Physiological cartilage tissue engineering effect of oxygen and biomechanics

Mechano-Hypoxia Conditioning of Engineered Human Meniscus

Meniscus fibrochondrocytes (MFCs) experience simultaneous hypoxia and mechanical loading in the knee joint. Experimental conditions based on these aspects of the native MFC environment may have promising applications in human meniscus tissue engineering. We hypothesized that in vitro “mechano-hypoxia conditioning” with mechanical loading such as dynamic compression (DC) and cyclic hydrostatic pressure (CHP) would enhance development of human meniscus fibrocartilage extracellular matrix in vitro. MFCs from inner human meniscus surgical discards were pre-cultured on porous type I collagen scaffolds with TGF-β3 supplementation to form baseline tissues with newly formed matrix that were used in a series of experiments. First, baseline tissues were treated with DC or CHP under hypoxia (HYP, 3% O₂) for 5 days. DC was the more effective load regime in inducing gene expression changes, and combined HYP/DC enhanced gene expression of fibrocartilage precursors. The individual treatments of DC and HYP regulated thousands of genes, such as chondrogenic markers SOX5/6, in an overwhelmingly additive rather than synergistic manner. Similar baseline tissues were then treated with a short course of DC (5 vs 60 min, 10–20% vs 30–40% strain) with different pre-culture duration (3 vs 6 weeks). The longer course of loading (60 min) had diminishing returns in regulating mechano-sensitive and inflammatory genes such as c-FOS and PTGS2, suggesting that as few as 5 min of DC was adequate. There was a dose-effect in gene regulation by higher DC strains, whereas outcomes were inconsistent for different MFC donors in pre-culture durations. A final set of baseline tissues was then cultured for 3 weeks with mechano-hypoxia conditioning to assess mechanical and protein-level outcomes. There were 1.8–5.1-fold gains in the dynamic modulus relative to baseline in HYP/DC, but matrix outcomes were equal or inferior to static controls. Long-term mechano-hypoxia conditioning was effective in suppressing hypertrophic markers (e.g., COL10A1 10-fold suppression vs static/normoxia). Taken together, these results indicate that appropriately applied mechano-hypoxia conditioning can support meniscus fibrocartilage development in vitro and may be useful as a strategy for developing non-hypertrophic articular cartilage using mesenchymal stem cells.

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.

A Novel Bioreactor System Capable of Simulating the In Vivo Conditions of Synovial Joints

Any significant in vitro evaluation of cartilage tissue engineering and cartilage repair strategies has to be performed under the harsh conditions encountered in vivo within synovial joints. To this end, we have developed a novel automated physiological robot reactor system (PRRS) that is capable of recapitulating complex physiological motions and load patterns within an environment similar to that found in the human knee. The PRRS consists of a mechanical stimulation unit (MSU) and an automatic sample changer (ASC) within an environment control box in which the humidity, temperature, and gas composition are tightly regulated. The MSU has three linear (orthogonal) axes and one rotational degree of freedom (around the z-axis). The ASC provides space for up to 24 samples, which can be allocated to individual stimulation patterns. Cell-seeded scaffolds and ex vivo tissue culture systems were established to demonstrate the applicability of the PRRS to the investigation of the effect of load and environmental conditions on engineering and maintenance of articular cartilage in vitro. The bioreactor is a flexible system that has the potential to be applied for culturing connective tissues other than cartilage, such as bone and intervertebral disc tissue, even though the mechanical and environmental parameters are very different.

Chasing Chimeras - The elusive stable chondrogenic phenotype

The choice of the best-suited cell population for the regeneration of damaged or diseased cartilage depends on the effectiveness of culture conditions (e.g. media supplements, three-dimensional scaffolds, mechanical stimulation, oxygen tension, co-culture systems) to induce stable chondrogenic phenotype. Herein, advances and shortfalls in in vitro, preclinical and clinical setting of various in vitro microenvironment modulators on maintaining chondrocyte phenotype or directing stem cells towards chondrogenic lineage are critically discussed. Chondrocytes possess low isolation efficiency, limited proliferative potential and rapid phenotypic drift in culture. Mesenchymal stem cells are relatively readily available, possess high proliferation potential, exhibit great chondrogenic differentiation capacity, but they tend to acquire a hypertrophic phenotype when exposed to chondrogenic stimuli. Embryonic and induced pluripotent stem cells, despite their promising in vitro and preclinical data, are still under-investigated. Although a stable chondrogenic phenotype remains elusive, recent advances in in vitro microenvironment modulators are likely to develop clinically- and commercially-relevant therapies in the years to come.

Evaluation of biocompatibility using in vitro methods: interpretation and limitations

The in vitro biocompatibility of novel materials has to be proven before a material can be used as component of a medical device. This must be done in cell culture tests according to internationally recognized standard protocols. Subsequently, preclinical and clinical tests must be performed to verify the safety of the new material and device. The present chapter focuses on the first step, the in vitro testing according to ISO 10993-5, and critically discusses its limited significance. Alternative strategies and a brief overview of activities to improve the current in vitro tests are presented in the concluding section.

