Matrix forming characteristics of inner and outer human meniscus cells on 3D collagen scaffolds under normal and low oxygen tensions

Suspension bath bioprinting and maturation of anisotropic meniscal constructs

Due to limited intrinsic healing capacity of the meniscus, meniscal injuries pose a significant clinical challenge. The most common method for treatment of damaged meniscal tissues, meniscectomy, leads to improper loading within the knee joint, which can increase the risk of osteoarthritis. Thus, there is a clinical need for the development of constructs for meniscal repair that better replicate meniscal tissue organization to improve load distributions and function over time. Advanced three-dimensional bioprinting technologies such as suspension bath bioprinting provide some key advantages, such as the ability to support the fabrication of complex structures using non-viscous bioinks. In this work, the suspension bath printing process is utilized to print anisotropic constructs with a unique bioink that contains embedded hydrogel fibers that align via shear stresses during printing. Constructs with and without fibers are printed and then cultured for up to 56 din vitroin a custom clamping system. Printed constructs with fibers demonstrate increased cell and collagen alignment, as well as enhanced tensile moduli when compared to constructs printed without fibers. This work advances the use of biofabrication to develop anisotropic constructs that can be utilized for the repair of meniscal tissue.

Time course of 3D fibrocartilage formation by expanded human meniscus fibrochondrocytes in hypoxia

Adult human meniscus fibrocartilage is avascular and nonhealing after injury. Meniscus tissue engineering aims to replace injured meniscus with lab-grown fibrocartilage. Dynamic culture systems may be necessary to generate fibrocartilage of sufficient mechanical properties for implantation; however, the optimal static preculture conditions before initiation of dynamic culture are unknown. This study thus investigated the time course of fibrocartilage formation by human meniscus fibrochondrocytes on a three-dimensional biomaterial scaffold under various static conditions. Human meniscus fibrochondrocytes from partial meniscectomy were expanded to passage 1 (P1) or P2 (3.0 ± 0.4 and 6.5 ± 0.6 population doublings), seeded onto type I collagen scaffolds, and grown in hypoxia (HYP, 3% O₂ ) or normoxia (NRX, 20% O₂ ) for 3, 6, and 9 weeks. Mechanical properties were not different between P1 and P2 cell-based constructs. Mechanical properties were lower in HYP, increased continually in NRX only, and were positively correlated with glycosaminoglycan content and accumulation of hyaline cartilage-like matrix components. The most mechanically competent tissues (NRX/9 weeks) reached 1/5 of the native meniscus instantaneous compression modulus but had an increasingly hypertrophic matrix-forming phenotype. HYP consistently suppressed the hypertrophic phenotype. The results provide baselines of engineered meniscus fibrocartilage properties under static conditions, which can be used to select a preculture strategy for dynamic culture depending on the desired combination of mechanical properties, hyaline cartilage-like matrix abundance, and hypertrophic phenotype.

Meniscus cell regional phenotypes: Dedifferentiation and reversal by biomaterial embedding

Meniscus injuries are common and a major cause of long-term joint degeneration and disability. Current treatment options are limited, so novel regenerative therapies or tissue engineering strategies are urgently needed. The development of new therapies is hindered by a lack of knowledge regarding the cellular biology of the meniscus and a lack of well-established methods for studying meniscus cells in vitro. The goals of this study were to (1) establish baseline expression profiles and dedifferentiation patterns of inner and outer zone primary meniscus cells, and (2) evaluate the utility of poly(ethylene glycol) diacrylate (PEGDA) and gelatin methacrylate (GelMA) polymer hydrogels to reverse dedifferentiation trends for long-term meniscus cell culture. Using reverse transcription-quantitative polymerase chain reaction, we measured expression levels of putative meniscus phenotype marker genes in freshly isolated meniscus tissue, tissue explant culture, and monolayer culture of inner and outer zone meniscus cells from porcine knees to establish baseline dedifferentiation characteristics, and then compared these expression levels to PEGDA/GelMA embedded passaged meniscus cells. COL1A1 showed robust upregulation, while CHAD, CILP, and COMP showed downregulation with monolayer culture. Expression levels of COL2A1, ACAN, and SOX9 were surprisingly similar between inner and outer zone tissue and were found to be less sensitive as markers of dedifferentiation. When embedded in PEGDA/GelMA hydrogels, expression levels of meniscus cell phenotype genes were significantly modulated by varying the ratio of polymer components, allowing these materials to be tuned for phenotype restoration, meniscus cell culture, and tissue engineering applications.

