Effect of proteoglycan removal on solute mobility in articular cartilage

Transport of anti-IL-6 antigen binding fragments into cartilage and the effects of injury

The efficacy of biological therapeutics against cartilage degradation in osteoarthritis is restricted by the limited transport of macromolecules through the dense, avascular extracellular matrix. The availability of biologics to cell surface and matrix targets is limited by steric hindrance of the matrix, and the microstructure of matrix itself can be dramatically altered by joint injury and the subsequent inflammatory response. We studied the transport into cartilage of a 48 kDa anti-IL-6 antigen binding fragment (Fab) using an in vitro model of joint injury to quantify the transport of Fab fragments into normal and mechanically injured cartilage. The anti-IL-6 Fab was able to diffuse throughout the depth of the tissue, suggesting that Fab fragments can have the desired property of achieving local delivery to targets within cartilage, unlike full-sized antibodies which are too large to penetrate beyond the cartilage surface. Uptake of the anti-IL-6 Fab was significantly increased following mechanical injury, and an additional increase in uptake was observed in response to combined treatment with TNFα and mechanical injury, a model used to mimic the inflammatory response following joint injury. These results suggest that joint trauma leading to cartilage degradation can further alter the transport of such therapeutics and similar-sized macromolecules.

Bath Concentration of Anionic Contrast Agents Does Not Affect Their Diffusion and Distribution in Articular Cartilage In Vitro

OBJECTIVE

Differences in contrast agent diffusion reflect changes in composition and structure of articular cartilage. However, in clinical application the contrast agent concentration in the joint capsule varies, which may affect the reliability of contrast enhanced cartilage tomography (CECT). In the present study, effects of concentration of x-ray contrast agents on their diffusion and equilibrium distribution in cartilage were investigated.

DESIGN

Full-thickness cartilage discs (d = 4.0 mm, n = 120) were detached from bovine patellae (n = 24). The diffusion of various concentrations of ioxaglate (5, 10, 21, 50 mM) and iodide (30, 60, 126, 300 mM) was allowed only through the articular surface. Samples were imaged with a clinical peripheral quantitative computed tomography scanner before immersion in contrast agent, and after 1, 5, 9, 16, 25, and 29 hours in the bath.

RESULTS

Diffusion and partition coefficients were similar between different contrast agent concentrations. The diffusion coefficient of iodide (473 ± 133 µm(2)/s) was greater (P ≤ 0.001) than that of ioxaglate (92 ± 46 µm(2)/s). In full-thickness cartilage, the partition coefficient (at 29 h) of iodide (71 ± 5%) was greater (P ≤ 0.02 with most concentrations) than that of ioxaglate (62 ± 6%).

CONCLUSIONS

Significant differences in partition and diffusion coefficient of two similarly charged (-1) contrast agents were detected, which shows the effect of steric interactions. However, the increase in solute concentration did not increase its partition coefficient. In clinical application, it is important that contrast agent concentration does not affect the interpretation of CECT imaging.

Ionizing radiation causes active degradation and reduces matrix synthesis in articular cartilage

PURPOSE

Little is known regarding radiation effects on adult articular (joint) cartilage, though joint damage has been reported following cancer treatment or occupational exposures. The aim of this study was to determine if radiation can reduce cartilage matrix production, induce cartilage degradation, or interfere with the anabolic effects of IGF-1.

MATERIALS AND METHODS

Isolated chondrocytes cultured in monolayers and whole explants harvested from ankles of human donors and knees of pigs were irradiated with 2 or 10 Gy γ-rays, with or without IGF-1 stimulation. Proteoglycan synthesis and IGF-1 signaling were examined at Day 1; cartilage degradation throughout the first 96 hours.

RESULTS

Human and pig cartilage responded similarly to radiation. Cell viability was unchanged. Basal and IGF-1 stimulated proteoglycan synthesis was reduced following exposure, particularly following 10 Gy. Both doses decreased IGF-induced Akt activation and IGF-1 receptor phosphorylation. Matrix metalloproteinases (ADAMTS5, MMP-1, and MMP-13) and proteoglycans were released into media after 2 and 10 Gy.

CONCLUSIONS

Radiation induced an active degradation of cartilage, reduced proteoglycan synthesis, and impaired IGF-1 signaling in human and pig chondrocytes. Lowered Akt activation could account for decreased matrix synthesis. Radiation may cause a functional decline of cartilage health in joints after exposure, contributing to arthropathy.

Imaging analysis of early DMP1 mediated dentine remineralization

OBJECTIVE

This study assessed the micro-morphological changes in demineralized dentine scaffold following incubation with recombinant dentine matrix protein 1 (rDMP1).

DESIGN

Extracted human molar crowns were sectioned into 6 beams (dimensions: 0.50mm×1.70mm×6.00mm), demineralized and incubated overnight in 3 different media (n=4): rDMP1 in bovine serum albumin (BSA), BSA and distilled water. Samples were placed in a chamber with simulated physiological concentrations of calcium and phosphate ions at constant pH 7.4. Samples were immediately processed for transmission electron microscopy (TEM) and field emission-scanning electron microscopy (FE-SEM) after 1 and 2 weeks.

RESULTS

Analysis of the scaffold showed that decalcification process retained the majority of endogenous proteoglycans and phosphoproteins. rDMP1 treated samples promoted deposition of amorphous calcium phosphate (ACP) precursors and needle shaped hydroxyapatite crystals surrounding collagen fibrils. The BSA group presented ACP bound to collagen with no needle-like apatite crystals. Samples kept in distilled water showed no evidence of ACP and crystal apatite. Results from rDMP1 immobilized on dentine matrix suggests that the acidic protein was able to bind to collagen fibrils and control formation of amorphous calcium phosphate and its subsequent transformation into hydroxyapatite crystals after 2 weeks.

CONCLUSION

These findings suggest a possible bio-inspired strategy to promote remineralization of dentine for reparative and regenerative purposes.

A feasibility study for evaluation of mechanical properties of articular cartilage with a two-electrode electrical impedance method

BACKGROUND

Since articular cartilage has important mechanical properties such as load-bearing, shock absorption and lubrication for activities in daily life, it is important to evaluate the mechanical properties of repaired cartilage in terms of whether those properties are the same as those of natural cartilage. The purpose of this study was to investigate the effectiveness of an electrical impedance method for quantitatively measuring the mechanical properties of cartilage.

METHODS

Cartilage specimens were harvested from porcine knee joint, and two kinds of enzyme-treated cartilages were prepared to investigate the correlation between mechanical and electrical properties resulting from changes in the structure of the extracellular matrix. Collagenase solution and hyaluronidase solution were used to digest the collagen fibril and proteoglycan, respectively. Electrical impedance measurement, indentation test and biochemical analysis were carried out for the enzyme-treated cartilages.

RESULTS

The water content increased with enzyme treatment time, and the permeability of the treated cartilages increased with decreasing glycosaminoglycan content for both types of enzyme-treated cartilages. The aggregate modulus and the electrical resistivity for both types of enzyme-treated cartilages decreased with increasing permeability after 12-h treatment. The aggregate modulus and the electrical resistivity for both types of treated cartilages decreased with increasing water content and permeability after 24-h treatment. The electrical resistivity and the aggregate modulus of articular cartilage depended not only on the water content, but also on the permeability, and the electrical resistivity for both types of enzyme-treated cartilages was found to be significantly linearly correlated with the aggregate modulus.

CONCLUSIONS

These results showed that the aggregate modulus of articular cartilage can be estimated by measuring its electrical impedance.

