Human chondrocyte migration behaviour to guide the development of engineered cartilage

Supramolecular Motion Enables Chondrogenic Bioactivity of a Cyclic Peptide Mimetic of Transforming Growth Factor-β1

Transforming growth factor (TGF)-β1 is a multifunctional protein that is essential in many cellular processes that include fibrosis, inflammation, chondrogenesis, and cartilage repair. In particular, cartilage repair is important to avoid physical disability since this tissue does not have the inherent capacity to regenerate beyond full development. We report here on supramolecular coassemblies of two peptide amphiphile molecules, one containing a TGF-β1 mimetic peptide, and another which is one of two constitutional isomers lacking bioactivity. Using human articular chondrocytes, we investigated the bioactivity of the supramolecular copolymers of each isomer displaying either the previously reported linear form of the mimetic peptide or a novel cyclic analogue. Based on fluorescence depolarization and 1H NMR spin-lattice relaxation times, we found that coassemblies containing the cyclic compound and the most dynamic isomer exhibited the highest intracellular TGF-β1 signaling and gene expression of cartilage extracellular matrix components. We conclude that control of supramolecular motion is emerging as an important factor in the binding of synthetic molecules to receptors that can be tuned through chemical structure.

Priming chondrocytes during expansion alters cell behavior and improves matrix production in 3D culture

OBJECTIVE

Cartilage tissue engineering strategies that use autologous chondrocytes require in vitro expansion of cells to obtain enough cells to produce functional engineered tissue. However, chondrocytes dedifferentiate during expansion culture, limiting their ability to produce chondrogenic tissue and their utility for cell-based cartilage repair strategies. The current study identified conditions that favor cartilage production and the mechanobiological mechanisms responsible for these benefits.

DESIGN

Chondrocytes were isolated from juvenile bovine knee joints and cultured with (primed) or without (unprimed) a growth factor cocktail. Gene expression, cell morphology, cell adhesion, cytoskeletal protein distribution, and cell mechanics were assessed. Following passage 5, cells were embedded into agarose hydrogels to evaluate functional properties of engineered cartilage.

RESULTS

Priming cells during expansion culture altered cell phenotype and chondrogenic tissue production. Unbiased ribonucleic acid-sequencing analysis suggested, and experimental studies confirmed, that growth factor priming delays dedifferentiation associated changes in cell adhesion and cytoskeletal organization. Priming also overrode mechanobiological pathways to prevent chondrocytes from remodeling their cytoskeleton to accommodate the stiff, monolayer microenvironment. Passage 1 primed cells deformed less and had lower yes associated protein 1 activity than unprimed cells. Differences in cell adhesion, morphology, and cell mechanics between primed and unprimed cells were mitigated by passage 5.

CONCLUSIONS

Priming suppresses mechanobiologic cytoskeletal remodeling to prevent chondrocyte dedifferentiation, resulting in more cartilage-like tissue-engineered constructs.

Addition of collagen type I in agarose created a dose-dependent effect on matrix production in engineered cartilage

Extracellular-matrix composition impacts mechanical performance in native and engineered tissues. Previous studies showed collagen type I-agarose blends increased cell-matrix interactions and extra-cellular matrix production. However, long-term impacts on protein production and mechanical properties of engineered cartilage are unknown. Our objective was to characterize the effect of collagen type I on matrix production of chondrocytes embedded in agarose hydrogels. We hypothesized that the addition of collagen would improve long-term mechanical properties and matrix production (e.g., collagen and glycosaminoglycans) through increased bioactivity. Agarose hydrogels (2% w/v) were mixed with varying concentrations of collagen type I (0, 2, 5 mg/mL). Juvenile bovine chondrocytes were added to the hydrogels to assess matrix production over 4 weeks through biochemical assays, and mechanical properties were assessed through unconfined compression. We observed a dose-dependent effect on cell bioactivity, where 2 mg/mL of collagen improved bioactivity, but 5 mg/mL had a negative impact on bioactivity. This resulted in higher modulus for scaffolds supplemented with lower collagen concentration as compared to the higher collagen concentration, but not when compared to the control. In conclusion, the addition of collagen to agarose constructs provided a dose-dependent impact on improving glycosaminoglycan production but did not improve collagen production or compressive mechanics.