Engineered human meniscus' matrix-forming phenotype is unaffected by low strain dynamic compression under hypoxic conditions

Low oxygen and mechanical loading may play roles in regulating the fibrocartilaginous phenotype of the human inner meniscus, but their combination in engineered tissues remains unstudied. Here, we investigated how continuous low oxygen (“hypoxia”) combined with dynamic compression would affect the fibrocartilaginous “inner meniscus-like” matrix-forming phenotype of human meniscus fibrochondrocytes (MFCs) in a porous type I collagen scaffold. Freshly-seeded MFC scaffolds were cultured for 4 weeks in either 3 or 20% O₂ or pre-cultured for 2 weeks in 3% O₂ and then dynamically compressed for 2 weeks (10% strain, 1 Hz, 1 h/day, 5 days/week), all with or without TGF-β3 supplementation. TGF-β3 supplementation was found necessary to induce matrix formation by MFCs in the collagen scaffold regardless of oxygen tension and application of the dynamic compression loading regime. Neither hypoxia under static culture nor hypoxia combined with dynamic compression had significant effects on expression of specific protein and mRNA markers for the fibrocartilaginous matrix-forming phenotype. Mechanical properties significantly increased over the two-week loading period but were not different between static and dynamic-loaded tissues after the loading period. These findings indicate that 3% O₂ applied immediately after scaffold seeding and dynamic compression to 10% strain do not affect the fibrocartilaginous matrix-forming phenotype of human MFCs in this type I collagen scaffold. It is possible that a delayed hypoxia treatment and an optimized pre-culture period and loading regime combination would have led to different outcomes.

Assessing the Resident Progenitor Cell Population and the Vascularity of the Adult Human Meniscus

PURPOSE

To identify, characterize, and compare the resident progenitor cell populations within the red-red, red-white, and white-white (WW) zones of freshly harvested human cadaver menisci and to characterize the vascularity of human menisci using immunofluorescence and 3-dimensional (3D) imaging.

METHODS

Fresh adult human menisci were harvested from healthy donors. Menisci were enzymatically digested, mononuclear cells isolated, and characterized using flow cytometry with antibodies against mesenchymal stem cell surface markers (CD105, CD90, CD44, and CD29). Cells were expanded in culture, characterized, and compared with bone marrow-derived mesenchymal stem cells. Trilineage differentiation potential of cultured cells was determined. Vasculature of menisci was mapped in 3D using a modified uDisco clearing and immunofluorescence against vascular markers CD31, lectin, and alpha smooth muscle actin.

RESULTS

There were no significant differences in the clonogenicity of isolated cells between the 3 zones. Flow cytometry showed presence of CD44⁺CD105⁺CD29⁺CD90⁺ cells in all 3 zones with high prevalence in the WW zone. Progenitors from all zones were found to be potent to differentiate to mesenchymal lineages. Larger vessels in the red-red zone of meniscus were observed spanning toward red-white, sprouting to smaller arterioles and venules. CD31⁺ cells were identified in all zones using the 3D imaging and co-localization of additional markers of vasculature (lectin and alpha smooth muscle actin) was observed.

CONCLUSIONS

The presence of resident mesenchymal progenitors was evident in all 3 meniscal zones of healthy adult donors without injury. In addition, our results demonstrate the presence of vascularization in the WW zone.

CLINICAL RELEVANCE

The existence of progenitors and presence of microvasculature in the WW zone of the meniscus suggests the potential for repair and biologic augmentation strategies in that zone of the meniscus in young healthy adults. Further research is necessary to fully define the functionality of the meniscal blood supply and its implications for repair.

Biomaterials and Advanced Biofabrication Techniques in hiPSCs Based Neuromyopathic Disease Modeling

Induced pluripotent stem cells (iPSCs) are reprogrammed somatic cells by defined factors, and have great application potentials in tissue regeneration and disease modeling. Biomaterials have been widely used in stem cell-based studies, and are involved in human iPSCs based studies, but they were not enough emphasized and recognized. Biomaterials can mimic the extracellular matrix and microenvironment, and act as powerful tools to promote iPSCs proliferation, differentiation, maturation, and migration. Many classic and advanced biofabrication technologies, such as cell-sheet approach, electrospinning, and 3D-bioprinting, are used to provide physical cues in macro-/micro-patterning, and in combination with other biological factors to support iPSCs applications. In this review, we highlight the biomaterials and fabrication technologies used in human iPSC-based tissue engineering to model neuromyopathic diseases, particularly those with genetic mutations, such as Duchenne Muscular Dystrophy (DMD), Congenital Heart Diseases (CHD) and Alzheimer's disease (AD).

