Interfacial Tissue Regeneration with Bone

PURPOSE OF REVIEW

Interfacial tissue exists throughout the body at cartilage-to-bone (osteochondral interface) and tendon-to-bone (enthesis) interfaces. Healing of interfacial tissues is a current challenge in regenerative approaches because the interface plays a critical role in stabilizing and distributing the mechanical stress between soft tissues (e.g., cartilage and tendon) and bone. The purpose of this review is to identify new directions in the field of interfacial tissue development and physiology that can guide future regenerative strategies for improving post-injury healing.

RECENT FINDINGS

Cues from interfacial tissue development may guide regeneration including biological cues such as cell phenotype and growth factor signaling; structural cues such as extracellular matrix (ECM) deposition, ECM, and cell alignment; and mechanical cues such as compression, tension, shear, and the stiffness of the cellular microenvironment. In this review, we explore new discoveries in the field of interfacial biology related to ECM remodeling, cellular metabolism, and fate. Based on emergent findings across multiple disciplines, we lay out a framework for future innovations in the design of engineered strategies for interface regeneration. Many of the key mechanisms essential for interfacial tissue development and adaptation have high potential for improving outcomes in the clinic.

Development of a method to investigate strain distribution across the cartilage-bone interface in guinea pig model of spontaneous osteoarthritis using lab-based contrast enhanced X-ray-computed tomography and digital volume correlation

OBJECTIVE

Strain changes at the cartilage-bone interface play a crucial role in osteoarthritis (OA) development. Contrast-Enhanced X-ray Computed Tomography (CECT) and Digital Volume Correlation (DVC) can measure 3D strain changes at the osteochondral interface. Using lab-based CT systems it is often difficult to visualise soft tissues such as articular cartilage without staining to enhance contrast. Contrast-Enhancing Staining Agents (CESAs), such as Phosphotungstic Acid (PTA) in 70% ethanol, can cause tissue shrinkage and alter tissue mechanics. The aims of this study were, firstly, to assess changes to the mechanical properties of osteochondral tissue after staining with a PTA/PBS solution, and secondly, to visualise articular cartilage during loading and with CECT imaging in order to compare strain across the interface in both healthy and OA joints using DVC.

DESIGN

Nanoindentation was used to assess changes to mechanical properties in articular cartilage and subchondral bone before and after staining. Hindlimbs from Dunkin-Hartley guinea pigs were stained with 1% PTA/PBS at room temperature for 6 days. Two consecutive CECT datasets were acquired for DVC error analysis. In-situ compression with a load corresponding to 2x body weight was applied, the specimen was re-imaged, and DVC was performed between the pre- and post-load tomograms.

RESULTS

Nanoindentation before and after PTA/PBS staining showed similar cartilage stiffness (p < 0.05), however, staining significantly decreased the stiffness of subchondral bone (∼9-fold; p = 0.0012). In severe OA specimens, third principal/compressive (εp3) strain was 141.7% higher and shear strain (γ) was 98.2% higher in tibial articular cartilage compared to non-OA (2 - month) specimens. A 23.1% increase in third principal stain strain and a 54.5% significant increase in the shear (γ) strain (p = 0.0027) was transferred into the mineralised regions of calcified cartilage and subchondral bone in severe OA specimens.

CONCLUSIONS

These results indicate the suitability of PTA in PBS as a contrast agent for the visualisation of cartilage during CECT imaging and allowed DVC computation of strain across the cartilage-bone interface. However, further research is needed to address the reduction in stiffness of subchondral bone after incubation in PBS.

3D bioprinting of tyramine modified hydrogels under visible light for osteochondral interface

Recent advancements in tissue engineering have demonstrated a great potential for the fabrication of three-dimensional tissue structures such as cartilage and bone. However, achieving structural integrity between different tissues and fabricating tissue interfaces are still great challenges. In this study, an in situ crosslinked hybrid, multi-material three-dimensional (3D) bioprinting approach was used for the fabrication of hydrogel structures based on an aspiration-extrusion microcapillary method. Different cell-laden hydrogels were aspirated in the same microcapillary glass and deposited in the desired geometrical and volumetric arrangement directly from a computer model. Alginate and carboxymethyl cellulose (CMC) were modified with tyramine to enhance cell bioactivity and mechanical properties of human bone marrow Mesenchymal Stem Cells (hBMSC)-laden bioinks. Hydrogels were prepared for extrusion by gelling in microcapillary glass utilizing an in situ crosslink approach with ruthenium (Ru) and sodium persulfate (SPS) photo-initiating mechanisms under visible light. The developed bioinks were then bioprinted in precise gradient composition for cartilage-bone tissue interface using microcapillary bioprinting technique. The biofabricated constructs were co-cultured in chondrogenic/osteogenic culture media for three weeks. After cell viability and morphology evaluations of the bioprinted structures, biochemical and histological analyses, and a gene expression analysis for the bioprinted structure were carried out. Analysis of cartilage and bone formation based on cell alignment and histological evaluation indicated that mechanical cues in conjunction with chemical cues successfully induced MSC differentiation into chondrogenic and osteogenic tissues with a controlled interface.