Aggrecan, an unusual polyelectrolyte: review of solution behavior and physiological implications

Aggrecan is a high-molecular-weight, bottlebrush-shaped, negatively charged biopolymer that forms supermolecular complexes with hyaluronic acid. In the extracellular matrix of cartilage, aggrecan-hyaluronic acid complexes are interspersed in a collagen meshwork and provide the osmotic properties required to resist deswelling under compressive load. In this review we compile aggrecan solution behavior from different experimental techniques, and discuss them in the context of concentration regimes that were identified in osmotic pressure experiments. At low concentrations, aggrecan exhibits microgel-like behavior. With increasing concentration, the bottlebrushes self-assemble into large complexes. In the physiological concentration range (2<c(aggrecan)<8% w/w), the physical properties of the solution are dominated by repulsive electrostatic interactions between aggrecan complexes. We discuss the consequences of the bottlebrush architecture on the polyelectrolyte characteristics of the aggrecan molecule, and its implications for cartilage properties and function.

The effect of transient proanthocyanidins preconditioning on the cross-linking and mechanical properties of demineralized dentin

Proanthocyanidin-based preconditioners were prepared by adding powdered proanthocyanidins-rich grape seed extract to various solvents at different concentrations. Demineralized dentin specimens were preconditioned for 20, 30, 60 or 120 s, followed by the evaluation of their cross-linking degree, mechanical properties and micromorphology. The cross-linking degree of the demineralized dentin collagen exhibited concentration- and time- dependent increase after preconditioning treatment, irrespective of the preconditioner and the solvent. When treated for the same exposure time, specimens after 15% proanthocyanidins preconditioning resulted in the highest mean ultimate tensile strength compared with all the other groups tested. Five percent glutaraldehyde control group produced the highest cross-linking degree, but the ultimate tensile strength was lower than that of 15% proanthocyanidins group. The field emission scanning electron microscopy confirmed that the demineralized dentin collagen was in a homogeneous and regular arrangement after preconditioning and maintained expanding, regardless of the surface moisture conditions.

Dynamic loading of immature epiphyseal cartilage pumps nutrients out of vascular canals

The potential influence of mechanical loading on transvascular transport in vascularized soft tissues has not been explored extensively. This experimental investigation introduced and explored the hypothesis that dynamic mechanical loading can pump solutes out of blood vessels and into the surrounding tissue, leading to faster uptake and higher solute concentrations than could otherwise be achieved under unloaded conditions. Immature epiphyseal cartilage was used as a model tissue system, with fluorescein (332 Da), dextran (3, 10, and 70 kDa) and transferrin (80 kDa) as model solutes. Cartilage disks were either dynamically loaded (± 10% compression over a 10% static offset strain, at 0.2 Hz) or maintained unloaded in solution for up to 20 h. Results demonstrated statistically significant solute uptake in dynamically loaded (DL) explants relative to passive diffusion (PD) controls for all solutes except unbound fluorescein, as evidenced by the DL:PD concentration ratios after 20 h (1.0 ± 0.2, 2.4 ± 1.1, 6.1 ± 3.3, 9.0 ± 4.0, and 5.5 ± 1.6 for fluorescein, 3, 10, and 70 kDa dextran, and transferrin). Significant uptake enhancements were also observed within the first 30s of loading. Termination of dynamic loading produced dissipation of enhanced solute uptake back to PD control values. Confocal images confirmed that solute uptake occurred from cartilage canals into their surrounding extracellular matrix. The incidence of this loading-induced transvascular solute pumping mechanism may significantly alter our understanding of the interaction of mechanical loading and tissue metabolism.

Characterization of biomodified dentin matrices for potential preventive and reparative therapies

Biomodification of existing hard tissue structures, specifically tooth dentin, is an innovative approach proposed to improve the biomechanical and biochemical properties of tissue for potential preventive or reparative therapies. The objectives of the study were to systematically characterize dentin matrices biomodified by proanthocyanidin-rich grape seed extract (GSE) and glutaraldehyde (GD). Changes to the biochemistry and biomechanical properties were assessed by several assays to investigate the degree of interaction, biodegradation rates, proteoglycan interaction, and effect of collagen fibril orientation and environmental conditions on the tensile properties. The highest degree of agent-dentin interaction was observed with GSE, which exhibited the highest denaturation temperature, regardless of the agent concentration. Biodegradation rates decreased remarkably following biomodification of dentin matrices after 24h collagenase digestion. A significant decrease in the proteoglycan content of GSE-treated samples was observed using a micro-assay for glycosaminoglycans and histological electron microscopy, while no changes were observed for GD and the control. The tensile strength properties of GD-biomodified dentin matrices were affected by dentin tubule orientation, most likely due to the orientation of the collagen fibrils. Higher and/or increased stability of the tensile properties of GD- and GSE-treated samples were observed following exposure to collagenase and 8 months water storage. Biomodification of dentin matrices using chemical agents not only affects the collagen biochemistry, but also involves interaction with proteoglycans. Tissue biomodifiers interact differently with dentin matrices and may provide the tissue with enhanced preventive and restorative/reparative abilities.

Transport and equilibrium uptake of a peptide inhibitor of PACE4 into articular cartilage is dominated by electrostatic interactions

The availability of therapeutic molecules to targets within cartilage depends on transport through the avascular matrix. We studied equilibrium partitioning and non-equilibrium transport into cartilage of Pf-pep, a 760 Da positively charged peptide inhibitor of the proprotein convertase PACE4. Competitive binding measurements revealed negligible binding of Pf-pep to sites within cartilage. Uptake of Pf-pep depended on glycosaminoglycan charge density, and was consistent with predictions of Donnan equilibrium given the known charge of Pf-pep. In separate transport experiments, the diffusivity of Pf-pep in cartilage was measured to be approximately 1 x 10(-6) cm(2)/s, close to other similarly-sized non-binding solutes. These results suggest that small positively charged therapeutics will have a higher concentration within cartilage than in the surrounding synovial fluid, a desired property for local delivery; however, such therapeutics may rapidly diffuse out of cartilage unless there is additional specific binding to intra-tissue substrates that can maintain enhanced intra-tissue concentration for local delivery.

Diffusion coefficients of articular cartilage for different CT and MRI contrast agents

In contrast enhanced magnetic resonance imaging (MRI) and computed tomography (CT), the equilibrium distribution of anionic contrast agent is expected to reflect the fixed charged density (FCD) of articular cartilage. Diffusion is mainly responsible for the transport of contrast agents into cartilage. In osteoarthritis, cartilage composition changes at early stages of disease, and solute diffusion is most likely affected. Thus, investigation of contrast agent diffusion could enable new methods for imaging of cartilage composition. The aim of this study was to determine the diffusion coefficient of four contrast agents (ioxaglate, gadopentetate, iodide, gadodiamide) in bovine articular cartilage. The contrast agents were different in molecular size and charge. In peripheral quantitative CT experiments, penetration of contrast agent into the tissue was allowed either through the articular surface or through deep cartilage. To determine diffusion coefficients, a finite element model based on Fick's law was fitted to experimental data. Diffusion through articular surface was faster than through deep cartilage with every contrast agent. Iodide, being of atomic size, diffused into the cartilage significantly faster (q<0.05) than the other three contrast agents, for either transport direction. The diffusion coefficients of all clinical contrast agents (ioxaglate, gadopentetate and gadodiamide) were relatively low (142.8-253.7 μm(2)/s). In clinical diagnostics, such slow diffusion may not reach equilibrium and this jeopardizes the determination of FCD by standard methods. However, differences between diffusion through articular surface and deep cartilage, that are characterized by different tissue composition, suggest that diffusion coefficients may correlate with cartilage composition. Present method could therefore enable image-based assessment of cartilage composition by determination of diffusion coefficients within cartilage tissue.

