Importance of Collagen Orientation and Depth-Dependent Fixed Charge Densities of Cartilage on Mechanical Behavior of Chondrocytes

The collagen network and proteoglycan matrix of articular cartilage are thought to play an important role in controlling the stresses and strains in and around chondrocytes, in regulating the biosynthesis of the solid matrix, and consequently in maintaining the health of diarthrodial joints. Understanding the detailed effects of the mechanical environment of chondrocytes on cell behavior is therefore essential for the study of the development, adaptation, and degeneration of articular cartilage. Recent progress in macroscopic models has improved our understanding of depth-dependent properties of cartilage. However, none of the previous works considered the effect of realistic collagen orientation or depth-dependent negative charges in microscopic models of chondrocyte mechanics. The aim of this study was to investigate the effects of the collagen network and fixed charge densities of cartilage on the mechanical environment of the chondrocytes in a depth-dependent manner. We developed an anisotropic, inhomogeneous, microstructural fibril-reinforced finite element model of articular cartilage for application in unconfined compression. The model consisted of the extracellular matrix and chondrocytes located in the superficial, middle, and deep zones. Chondrocytes were surrounded by a pericellular matrix and were assumed spherical prior to tissue swelling and load application. Material properties of the chondrocytes, pericellular matrix, and extracellular matrix were obtained from the literature. The loading protocol included a free swelling step followed by a stress-relaxation step. Results from traditional isotropic and transversely isotropic biphasic models were used for comparison with predictions from the current model. In the superficial zone, cell shapes changed from rounded to elliptic after free swelling. The stresses and strains as well as fluid flow in cells were greatly affected by the modulus of the collagen network. The fixed charge density of the chondrocytes, pericellular matrix, and extracellular matrix primarily affected the aspect ratios (height/width) and the solid matrix stresses of cells. The mechanical responses of the cells were strongly location and time dependent. The current model highlights that the collagen orientation and the depth-dependent negative fixed charge densities of articular cartilage have a great effect in modulating the mechanical environment in the vicinity of chondrocytes, and it provides an important improvement over earlier models in describing the possible pathways from loading of articular cartilage to the mechanical and biological responses of chondrocytes.

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Functional grading of pericellular matrix surrounding chondrocytes: potential roles in signaling and fluid transport

10.1101/365569 ◽

2018 ◽

Author(s):

F. Saadat ◽

M.J. Lagieski ◽

V. Birman ◽

S. Thomopoulos ◽

G.M. Genin

Keyword(s):

Mechanical Properties ◽

Extracellular Matrix ◽

Articular Cartilage ◽

Cell Surface ◽

Fluid Transport ◽

Pericellular Matrix ◽

Mechanical Environment ◽

Spatial Gradients ◽

Linearized Model ◽

Mechanical Microenvironment

AbstractThe extracellular matrix surrounding chondrocytes within cartilage and fibrocartilage has spatial gradients in mechanical properties. Although the function of these gradients is unknown, the potential exists for cells to tailor their mechanical microenvironment through these gradients. We hypothesized that these gradients enhance fluid transport around the cell during the slow loading cycles that occur over the course of a day, and that this enhancement changes the nature of the mechanical signals received at the surface of the cell. To test this hypothesis, we studied the effect of these gradients on the mechanical environment around a chondrocyte using a closed form, linearized model. Results demonstrated that functional grading of the character observed around chondrocytes in articular cartilage enhances fluid transport, and furthermore inverts compressive radial strains to provide tensile signals at the cell surface. The results point to several potentially important roles for functional grading of the pericellular matrix.

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Chondrons from articular cartilage. V. Immunohistochemical evaluation of type VI collagen organisation in isolated chondrons by light, confocal and electron microscopy

Journal of Cell Science ◽

10.1242/jcs.103.4.1101 ◽

1992 ◽

Vol 103 (4) ◽

pp. 1101-1110 ◽

Cited By ~ 2

Author(s):

C.A. Poole ◽

S. Ayad ◽

R.T. Gilbert

Keyword(s):