TGF-β2 increases cell-cell communication in chondrocytes via p-Smad3 signalling

Connexin 43 (Cx43)-mediated gap junction intercellular communication (GJIC) plays a crucial role in the pathology and physiology of joint tissues. Transforming growth factor-β2 (TGF-β2), one of the potent regulatory factors in chondrocytes, plays a key role in the regulation of cell cycle and development of joint diseases. However, it is still unknown how TGF-β2 mediates GJIC in chondrocytes. The aim of this study was to explore the potential mechanism by which TGF-β2 regulates GJIC in chondrocytes. CCK-8 assays and scratch assays were performed to define the role of TGF-β2 on cell proliferation and migration. The scrape loading/dye transfer assay and scanning electron microscopy (SEM) were used to verify the effect of TGF-β2 on GJIC between chondrocytes. qPCR was performed to analyse the expression of genes in the gap junction protein family in chondrocytes. The expression of the Cx43 protein and phosphorylated Smad3 (p-Smad3) was evaluated by western blot assay. Immunofluorescence staining was used to explore p-Smad3 signalling pathway activation and Cx43 distribution. From these experiments, we found that the Cx43 protein was the most highly expressed member of the gap junction protein family in chondrocytes. We also found that TGF-β2 facilitated cell-to-cell communication in chondrocytes by upregulating Cx43 expression in chondrocytes. Finally, we found that TGF-β2 activated Smad3 signalling and promoted the nuclear aggregation of p-Smad3. Inhibition experiments by SIS3 also confirmed that TGF-β2-mediated GJIC through p-Smad3 signalling. For the first time, this study confirmed that TGF-β2 could regulate the formation of Cx43-mediated GJIC in chondrocytes via the canonical p-Smad3 signalling pathway.

Probing Multicellular Tissue Fusion of Cocultured Spheroids-A 3D-Bioassembly Model

While decades of research have enriched the knowledge of how to grow cells into mature tissues, little is yet known about the next phase: fusing of these engineered tissues into larger functional structures. The specific effect of multicellular interfaces on tissue fusion remains largely unexplored. Here, a facile 3D-bioassembly platform is introduced to primarily study fusion of cartilage-cartilage interfaces using spheroids formed from human mesenchymal stromal cells (hMSCs) and articular chondrocytes (hACs). 3D-bioassembly of two adjacent hMSCs spheroids displays coordinated migration and noteworthy matrix deposition while the interface between two hAC tissues lacks both cells and type-II collagen. Cocultures contribute to increased phenotypic stability in the fusion region while close initial contact between hMSCs and hACs (mixed) yields superior hyaline differentiation over more distant, indirect cocultures. This reduced ability of potent hMSCs to fuse with mature hAC tissue further underlines the major clinical challenge that is integration. Together, this data offer the first proof of an in vitro 3D-model to reliably study lateral fusion mechanisms between multicellular spheroids and mature cartilage tissues. Ultimately, this high-throughput 3D-bioassembly model provides a bridge between understanding cellular differentiation and tissue fusion and offers the potential to probe fundamental biological mechanisms that underpin organogenesis.