Anisotropic solute diffusion tensor in porcine TMJ discs measured by FRAP with spatial Fourier analysis

A new method solely based on spatial Fourier analysis (SFA) was developed to completely determine a two-dimensional (2D) anisotropic diffusion tensor in fibrous tissues using fluorescence recovery after photobleaching (FRAP). The accuracy and robustness of this method was validated using computer-simulated FRAP experiments. This method was applied to determine the region-dependent anisotropic diffusion tensor in porcine temporomandibular joint (TMJ) discs. The average characteristic diffusivity of 4 kDa FITC-Dextran across the disc was 26.05 ± 4.32 μm²/s which is about 16% of its diffusivity in water. In the anteroposterior direction, the anterior region (30.99 ± 5.93 μm²/s) had significantly higher characteristic diffusivity than the intermediate region (20.49 ± 5.38 μm²/s) and posterior region (20.97 ± 2.46 μm²/s). The ratio of the two principal diffusivities represents the anisotropy of the diffusion and ranged between 0.45 and 0.51 (1.0 = isotropic). Our results indicated that the solute diffusion in TMJ discs is inhomogeneous and anisotropic. These findings suggested that diffusive transport in the TMJ disc is dependent on tissue composition (e.g., water content) and structure (e.g., collagen orientation). This study provides a new method to quantitatively investigate the relationship between solute transport properties and tissue composition and structure.

Nondestructive assessment of sGAG content and distribution in normal and degraded rat articular cartilage via EPIC-microCT

OBJECTIVE

The objective of this study was to evaluate the feasibility of quantifying the Equilibrium Partitioning of an Ionic Contrast agent via Microcomputed Tomography (EPIC-microCT) to nondestructively assess sulfated glycosaminoglycan (sGAG) content and distribution in rat articular cartilage ex vivo, and in doing so to establish a paradigm for extension of this technique to other small animal models.

DESIGN

After determination of an appropriate incubation time for the anionic contrast agent, EPIC-microCT was used to examine age-related differences in cartilage sGAG content between 4-, 8-, and 16-week old (n=5 each) male Wistar rats and to evaluate sGAG depletion in the right femora of each age group after 60 min of digestion with chondroitinase ABC. The EPIC-microCT measurements were validated by histological safranin-O staining, and reproducibility was evaluated by triplicate scans of six femora.

RESULTS

Cartilage attenuation gradually increased with cumulative digestion time and reached a plateau at approximately 60 min with a 16.0% temporal increase (P<0.01). Average femoral articular cartilage attenuation increased by 14.2% from 4- to 8-weeks of age (P<0.01) and further increased by 2.5% from 8 to 16 weeks (P<0.05). After 60 min of digestion, femoral articular cartilage attenuations increased by 15-17% in each age group (P<0.01). Correspondingly, sGAG optical density decreased with age and digestion, and showed a linear correlation (r=-0.88, slope=-1.26, P<0.01, n=30) with EPIC-microCT cartilage attenuation. High reproducibility was indicated by a low coefficient of variation (1.5%) in cartilage attenuation.

CONCLUSIONS

EPIC-microCT imaging provides high spatial resolution and sensitivity to assess sGAG content and three-dimensional distribution in rat femoral articular cartilage.

Effect of dynamic loading on the transport of solutes into agarose hydrogels

In functional tissue engineering, the application of dynamic loading has been shown to improve the mechanical properties of chondrocyte-seeded agarose hydrogels relative to unloaded free swelling controls. The goal of this study is to determine the effect of dynamic loading on the transport of nutrients in tissue-engineered constructs. To eliminate confounding effects, such as nutrient consumption in cell-laden disks, this study examines the response of solute transport due to loading using a model system of acellular agarose disks and dextran in phosphate-buffered saline (3 and 70 kDa). An examination of the passive diffusion response of dextran in agarose confirms the applicability of Fick's law of diffusion in describing the behavior of dextran. Under static loading, the application of compressive strain decreased the total interstitial volume available for the 70 kDa dextran, compared to free swelling. Dynamic loading significantly enhanced the rate of solute uptake into agarose disks, relative to static loading. Moreover, the steady-state concentration under dynamic loading was found to be significantly greater than under static loading, for larger-molecular-mass dextran (70 kDa). This experimental finding confirms recent theoretical predictions that mechanical pumping of a porous tissue may actively transport solutes into the disk against their concentration gradient. The results of this study support the hypothesis that the application of dynamic loading in the presence of growth factors of large molecular weight may result in both a mechanically and chemically stimulating environment for tissue growth.

TRANSPORT PROPERTIES OF CARTILAGINOUS TISSUES

Cartilaginous tissues, such as articular cartilage and intervertebral disc, are avascular tissues which rely on transport for cellular nutrition. Comprehensive knowledge of transport properties in such tissues is therefore necessary in the understanding of nutritional supply to cells. Furthermore, poor cellular nutrition in cartilaginous tissues is believed to be a primary source of tissue degeneration, which may result in osteoarthritis (OA) or disc degeneration. In this mini-review, we present an overview of the current status of the study of transport properties and behavior in cartilaginous tissues. The mechanisms of transport in these tissues, as well as experimental approaches to measuring transport properties and results obtained are discussed. The current status of bioreactors used in cartilage tissue engineering is also presented.

Microscale diffusion properties of the cartilage pericellular matrix measured using 3D scanning microphotolysis

Chondrocytes, the cells in articular cartilage, are enclosed within a pericellular matrix (PCM) whose composition and structure differ from those of the extracellular matrix (ECM). Since the PCM surrounds each cell, molecules that interact with the chondrocyte must pass through the pericellular environment. A quantitative understanding of the diffusional properties of the PCM may help in elucidating the regulatory role of the PCM in controlling transport to and from the chondrocyte. The diffusivities of fluorescently labeled 70 kDa and 500 kDa dextrans were quantified within the PCM of porcine articular cartilage using a newly developed mathematical model of scanning microphotolysis (SCAMP). SCAMP is a rapid line photobleaching method that accounts for out-of-plane bleaching attributable to high magnification. Data were analyzed by a best-fit comparison to simulations generated using a discretization of the diffusion-reaction equation in conjunction with the microscope-specific three-dimensional excitation and detection profiles. The diffusivity of the larger molecule (500 kDa dextran) was significantly lower than that of the smaller molecule (70 kDa dextran), and values were consistent with those reported previously using standard techniques. Furthermore, for both dextran sizes, the diffusion coefficient was significantly lower in the PCM than in the ECM; however, this difference was not detected in early-stage arthritic tissue. We have successfully modified the SCAMP technique to measure diffusion coefficients within the small volume of the PCM using confocal laser scanning microscopy. Our results support the hypothesis that diffusivity within the PCM of healthy articular cartilage is lower than that within the ECM, presumably due to differences in proteoglycan content.

Site-specific effects of compression on macromolecular diffusion in articular cartilage

Articular cartilage is the connective tissue that lines joints and provides a smooth surface for joint motion. Because cartilage is avascular, molecular transport occurs primarily via diffusion or convection, and cartilage matrix structure and composition may affect diffusive transport. Because of the inhomogeneous compressive properties of articular cartilage, we hypothesized that compression would decrease macromolecular diffusivity and increase diffusional anisotropy in a site-specific manner that depends on local tissue strain. We used two fluorescence photobleaching methods, scanning microphotolysis and fluorescence imaging of continuous point photobleaching, to measure diffusion coefficients and diffusional anisotropy of 70 kDa dextran in cartilage during compression, and measured local tissue strain using texture correlation. For every 10% increase in normal strain, the fractional change in diffusivity decreased by 0.16 in all zones, and diffusional anisotropy increased 1.1-fold in the surface zone and 1.04-fold in the middle zone, and did not change in the deep zone. These results indicate that inhomogeneity in matrix structure and composition may significantly affect local diffusive transport in cartilage, particularly in response to mechanical loading. Our findings suggest that high strains in the surface zone significantly decrease diffusivity and increase anisotropy, which may decrease transport between cartilage and synovial fluid during compression.