Surgical and tissue engineering strategies for articular cartilage and meniscus repair

Injuries to articular cartilage and menisci can lead to cartilage degeneration that ultimately results in arthritis. Different forms of arthritis affect ~50 million people in the USA alone, and it is therefore crucial to identify methods that will halt or slow the progression to arthritis, starting with the initiating events of cartilage and meniscus defects. The surgical approaches in current use have a limited capacity for tissue regeneration and yield only short-term relief of symptoms. Tissue engineering approaches are emerging as alternatives to current surgical methods for cartilage and meniscus repair. Several cell-based and tissue-engineered products are currently in clinical trials for cartilage lesions and meniscal tears, opening new avenues for cartilage and meniscus regeneration. This Review provides a summary of surgical techniques, including tissue-engineered products, that are currently in clinical use, as well as a discussion of state-of-the-art tissue engineering strategies and technologies that are being developed for use in articular cartilage and meniscus repair and regeneration. The obstacles to clinical translation of these strategies are also included to inform the development of innovative tissue engineering approaches.

Gene expression profiles of the meniscus avascular phenotype: A guide for meniscus tissue engineering

Avascular (Avas) meniscus regeneration remains a challenge, which is partly a consequence of our limited knowledge of the cells that maintain this tissue region. In this study, we utilized microarrays to characterize gene expression profiles of intact human Avas meniscus tissue and of cells following culture expansion. Using these data, we examined various 3D culture conditions to redifferentiate Avas cells toward the tissue phenotype. RNA was isolated from either the tissue directly or following cell isolation and 2 weeks in monolayer culture. RNA was hybridized on human genome arrays. Differentially expressed (DE) genes were identified by ranking analysis. DAVID pathway analysis was performed and visualized using STRING analysis. Quantitative PCR (qPCR) on additional donor menisci (tissues and cells) were used to validate array data. Avas cells cultured in 3D were subjected to qPCR to compare with the array-generated data. A total of 387 genes were DE based on differentiation state (>3-fold change; p < 0.01). In Avas-cultured cells, the upregulated pathways included focal adhesion, ECM-receptor interaction, regulation of actin cytoskeleton, and PDGF Signaling. In 3D-cultured Avas cells, TGFβ1 or combinations of TGFβ1 and BMP6 were most effective to promote an Avas tissue phenotype. THBS2 and THBS4 expression levels were identified as a means to denote meniscus cell phenotype status. We identified the key gene expression profiles, new markers and pathways involved in characterizing the Avas meniscus phenotype in the native state and during in vitro dedifferentiation and redifferentiation. These data served to screen 3D conditions to generate meniscus-like neotissues. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1947-1958, 2018.

Plasticity of Human Meniscus Fibrochondrocytes: A Study on Effects of Mitotic Divisions and Oxygen Tension

Meniscus fibrochondrocytes (MFCs) may be the optimal cell source to repair non-healing meniscus injuries using tissue engineering strategies. In this study, we investigated the effects of mitotic divisions and oxygen tension on the plasticity of adult human MFCs. Our assessment techniques included gene expression, biochemical, histological, and immunofluorescence assays. MFCs were expanded in monolayer culture with combined growth factors TGFβ1 and FGF-2 (T1F2) under normoxia (21% O₂). Trilineage (adipogenesis, chondrogenesis and osteogenesis) differentiation was performed under both normoxic (21% O₂) and hypoxic (3% O₂) conditions. The data demonstrated that MFCs with a mean total population doubling of 10 can undergo adipogenesis and chondrogenesis. This capability was enhanced under hypoxic conditions. The MFCs did not undergo osteogenesis. In conclusion, our findings suggest that extensively expanded human MFCs have the capacity to generate tissues with the functional matrix characteristics of avascular meniscus. To this end, expanded MFCs may be an ideal cell source for engineering functional constructs for the replacement or repair of avascular meniscus.

A review of in-vitro fibrocartilage tissue engineered therapies with a focus on the temporomandibular joint

The inability of fibrocartilage, specifically the temporomandibular joint (TMJ) disc, to regenerate and remodel following injury presents a unique problem for clinicians. Tissue engineering then offers a potential regenerative therapy. In vitro testing provides a valuable screening tool for potential tissue engineered solutions. The conclusions drawn for TMJ in vitro research were compared against state of the art fibrocartilage studies in the knee meniscus, and annulus fibrosus of the intervertebral disc (IVD). For TMJ disc regeneration, in vitro tissue engineered approaches, focused on cellular therapies with fibrochondrocytes, have displayed an inability to produce enough collagen, as well as an inability to recapitulate native mechanical properties. Biomaterial approaches have recapitulated the native properties of the TMJ disc, but their in vivo efficacy has yet to be determined. By comparison, the knee meniscus field is the most progressive in the use of stem cells as a cell source. The knee meniscus field has moved away from measuring mechanical properties, and are instead more focused on biochemistry and gene expression. IVD studies mainly use electrospun scaffolds, and have produced the best success in mechanical properties. The TMJ field, in comparison to knee meniscus and IVD, needs to employ stem cell therapies, new biomaterials and manufacturing techniques, and cutting edge molecular assays, in future in vitro approaches to screen for viable technologies to move to in vivo studies.

