Co-aligned chondrocytes: Zonal morphological variation and structured arrangement of cell lacunae in tessellated cartilage

Skeletal calcification patterns of batoid, teleost, and mammalian models: Calcified cartilage versus bone matrix

This study compares the skeletal calcification pattern of batoid Raja asterias with the endochondral ossification model of mammalians Homo sapiens and teleost Xiphias gladius. Skeletal mineralization serves to stiffen the mobile elements for locomotion. Histology, histochemistry, heat deproteination, scanning electron microscopy (SEM)/EDAX analysis, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier transform infrared spectrometry (FTIR) have been applied in the study. H. sapiens and X. gladius bone specimens showed similar profiles, R. asterias calcified cartilage diverges for higher water release and more amorphous bioapatite. In endochondral ossification, fetal calcified cartilage is progressively replaced by bone matrix, while R. asterias calcified cartilage remains un-remodeled throughout the life span. Ca2+ and PO4 3- concentration in extracellular matrix is suggested to reach the critical salts precipitation point through H2 O recall from extracellular matrix into both chondroblasts or osteoblasts. Cartilage organic phase layout and incomplete mineralization allow interstitial fluids diffusion, chondrocytes survival, and growth in a calcified tissue lacking of a vascular and canalicular system. HIGHLIGHTS: Comparative physico-chemical characterization (TGA, DTG and DSC) testifies the mass loss due to water release, collagen and carbonate decomposition of the three tested matrices. R. asterias calcified cartilage water content is higher than that of H. sapiens and X. gladius, as shown by the respectively highest dehydration enthalpy values. Lower crystallinity degree of R. asterias calcified cartilage can be related to the higher amount of collagen in amorphous form than in bone matrix. These data can be discussed in terms of the mechanostat theory (Frost, 1966) or by organic/inorganic phase transformation in the course evolution from fin to limbs. Mineral analysis documented different charactersof R. asterias vs H. sapiens and X. gladius calcified matrix.

Microenvironmental Stiffness Directs Chondrogenic Lineages of Stem Cells from the Human Apical Papilla via Cooperation between ROCK and Smad3 Signaling

Cell-based cartilage tissue engineering faces a great challenge in the repair process, partly due to the special physical microenvironment. Human stem cell from apical papilla (hSCAP) shows great potential as seed cells because of its versatile differentiation capacity. However, whether hSCAP has potent chondrogenic differentiation ability in the physical microenvironment of chondroid remains unknown. In this study, we fabricated poly(dimethylsiloxane) (PDMS) substrates with different stiffnesses and investigated the chondrogenic differentiation potential of hSCAPs. First, we found that hSCAPs cultured on soft substrates spread more narrowly accompanied by cortical actin organization, a hallmark of differentiated chondrocytes. On the contrary, stiff substrates were favorable for cell spreading and stress fiber formation. More importantly, the increased chondrogenic differentiation of hSCAPs seeded on soft substrates was confirmed by characterizing increased extracellular proteoglycan aggregation through Alcian blue staining and Safranin O staining and enhanced markers toward chondrogenic differentiation including SRY-box transcription factor 9 (Sox9), type II collagen (Col2), and aggrecan in both normal α-minimum essential medium (αMEM) and specific chondrogenic medium (CM) culture conditions. Then, we investigated the mechanosensing/mechanotransduction governing the chondrogenic differentiation of hSCAPs in response to different stiffnesses and found that stiffness-sensitive integrin β1 and focal adhesion kinase (FAK) were essential for mechanical signal perception and were oriented at the start of mechanotransduction induced by matrix stiffness. We next showed that the increased nuclear accumulation of Smad3 signaling and target Sox9 facilitated the chondrogenic differentiation of hSCAPs on the soft substrates and further verified the importance of Rho-associated protein kinase (ROCK) signaling in regulating chondrogenic differentiation and its driving factors, Smad3 and Sox9. By using SIS3, the specific inhibitor of p-Smad3, and miRNA targeting Rho-associated protein kinase 1 (ROCK-1), we finally confirmed the importance of ROCK/Smad3/Sox9 axis in the chondrogenic differentiation of hSCAPs in response to substrate stiffness. These results help us to increase the understanding of how microenvironmental stiffness directs chondrogenic differentiation from the aspects of mechanosensing, mechanotransduction, and cell fate decision, which will be of great value in the application of hSCAPs in cartilage tissue engineering.