Glycosaminoglycan composition of the vocal fold lamina propria in relation to function

OBJECTIVES

This study was designed to quantify the specific glycosaminoglycans (GAGs) in the midmembranous vocal fold (VF) lamina propria (LP) and to interpret their presence in relation to the known stresses borne by each LP layer.

METHODS

GAGs from normal human LP and from both normal and scarred canine LPs were analyzed by fluorophore-assisted carbohydrate electrophoresis (FACE). Immunostaining was conducted to give insight into the spatial distribution of each GAG type.

RESULTS

Hyaluronan composes roughly 0.64% +/- 0.41% of the human LP as measured relative to tissue total protein. Chondroitin sulfate and/or dermatan sulfate (CS/DS), keratan sulfate, and heparan sulfate chains constitute approximately 23.9% +/- 12.1%, 14.7% +/- 6.1%, and 61.4% +/- 13.6%, respectively, of human LP sulfated GAGs.

CONCLUSIONS

Observed CS/DS sulfation patterns imply that versican is a major contributor to human LP CS levels. In addition, examination of LP GAG with respect to gender revealed a significant variation in total levels of CS/DS and a potential difference in the levels of versican relative to decorin and biglycan. In dogs, LP scarring appeared to result in a reduction in hyaluronan and CS/DS. These FACE results were combined with histologic data to update current descriptive models linking LP microstructure with the regional variations in LP loading.

Biofunctional polymer nanoparticles for intra-articular targeting and retention in cartilage

The extracellular matrix of dense, avascular tissues presents a barrier to entry for polymer-based therapeutics, such as drugs encapsulated within polymeric particles. Here, we present an approach by which polymer nanoparticles, sufficiently small to enter the matrix of the targeted tissue, here articular cartilage, are further modified with a biomolecular ligand for matrix binding. This combination of ultrasmall size and biomolecular binding converts the matrix from a barrier into a reservoir, resisting rapid release of the nanoparticles and clearance from the tissue site. Phage display of a peptide library was used to discover appropriate targeting ligands by biopanning on denuded cartilage. The ligand WYRGRL was selected in 94 of 96 clones sequenced after five rounds of biopanning and was demonstrated to bind to collagen II alpha1. Peptide-functionalized nanoparticles targeted articular cartilage up to 72-fold more than nanoparticles displaying a scrambled peptide sequence following intra-articular injection in the mouse.

Transport of neutral solute in articular cartilage: effect of microstructure anisotropy

Due to the avascular nature of articular cartilage, solute transport through its extracellular matrix is critical for the maintenance and the functioning of the tissue. What is more, diffusion of macromolecules may be affected by the microstructure of the extracellular matrix in both undeformed and deformed cartilage and experiments demonstrate diffusion anisotropy in the case of large solute. However, these phenomena have not received sufficient theoretical attention to date. We hypothesize here that the diffusion anisotropy of macromolecules is brought about by the particular microstructure of the cartilage network. Based on this hypothesis, we then propose a mathematical model that correlates the diffusion coefficient tensor with the structural orientation tensor of the network. This model is shown to be successful in describing anisotropic diffusion of macromolecules in undeformed tissue and is capable of clarifying the effects of network reorientation as the tissue deforms under mechanical load. Additionally, our model explains the anomaly that at large strain, in a cylindrical plug under unconfined compression, solute diffusion in the radial direction increases with strain. Our results indicate that in cartilage the degree of diffusion anisotropy is site specific, but depends also on the size of the diffusing molecule. Mechanical loading initiates and/or further exacerbates this anisotropy. At small deformation, solute diffusion is near isotropic in a tissue that is isotropic in its unstressed state, becoming anisotropic as loading progresses. Mechanical loading leads to an attenuation of solute diffusion in all directions when deformation is small. However, loading, if it is high enough, enhances solute transport in the direction perpendicular to the load line, instead of inhibiting it.

Solute transport in growth plate cartilage: in vitro and in vivo

Bone elongation originates from cartilaginous discs (growth plates) at both ends of a growing bone. Here chondrocytes proliferate and subsequently enlarge (hypertrophy), laying down a matrix that serves as the scaffolding for subsequent bone matrix deposition. Because cartilage is generally avascular, all nutrients, oxygen, signaling molecules, and waste must be transported relatively long distances through the tissue for it to survive and function. Here we examine the transport properties of growth plate cartilage. Ex vivo, fluorescence photobleaching recovery methods are used in tissue explants. In vivo, multiphoton microscopy is used to image through an intact perichondrium and into the cartilage of anesthetized mice. Systemically introduced fluorescent tracers are monitored directly as they move from the vasculature into the cartilage. We demonstrate the existence of a relatively permissive region at the midplane of the growth plate, where chondrocytes transition from late proliferative to early hypertrophic stages and where paracrine communication is known to occur between chondrocytes and cells in the surrounding perichondrium. Transport in the living mouse is also significantly affected by fluid flow from the two chondro-osseus junctions, presumably resulting from a pressure difference between the bone vasculature and the cartilage.

Tensorial electrokinetics in articular cartilage

Electrokinetic phenomena contribute to biomechanical functions of articular cartilage and underlie promising methods for early detection of osteoarthritic lesions. Although some transport properties, such as hydraulic permeability, are known to become anisotropic with compression, the direction-dependence of cartilage electrokinetic properties remains unknown. Electroosmosis experiments were therefore performed on adult bovine articular cartilage samples, whereby fluid flows were driven by electric currents in directions parallel and perpendicular to the articular surface of statically compressed explants. Magnitudes of electrokinetic coefficients decreased slightly with compression (from approximately -7.5 microL/As in the range of 0-20% compression to -6.0 microL/As in the 35-50% range) consistent with predictions of microstructure-based models of cartilage material properties. However, no significant dependence on direction of the electrokinetic coupling coefficient was detected, even for conditions where the hydraulic permeability tensor is known to be anisotropic. This contrast may also be interpreted using microstructure-based models, and provides insights into structure-function relationships in cartilage extracellular matrix and physical mediators of cell responses to tissue compression. Findings support the use of relatively simple isotropic modeling approaches for electrokinetic phenomena in cartilage and related materials, and indicate that measurement of electrokinetic properties may provide particularly robust means for clinical evaluation of cartilage matrix integrity.

Microstructural Modeling of Collagen Network Mechanics and Interactions with the Proteoglycan Gel in Articular Cartilage

Cartilage matrix mechanical function is largely determined by interactions between the collagen fibrillar network and the proteoglycan gel. Although the molecular physics of these matrix constituents have been characterized and modern imaging methods are capable of localized measurement of molecular densities and orientation distributions, theoretical tools for using this information for prediction of cartilage mechanical behavior are lacking. We introduce a means to model collagen network contributions to cartilage mechanics based upon accessible microstructural information (fibril density and orientation distributions) and which self-consistently follows changes in microstructural geometry with matrix deformations. The interplay between the molecular physics of the collagen network and the proteoglycan gel is scaled up to determine matrix material properties, with features such as collagen fibril pre-stress in free-swelling cartilage emerging naturally and without introduction of ad hoc parameters. Methods are developed for theoretical treatment of the collagen network as a continuum-like distribution of fibrils, such that mechanical analysis of the network may be simplified by consideration of the spherical harmonic components of functions of the fibril orientation, strain, and stress distributions. Expressions for the collagen network contributions to matrix stress and stiffness tensors are derived, illustrating that only spherical harmonic components of orders 0 and 2 contribute to the stress, while orders 0, 2, and 4 contribute to the stiffness. Depth- and compression-dependent equilibrium mechanical properties of cartilage matrix are modeled, and advantages of the approach are illustrated by exploration of orientation and strain distributions of collagen fibrils in compressed cartilage. Results highlight collagen-proteoglycan interactions, especially for very small physiological strains where experimental data are relatively sparse. These methods for determining matrix mechanical properties from measurable quantities at the microscale (composition, structure, and molecular physics) may be useful for investigating cartilage structure-function relationships relevant to load-bearing, injury, and repair.