Reversibly Immortalized Mouse Articular Chondrocytes Acquire Long-Term Proliferative Capability While Retaining Chondrogenic Phenotype

Cartilage tissue engineering holds great promise for treating cartilaginous pathologies including degenerative disorders and traumatic injuries. Effective cartilage regeneration requires an optimal combination of biomaterial scaffolds, chondrogenic seed cells, and biofactors. Obtaining sufficient chondrocytes remains a major challenge due to the limited proliferative capability of primary chondrocytes. Here we investigate if reversibly immortalized mouse articular chondrocytes (iMACs) acquire long-term proliferative capability while retaining the chondrogenic phenotype. Primary mouse articular chondrocytes (MACs) can be efficiently immortalized with a retroviral vector-expressing SV40 large T antigen flanked with Cre/loxP sites. iMACs exhibit long-term proliferation in culture, although the immortalization phenotype can be reversed by Cre recombinase. iMACs express the chondrocyte markers Col2a1 and aggrecan and produce chondroid matrix in micromass culture. iMACs form subcutaneous cartilaginous masses in athymic mice. Histologic analysis and chondroid matrix staining demonstrate that iMACs can survive, proliferate, and produce chondroid matrix. The chondrogenic growth factor BMP2 promotes iMACs to produce more mature chondroid matrix resembling mature articular cartilage. Taken together, our results demonstrate that iMACs acquire long-term proliferative capability without losing the intrinsic chondrogenic features of MACs. Thus, iMACs provide a valuable cellular platform to optimize biomaterial scaffolds for cartilage regeneration, to identify biofactors that promote the proliferation and differentiation of chondrogenic progenitors, and to elucidate the molecular mechanisms underlying chondrogenesis.

Diversity of intervertebral disc cells: phenotype and function

The intervertebral disc (IVD) is a moderately moving joint that is located between the bony vertebrae and provides flexibility and load transmission throughout the spinal column. The disc is composed of different but interrelated tissues, including the central highly hydrated nucleus pulposus (NP), the surrounding elastic and fibrous annulus fibrosus (AF), and the cartilaginous endplate (CEP), which provides the connection to the vertebral bodies. Each of these tissues has a different function and consists of a specific matrix structure that is maintained by a cell population with distinct phenotype. Although the healthy IVD is able to balance the slow matrix turnover of synthesis and degradation, this balance is often disturbed, leading to degenerative disorders. Successful therapeutic management of IVD degeneration requires a profound understanding of the cellular and molecular characteristics of the functional IVD. Hence, the phenotype of IVD cells has been of significant interest from multiple perspectives, including development, growth, remodelling, degeneration and repair. One major challenge that complicates our understanding of the disc cells is that both the cellular phenotype and the extracellular matrix strongly depend on disc maturity and health and as a consequence are continuously evolving. This review delineates the diversity of the cell types found in the intervertebral disc, with emphasis on human, but with reference to other species. The cells of the NP appear rounded and express a proteoglycan-rich matrix, whereas the more elongated AF cells are embedded in a collagen fibre matrix and the CEPs represent a layer of cartilage. Even though all disc cells have often been referred to as 'intervertebral disc chondrocytes', distinct phenotypical differences in comparison with articular chondrocytes exist and have been reported recently. The availability of more specific markers has also improved our understanding of progenitor cell differentiation towards an IVD cell phenotype. Ultimately, new cell- and tissue-engineering approaches to regenerative therapies will only be successful if the specific characteristics of the individual tissues and their context in the function of the whole organ, are taken into consideration.

Chitosan, hyaluronan and chondroitin sulfate in tissue engineering for cartilage regeneration: a review

Injection of hyaluronan into osteoarthritic joints restores the viscoelasticity, augments the flow of joint fluid, normalizes endogenous hyaluronan synthesis, and improves joint function. Chitosan easily forms polyelectrolyte complexes with hyaluronan and chondroitin sulfate. Synergy of chitosan with hyaluronan develops enhanced performances in regenerating hyaline cartilage, typical results being structural integrity of the hyaline-like neocartilage, and reconstitution of the subchondral bone, with positive cartilage staining for collagen-II and GAG in the treated sites. Chitosan qualifies for the preparation of scaffolds intended for the regeneration of cartilage: it yields mesoporous cryogels; it provides a friendly environment for chondrocytes to propagate, produce typical ECM, and assume the convenient phenotype; it is a good carrier for growth factors; it inactivates metalloproteinases thus preventing collagen degradation; it is suitable for the induction of the chondrogenic differentiation of mesenchymal stem cells; it is a potent means for hemostasis and platelet delivery.