Three-dimensional mapping of mineral in intact shark centra with energy dispersive x-ray diffraction

The centra of shark vertebrae consist of cartilage mineralized by a bioapatite similar to bone's carbonated hydroxyapatite, and, without a repair mechanism analogous to remodeling in bone, these structures still survive millions of cycles of high-strain loading. The main structures of the centrum are an hourglass-shaped double cone and the intermedialia which supports the cones. Little is known about the nanostructure of shark centra, specifically the relationship between bioapatite and cartilage fibers, and this study uses energy dispersive diffraction (EDD) with polychromatic synchrotron x-radiation to study the spatial organization of the mineral phase and its crystallographic texture. The unique energy-sensitive detector array at beamline 6-BM-B, the Advanced Photon Source, enables EDD to quantify the texture within each sampling volume with one exposure while constructing 3D maps via specimen translation across the sampling volume. This study maps a centrum from two shark orders, a carcharhiniform and a lamniform, with different intermedialia structures. In the blue shark (Prionace glauca, Carcharhiniformes), the bioapatite's c-axes are oriented laterally within the centrum's cone walls but axially within the wide wedges of the intermedialia; the former is interpreted to resist lateral deformation, the latter to support axial loads. In the shortfin mako (Isurus oxyrinchus, Lamniformes), there is some tendency for c-axis variation with position, but the situation is unclear because one dimension of the sampling volume is considerably larger than the thickness and spacing of the intermedialia's radially-oriented lamellae. Because elastic modulus in collagen plus bioapatite mineralized tissues varies significantly with both volume fraction of bioapatite and crystallographic texture, the present 3D EDD-derived maps should inform future 3D numerical models of shark centra under applied load.

Bioapatite in shark centra studied by wide-angle and by small-angle X-ray scattering

Members of subclass Elasmobranchii possess cartilage skeletons; the centra of many species are mineralized with a bioapatite, but virtually nothing is known about the mineral's organization. This study employed high-energy, small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS, i.e. X-ray diffraction) to investigate the bioapatite crystallography within blocks cut from centra of four species (two carcharhiniform families, one lamniform family and 1-ID of the Advanced Photon Source). All species' crystallographic quantities closely matched and indicated a bioapatite closely related to that in bone. The centra's lattice parameters a and c were somewhat smaller and somewhat larger, respectively, than in bone. Nanocrystallite sizes (WAXS peak widths) in shark centra were larger than typical of bone, and little microstrain was observed. Compared with bone, shark centra exhibited SAXS D-period peaks with larger D magnitudes, and D-period arcs with narrower azimuthal widths. The shark mineral phase, therefore, is closely related to that in bone but does possess real differences which probably affect mechanical property and which are worth further study.

Mechanical stress reduces secreted frizzled-related protein expression and promotes temporomandibular joint osteoarthritis via Wnt/β-catenin signaling

AIMS

Mechanical stress overload in the temporomandibular joint (TMJ) is an important cause of TMJ osteoarthritis (TMJOA). Whether secreted frizzled-related proteins (SFRPs) play important roles in the development of mechanical stress-induced TMJOA remains controversial. In this study, we investigated the roles of the Wnt/β-catenin signaling and SFRPs in the progression of mechanical stress-induced TMJOA.