The PTHrP-Ihh feedback loop in the embryonic growth plate allows PTHrP to control hypertrophy and Ihh to regulate proliferation

Growth plate and long bone development is governed by biochemical signaling pathways of which the PTHrP-Ihh system is the best known. Other factors, such as BMPs, FGFs and mechanical loading, may interact with this system. This study aims at elucidating the relative importance of PTHrP and Ihh for controlling proliferation, and hypertrophy in fetal growth plate cartilage. We assessed the question why reduced Ihh expression leads to more pronounced effects on the number of non-hypertrophic cells and total bone formation, compared to PTHrP down-regulation. Using few basic equations, constituted from literature data, this paper shows how the PTHrP-Ihh feedback system can control different aspects of tissue differentiation at distinct locations. In particular, it is shown that (mechanical or biochemical) perturbations will affect proliferation via Ihh-related parameters, whereas changes in PTHrP-related parameters selectively interact with hypertrophy. This is contra-intuitive, since PTHrP acts to keep cells proliferating. In this context, the critical PTHrP level for keeping cells proliferating has been reconsidered. In addition, an explanation is provided for the aforementioned difference in effect between reduced Ihh and PTHrP expression.

Dynamic compression augments interstitial transport of a glucose-like solute in articular cartilage

Solute transport through the extracellular matrix is essential for cellular activities in articular cartilage. Increased solute transport via fluid convection may be a mechanism by which dynamic compression stimulates chondrocyte metabolism. However, loading conditions that optimally augment transport likely vary for different solutes. To investigate effects of dynamic loading on transport of a bioactive solute, triangular mechanical loading waveforms were applied to cartilage explants disks while interstitial transport of a fluorescent glucose analog was monitored. Peak-to-peak compression amplitudes varied from 5-50% and frequencies varied from 0.0006-0.1 Hz to alter the spatial distribution and magnitude of oscillatory fluid flow. Solute transport was quantified by monitoring accumulation of fluorescence in a saline bath circulated around the explant. Individual explants were subjected to a series of compression protocols, so that effects of loading on solute desorption could be observed directly. Maximum increases in solute transport were obtained with 10-20% compression amplitudes at 0.1 Hz; similar loading protocols were previously found to stimulate chondrocyte metabolism in vitro. Results therefore support hypotheses relating to increased solute transport as a mediator of the cartilage biological response to dynamic compression, and may have application in mechanical conditioning of cartilage constructs for tissue engineering.

Diffusional anisotropy in collagenous tissues: fluorescence imaging of continuous point photobleaching

Molecular transport in avascular collagenous tissues such as articular cartilage occurs primarily via diffusion. The presence of ordered structures in the extracellular matrix may influence the local transport of macromolecules, leading to anisotropic diffusion depending on the relative size of the molecule and that of extracellular matrix structures. Here we present what we believe is a novel photobleaching technique for measuring the anisotropic diffusivity of macromolecules in collagenous tissues. We hypothesized that macromolecular diffusion is anisotropic in collagenous tissues, depending on molecular size and the local organization of the collagen structure. A theoretical model and experimental protocol for fluorescence imaging of continuous point photobleaching was developed to measure diffusional anisotropy. Significant anisotropy was observed in highly ordered collagenous tissues such as ligament, with diffusivity ratios>2 along the fiber direction compared to the perpendicular direction. In less-ordered tissues such as articular cartilage, diffusional anisotropy was dependent on site in the tissue and size of the diffusing molecule. Anisotropic diffusion was also dependent on the size of the diffusing molecule, with greatest anisotropy observed for larger molecules. These findings suggest that diffusional transport of macromolecules is anisotropic in collagenous tissues, with higher rates of diffusion along primary orientation of collagen fibers.

Composition and transport properties of human ankle and knee cartilage

The incidence of osteoarthritis is significantly higher in the knee as compared to the ankle, suggesting that differences in the properties of cartilage from these joints may contribute to the development of osteoarthritis. As an avascular tissue, articular cartilage depends primarily upon diffusion for molecular transport. The goal of this study was to determine if differences in the structure and composition between ankle and knee cartilage were also reflected as differences in solute transport properties. The diffusion coefficient and partition coefficient of a 70-kDa dextran molecule were measured in human ankle and knee articular cartilage using fluorescence recovery after photobleaching (FRAP) and were compared to the proteoglycan, collagen, water, and DNA contents within each zone. The mean partition coefficient was significantly lower in the ankle compared to the knee (0.010+/-0.002 vs. 0.022+/-0.003, p<0.01), but no differences in the diffusion coefficients were observed (34.6 +/- 0.9 microm(2)s(-1) vs. 35.4+/-2.4 microm(2)s(-1), p=0.70). Ankle cartilage exhibited higher proteoglycan content as well as a trend toward lower water content, suggesting that ankle cartilage has a smaller effective pore size than knee cartilage. These findings suggest that differences in the composition of ankle and knee cartilage contribute to a difference in the partition coefficient. The results of this study provide further support for the hypothesis that the transport properties of cartilage may play a role in the differences in the incidence of osteoarthritis in these joints by altering the effective concentration of growth factors and cytokines to which chondrocytes are exposed.

Effects of 24R,25- and 1α,25-dihydroxyvitamin D3 on mineralizing growth plate chondrocytes

Time- and dosage-dependent effects of 1,25(OH)(2)D(3) and 24,25(OH)(2)D(3) on primary cultures of pre- and post-confluent avian growth plate (GP) chondrocytes were examined. Cultures were grown in either a serum-containing culture medium designed to closely mimic normal GP extracellular fluid (DATP5) or a commercially available serum-free media (HL-1) frequently used for studying skeletal cells. Hoechst DNA, Lowry protein, proteoglycan (PG), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP) activity and calcium and phosphate mineral deposition in the extracellular matrix were measured. In preconfluent cultures grown in DATP5, physiological levels of 24,25(OH)(2)D(3) (0.10-10 nM) increased DNA, protein, and LDH activity significantly more than did 1,25(OH)(2)D(3) (0.01-1.0 nM). However, in HL-1, the reverse was true. Determining ratios of LDH and PG to DNA, protein, and each other, revealed that 1,25(OH)(2)D(3) specifically increased PG, whereas 24,25(OH)(2)D(3) increased LDH. Post-confluent cells were generally less responsive, especially to 24,25(OH)(2)D(3). The positive anabolic effects of 24,25(OH)(2)D(3) required serum-containing GP-fluid-like culture medium. In contrast, effects of 1,25(OH)(2)D(3) were most apparent in serum-free medium, but were still significant in serum-containing media. Administered to preconfluent cells in DATP5, 1,25(OH)(2)D(3) caused rapid, powerful, dosage-dependent inhibition of Ca(2+) and Pi deposition. The lowest level tested (0.01 nM) caused >70% inhibition during the initial stages of mineral deposition; higher levels of 1,25(OH)(2)D(3) caused progressively more profound and persistent reductions. In contrast, 24,25(OH)(2)D(3) increased mineral deposition 20-50%; it required >1 week, but the effects were specific, persistent, and largely dosage-independent. From a physiological perspective, these effects can be explained as follows: 1,25(OH)(2)D(3) levels rise in hypocalcemia; it stimulates gut absorption and releases Ca(2+) from bone to correct this deficiency. We now show that 1,25(OH)(2)D(3) also conserves Ca(2+) by inhibiting mineralization. The slow anabolic effects of 24,25(OH)(2)D(3)are consistent with its production under eucalcemic conditions which enable bone formation. These findings, which implicate serum-binding proteins and accumulation of PG in modulating accessibility of the metabolites to GP chondrocytes, also help explain some discrepancies previously reported in the literature.