Progress in Osteochondral Regeneration with Engineering Strategies

Osteoarthritis, the main cause of disability worldwide, involves not only cartilage injury but also subchondral bone injury, which brings challenges to clinical repair. Tissue engineering strategies provide a promising solution to this degenerative disease. Articular cartilage connects to subchondral bone through the osteochondral interfacial tissue, which has a complex anatomical architecture, distinct cell distribution and unique biomechanical properties. Forming a continuous and stable osteochondral interface between cartilage tissue and subchondral bone is challenging. Thus, successful osteochondral regeneration with engineering strategies requires intricately coordinated interplay between cells, materials, biological factors, and physical/chemical factors. This review provides an overview of the anatomical composition, microstructure, and biomechanical properties of the osteochondral interface. Additionally, the latest research on the progress related to osteochondral regeneration is reviewed, especially discussing the fabrication of biomimetic scaffolds and the regulation of biological factors for osteochondral defects.

A technique for preparing undecalcified osteochondral fresh frozen sections for elemental mapping and understanding disease etiology

The anatomy of the osteochondral junction is complex because several tissue components exist as a unit, including uncalcified cartilage (with superficial, middle, and deep layers), calcified cartilage, and subchondral bone. Furthermore, it is difficult to study because this region is made up of a variety of cell types and extracellular matrix compositions. Using X-ray fluorescence microscopy, we present a protocol for simultaneous elemental detection on fresh frozen samples. We transferred the osteochondral sample using a tape-assisted system and successfully tested it in synchrotron X-ray fluorescence. This protocol elucidates the distinct distribution of elements at the human knee’s osteochondral junction, making it a useful tool for analyzing the co-distribution of various elements in both healthy and diseased states.

Bio-inspired zonal-structured matrices for bone-cartilage interface engineering

Design and development of scaffold structures for osteochondral (OC) interface regeneration is a significant engineering challenge. Recent efforts are aimed at recapitulating the unique compositional and hierarchical structure of an OC interface. Conventional scaffold fabrication techniques often have limited design control and reproducibility, and the development of OC scaffolds with zonal hierarchy and structural integrity between zones is especially challenging. In this study, a series of multi-zonal and gradient structures were designed and fabricated using three-dimensional (3D) bioprinting. We developed OC scaffolds with bi-phasic and tri-phasic configurations to support the zonal structure of OC tissue, and gradient scaffold configurations to enable smooth transitions between the zones to more closely mimic a bone-cartilage interface. A biodegradable polymer, polylactic acid (PLA), was used for the fabrication of zonal/gradient scaffolds to provide mechanical strength and support OC function. The formation of the multi-zonal and gradient scaffolds was confirmed through SEM imaging and micro-CT scanning. Precisely controlled hierarchy with tunable porosity along the scaffold length established the formation of the bio-inspired scaffolds with different zones/gradient structure. In addition, we also developed a novel bioprinting method to selectively introduce cells into desired scaffold zones of the zonal/gradient scaffolds via concurrent printing of a cell-laden hydrogel within the porous template. Live/dead staining of the cell-laden hydrogel introduced in the cartilage zone showed uniform cell distribution with high cell viability. Overall, our study developed bio-inspired scaffold structures with structural hierarchy and mechanical integrity for bone-cartilage interface engineering.

Regulation of chondrocyte hypertrophy in an osteochondral interface mimicking gel matrix

Calcified cartilage extracellular matrix (ECM) is a critical interface at the osteochondral junction which plays an important role in maintaining the structural continuity between articular cartilage and subchondral bone. This mineralized network is primarily composed of glycosaminoglycans (GAGs) and collagen type II (col II) and hosts hypertrophic chondrocytes. This work aimed to investigate the effect of gel composition and collagen II content on the behavior and hypertrophic differentiation of ATDC5 cells for regeneration of calcified cartilage tissue. For this purpose, chitosan/collagen type II/nanohydroxyapatite (chi/col II/nHA) composite hydrogels were prepared to mimic the calcified cartilage ECM. ATDC5 cells were encapsulated within the composite gels and the viability, ECM production and hypertrophic gene expression were assessed during culture. All composites were favorable for ATDC5 viability and proliferation, whereas specific ECM production and hypertrophic differentiation were dependent on gel composition. Chitosan: collagen II ratio had an impact on ATDC5 cell fate. Hypertrophic differentiation was best pronounced in chi/col II/nHA 70:30 composition. The results obtained from this study offers a scaffold-based approach for calcified cartilage regeneration and provide an insight for biomimetic design and preparation of more complicated gradient osteochondral units.