METHODS

We investigated the progression of mechanical stress-induced TMJOA using an in vivo model via modified increased occlusal vertical dimension (iOVD) malocclusion and an in vitro model in which isolated chondrocytes were subjected to mechanical stress. The effects of inhibition of Wnt/β-catenin signal on TMJOA induced by mechanical stress were studied by in vitro drug added and in vivo intra-articular injection of XAV-939. TMJOA progression, Wnt/β-catenin signaling and SFRPs was assessed by Cone beam computed tomography (CBCT) analysis, histochemical and immunohistochemical (IHC) staining, quantitative real-time PCR (qRT-PCR), Western blotting (WB), and immunofluorescence (IF) staining.

RESULTS

Our in vivo results showed that iOVD-induced mechanical stress in the TMJ disrupted mandible growth, induced OA-like changes in TMJ cartilage, and increased OA-related cytokine expression. In addition, iOVD activated Wnt/β-catenin signaling and suppressed Sfrp1, Sfrp3, and Sfrp4 expression in condylar cartilage. Moreover, our in vitro study showed that stress disrupted homeostasis, activated Wnt/β-catenin signaling and inhibited SFRP3 and SFRP4 expression in chondrocytes. Suppression of Wnt/β-catenin signaling with XAV-939 promoted SFRP3 and SFRP4 expression and rescued mechanical stress-induced cartilage degeneration in vivo and in vitro.

CONCLUSIONS

Our work suggests that mechanical stress reduces SFRPs expression both in vivo and in vitro and promotes TMJOA via Wnt/β-catenin signaling. Suppression of Wnt/β-catenin signaling promotes SFRPs expression, especially SFRP3 and SFRP4 expression, and rescues mechanical stress-induced cartilage degeneration. Wnt/β-catenin signaling and SFRPs may represent potential therapeutic targets for TMJOA.

Shark centra microanatomy and mineral density variation studied with laboratory microComputed Tomography

Centra of shark vertebrae from three species of Lamniformes (Alopias vulpinus, Carcharodon carcharias and Isurus oxyrinchus) and three species of Carcharhiniformes (Carcharhinus plumbeus, Carcharhinus obscurus and Prionace glauca) were imaged with laboratory microcomputed Tomography (microCT) using volume element (voxel) sizes between 16 and 24 µm. Linear attenuation coefficients were the same in the corpus calcarea (hour-glass-shaped cone) and intermedialia of the lamniforms but were smaller in the intermedialia than in the corpus calcarea of the carcharhiniforms. All centra contained growth bands which were visible as small changes in linear attenuation coefficient. In all six cases, the cross-sections of the cones were close to circular, and the cone angles matched those reported in the literature. Cartilage canals were a prominent structure in the intermedialia of all species, 3D renderings of centra of C. obscurus and I. oxyrinchus diameters showed these canals ran radially outward from the cone walls, and canal diameters were consistent with the limited numerical values in the literature. Somewhat higher calcification levels around the periphery of cartilage canals and of outer surfaces of the intermedialia and corpus calcerea suggest microstructural variation exists at scale below that which can be resolved in the present data sets.

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.

Local anisotropy in mineralized fibrocartilage and subchondral bone beneath the tendon-bone interface