The role of computational models in the search for the mechanical behavior and damage mechanisms of articular cartilage

Articular cartilage plays a vital role in the function of diarthrodial joints. Due to osteoarthritis degeneration of articular cartilage occurs. The initial event that triggers the pathological process of cartilage degeneration is still unknown. Cartilage damage due to osteoarthritis is believed to be mechanically induced. Hence, to investigate the initiation of osteoarthritis the stresses and strains in the cartilage must be determined. So far the most common method to accomplish that is finite element analysis. This paper provides an overview of computational descriptions developed for this purpose, and what they can be used for. Articular cartilage composition and structure are discussed in relation with degenerative changes, and how these affect mechanical properties.

Solute diffusivity correlates with mechanical properties and matrix density of compressed articular cartilage

The biomechanical functions of articular cartilage are governed largely by the composition and density of its specialized extracellular matrix. Relationships between matrix density and functional indices such as mechanical properties or interstitial solute diffusivities have been previously explored. However, direct correlations between mechanical properties and solute transport parameters have received less attention, despite potential application of this information for cartilage functional assessment both in vivo and in vitro. The objective of this study was therefore to examine relationships among solute diffusivities, mechanical properties, and matrix density of compressed articular cartilage. Matrix density varied due to natural variation among explants and due to applied static compression. Matrix density of statically compressed cartilage explants was characterized by glycoaminoglycan (GAG) weight fraction and fluid volume fraction, while diffusion coefficients of a wide range of solutes were measured to characterize the transport environment. Explant mechanical properties were characterized by a non-linear Young's modulus (axial stress-strain ratio) and a non-linear Poisson's ratio (radial-to-axial strain ratio). Solute diffusivities were consistently correlated with Young's modulus, as well as with explant GAG weight and fluid volume fractions. Therefore, in vitro mechanical tests may provide a means of assessing transport environments in cartilage-like materials, while in vivo measurements of solute transport (for example with magnetic resonance imaging) may be a useful complement in identifying localized differences in matrix density and mechanical properties.

Determining diffusion coefficients in inhomogeneous tissues using fluorescence recovery after photobleaching

Diffusion plays an important role in the transport of nutrients and signaling molecules in cartilaginous tissues. Diffusion coefficients can be measured by fluorescence recovery after photobleaching (FRAP). Available methods to analyze FRAP data, however, assume homogeneity in the environment of the bleached area and neglect geometrical restrictions to diffusion. Hence, diffusion coefficients in inhomogeneous materials, such as most biological tissues, cannot be assessed accurately. In this study, a new method for analyzing data from FRAP measurements has been developed, which is applicable to inhomogeneous tissues. It is based on a fitting procedure of the intensity recovery after photobleaching with a two-dimensional finite element analysis, which includes Fick's law for diffusion. The finite element analysis can account for distinctive diffusivity in predefined zones, which allows determining diffusion coefficients in inhomogeneous samples. The method is validated theoretically and experimentally in both homogeneous and inhomogeneous tissues and subsequently applied to the proliferation zone of the growth plate. Finally, the importance of accounting for inhomogeneities, for appropriate assessment of diffusivity in inhomogeneous tissues, is illustrated.

Solute convection in dynamically compressed cartilage

Chondrocytes depend upon solute transport within the avascular extracellular matrix of articular cartilage for many of their biological activities. Alterations to solute transport parameters may therefore mediate the cell response to tissue compression. While interstitial solute transport may be supplemented by convection during dynamic tissue compression, matrix compression is also associated with decreased diffusivities. Such trade-offs between increased convection and decreased diffusivities of solutes in dynamically compressed cartilage remain largely unexplored. We measured diffusion and convection coefficients of a wide range of solutes in mature bovine cartilage explant disks subjected to radially unconfined axial ramp compression and release. Solutes included approximately 500 Da fluorophores bearing positive and negative charges, and 10 kDa dextrans bearing positive, neutral, and negative charges. Significantly positive values of convection coefficients were measured for several different solutes. Findings therefore support a role for solute convection in mediating the cartilage biological response to dynamic compression.

Adjacent tissues (cartilage, bone) affect the functional integration of engineered calf cartilage in vitro

OBJECTIVE

An in vitro model was used to test the hypothesis that culture time and adjacent tissue structure and composition affected chondrogenesis and integrative repair in engineered cartilage.

METHOD

Engineered constructs made of bovine calf chondrocytes and hyaluronan benzyl ester non-woven mesh were press-fitted into adjacent tissue rings made of articular cartilage (AC), devitalized bone (DB), or vital bone (VB) and cultured in rotating bioreactors for up to 8 weeks. Structure (light and electron microscopy), biomechanical properties (interfacial adhesive strength, construct compressive modulus), biochemical composition (construct glycosaminoglycans (GAG), collagen, and cells), and adjacent tissue diffusivity were assessed.

RESULTS

Engineered constructs were comprised predominately of hyaline cartilage, and appeared either closely apposed to adjacent cartilage or functionally interdigitated with adjacent bone due to interfacial deposition of extracellular matrix. An increase in culture time significantly improved construct adhesive strength (P<0.001), modulus (P=0.02), GAG (P=0.04) and cellularity (P<0.001). The type of adjacent tissue significantly affected construct adhesion (P<0.001), modulus (P<0.001), GAG (P<0.001) and collagen (P<0.001). For constructs cultured in rings of cartilage, negative correlations were observed between ring GAG content (log transformed) and construct adhesion (R2=0.66, P<0.005), modulus (R2=0.49, P<0.05) and GAG (R2=0.44, P<0.05). Integrative repair was better for constructs cultured adjacent to bone than cartilage, in association with its solid architectural structure and high GAG content, and best for constructs cultured adjacent to DB, in association with its high diffusivity.

CONCLUSIONS

Chondrogenesis and integrative repair in engineered cartilage improved with time and depended on adjacent tissue architecture, composition, and transport properties.

Dynamic depth-dependent osmotic swelling and solute diffusion in articular cartilage monitored using real-time ultrasound

The objective of this study was to investigate the feasibility of ultrasonic monitoring for the transient depth-dependent osmotic swelling and solute diffusion in normal and degenerated articular cartilage (artC) tissues. Full-thickness artC specimens were collected from fresh bovine patellae. The artC specimens were continuously monitored using a focused beam of 50 MHz ultrasound (US) during sequential changes of the bathing solution from 0.15 mol/L to 2 mol/L saline, 0.15 mol/L saline, 1 mg/mL trypsin solution, 0.15 mol/L saline, 2 mol/L saline and back to 0.15 mol/L saline. The transient displacements of US echoes from the artC tissues at different depths were used to represent the tissue deformation and the NaCl diffusion. The trypsin solution was used selectively to digest the proteoglycans in artC. It was demonstrated that high-frequency US was feasible for monitoring the transient osmotic swelling, solute transport and progressive degeneration of artC in real-time. Preliminary results showed that the normal bovine patellar artC shrank during the first several minutes and then recovered to its original state in approximately 1 h when the solution was changed from 0.15 mol/L to 2 mol/L saline. Degenerated artC showed neither shrinkage nor recovery during the same process. In addition, a dehydrated-hydrated artC specimen showed much stronger shrinkage and it resumed the original state when the solution was changed from 2 mol/L back to 0.15 mol/L saline. The diffusion of NaCl and the digestion process of proteoglycans induced by trypsin were also successfully monitored in real-time.

