Shape of chondrocytes within articular cartilage affects the solid but not the fluid microenvironment under unconfined compression

Application of Alginate Hydrogels for Next-Generation Articular Cartilage Regeneration

The articular cartilage has insufficient intrinsic healing abilities, and articular cartilage injuries often progress to osteoarthritis. Alginate-based scaffolds are attractive biomaterials for cartilage repair and regeneration, allowing for the delivery of cells and therapeutic drugs and gene sequences. In light of the heterogeneity of findings reporting the benefits of using alginate for cartilage regeneration, a better understanding of alginate-based systems is needed in order to improve the approaches aiming to enhance cartilage regeneration with this compound. This review provides an in-depth evaluation of the literature, focusing on the manipulation of alginate as a tool to support the processes involved in cartilage healing in order to demonstrate how such a material, used as a direct compound or combined with cell and gene therapy and with scaffold-guided gene transfer procedures, may assist cartilage regeneration in an optimal manner for future applications in patients.

The Protective Function of Directed Asymmetry in the Pericellular Matrix Enveloping Chondrocytes

The specialized pericellular matrix (PCM) surrounding chondrocytes within articular cartilage is critical to the tissue's health and longevity. Growing evidence suggests that PCM alterations are ubiquitous across all trajectories of osteoarthritis, a crippling and prevalent joint disease. The PCM geometry is of particular interest as it influences the cellular mechanical environment. Observations of asymmetrical PCM thickness have been reported, but a quantified characterization is lacking. To this end, a novel microscopy protocol was developed and applied to acquire images of the PCM surrounding live cells. Morphometric analysis indicated a statistical bias towards thicker PCM on the inferior cellular surface. The mechanical effects of this bias were investigated with multiscale modelling, which revealed potentially damaging, high tensile strains in the direction perpendicular to the membrane and localized on the inferior surface. These strains varied substantially between PCM asymmetry cases. Simulations with a thicker inferior PCM, representative of the observed geometry, resulted in strain magnitudes approximately half of those calculated for a symmetric geometry, and a third of those with a thin inferior PCM. This strain attenuation suggests that synthesis of a thicker inferior PCM may be a protective adaptation. PCM asymmetry may thus be important in cartilage development, pathology, and engineering.

Tissue Engineering of Canine Cartilage from Surgically Debrided Osteochondritis Dissecans Fragments

This study in dogs explored the feasibility of using cartilage fragments removed and discarded during routine palliative surgery for osteochondritis dissecans (OCD) as a source of primary chondrocytes for scaffold-free cartilage tissue-engineering. Primary chondrocytes were obtained from three OCD donors and one age-matched healthy articular cartilage (HAC) donor. After monolayer expansion of primary cells, a three-dimensional spherical suspension culture was implemented. Following this stage, cells were seeded at a high density into custom-made agarose molds that allowed for size and shape-specific constructs to be generated via a method of cellular self-assembling in a scaffold-free environment. Fifty-eight neocartilage constructs were tissue-engineered using this methodology. Neocartilage constructs and native cartilage from shoulder joint were subjected to histological, mechanical, and biochemical testing. OCD and HAC chondrocytes-sourced constructs had uniformly flat morphology and histology consistent with cartilage tissue. Constructs sourced from OCD chondrocytes were 1.5-times (32%) stiffer in compression and 1.3 times (23%) stronger in tension than constructs sourced from HAC chondrocytes and only 8.7-times (81%) less stiff in tension than native tissue. Constructs from both cell sources consistently had lower collagen content than native tissue (22.9%/dry weight [DW] for OCD and 4.1%/DW for HAC vs. 51.1%/DW native tissue). To improve the collagen content and mechanical properties of neocartilage, biological and mechanical stimuli, and thyroid hormone (tri-iodothyronine) were applied to the chondrocytes during the self-assembling stage in two separate studies. A 2.6-fold (62%) increase in compressive stiffness was detected with supplementation of biological stimuli alone and 5-fold (81%) increase with combined biological and mechanical stimuli at 20% strain. Application of thyroid hormone improved collagen content (1.7-times, 33%), tensile strength (1.8-times, 43%), and stiffness (1.3-times, 21%) of constructs, relative to untreated controls. Collectively, these data suggest that OCD chondrocytes can serve as a reliable cell source for cartilage tissue-engineering and that canine chondrocytes respond favorably to biological and mechanical stimuli that have been shown effective in chondrocytes from other animal species, including humans.