The enthesis allows the insertion of tendon into bone thanks to several remarkable strategies. This complex and clinically relevant location often features a thin layer of fibrocartilage sandwiched between tendon and bone to cope with a highly heterogeneous mechanical environment. The main purpose of this study was to investigate whether mineralized fibrocartilage and bone close to the enthesis show distinctive three-dimensional microstructural features, possibly to enable load transfer from tendon to bone. As a model, the Achilles tendon-calcaneus bone system of adult rats was investigated with histology, backscattered electron imaging and micro-computed tomography. The microstructural porosity of bone and mineralized fibrocartilage in different locations including enthesis fibrocartilage, periosteal fibrocartilage and bone away from the enthesis was characterized. We showed that calcaneus bone presents a dedicated protrusion of low porosity where the tendon inserts. A spatially resolved analysis of the trabecular network suggests that such protrusion may promote force flow from the tendon to the plantar ligament, while partially relieving the trabecular bone from such a task. Focusing on the tuberosity, highly specific microstructural aspects were highlighted. Firstly, the interface between mineralized and unmineralized fibrocartilage showed the highest roughness at the tuberosity, possibly to increase failure resistance of a region carrying large stresses. Secondly, fibrochondrocyte lacunae inside mineralized fibrocartilage, in analogy with osteocyte lacunae in bone, had a predominant alignment at the enthesis and a rather random organization away from it. Finally, the network of subchondral channels inside the tuberosity was highly anisotropic when compared to contiguous regions. This dual anisotropy of subchondral channels and cell lacunae at the insertion may reflect the alignment of the underlying collagen network. Our findings suggest that the microstructure of fibrocartilage may be linked with the loading environment. Future studies should characterize those microstructural aspects in aged and or diseased conditions to elucidate the poorly understood role of bone and fibrocartilage in enthesis-related pathologies.

Diversity and Evolution of Mineralized Skeletal Tissues in Chondrichthyans

The diversity of skeletal tissues in extant vertebrates includes mineralized and unmineralized structures made of bone, cartilage, or tissues of intermediate nature. This variability, together with the diverse nature of skeletal tissues in fossil species question the origin of skeletonization in early vertebrates. In particular, the study of skeletal tissues in cartilaginous fishes is currently mostly restrained to tessellated cartilage, a derived form of mineralized cartilage that evolved at the origin of this group. In this work, we describe the architectural and histological diversity of neural arch mineralization in cartilaginous fishes. The observed variations in the architecture include tessellated cartilage, with or without more massive sites of mineralization, and continuously mineralized neural arches devoid of tesserae. The histology of these various architectures always includes globular mineralization that takes place in the cartilaginous matrix. In many instances, the mineralized structures also include a fibrous component that seems to emerge from the perichondrium and they may display intermediate features, ranging from partly cartilaginous to mostly fibrous matrix, similar to fibrocartilage. Among these perichondrial mineralized tissues is also found, in few species, a lamellar arrangement of the mineralized extracellular matrix. The evolution of the mineralized tissues in cartilaginous fishes is discussed in light of current knowledge of their phylogenetic relationships.

The multiscale architecture of tessellated cartilage and its relation to function

When describing the architecture and ultrastructure of animal skeletons, introductory biology, anatomy and histology textbooks typically focus on the few bone and cartilage types prevalent in humans. In reality, cartilage and bone are far more diverse in the animal kingdom, particularly within fishes (Chondrichthyes and Actinopterygii), where cartilage and bone types are characterized by features that are anomalous or even pathological in human skeletons. This review discusses the curious and complex architectures of shark and ray tessellated cartilage, highlighting similarities and differences with their mammalian skeletal tissue counterparts. By synthesizing older anatomical literature with recent high-resolution structural and materials characterization work, this review frames emerging pictures of form-function relationships in this tissue and of the evolution and true diversity of cartilage and bone.

Mineralization of the Callorhinchus Vertebral Column (Holocephali; Chondrichthyes)

Members of the Chondrichthyes (Elasmobranchii and Holocephali) are distinguished by their largely cartilaginous endoskeletons, which comprise an uncalcified core overlain by a mineralized layer; in the Elasmobranchii (sharks, skates, rays) most of this mineralization takes the form of calcified polygonal tiles known as tesserae. In recent years, these skeletal tissues have been described in ever increasing detail in sharks and rays, but those of Holocephali (chimaeroids) have been less well-studied, with conflicting accounts as to whether or not tesserae are present. During embryonic ontogeny in holocephalans, cervical vertebrae fuse to form a structure called the synarcual. The synarcual mineralizes early and progressively, anteroposteriorly and dorsoventrally, and therefore presents a good skeletal structure in which to observe mineralized tissues in this group. Here, we describe the development and mineralization of the synarcual in an adult and stage 36 elephant shark embryo (Callorhinchus milii). Small, discrete, but irregular blocks of cortical mineralization are present in stage 36, similar to what has been described recently in embryos of other chimaeroid taxa such as Hydrolagus, while in Callorhinchus adults, the blocks of mineralization are more irregular, but remain small. This differs from fossil members of the holocephalan crown group (Edaphodon), as well as from stem group holocephalans (e.g., Symmorida, Helodus, Iniopterygiformes), where tesserae are notably larger than in Callorhinchus and show similarities to elasmobranch tesserae, for example with respect to polygonal shape.