Icariin conjugated hyaluronic acid/collagen hydrogel for osteochondral interface restoration

Over the past decades, numerous tissue-engineered constructs have been investigated for the osteochondral repair. However, it still remains a challenge to regenerate the functionalized calcified layer. In this study, the potential of icariin (Ica) conjugated hyaluronic acid/collagen (Ica-HA/Col) hydrogel to promote the osteochondral interface restoration was investigated. Compared with HA/Col hydrogel, Ica-HA/Col hydrogel simultaneously facilitated chondrogenesis and osteogenesis in vitro. The cells encapsulated in Ica-HA/Col hydrogel tended to aggregate into bigger clusters. The chondrogenic genes' expression level was remarkably up-regulated, and the matrix synthesis of sGAG and type II collagen was significantly enhanced. Similarly, the osteogenic genes, including RUNX2, ALP, and OCN were also up-regulated at early stage. Consequently, more calcium deposition was observed in the Ica-HA/Col hydrogel construct. Moreover, the gene expression and matrix synthesis of type X collagen, an important marker for the formation of calcified layer; were significantly higher in the Ica-HA/Col hydrogel. Furthermore, the in vivo study showed that Ica-HA/Col constructs facilitated the reconstruction of osteochondral interface in rabbit subchondral defects. In the Ica-HA/Col group, the neo-cartilage layer contained more type II collagen and the newly formed subchondral bone deposited more abundant type I collagen. Overall, the results indicated that Ica-HA/Col hydrogel might be a promising scaffold to reconstruct an osteochondral interface, therefore promoting restoring of osteochondral defect.

STATEMENT OF SIGNIFICANCE

The osteochondral defect restoration not only involves the repair of damaged cartilage and the subchondral bone, but also the reconstruction of osteochondral interface (the functional calcified layer). The calcified layer regeneration is essential for integrative and functional osteochondral repair. Over the past decade, numerous tissue engineered constructs have been investigated for the osteochondral repair. However, it still remains a challenge to regenerate a functionalized calcified layer. The present study demonstrates that Ica-HA/Col hydrogel facilitates deposition of matrix related to calcified layer in mixed chondrogenic/osteogenic inductive media and restoration of osteochondral defect in vivo. Since, Ica-HA/Col hydrogel as is cheaper, easier and more efficient, it might be a desired scaffold for the osteochondral defects restoration.

Micro/Nano Scaffolds for Osteochondral Tissue Engineering

To develop an osteochondral tissue regeneration strategy it is extremely important to take into account the multiscale organization of the natural extracellular matrix. The structure and gradients of organic and inorganic components present in the cartilage and bone tissues must be considered together. Another critical aspect is an efficient interface between both tissues. So far, most of the approaches were focused on the development of multilayer or stratified scaffolds which resemble the structural composition of bone and cartilage, not considering in detail a transitional interface layer. Typically, those scaffolds have been produced by the combined use of two or more processing techniques (microtechnologies and nanotechnologies) and materials (organic and inorganic). A significant number of works was focused on either cartilage or bone, but there is a growing interest in the development of the osteochondral interface and in tissue engineering models of composite constructs that can mimic the cartilage/bone tissues. The few works that give attention to the interface between cartilage and bone, as well as to the biochemical gradients observed at the osteochondral unit, are also herein described.

Solute transport at the interface of cartilage and subchondral bone plate: Effect of micro-architecture

Cross-talk of subchondral bone and articular cartilage could be an important aspect in the etiology of osteoarthritis. Previous research has provided some evidence of transport of small molecules (~370Da) through the calcified cartilage and subchondral bone plate in murine osteoarthritis models. The current study, for the first time, uses a neutral diffusing computed tomography (CT) contrast agent (iodixanol, ~1550Da) to study the permeability of the osteochondral interface in equine and human samples. Sequential CT monitoring of diffusion after injecting a finite amount of contrast agent solution onto the cartilage surface using a micro-CT showed penetration of the contrast molecules across the cartilage-bone interface. Moreover, diffusion through the cartilage-bone interface was affected by thickness and porosity of the subchondral bone as well as the cartilage thickness in both human and equine samples. Our results revealed that porosity of the subchondral plate contributed more strongly to the diffusion across osteochondral interface compared to other morphological parameters in healthy equine samples. However, thickness of the subchondral plate contributed more strongly to the diffusion in slightly osteoarthritic human samples.