Recellularization and Integration of Dense Extracellular Matrix by Percolation of Tissue Microparticles

Cells embedded in the extracellular matrix of tissues play a critical role in maintaining homeostasis while promoting integration and regeneration following damage or disease. Emerging engineered biomaterials utilize decellularized extracellular matrix as a tissue-specific support structure; however, many dense, structured biomaterials unfortunately demonstrate limited formability, fail to promote cell migration, and result in limited tissue repair. Here, we developed a reinforced composite material of densely packed acellular extracellular matrix microparticles in a hydrogel, termed tissue clay, that can be molded and crosslinked to mimic native tissue architecture. We utilized hyaluronic acid-based hydrogels, amorphously packed with acellular articular cartilage tissue particulated to ~125-250 microns in diameter and defined a percolation threshold of 0.57 (v/v) beyond which the compressive modulus exceeded 300kPa. Remarkably, primary chondrocytes recellularized particles within 48 hours, a process driven by chemotaxis, exhibited distributed cellularity in large engineered composites, and expressed genes consistent with native cartilage repair. We additionally demonstrated broad utility of tissue clays through recellularization and persistence of muscle, skin, and cartilage composites in a subcutaneous in vivo mouse model. Our findings suggest optimal strategies and material architectures to balance concurrent demands for large-scale mechanical properties while also supporting recellularization and integration of dense musculoskeletal and connective tissues.

TABLE OF CONTENTS ENTRY

We present a new design framework for regenerative articular cartilage scaffolds using acellular extracellular matrix particles, packed beyond a percolation threshold, and crosslinked within chondroinductive hydrogels. Our results suggest that the architecture and the packing, rather than altering the individual components, creates a composite material that can balance mechanics, porosity to enable migration, and tissue specific biochemical interactions with cells. Moreover, we provide a technique that we show is applicable to other tissue types.

Endogenous Repair and Regeneration of Injured Articular Cartilage: A Challenging but Promising Therapeutic Strategy

Articular cartilage (AC) has a very limited intrinsic repair capacity after injury or disease. Although exogenous cell-based regenerative approaches have obtained acceptable outcomes, they are usually associated with complicated procedures, donor-site morbidities and cell differentiation during ex vivo expansion. In recent years, endogenous regenerative strategy by recruiting resident mesenchymal stem/progenitor cells (MSPCs) into the injured sites, as a promising alternative, has gained considerable attention. It takes full advantage of body's own regenerative potential to repair and regenerate injured tissue while avoiding exogenous regenerative approach-associated limitations. Like most tissues, there are also multiple stem-cell niches in AC and its surrounding tissues. These MSPCs have the potential to migrate into injured sites to produce replacement cells under appropriate stimuli. Traditional microfracture procedure employs the concept of MSPCs recruitment usually fails to regenerate normal hyaline cartilage. The reasons for this failure might be attributed to an inadequate number of recruiting cells and adverse local tissue microenvironment after cartilage injury. A strategy that effectively improves local matrix microenvironment and recruits resident MSPCs may enhance the success of endogenous AC regeneration (EACR). In this review, we focused on the reasons why AC cannot regenerate itself in spite of potential self-repair capacity and summarized the latest developments of the three key components in the field of EACR. In addition, we discussed the challenges facing in the present EACR strategy. This review will provide an increasing understanding of EACR and attract more researchers to participate in this promising research arena.

Electric field stimulation for tissue engineering applications

Electric fields are involved in numerous physiological processes, including directional embryonic development and wound healing following injury. To study these processes in vitro and/or to harness electric field stimulation as a biophysical environmental cue for organised tissue engineering strategies various electric field stimulation systems have been developed. These systems are overall similar in design and have been shown to influence morphology, orientation, migration and phenotype of several different cell types. This review discusses different electric field stimulation setups and their effect on cell response.

Transcriptome analyses identify key genes and potential mechanisms in a rat model of osteoarthritis

BACKGROUND

Osteoarthritis (OA) is one of the most common degenerative diseases of the joints worldwide, but still the pathogenesis of OA is largely unknown. The purpose of our study is to clarify key candidate genes and relevant signaling pathways in a surgical-induced OA rat model.

METHODS

The microarray raw data of GSE8077 was downloaded from GEO datasets. GeoDiver were employed to screen differentially-expressed genes (DEGs). Enrichment analyses of DEGs were performed using Metascape. Construction of protein-protein interaction (PPI) network and identification of key genes were conducted using STRING, Cytoscape v3.6.0, and Centiscape2.2. Furthermore, miRDB and Cytoscape v3.6.0 were used for visualization of miRNA-mRNA regulatory network. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis for predicted miRNAs was undertaken using DIANA-miRPath v3.0.

