation of canine chondrocytes. Few surviving chondrocytes were observed in the agarose and no accumulation of blood cells was observed in the inner parts of the transplants. Localizations of IgG and complements were noted in areas of agarose, and also in the devitalized cells embedded within the agarose. Even if we had inhibited the proximity of the blood cells to the transplanted cells, the survival of the cells could not be secured. We suggest that these cytotoxic mechanisms seem to be associated not only with macrophages but also with soluble factors, including antibodies and complements.

Effects of hypertonic (NaCl) two-dimensional and three-dimensional culture conditions on the properties of cartilage tissue engineered from an expanded mature bovine chondrocyte source

Clinically relevant mature cartilage cells (chondrocytes) present challenges for use in cartilage tissue engineering applications, given their low capacity for cell division and tissue production. Since the in situ environment of chondrocytes is hypertonic relative to standard culture medium conditions, in this study we tested the hypothesis that using culture medium of a hypertonic, more physiologic osmolarity during both two-dimensional (2D) expansion of mature bovine chondrocytes (MBCs) and their subsequent encapsulation culture in three-dimensional (3D) agarose hydrogel constructs produces improved engineered tissue construct mechanical and biochemical properties. Results demonstrate that 2D expansion of MBCs in hypertonic (NaCl) medium before encapsulation yielded improved construct mechanical properties. However, 3D encapsulation culture of cells in hypertonic (NaCl) medium yielded poorer construct mechanical properties. Osmolarity-related differences in construct biochemical content and organization may have contributed to differences in mechanical properties, as construct glycosaminoglycan content correlated moderately with construct mechanical properties, and construct collagen distribution varied between 3D osmotic culture groups. Results of this study suggest that application of hypertonic (NaCl) medium during 2D mature chondrocyte expansion, but not 3D encapsulated chondrocyte culture, may serve as a convenient and inexpensive method for improving mechanical properties of expanded cell-seeded constructs.

Transient supplementation of anabolic growth factors rapidly stimulates matrix synthesis in engineered cartilage

The purpose of the presented work is to examine the response of engineered cartilage to a transient, 2-week application of anabolic growth factors compared to continuous exposure in in vitro culture. Immature bovine chondrocytes were suspended in agarose hydrogel and cultured for 28 days (Study 1) or 42 days (Study 2) in chondrogenic media with TGF-β1, TGF-β3, or IGF-I either added for only the first 14 days in culture or added to the media for the entire study period. In both studies, there were no statistical differences in tissue mechanical or biochemical properties between the growth factors on day 14. In Study 1, growth factor removal led to a significant and drastic increase in Young's modulus and glycosaminoglycans content compared to continuously exposed controls on day 28. In Study 2, both TGF-β1 and β3 led to significantly higher mechanical properties and collagen content vs. IGF-I on day 42. These results indicate that the rapid rise in tissue properties (previously observed with TGF-β3 only) is not dependent on the type but rather the temporal application of the anabolic growth factor. These findings shed light on possible techniques to rapidly develop engineered cartilage tissue for the future treatment of osteoarthritis.

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.

Growth factor priming of synovium-derived stem cells for cartilage tissue engineering

This study investigated the potential use of synovium-derived stem cells (SDSCs) as a cell source for cartilage tissue engineering. Harvested SDSCs from juvenile bovine synovium were expanded in culture in the presence (primed) or absence (unprimed) of growth factors (1 ng/mL transforming growth factor-β(1), 10 ng/mL platelet-derived growth factor-ββ, and 5 ng/mL basic fibroblast growth factor-2) and subsequently seeded into clinically relevant agarose hydrogel scaffolds. Constructs seeded with growth factor-primed SDSCs that received an additional transient application of transforming growth factor-β(3) for the first 21 days (release) exhibited significantly better mechanical and biochemical properties compared to constructs that received sustained growth factor stimulation over the entire culture period (continuous). In particular, the release group exhibited a Young's modulus (267±96 kPa) approaching native immature bovine cartilage levels, with corresponding glycosaminoglycan content (5.19±1.45%ww) similar to native values, within 7 weeks of culture. These findings suggest that SDSCs may serve as a cell source for cartilage tissue engineering applications.