Modeling of neutral solute transport in a dynamically loaded porous permeable gel: implications for articular cartilage biosynthesis and tissue engineering

A primary mechanism of solute transport in articular cartilage is believed to occur through passive diffusion across the articular surface, but cyclical loading has been shown experimentally to enhance the transport of large solutes. The objective of this study is to examine the effect of dynamic loading within a theoretical context, and to investigate the circumstances under which convective transport induced by dynamic loading might supplement diffusive transport. The theory of incompressible mixtures was used to model the tissue (gel) as a mixture of a gel solid matrix (extracellular matrix/scaffold), and two fluid phases (interstitial fluid solvent and neutral solute), to solve the problem of solute transport through the lateral surface of a cylindrical sample loaded dynamically in unconfined compression with frictionless impermeable platens in a bathing solution containing an excess of solute. The resulting equations are governed by nondimensional parameters, the most significant of which are the ratio of the diffusive velocity of the interstitial fluid in the gel to the solute diffusivity in the gel (Rg), the ratio of actual to ideal solute diffusive velocities inside the gel (Rd), the ratio of loading frequency to the characteristic frequency of the gel (f), and the compressive strain amplitude (epsilon 0). Results show that when Rg > 1, Rd < 1, and f > 1, dynamic loading can significantly enhance solute transport into the gel, and that this effect is enhanced as epsilon 0 increases. Based on representative material properties of cartilage and agarose gels, and diffusivities of various solutes in these gels, it is found that the ranges Rg > 1, Rd < 1, correspond to large solutes, whereas f > 1 is in the range of physiological loading frequencies. These theoretical predictions are thus in agreement with the limited experimental data available in the literature. The results of this study apply to any porous hydrated tissue or material, and it is therefore plausible to hypothesize that dynamic loading may serve to enhance solute transport in a variety of physiological processes.

MR imaging of normal and matrix-depleted cartilage: correlation with biomechanical function and biochemical composition

PURPOSE

To correlate articular cartilage function, as reflected in biomechanical properties and biochemical composition, with magnetic resonance (MR) imaging parameters of normal articular cartilage and cartilage partially depleted of matrix components.

MATERIALS AND METHODS

Normal articular cartilage from 12 porcine patellae was evaluated biomechanically, biochemically, and with MR imaging (with and without gadolinium enhancement). The patellae were then enzymatically treated to deplete the matrix of either collagen or proteoglycan and then reevaluated biomechanically, biochemically, and with MR imaging. Correlations between cartilaginous tissue function and MR imaging parameters were made. Analysis of variance was performed to assess the effect of enzymatic treatment on measured parameters. Linear correlations among the MR imaging, biochemical, and biomechanical parameters were performed to determine the strengths of the relationships. P <.05 indicated statistically significant differences.

RESULTS

Biochemical, biomechanical, and MR analyses enabled detection of changes caused by matrix depletion (P <.05). T2 was the most useful MR imaging parameter for distinguishing proteoglycan loss from collagen loss. T2 correlated significantly with both biomechanical modulus (indicative of cartilage stiffness; P <.001, R2 = 0.51) and biochemical proteoglycan content (P <.001, R2 = 0.44). Differentiation between proteoglycan loss and collagen loss in terms of T1 improved with gadolinium enhancement. With gadolinium enhancement, proteoglycan depletion was associated with a greater decrease in T1 than collagen depletion (P <.05).

CONCLUSION

An association between biochemical and biomechanical functional status and MR imaging parameters of articular cartilage was demonstrated. Linear correlations existed between modulus and proteoglycan content in terms of T2. Additionally, proteoglycan loss and collagen loss had differing effects on gadolinium-enhanced T1 when it was expressed as the ratio of T1 after gadolinium enhancement/T1 before gadolinium enhancement.

Self-diffusion of polymers in cartilage as studied by pulsed field gradient NMR

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) was used to investigate the self-diffusion behaviour of polymers in cartilage. Polyethylene glycol and dextran with different molecular weights and in different concentrations were used as model compounds to mimic the diffusion behaviour of metabolites of cartilage. The polymer self-diffusion depends extremely on the observation time: The short-time self-diffusion coefficients (diffusion time Delta approximately 15 ms) are subjected to a rather non-specific obstruction effect that depends mainly on the molecular weights of the applied polymers as well as on the water content of the cartilage. The observed self-diffusion coefficients decrease with increasing molecular weights of the polymers and with a decreasing water content of the cartilage. In contrast, the long-time self-diffusion coefficients of the polymers in cartilage (diffusion time Delta approximately 600 ms) reflect the structural properties of the tissue. Measurements at different water contents, different molecular weights of the polymers and varying observation times suggest that primarily the collagenous network of cartilage but also the entanglements of the polymer chains themselves are responsible for the observed restricted diffusion. Additionally, anomalous restricted diffusion was shown to occur already in concentrated polymer solutions.

Surface fissures in articular cartilage: new concepts, hypotheses and modeling

OBJECTIVE

Clarification and mesomechanical modeling of the inception of fissures at the surface of articular cartilage.

DESIGN

Articular cartilage is described as a macroscopically heterogeneous medium consisting of zones - layers - with different orientation of collagen fibers.

BACKGROUND

Degradation of mechanical properties of cartilage is a serious, still not fully clarified problem that deserves attention.

METHODS

Theoretical analysis based on a survey of known experimental findings related to the subject. The general author's mesomechanical concept of modeling heterogeneous media is applied to the elucidation and description of the formation of fissures at the surface of articular cartilage.

RESULTS

Our model clarifies how the high tensile stresses in the collagen fibers of the superficial tangential zone depend on the rate of loading.

CONCLUSIONS

The superficial cracks are caused predominantly by a very quick loading. This explains among others the high incidence of post-traumatic osteoarthritis of the lower extremity after accidents and injuries in sports.

RELEVANCE

Superficial fissures in articular cartilage are observed in joints with primary osteoarthritis. The current study specifies the kinds of loading that lead to their inception.

Measurement of bioreactor perfusion using dynamic contrast agent-enhanced magnetic resonance imaging

Dynamic magnetic resonance imaging was used to monitor solute diffusion through aggregates of Chinese hamster ovary cells growing on macroporous carriers in a fixed-bed bioreactor. Diffusion-weighted (1)H magnetic resonance imaging (MRI) and scanning electron microscopy demonstrated that cell growth in the bioreactor was heterogeneous, with the highest cell densities being found at the periphery of the carriers. T(1)-weighted magnetic resonance imaging measurements of the inflow of a commonly used magnetic resonance contrast agent, gadolinium-diethylenetriaminopentaacetic acid (Gd-DTPA), showed that migration of the agent through the peripheral cell masses could be explained by diffusion. However, appearance of the contrast agent in the center of the carriers was too fast to be explained by simple diffusion and indicated that these regions were perfused by convective flow. The average diffusivity of Gd-DTPA through the cell mass was found to be (2.4 +/- 0.2) x 10(-10) m(2) sec(-) (mean +/- SEM). This technique will be useful in the characterization and development of high-cell-density bioreactor systems, in which solute transport plays a critical role in cell growth and physiology.