RESULTS

Several DEGs (188 in comparison between OA and sham-operated group and 160 in comparison between OA and contralateral group) were identified. DEGs mainly enriched in vasculature development, regulation of cell migration, response to growth factor (Gene ontology), and ECM-receptor interaction (KEGG). Two comparison cohorts shared 79 intersection genes, and of these, Ccl2, Col4a1, Col1a1, Aldh1a3, and Itga8 were defined as the hub genes. Predicted miRNAs of seven DEGs from sub-networks mainly enriched in MAPK signaling pathway.

CONCLUSION

The current study shows that some key genes and pathways, such as Ccl2, Col4a1, Col1a1, Aldh1a3, Itga8, ECM-receptor interaction, and MAPK signaling pathway may be associated with OA progression and act as potential biomarkers and therapeutic targets for OA.

Hyaluronic Acid Dictates Chondrocyte Morphology and Migration in Composite Gels

Tissue equivalent collagen-hyaluronic acid-based hydrogels are widely used for cartilage tissue engineering; however, not much importance has been given to investigate how cellular responses are altered with varying concentrations of hyaluronic acid in gels. In this study, different concentrations of hyaluronic acid dialdehyde (HAD) were combined with collagen to fabricate collagen-HAD composite (CH) gels, and the influence of HAD on cell shape, migration, viability, cytoskeletal organization, and gel contraction was examined. The microstructure and the mechanical strength of the composite gels were altered by varying HAD concentrations. Morphology of chondrocytes cultured on CH gels showed a significant increase in their aspect ratio and decrease in number of cell protrusions with increase in concentration of HAD. The organization of the cytoskeleton at the cellular protrusions was vimentin localized at the base, microtubules at the tip, and actin localized throughout the cell body. Changes in HAD concentrations altered hydrogel mechanical strength, cytoskeletal organization, and formation of cellular protrusions, all of which contributed to changes in cell morphology and migration. These changes were more evident in 3D cell-encapsulated gels than chondrocytes cultured over the 2D gels. However, viability of cells and matrix contraction, staining for adhesion protein vinculin, and hyaluronic acid receptor CD44 remained similar in all CH compositions. The changes in cell responses further influenced extracellular matrix deposition during in vitro culture. Cell responses in low HAD gels mimic the cellular behavior in damaged cartilage, whereas those in high HAD gels resembled the behavior in healthy cartilage tissue. Our study illustrates the importance of careful formulations of hydrogel compositions in designing biomimetic matrices that are used as in vitro models to study chondrocyte behavior.

Redifferentiation of aged human articular chondrocytes by combining bone morphogenetic protein-2 and melanoma inhibitory activity protein in 3D-culture

Melanoma inhibitory activity (MIA) affects the differentiation to hyaline cartilage and can inhibit the osteogenic potential of bone morphogenetic protein (BMP)-2 in mesenchymal stem cells (MSC). The aim of this study was to investigate if MIA also inhibits the osteogenic potential of BMP-2 in human articular chondrocytes during redifferentiation, which may lead to a higher grade of differentiation without calcification. HAC of four female patients (mean age: 73.75 ±6.98) were seeded into 3D culture for 28 days; after adding the recombinant proteins, four groups were formed (Control, BMP-2, MIA, BMP-2+MIA). Samples were analysed for gene expression, glycosaminoglycan (GAG) content and histology on day 0, 14 and 28. Collagen type 2 (COL2A1) was significantly increased in the BMP-2 containing groups on day 28; BMP-2 (100-fold, p = 0.001), BMP-2+MIA (65-fold, p = 0.009) and similar to the level of native cartilage. Higher aggrecan (Agg) levels were present in the BMP-2 (3-fold, p = 0.007) and BMP-2+MIA (4-fold, p = 0.002) group after 14 days and in the BMP-2 (9-fold, p = 0.001) group after 28 days. Collagen type 10 (COL10A1) was increased in the BMP-2 containing groups (6-fold, p = 0.006) but these levels were significantly below native cartilage. Alkaline phosphatase (ALP), collagen type 1 (COL1A1) and the glycosaminoglycan (GAG) content did not reveal any relevant differences between groups. BMP-2 is a potent inducer for differentiation of HAC. A significant enhancement of this effect in combination with MIA could not be observed. Furthermore no significant reduction of osteogenic markers during re-differentiation of chondrocytes was present combining BMP-2 and MIA.