Electron Microscopy ◽

Extracellular Matrix ◽

Articular Cartilage ◽

Specific Cell ◽

Potential Contribution ◽

Collagen Network ◽

Tail Region ◽

Type Vi Collagen ◽

Surface Receptors ◽

Basal Pole

The pericellular microenvironment around articular cartilage chondrocytes must play a key role in regulating the interaction between the cell and its extracellular matrix. The potential contribution of type VI collagen to this interaction was investigated in this study using isolated canine tibial chondrons embedded in agarose monolayers. The immunohistochemical distribution of an anti-type VI collagen antibody was assessed in these preparations using fluorescence, peroxidase and gold particle probes in combination with light, confocal and transmission electron microscopy. Light and confocal microscopy both showed type VI collagen concentrated in the pericellular capsule and matrix around the chondrocyte with reduced staining in the tail region and the interconnecting segments between adjacent chondrons. Minimal staining was recorded in the territorial and interterritorial matrices. At higher resolution, type VI collagen appeared both as microfibrils and as amorphous deposits that accumulated at the junction of intersecting capsular fibres and microfibrils. Electron microscopy also showed type VI collagen anchored to the chondrocyte membrane at the articular pole of the pericellular capsule and tethered to the radial collagen network through the tail at the basal pole of the capsule. We suggest that type VI collagen plays a dual role in the maintenance of chondron integrity. First, it could bind to the radial collagen network and stabilise the collagens, proteoglycans and glycoproteins of the pericellular microenvironment. Secondly, specific cell surface receptors exist, which could mediate the interaction between the chondrocyte and type VI collagen, providing firm anchorage and signalling potentials between the pericellular matrix and the cell nucleus. In this way type VI collagen could provide a close functional interrelationship between the chondrocyte, its pericellular microenvironment and the load bearing extracellular matrix of adult articular cartilage.

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Zonal Poisson Effect on the Chondron Deformation in Articular Cartilage under Compression

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.342-343.133 ◽

2007 ◽

Vol 342-343 ◽

pp. 133-136

Author(s):

Jae Bong Choi

Keyword(s):

Extracellular Matrix ◽

Articular Cartilage ◽

Mechanical Response ◽

Three Dimensional ◽

Pericellular Matrix ◽

Poisson Effect ◽

Type Vi Collagen ◽

Confocal Images ◽

Three Dimensional Models

The objective of this study was to quantify the zonal difference of the in situ chondron’s Poisson effect under different magnitudes of compression. Fluorescence immunolabeling for type VI collagen was used to identify the pericellular matrix (PCM) and chondron, and a series of fluorescent confocal images were recorded and reconstructed to form quantitative three-dimensional models. The zonal variations in the mechanical response of the chondron do not appear to be due to zonal differences in PCM properties, but rather seem to result from significant inhomogeneities in relative stiffnesses of the extracellular matrix (ECM) and PCM with depth.

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A Triphasic Theory for the Swelling and Deformation Behaviors of Articular Cartilage

Journal of Biomechanical Engineering ◽

10.1115/1.2894880 ◽

1991 ◽

Vol 113 (3) ◽

pp. 245-258 ◽

Cited By ~ 743

Author(s):

W. M. Lai ◽

J. S. Hou ◽

V. C. Mow

Keyword(s):

Articular Cartilage ◽

Charge Density ◽

Fixed Charge ◽

Solid Matrix ◽

Confined Compression ◽

Fixed Charge Density ◽

Chemical Expansion ◽

Chemical Potentials ◽

Free Swelling ◽

Triphasic Theory

Swelling of articular cartilage depends on its fixed charge density and distribution, the stiffness of its collagen-proteoglycan matrix, and the ion concentrations in the interstitium. A theory for a tertiary mixture has been developed, including the two fluid-solid phases (biphasic), and an ion phase, representing cation and anion of a single salt, to describe the deformation and stress fields for cartilage under chemical and/or mechanical loads. This triphasic theory combines the physico-chemical theory for ionic and polyionic (proteoglycan) solutions with the biphasic theory for cartilage. The present model assumes the fixed charge groups to remain unchanged, and that the counter-ions are the cations of a single salt of the bathing solution. The momentum equation for the neutral salt and for the intersitial water are expressed in terms of their chemical potentials whose gradients are the driving forces for their movements. These chemical potentials depend on fluid pressure p, salt concentration c, solid matrix dilatation e and fixed charge density cF. For a uni-uni valent salt such as NaCl, they are given by μi = μoi + (RT/Mi)ln[γ±2c (c + c F)] and μW = μow + [p − RTφ(2c + cF) + Bwe]/ρTw, where R, T, Mi, γ±, φ, ρTw and Bw are universal gas constant, absolute temperature, molecular weight, mean activity coefficient of salt, osmotic coefficient, true density of water, and a coupling material coefficient, respectively. For infinitesimal strains and material isotropy, the stress-strain relationship for the total mixture stress is σ = − pI − TcI + λs(trE)I + 2μsE, where E is the strain tensor and (λs,μs) are the Lame´ constants of the elastic solid matrix. The chemical-expansion stress (− Tc) derives from the charge-to-charge repulsive forces within the solid matrix. This theory can be applied to both equilibrium and non-equilibrium problems. For equilibrium free swelling problems, the theory yields the well known Donnan equilibrium ion distribution and osmotic pressure equations, along with an analytical expression for the “pre-stress” in the solid matrix. For the confined-compression swelling problem, it predicts that the applied compressive stress is shared by three load support mechanisms: 1) the Donnan osmotic pressure; 2) the chemical-expansion stress; and 3) the solid matrix elastic stress. Numerical calculations have been made, based on a set of equilibrium free-swelling and confined-compression data, to assess the relative contribution of each mechanism to load support. Our results show that all three mechanisms are important in determining the overall compressive stiffness of cartilage.