Contrast-enhanced CT imaging as a non-destructive tool for ex vivo examination of the biochemical content and structure of the human meniscus

The biochemical and histopathological techniques used to investigate meniscal content and structure are destructive and time-consuming. Therefore, this study evaluated whether contrast-enhanced computed tomography (CECT) attenuation and contrast agent flux using the iodinated contrast agents CA4+ and ioxaglate correlate with the glycosaminoglycan (GAG) content/distribution and water content in human menisci. The optimal ioxaglate and CA4+ contrast agent concentrations for mapping meniscal GAG distribution were qualitatively determined by comparison of CECT color maps with Safranin-O stained histological sections. The associations between CECT attenuation and GAG content, CECT attenuation and water content, and flux and water content at various time points were determined using both contrast agents. Depth-wise analyses were also performed through each of the native surfaces to examine differences in contrast agent diffusion kinetics and equilibrium partitioning. The optimal concentrations for GAG depiction for ioxaglate and CA4+ were ≥80 and 12 mgI/ml, respectively. Using these concentrations, weak to moderate associations were found between ioxaglate attenuation and GAG content at all diffusion time points (1-48 h), while strong and significant associations were observed between CA4+ attenuation and GAG content as early as 7 h (R²  ≥ 0.67), being strongest at the equilibrium time point (48 h, R²  = 0.81). CECT attenuation for both agents did not significantly correlate with water content, but CA4+ flux correlated with water content (R²  = 0.56-0.64). CECT is a promising, non-destructive imaging technique for ex vivo assessment of meniscal GAG concentration and water content compared to traditional biochemical and histopathological methods. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1018-1028, 2017.

Riboflavin-induced photo-crosslinking of collagen hydrogel and its application in meniscus tissue engineering

A meniscus tear is a common knee injury, but its regeneration remains a clinical challenge. Recently, collagen-based scaffolds have been applied in meniscus tissue engineering. Despite its prevalence, application of natural collagen scaffold in clinical setting is limited due to its extremely low stiffness and rapid degradation. The purpose of the present study was to increase the mechanical properties and delay degradation rate of a collagen-based scaffold by photo-crosslinking using riboflavin (RF) and UV exposure. RF is a biocompatible vitamin B2 that showed minimal cytotoxicity compared to conventionally utilized photo-initiator. Furthermore, collagen photo-crosslinking with RF improved mechanical properties and delayed enzyme-triggered degradation of collagen scaffolds. RF-induced photo-crosslinked collagen scaffolds encapsulated with fibrochondrocytes resulted in reduced scaffold contraction and enhanced gene expression levels for the collagen II and aggrecan. Additionally, hyaluronic acid (HA) incorporation into photo-crosslinked collagen scaffold showed an increase in its retention. Based on these results, we demonstrate that photo-crosslinked collagen-HA hydrogels can be potentially applied in the scaffold-based meniscus tissue engineering.

Redifferentiation of High-Throughput Generated Fibrochondrocyte Micro-Aggregates: Impact of Low Oxygen Tension

In meniscus tissue engineering strategies, enhancing the matrix quality of the neomeniscal tissue is important. When the differentiated phenotype of fibrochondrocytes is lost, the quality of the matrix becomes compromised. The objective of this study was to produce uniform fibrochondrocyte micro-aggregates with desirable phenotype and tissue homogeneity in large quantities using a simple and reproducible method. Furthermore, we investigated if hypoxia could enhance the matrix quality. Porcine fibrochondrocytes were expanded at 21% oxygen until passage 3 (P3) and a gene expression profile was determined. P3 fibrochondrocytes were cultivated in chondrogenic medium at 5 and 21% oxygen in high-throughput agarose chips containing 2,865 microwells 200 µm in diameter. Evaluation included live/dead staining, histological examination, immunohistochemistry, dimethylmethylene blue assay and real-time reverse transcriptase quantitative polymerase chain reaction of the micro-aggregates. Gene expression analysis showed a drastic decline in collagen II and high expression of collagen I during monolayer culture. After 4 days, uniform and stable micro-aggregates could be produced. The redifferentiation and matrix quality of the hypoxic cultured micro-aggregates were enhanced relative to the normoxic cultures. Sulfated glycosaminoglycan synthesis was significantly higher, and collagen II expression and the collagen II/collagen I ratio were significantly upregulated in the hypoxic cultures. High-throughput production of uniform microtissues holds promise for the generation of larger-scale tissue engineering constructs or optimization of redifferentiation mechanisms for clinical applications.