Chondrocytes from congenital microtia possess an inferior capacity for in vivo cartilage regeneration to healthy ear chondrocytes

The remnant auricular cartilage from microtia has become a valuable cell source for ear regeneration. It is important to clarify the issue of whether the genetically defective microtia chondrocytes could engineer cartilage tissue comparable to healthy ear chondrocytes. In the current study, the histology and cell yield of native microtia and normal ear cartilage were investigated, and the biological characteristics of derived chondrocytes examined, including proliferation, chondrogenic phenotype and cell migration. Furthermore, the in vivo cartilage-forming capacity of passaged microtia and normal auricular chondrocytes were systematically compared by seeding them onto polyglycolic acid/polylactic acid scaffold to generate tissue engineered cartilage in nude mice. Through histological examinations and quantitative analysis of glycosaminoglycan, Young's modulus, and the expression of cartilage-related genes, it was found that microtia chondrocytes had a slower dedifferentiation rate with the decreased expression of stemness-related genes, and weaker migration ability than normal ear chondrocytes, and the microtia chondrocytes-engineered cartilage was biochemically and biomechanically inferior to that constructed using normal ear chondrocytes. This study provides valuable information for the clinical application of the chondrocytes derived from congenital microtia to engineer cartilage. Copyright © 2016 John Wiley & Sons, Ltd.

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.

Bioengineering pediatric scaffold-free auricular cartilaginous constructs

OBJECTIVE

The use of exogenous materials as scaffolds in cartilage tissue engineering has limited the clinical application of resultant constructs due to the risk of postoperative complications. In an effort to minimize such complications, we aim to generate human, scaffold-free auricular cartilaginous constructs.

STUDY DESIGN

Laboratory study using pediatric auricular cartilage.

METHODS

Remnant, normal pediatric auricular cartilage samples that would have otherwise been discarded were collected and digested to free cells. Harvested cells were cultured and expanded in vitro for two passages and plated as micromass cultures. The culture medium was replaced with a chemically defined chondrogenic medium, and cellular monolayers surrounding micromass cultures were continuously scraped off. Constructs were allowed to mature for a period of 8 weeks.

RESULTS

Micromass constructs showed mechanical stability and structurally resembled native auricular tissue, with a perichondrium-like layer of cells surrounding the inner cartilaginous zone. Constructs accumulated equivalent sulphated glycosaminoglycan and 50% of collagen content compared to native auricular cartilage by mass, while displaying 156% more cellularity.

CONCLUSIONS

High-density micromass cultures of pediatric auricular chondrocytes can generate stable cartilaginous constructs following prolonged chondrogenic inductions in vitro. This technique is an essential step toward the development of three-dimensional constructs to recreate clinically applicable auricular cartilaginous constructs.

LEVEL OF EVIDENCE

NA. Laryngoscope, 127:E153-E158, 2017.

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.