3D printed silk-gelatin hydrogel scaffold with different porous structure and cell seeding strategy for cartilage regeneration

Hydrogel scaffolds are attractive for tissue defect repair and reorganization because of their human tissue-like characteristics. However, most hydrogels offer limited cell growth and tissue formation ability due to their submicron- or nano-sized gel networks, which restrict the supply of oxygen, nutrients and inhibit the proliferation and differentiation of encapsulated cells. In recent years, 3D printed hydrogels have shown great potential to overcome this problem by introducing macro-pores within scaffolds. In this study, we fabricated a macroporous hydrogel scaffold through horseradish peroxidase (HRP)-mediated crosslinking of silk fibroin (SF) and tyramine-substituted gelatin (GT) by extrusion-based low-temperature 3D printing. Through physicochemical characterization, we found that this hydrogel has excellent structural stability, suitable mechanical properties, and an adjustable degradation rate, thus satisfying the requirements for cartilage reconstruction. Cell suspension and aggregate seeding methods were developed to assess the inoculation efficiency of the hydrogel. Moreover, the chondrogenic differentiation of stem cells was explored. Stem cells in the hydrogel differentiated into hyaline cartilage when the cell aggregate seeding method was used and into fibrocartilage when the cell suspension was used. Finally, the effect of the hydrogel and stem cells were investigated in a rabbit cartilage defect model. After implantation for 12 and 16 weeks, histological evaluation of the sections was performed. We found that the enzymatic cross-linked and methanol treatment SF5GT15 hydrogel combined with cell aggregates promoted articular cartilage regeneration. In summary, this 3D printed macroporous SF-GT hydrogel combined with stem cell aggregates possesses excellent potential for application in cartilage tissue repair and regeneration.

Multi-scale mechanical investigation of articular cartilage suffered progressive pseudorheumatoid dysplasia

BACKGROUND

Progressive pseudorheumatoid dysplasia is a rare skeletal dysplasia mainly caused by abnormal autosomal recessive inheritance. Although the main function of cartilage is mechanical support and the characteristics of this disease is the degradation of AC, previous studies on it had been mainly focused on clinical and genetic aspects and the mechanical behavior of the cartilage affected by PPRD is still ambiguous. In this study, we investigate the mechanics and structure of the cartilage suffered disease at multi-scale, from individual chondrocytes to the bulk-scale tissue.

METHODS

Depth-sensing indenter were employed to investigate the mechanics of cartilage; we performed atomic force microscope nanoindentation to investigate the cell mechanics and scanning electron microscopy were used to explore the structure feature and chemical composition.

FINDINGS

The elastic modulus of chondrocytes harvested from cartilage suffered from progressive pseudorheumatoid dysplasia is significantly higher than from normal cartilage, same trend were also found in tissue level. Moreover, denser collagen meshwork and matrix calcification were also observed.

INTERPRETATION

The elastic modulus of cartilage should closely related to its denser structure and the calcification, and may potentially be an indicator for clinical diagnosis. The stiffening of chondrocytes during PPRD progression should play a rather important role in its pathogenesis.