Hypoxic culture conditions induce increased metabolic rate and collagen gene expression in ACL-derived cells

There has been substantial effort directed toward the application of bone marrow and adipose-derived mesenchymal stromal cells (MSCs) in the regeneration of musculoskeletal tissue. Recently, resident tissue-specific stem cells have been described in a variety of mesenchymal structures including ligament, tendon, muscle, cartilage, and bone. In the current study, we systematically characterize three novel anterior cruciate ligament (ACL)-derived cell populations with the potential for ligament regeneration: ligament-forming fibroblasts (LFF: CD146(neg) , CD34(neg) CD44(pos) , CD31(neg) , CD45(neg) ), ligament perivascular cells (LPC: CD146(pos) CD34(neg) CD44(pos) , CD31(neg) , CD45(neg) ) and ligament interstitial cells (LIC: CD34(pos) CD146(neg) , CD44(pos) , CD31(neg) , CD45(neg) )-and describe their proliferative and differentiation potential, collagen gene expression and metabolism in both normoxic and hypoxic environments, and their trophic potential in vitro. All three groups of cells (LIC, LPC, and LFF) isolated from adult human ACL exhibited progenitor cell characteristics with regard to proliferation and differentiation potential in vitro. Culture in low oxygen tension enhanced the collagen I and III gene expression in LICs (by 2.8- and 3.3-fold, respectively) and LFFs (by 3- and 3.5-fold, respectively) and increased oxygen consumption rate and extracellular acidification rate in LICs (by 4- and 3.5-fold, respectively), LFFs (by 5.5- and 3-fold, respectively), LPCs (by 10- and 4.5-fold, respectively) as compared to normal oxygen concentration. In summary, this study demonstrates for the first time the presence of three novel progenitor cell populations in the adult ACL that demonstrate robust proliferative and matrix synthetic capacity; these cells may play a role in local ligament regeneration, and consequently represent a potential cell source for ligament engineering applications. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:985-994, 2016.

Role of mesenchymal stem cells in meniscal repair

Meniscus integrity is the key for joint health of the knee. Therefore, the main goal of every meniscus treatment should be the maintenance of as much meniscus tissue as possible.

Repair of meniscus tears can be achieved by meniscus suture. However, in a recently published meta-analysis, the long-term outcome of meniscus repair showed a mean failure rate of 24%.

In a preclinical trial, locally applied mesenchymal stem cells produced differentiated meniscus-like tissue in meniscus tears indicating that mesenchymal-based cells, harvested from the bone marrow, enhance meniscus healing in critical-size meniscus tears.

Symptomatic meniscus defects offer the option for meniscus transplantation with porous cell free biomaterials, when a complete meniscus rim is available. Cell-free biomaterials, which are actually in clinical application, reveal variable outcome in mid-term results from complete failure to regeneration with meniscus-like tissue.

In several preclinical studies with different critical-size defects in the meniscus, the application of mesenchymal stem cells could significantly enhance meniscus regeneration compared to empty defects or to cell-free biomaterials.

Regenerative treatment of meniscus with mesenchymal stem cells seems to be a promising approach to treat meniscal tears and defects. However it is still not clear, whether the stem cell effect is a direct action of the mesenchymal-based cells or is rather mediated by secretion of certain stimulating factors. The missing knowledge of the underlying mechanism is one of the reasons for regulatory burdens to permit these stem cell-based strategies in clinical practice. Other limitations are the necessity to expand cells prior to transplantation resulting in high treatment costs. Alternative treatment modalities, which use growth factors concentrated from peripheral blood aspirates or mononucleated cells concentrated from bone marrow aspirates, are currently in development in order to allow an attractive one-step procedure without the need for cell expansion in cultures and thus lower efforts and costs.

In summary, Tissue Engineering of meniscus with mesenchymal based cells seems to be a promising approach to treat meniscal tears and defects in order to restore native meniscus tissue. However, advances of the technology are necessary to allow clinical application of this modern regenerative therapy.