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Collagen network and fixed charge density of articular cartilage modulate time- and depth-dependent behavior of chondrocytes

Journal of Biomechanics ◽

10.1016/s0021-9290(06)82962-3 ◽

2006 ◽

Vol 39 ◽

pp. S24

Author(s):

R.K. Korhonen ◽

P. Julkunen ◽

W. Herzog

Keyword(s):

Articular Cartilage ◽

Charge Density ◽

Fixed Charge ◽

Collagen Network ◽

Fixed Charge Density ◽

Dependent Behavior

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An Axisymmetric Elastic Layered Half-Space Model for Micropipette Aspiration of the Chondrocyte Pericellular Matrix

10.1115/imece2001/bed-23156 ◽

2001 ◽

Author(s):

Leonidas G. Alexopoulos ◽

Mansoor A. Haider ◽

Farshid Guilak

Keyword(s):

Extracellular Matrix ◽

Pericellular Matrix ◽

Bearing Surface ◽

Single Population ◽

Mechanical Environment ◽

Space Model ◽

Cartilage Homeostasis ◽

Diarthrodial Joints ◽

Genetic And Environmental Factors ◽

Layered Half Space

Abstract Articular cartilage is an aneural, avascular connective tissue that serves as the resilient load-bearing surface at the articulating ends of diarthrodial joints. A sparse single population of cells known as chondrocytes maintains the extracellular matrix (ECM) of this tissue through a balance of anabolic and catabolic activities. The mechanical environment of chondrocytes, in conjunction with other genetic and environmental factors (e.g., growth factors, cytokines), plays an important role in regulating cartilage homeostasis and, as a consequence, the health of the joint.

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Effects of Inhomogeneous Fixed Charge Density on the Electrical Signals for Chondrocytes in Cartilage

10.1115/imece2000-1931 ◽

2000 ◽

Author(s):

W. M. Lai ◽

D. D. Sun ◽

G. A. Ateshian ◽

X. E. Guo ◽

V. C. Mow

Keyword(s):

Signal Transduction ◽

Articular Cartilage ◽

Charge Density ◽

Streaming Potential ◽

Potential Difference ◽

Fixed Charge ◽

Diffusion Potential ◽

Solid Matrix ◽

Fixed Charge Density ◽

Transduction Mechanisms

Abstract An important step toward understanding the signal transduction mechanisms that modulate cellular activities is the accurate prediction of the mechanical and electro-chemical environment of the cells in well-defined experimental configurations. One such configuration is the steady permeation experiment (e.g., bioreactors) in the open circuit condition. Using our triphasic theory, we have calculated the strain, velocity and the electric potential fields inside a layer of charged articular cartilage, through which a uni-univalent salt (e.g., NaCl) solution permeates under a constant pressure difference across the layer. The fluid flow through the tissue gives rise to an electrical potential difference across the tissue. This potential difference is the well-known “streaming potential” that is measured by Ag/AgCl electrodes placed across the tissue on the outside. Our results show that inside the tissue, in addition to the streaming potential caused by fluid convection, there is also a “diffusion potential” caused by cation and anion concentration gradients that are induced by the gradient of fixed charge density (FCD) inside the tissue. The gradient of FCD may be intrinsic, i.e., the tissue has an inhomogeneous FCD distribution, or it may also be caused by a non-uniform compaction of the solid matrix as is the case in steady permeation where the drag force exerted by the permeating fluid onto the solid matrix causes a compressive strain field inside the tissue. In this experimental configuration, the diffusion potential would compete against the streaming potential. The magnitude and the polarity of the electric field depend, amongst other material parameters, on the compressive stiffness of the tissue. For softer tissue (e.g., aggregate modulus <0.54 MPa for a set of realistic material and testing parameters), the diffusion potential dominates over the streaming potential and vice versa for stiffer tissue. For articular cartilage what the cells see in situ is the combined electrical effect of intrinsic and deformation induced inhomogeneity of FCD. The present results provide not only quantitative information, but also new insight into an important problem in biotechnology. These results also demonstrate that for proper interpretation of the mechano-electrochemical signal transduction mechanisms that is needed for modulating cellular biosynthetic activities, one must not ignore the important effects of diffusion potential.