Local Strain Distribution and Increased Intracellular Ca2+ Signaling in Bovine Articular Cartilage Exposed to Compressive Strain

Articular cartilage is exposed to compressive strain of approximately 10% under physiological loads in vivo, and intracellular Ca2+ signaling is one of the earliest responses in chondrocytes under this physical stimulation. However, it remains unknown whether compressive strain itself evokes intracellular Ca2+ signaling in chondrocytes located within each layer (from surface to deep) in an equal manner with physiological levels of strain. The purpose of this study, therefore, was to determine the distribution of local strain and increased intracellular Ca2+ signaling in layer-dependent cell populations in response to 10% compressive strain loading. For this purpose, the time course of strain was measured in each layer to calculate layer-specific deformation properties. In addition, layer-specific changes in chondrocyte intracellular Ca2+ signals were recorded over time using a fluorescent Ca2+ indicator, Fluo-3, to establish ratios of cells with increased Ca2+ signaling at each depth of cartilage under static conditions or exposed to compression. The results showed that the surface layer was compressed with a larger strain compared with other layers. Few cells with Ca2+ signaling were observed under static conditions. Percentages of responsive cells within compressed cartilage were higher than those within cartilage under static conditions. However, increased intracellular Ca2+ signals were observed in a prominent number of chondrocytes within the deep layer, but not the surface layer, of compressed cartilage. Our results suggest that at a physiological compression level, Ca2+ is upregulated, but the stimulation of Ca2+ signaling in articular cartilage is not simply defined by local deformation.

Effect of interface mechanical discontinuities on scaffold-cartilage integration

A consistent lack of lateral integration between scaffolds and adjacent articular cartilage has been exhibited in vitro and in vivo. Given the mismatch in mechanical properties between scaffolds and articular cartilage, the mechanical discontinuity that occurs at the interface has been implicated as a key factor, but remains inadequately studied. Our objective was to investigate how the mechanical environment within a mechanically loaded scaffold-cartilage construct might affect integration. We hypothesized that the magnitude of the mechanical discontinuity at the scaffold-cartilage interface would be related to decreased integration. To test this hypothesis, chondrocyte seeded scaffolds were embedded into cartilage explants, pre-cultured for 14 days, and then mechanically loaded for 28 days at either 1N or 6N of applied load. Constructs were kept either peripherally confined or unconfined throughout the duration of the experiment. Stress, strain, fluid flow, and relative displacements at the cartilage-scaffold interface and within the scaffold were quantified using biphasic, inhomogeneous finite element models (bFEMs). The bFEMs indicated compressive and shear stress discontinuities occurred at the scaffold-cartilage interface for the confined and unconfined groups. The mechanical strength of the scaffold-cartilage interface and scaffold GAG content were higher in the radially confined 1N loaded groups. Multivariate regression analyses identified the strength of the interface prior to the commencement of loading and fluid flow within the scaffold as the main factors associated with scaffold-cartilage integration. Our study suggests a minimum level of scaffold-cartilage integration is needed prior to the commencement of loading, although the exact threshold has yet to be identified. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Osteoarthritis year in review 2016: mechanics

Inappropriate biomechanics, namely wear-and-tear, has been long believed to be a main cause of osteoarthritis (OA). However, this view is now being re-evaluated, especially when examined alongside mechanobiology and new biomechanical studies. These are multiscale experimental and computational studies focussing on cell- and tissue-level mechanobiology through to organ- and whole-body-level biomechanics, which focuses on the biomechanical and biochemical environment of the joint tissues. This review examined papers from April 2015 to April 2016, with a focus on multiscale experimental and computational biomechanical studies of OA. Assessing the onset or progression of OA at organ- and whole-body-levels, gait analysis, medical imaging and neuromusculoskeletal modelling revealed the extent to which tissue damage changes the view of inappropriate biomechanics. Traditional gait analyses studies reported that conservative treatments can alter joint biomechanics, thereby improving pain and function experienced by those with OA. Results of animal models of OA were consistent with these human studies, showing interactions among bone, cartilage and meniscus biomechanics and the onset and/or progression OA. Going down size scales, experimental and computational studies probed the nanosize biomechanics of molecules, cells and extracellular matrix, and demonstrated how the interactions between biomechanics and morphology affect cartilage dynamic poroelastic behaviour and pathways to OA. Finally, integration of multiscale experimental data and computational models were proposed to predict cartilage extracellular matrix remodelling and the development of OA. Summarising, experimental and computational methods provided a nuanced biomechanical understanding of the sub-cellular, cellular, tissue, organ and whole-body mechanisms involved in OA.