Endoskeletal mineralization in chimaera and a comparative guide to tessellated cartilage in chondrichthyan fishes (sharks, rays and chimaera)

An accepted uniting character of modern cartilaginous fishes (sharks, rays, chimaera) is the presence of a mineralized, skeletal crust, tiled by numerous minute plates called tesserae. Tesserae have, however, never been demonstrated in modern chimaera and it is debated whether the skeleton mineralizes at all. We show for the first time that tessellated cartilage was not lost in chimaera, as has been previously postulated, and is in many ways similar to that of sharks and rays. Tesserae in Chimaera monstrosa are less regular in shape and size in comparison to the general scheme of polygonal tesserae in sharks and rays, yet share several features with them. For example, Chimaera tesserae, like those of elasmobranchs, possess both intertesseral joints (unmineralized regions, where fibrous tissue links adjacent tesserae) and recurring patterns of local mineral density variation (e.g. Liesegang lines, hypermineralized ‘spokes’), reflecting periodic accretion of mineral at tesseral edges as tesserae grow. Chimaera monstrosa's tesserae, however, appear to lack the internal cell networks that characterize tesserae in elasmobranchs, indicating fundamental differences among chondrichthyan groups in how calcification is controlled. By compiling and comparing recent ultrastructure data on tesserae, we also provide a synthesized, up-to-date and comparative glossary on tessellated cartilage, as well as a perspective on the current state of research into the topic, offering benchmark context for future research into modern and extinct vertebrate skeletal tissues.

Image analysis pipeline for segmentation of a biological porosity network, the lacuno-canalicular system in stingray tesserae

A prerequisite for many analysis tasks in modern comparative biology is the segmentation of 3-dimensional (3D) images of the specimens being investigated (e.g. from microCT data). Depending on the specific imaging technique that was used to acquire the images and on the image resolution, different segmentation tools are required. While some standard tools exist that can often be applied for specific subtasks, building whole processing pipelines solely from standard tools is often difficult. Some tasks may even necessitate the implementation of manual interaction tools to achieve a quality that is sufficient for subsequent analysis. In this work, we present a pipeline of segmentation tools that can be used for the semiautomatic segmentation and quantitative analysis of voids in tissue (i.e. internal structural porosity). We use this pipeline to analyze lacuno-canalicular networks in stingray tesserae from 3D images acquired with synchrotron microCT.•

The first step of this pipeline, the segmentation of the tesserae, was performed using standard marker-based watershed segmentation.

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The efficient processing of the next two steps, that is, the segmentation of all lacunae spaces belonging to a specific tessera and the separation of these spaces into individual lacunae required recently developed, novel tools.

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For error correction, we developed an interactive method that allowed us to quickly split lacunae that were accidentally merged, and to merge lacunae that were wrongly split.

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Finally, the tesserae and their corresponding lacunae were subdivided into structural wedges (i.e. specific anatomical regions) using a semi-manual approach.

With this processing pipeline, analysis of a variety of interconnected structural networks (e.g. vascular or lacuno-canalicular networks) can be achieved in a comparatively high-throughput fashion. In our study system, we were able to efficiently segment more than 12,000 lacunae in high-resolution scans of nine tesserae, providing a robust data set for statistical analysis.