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Biomechanical Properties of the Canine Knee Articular Cartilage as Related to Matrix Proteoglycans and Collagen

Engineering in Medicine ◽

10.1243/emed_jour_1988_017_042_02 ◽

1988 ◽

Vol 17 (4) ◽

pp. 157-162 ◽

Cited By ~ 47

Author(s):

J Jurvelin ◽

A-M Säämänen ◽

J Arokoski ◽

H J Helminen ◽

I Kiviranta ◽

...

Keyword(s):

Articular Cartilage ◽

Biomechanical Properties ◽

Constant Load ◽

Load Application ◽

Solid Matrix ◽

Indentation Creep ◽

Collagen Network ◽

Tibial Cartilage ◽

Equilibrium Modulus

The instant, creep and equilibrium responses of canine knee articular cartilages were determined after a constant load application with an in situ indentation creep test and related to the chemical composition of the tissue. Instantly, the cartilage stiffness correlated inversely with the proportion of proteoglycans (PGs) extractable with guanidium chloride. The tibial cartilage, rich in PGs but relatively poor in collagen, showed a low resistance to instant rearrangement of the solid matrix after load application. However, the resistance of the tibial cartilage to water flow during creep deformation was similar or even higher than in the femur. The rate of creep correlated inversely with the PG content. The equilibrium modulus of the femoral cartilage (0.40 MPa), 29 per cent higher than in the tibia (0.31 MPa), was related to the content of PGs, while in the tibia the direct correlation between PGs and modulus was not observed. Our results suggest that while PGs control the fluid flow in articular cartilage, a high PG content alone does not guarantee high stiffness of the cartilage. Instead, the properties of the collagen network are suggested to control particularly the instant shape alterations of the articular cartilage under compression.

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Study of the Mechanical Environment of Chondrocytes in Articular Cartilage Defects Repaired Area under Cyclic Compressive Loading

Journal of Healthcare Engineering ◽

10.1155/2017/1308945 ◽

2017 ◽

Vol 2017 ◽

pp. 1-10

Author(s):

Hai-Ying Liu ◽

Hang-Tian Duan ◽

Chun-Qiu Zhang ◽

Wei Wang

Keyword(s):

Articular Cartilage ◽

Cyclic Loading ◽

Response Curve ◽

Fluid Pressure ◽

Cartilage Defects ◽

Loading Frequency ◽

Liquid Pressure ◽

Mechanical Environment ◽

Finite Element Software ◽

Maximum Value

COMSOL finite element software was used to establish a solid-liquid coupling biphasic model of articular cartilage and a microscopic model of chondrocytes, using modeling to take into account the shape and number of chondrocytes in cartilage lacuna in each layer. The effects of cyclic loading at different frequencies on the micromechanical environment of chondrocytes in different regions of the cartilage were studied. The results showed that low frequency loading can cause stress concentration of superficial chondrocytes. Moreover, along with increased frequency, the maximum value of stress response curve of chondrocytes decreased, while the minimum value increased. When the frequency was greater than 0.2 Hz, the extreme value stress of response curve tended to be constant. Cyclic loading had a large influence on the distribution of liquid pressure in chondrocytes in the middle and deep layers. The concentration of fluid pressure changed alternately from intracellular to peripheral in the middle layer. Both the range of liquid pressure in the upper chondrocytes and the maximum value of liquid pressure in the lower chondrocytes in the same lacunae varied greatly in the deep layer. At the same loading frequency, the elastic modulus of artificial cartilage had little effect on the mechanical environment of chondrocytes.

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