Mechanical Stress Inhibits Early Stages of Endogenous Cell Migration: A Pilot Study in an Ex Vivo Osteochondral Model

A rabbit osteochondral defect (OCD) model for evaluation of tissue engineered implants on their biosafety and efficacy in osteochondral repair

Osteochondral defect (OCD) is a common but challenging condition in orthopaedics that imposes huge socioeconomic burdens in our aging society. It is imperative to accelerate the R&D of regenerative scaffolds using osteochondral tissue engineering concepts. Yet, all innovative implant-based treatments require animal testing models to verify their feasibility, biosafety, and efficacy before proceeding to human trials. Rabbit models offer a more clinically relevant platform for studying OCD repair than smaller rodents, while being more cost-effective than large animal models. The core-decompression drilling technique to produce full-thickness distal medial femoral condyle defects in rabbits can mimic one of the trauma-relevant OCD models. This model is commonly used to evaluate the implant's biosafety and efficacy of osteochondral dual-lineage regeneration. In this article, we initially indicate the methodology and describe a minimally-invasive surgical protocol in a step-wise manner to generate a standard and reproducible rabbit OCD for scaffold implantation. Besides, we provide a detailed procedure for sample collection, processing, and evaluation by a series of subsequent standardized biochemical, radiological, biomechanical, and histological assessments. In conclusion, the well-established, easy-handling, reproducible, and reliable rabbit OCD model will play a pivotal role in translational research of osteochondral tissue engineering.

Bridging bench to body: ex vivo models to understand articular cartilage repair

With little to no ability to self-regenerate, human cartilage defects of the knee remain a major clinical challenge. Tissue engineering strategies include delivering specific types of cells and biomaterials to the injured cartilage for restoration of architecture and function. Pre-clinical models to test the efficacy of the therapies come with high costs and ethical issues, and imperfect prediction of performance in humans. Ex vivo models represent an alternative avenue to trial cartilage tissue engineering. Defined as viable explanted cartilage samples, ex vivo models can be cultured with a cell-laden biomaterial or tissue-engineered construct to evaluate cartilage repair. Though human and animal ex vivo models are currently used in the field, there is a need for alternative methods to assess the strength of integration, to increase throughput and manage variability and to optimise and standardise culture conditions, enhancing the utility of these models overall.

Osteochondral Explant Isolation and Culture Under a Compression and Shear Bioreactor

Osteochondral explants harvested from different species are valuable preclinical ex vivo models for tissue engineering research. In this chapter, we describe the isolation of osteochondral plugs from bovine stifle joints, followed by defect creation, and plug preparation in a straightforward manner before mechanical loading using a compression and shear bioreactor. The method can be adapted to isolate osteochondral plugs from any animal species and to load explants in any type of bioreactor.

Modeling early changes associated with cartilage trauma using human-cell-laden hydrogel cartilage models

BACKGROUND

Traumatic impacts to the articular joint surface are known to lead to cartilage degeneration, as in post-traumatic osteoarthritis (PTOA). Limited progress in the development of disease-modifying OA drugs (DMOADs) may be due to insufficient mechanistic understanding of human disease onset/progression and insufficient in vitro models for disease and therapeutic modeling. In this study, biomimetic hydrogels laden with adult human mesenchymal stromal cells (MSC) are used to examine the effects of traumatic impacts as a model of PTOA. We hypothesize that MSC-based, engineered cartilage models will respond to traumatic impacts in a manner congruent with early PTOA pathogenesis observed in animal models.

METHODS

Engineered cartilage constructs were fabricated by encapsulating adult human bone marrow-derived mesenchymal stem cells in a photocross-linkable, biomimetic hydrogel of 15% methacrylated gelatin and promoting chondrogenic differentiation for 28 days in a defined medium and TGF-β3. Constructs were subjected to traumatic impacts with different strains or 10 ng/ml IL-1β, as a common comparative method of modeling OA. Cell viability and metabolism, elastic modulus, gene expression, matrix protein production and activation of catabolic enzymes were assessed.

RESULTS

Cell viability staining showed that traumatic impacts of 30% strain caused an appropriate level of cell death in engineered cartilage constructs. Gene expression and histo/immunohistochemical analyses revealed an acute decrease in anabolic activities, such as COL2 and ACAN expression, and a rapid increase in catabolic enzyme expression, e.g., MMP13, and inflammatory modulators, e.g., COX2. Safranin O staining and GAG assays together revealed a transient decrease in matrix production 24 h after trauma that recovered within 7 days. The decrease in elastic modulus of engineered cartilage constructs was coincident with GAG loss and mediated by the encapsulated cells. The acute and transient changes observed after traumatic impacts contrasted with progressive changes observed using continual IL-1β treatment.

CONCLUSIONS

Traumatic impacts delivered to engineered cartilage constructs induced PTOA-like changes in the encapsulated cells. While IL-1b may be appropriate in modeling OA pathogenesis, the results of this study indicate it may not be appropriate in understanding the etiology of PTOA. The development of a more physiological in vitro PTOA model may contribute to the more rapid development of DMOADs.

Ex Vivo Systems to Study Chondrogenic Differentiation and Cartilage Integration

Articular cartilage injury and repair is an issue of growing importance. Although common, defects of articular cartilage present a unique clinical challenge due to its poor self-healing capacity, which is largely due to its avascular nature. There is a critical need to better study and understand cellular healing mechanisms to achieve more effective therapies for cartilage regeneration. This article aims to describe the key features of cartilage which is being modelled using tissue engineered cartilage constructs and ex vivo systems. These models have been used to investigate chondrogenic differentiation and to study the mechanisms of cartilage integration into the surrounding tissue. The review highlights the key regeneration principles of articular cartilage repair in healthy and diseased joints. Using co-culture models and novel bioreactor designs, the basis of regeneration is aligned with recent efforts for optimal therapeutic interventions.