Static compression of articular cartilage can reduce solute diffusivity and partitioning: implications for the chondrocyte biological response

Chondrocytes depend upon solute transport within the avascular extracellular matrix of adult articular cartilage for many of their biological activities. Alterations to bioactive solute transport may, therefore, represent a mechanism by which cartilage compression is transduced into cellular metabolic responses. We investigated the effects of cartilage static compression on diffusivity and partitioning of a range of model solutes including dextrans of molecular weights 3 and 40 kDa, and tetramethylrhodamine (a 430 Da fluorophore). New fluorescence methods were developed for real-time visualization and measurement of transport within compressed cartilage explants. Experimental design allowed for multiple measurements on individual explants at different compression levels in order to minimize confounding influences of compositional variations. Results demonstrate that physiological levels of static compression may significantly decrease solute diffusivity and partitioning in cartilage. Effects of compression were most dramatic for the relatively high molecular weight solutes. For 40 kDa dextran, diffusivity decreased significantly (p<0.01) between 8% and 23% compression, while partitioning of 3 and 40 kDa dextran decreased significantly (p<0.01) between free-swelling conditions and 8% compression. Since diffusivity and partitioning can influence pericellular concentrations of bioactive solutes, these observations support a role for perturbations to solute transport in mediating the cartilage biological response to compression.

Static compression is associated with decreased diffusivity of dextrans in cartilage explants

The chondrocytes of adult articular cartilage rely upon transport phenomena within their avascular extracellular matrix for many biological activities. Therefore, changes in matrix structure which influence cytokine transport parameters may be an important mechanism involved in the chondrocyte response to tissue compression. With this hypothesis in mind, partitioning and diffusion of 3-, 10-, and 40-kDa dextrans conjugated to tetramethylrhodamine, and 430-Da tetramethylrhodamine itself, were measured within statically compressed bovine articular cartilage explants using a novel experimental apparatus and desorption fluorescence method. Partitioning and diffusion were examined as functions of solute molecular weight and matrix proteoglycan density, and diffusion was measured versus static compression up to 35% volumetric strain. In general, partition coefficients and diffusivities were found to decrease with increasing solute molecular weight. In addition, for a given solute, diffusivities decreased significantly with increasing static compression. Results therefore suggest a possible role for transport limitations of relatively large molecular weight solutes within the extracellular matrix in mediating the biological response of chondrocytes to cartilage compression.

Scanning electrochemical microscopy as a local probe of oxygen permeability in cartilage

The use of scanning electrochemical microscopy, a high-resolution chemical imaging technique, to probe the distribution and mobility of solutes in articular cartilage is described. In this application, a mobile ultramicroelectrode is positioned close ( approximately 1 microm) to the cartilage sample surface, which has been equilibrated in a bathing solution containing the solute of interest. The solute is electrolyzed at a diffusion-limited rate, and the current response measured as the ultramicroelectrode is scanned across the sample surface. The topography of the samples was determined using Ru(CN)(6)(4-), a solute to which the cartilage matrix was impermeable. This revealed a number of pit-like depressions corresponding to the distribution of chondrocytes, which were also observed by atomic force and light microscopy. Subsequent imaging of the same area of the cartilage sample for the diffusion-limited reduction of oxygen indicated enhanced, but heterogeneous, permeability of oxygen across the cartilage surface. In particular, areas of high permeability were observed in the cellular and pericellular regions. This is the first time that inhomogeneities in the permeability of cartilage toward simple solutes, such as oxygen, have been observed on a micrometer scale.

Analysis of cell growth kinetics and substrate diffusion in a polymer scaffold

The cultivation of cartilage cells (chondrocytes) in polymer scaffolds leads to implants that may potentially be used to repair damaged joint cartilage or for reconstructive surgery. For this technique to be medically applicable, the physical parameters that govern cell growth in a polymer scaffold must be understood. This understanding of cell behavior under in vitro conditions, where diffusion is the primary mode of transport of nutrients, may aid in the scale-up of the cartilage generation process. A mathematical model of chondrocyte generation and nutrient consumption is developed here to analyze the behavior of cell growth in a biodegradable polymer matrix for a series of different thickness polymers. Recent literature has implied that the diffusion of nutrients is a major factor that limits cell growth (Freed et al., 1994). In the present paper, a mathematical model is developed to directly relate the effects of increasing cell mass in the polymer matrix on the transport of nutrients. Reaction and diffusion of nutrients in the cell-polymer system are described using the fundamental species continuity equations and the volume averaging method. The volume averaging method is utilized to derive a single averaged nutrient continuity equation that includes the effective transport properties. This approach allows for the derivation of effective diffusion and rate coefficients as functions of the cell volume fraction. The cell volume fraction as a function of time is determined by solution of a material balance on cell mass. Growth functions including the Moser, a modified Contois, and an nth-order heterogeneous growth kinetic model are evaluated through a parameter analysis, and the results are compared to experimental data found in the literature. The results indicate that cellular functions in conjunction with mass transfer processes can account partially for the general trends in the cell growth behavior for various thickness polymers. The Contois growth function appeared to describe the data more accurately in terms of the lag period at early times and the long time limits. However, all kinetic growth functions required variations in the kinetic parameters to fully describe the effects of polymer thickness. This result implies that restricted diffusion of nutrients is not the sole factor limiting cell growth when the thickness of the polymer is changed. Therefore, further experimental data and model improvements are needed to accurately describe the cell growth process.

Recombinant proteins for genetic disease

The era of molecular biology has led to the development of powerful tools capable of generating therapeutics for genetic disorders. Although there is much current emphasis placed on the development of 'gene therapy' for human disease, developments in the production and availability of recombinant proteins are likely to have a more substantial impact on genetic disease in the short term. The clinical evaluation of recombinant or purified proteins serves as an initial 'proof of principle' of gene-based therapies and thus should expedite advances in this area. Current examples include the use of bovine adenosine deaminase for a form of severe combined immune deficiency (SCID) (Hilman BC, Sorensen RU. Management options: SCIDS with adenosine deaminase deficiency. Ann Allergy 72: 1994: 395-403) and glucocerebrosidase for Gaucher disease (Niederau C, vom Dahl S, Haussinger D. First long-term results of imiglucerase therapy of type 1 Gaucher disease. Eur J Med Res 1998: 3: 25-30). The success of these two enzyme replacement regimes in human clinical trials has been a main impetus for the development of gene-based therapies for these disorders. The 'molecularization of medicine' has led to a more thorough understanding of the molecular basis of disease and disease pathogenesis. The availability of recombinant proteins and the development of appropriate delivery systems will result in more widespread use of these agents. Protein engineering will generate agents with novel functions as is already witnessed with the generation of fusion proteins. This review will highlight advances in the use of recombinant proteins for genetic disease and future potential uses of recombinant proteins.

Diffusive properties of immature articular cartilage

The diffusive properties of immature bovine articular cartilage were determined using two different-sized, uncharged solutes (glucose 180 Da, and dextran 10k Da). Radioactively tagged glucose and dextran were diffused into the cartilage for transport times of 5, 15, and 60 min, and the diffusion and partition coefficients were calculated by fitting the experimental data to a one-dimensional diffusion model. The diffusion and partition coefficients for the two solutes averaged 6.08 +/- 2.19 and 5.09 +/- 2.51 (x 10(-6) cm2/s) and 0.712 +/- 0.149 and 0.615 +/- 0.120, respectively. Both coefficients were significantly greater for glucose compared to the larger dextran. While no statistical differences could be found in the diffusive properties of these solutes in immature cartilage compared to their diffusive properties in mature cartilage, there was some evidence that the larger dextran solute might diffuse faster in the earlier time periods. Finally, the bulk fluid contents between the two types of cartilage were not different even though the immature tissue was significantly thicker (1.6 times) than the mature tissue. Our results indicate that the solute diffusion properties of articular cartilage, at least with respect to uncharged solutes, do not change during skeletal maturation.