In vitro effects of platelet-rich gel supernatants on histology and chondrocyte apoptosis scores, hyaluronan release and gene expression of equine cartilage explants challenged with lipopolysaccharide

Background

Platelet-rich plasma (PRP) preparations are a common treatment in equine osteoarthritis (OA). However, there are controversies regarding the ideal concentration of platelets and leukocytes in these biological substances necessary to induce an adequate anti-inflammatory and anabolic response in articular cartilage. The aims were to study the influence of leukocyte- and platelet-rich gel (L-PRG) and pure platelet-rich gel (P-PRG) supernatants on the histological changes of cartilage, the degree of chondrocyte apoptosis, the production of hyaluronan (HA) and the gene expression of nuclear factor kappa beta (NFkβ), matrix metalloproteinase 13 (MMP-13), a disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS-4), collagen type I alpha 1 (COL1A1), collagen type II alpha 1 (COL2A1) and cartilage oligomeric matrix protein (COMP) in normal cartilage explants (CEs) challenged with lipopolysaccharide (LPS).

Results

Overall, 25 % L-PRG supernatant (followed in order of importance by, 50 % P-PRG, 25 % P-PRG and 50 % L-PRG) represented the substance with the most important anti-inflammatory and anabolic effect. 25 % P-PRG supernatant presented important anabolic effects, but it induced a more severe chondrocyte apoptosis than the other evaluated substances.

Conclusions

25 % L-PRG supernatant presented the best therapeutic profile. Our results demonstrate that the biological variability of PRP preparations makes their application rather challenging. Additional in vivo research is necessary to know the effect of PRP preparations at different concentrations.

Optimizing nutrient channel spacing and revisiting TGF-beta in large engineered cartilage constructs

Cartilage tissue engineering is a promising approach to treat osteoarthritis. However, current techniques produce tissues too small for clinical relevance. Increasingly close-packed channels have helped overcome nutrient transport limitations in centimeter-sized chondrocyte-agarose constructs, yet optimal channel spacings to recapitulate native cartilage compositional and mechanical properties in constructs this large have not been identified. Transient active TGF-β treatment consistently reproduces native compressive Young׳s modulus (EY) and glycosaminoglycan (GAG) content in constructs, but standard dosages of 10ng/mL exacerbate matrix heterogeneity. To ultimately produce articular layer-sized constructs, we must first optimize channel spacing and investigate the role of TGF-β in the utility of channels. We cultured ∅10mm constructs with 0, 12, 19, or 27 nutrient channels (∅1mm) for 6-8 weeks with 0, 1, or 10ng/mL TGF-β; subsequently we analyzed them mechanically, biochemically, and histologically. Constructs with 12 or 19 channels grew the most favorably, reaching EY=344±113kPa and GAG and collagen contents of 10.8±1.2% and 2.2±0.2% of construct wet weight, respectively. Constructs with 27 channels had significantly less deposited GAG than other groups. Channeled constructs given 1 or 10ng/mL TGF-β developed similar properties. Without TGF-β, constructs with 0 or 12 channels exhibited properties that were indistinguishable, and lower than TGF-β-supplemented constructs. Taken together, these results emphasize that nutrient channels are effective only in the presence of TGF-β, and indicate that spacings equivalent to 12 channels in ∅10mm constructs can be employed in articular-layer-sized constructs with reduced dosages of TGF-β.

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.

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.

Tissue Engineering a Biological Repair Strategy for Lumbar Disc Herniation

The intervertebral disc is a critical part of the intersegmental soft tissue of the spinal column, providing flexibility and mobility, while absorbing large complex loads. Spinal disease, including disc herniation and degeneration, may be a significant contributor to low back pain. Clinically, disc herniations are treated with both nonoperative and operative methods. Operative treatment for disc herniation includes removal of the herniated material when neural compression occurs. While this strategy may have short-term advantages over nonoperative methods, the remaining disc material is not addressed and surgery for mild degeneration may have limited long-term advantage over nonoperative methods. Furthermore, disc herniation and surgery significantly alter the mechanical function of the disc joint, which may contribute to progression of degeneration in surrounding tissues. We reviewed recent advances in tissue engineering and regenerative medicine strategies that may have a significant impact on disc herniation repair. Our review on tissue engineering strategies focuses on cell-based and inductive methods, each commonly combined with material-based approaches. An ideal clinically relevant biological repair strategy will significantly reduce pain and repair and restore flexibility and motion of the spine.