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PubMed2006

Modeling cell and matrix anisotropy in fibroblast populated collagen vessels.

Microstructurally based models for bio-artificial tissues are needed to predict in vivo mechanical behavior and to validate assumptions for models of biologic tissues. We develop a microstructural model, based on on Zahalak et al. (2000) [Biophys 79(5):2369-2381], to describe matrix and tissue anisotropy observed in recent biaxial tests of fibroblast populated collagen vessels (FPCVs) with different cell orientations (Wagenseil et al. in Ann Biomed Eng 32(5):720-731 2004). The model includes pseudo-elastic cell behavior and pseudo-elastic, non-linear matrix behavior with recruitment of initially buckled collagen fibers. We obtained estimates of collagen matrix parameters from measurements of FPCVs treated with 2x 10(-6) M Cytochalasin D and used these estimates to determine cell parameters in FPCVs activated with 5% fetal calf serum. The estimated stiffness of individual fibroblasts was 41-1,165 kPa. Parameter estimates for both cell and matrix were influenced by the non-linearity of the biaxial test data, making it difficult to obtain unique parameter values for some experiments. Additional microstructural measurements of the collagen matrix may help to more precisely determine the relative contributions of cells and matrix.

Animals↗
PubMed2004

Direct measurements of the compressive properties of single proteoglycan aggregates.

Proteoglycan aggregate is a major component of the extracellular matrix in articular cartilage and is considered to be responsible for the resistance to compression of this tissue. The reduced stiffness of articular cartilage due to the loss of proteoglycan aggregate has been reported in osteoarthritis. In order to understand the mechanical properties of extracellular matrix in articular cartilage at molecular level, the compressive properties of 36 single molecules of proteoglycan aggregate were directly measured using a laser tweezers/interferometer system. The proteoglycan aggregates showed resistance when compressed to approximately 30% of their contour length. The stiffness of proteoglycan aggregates increased non-linearly from 2.6+/-3.8 pN/microm (compressed to 30-35% of their contour length) to 115.5+/-30.9 pN/microm (compressed to 2.5-5% of their contour length).

Compressive Strength↗
PubMed1994

Fibre reinforcing by collagen in cartilage and soft connective tissues.

Mechanical support in animals is performed by connective tissues. The soft tissues consist of collagen fibres embedded in a highly hydrated proteoglycan gel. By considering them as fibrous composite materials, a unifying theme can be found to explain their mechanical behaviour in terms of their structure and composition. Interactions between fibres and matrix are essential to their functioning in this way. Calculations are made of the maximum stress transfer per D-period required to enable collagen fibres of a given axial ratio to provide effective reinforcing. Weak non-specific interactions are shown to be sufficient. A mechanical function is proposed for type X collagen in the epiphyseal growth plate on the basis that it modifies and supplements the properties of the type II fibrils. This provides extra reinforcing and, hence, a greater stiffness to the cartilage to compensate for the reduced amount of extracellular matrix.

Animals↗
PubMed1995

Changes in cartilage composition and physical properties due to stromelysin degradation.

OBJECTIVE: To determine the effects of stromelysin treatment on biochemical, histologic, and swelling characteristics of intact cartilage explants and to correlate these effects with changes in the functional physical properties of the tissue. METHODS: Bovine articular cartilage explants were cultured for up to 3 days in the presence or absence of recombinant human stromelysin (SLN). Damage to matrix proteoglycans and collagens was assessed and characterized by N-terminal sequencing and Western blot analysis, respectively. Explants were mechanically tested to assess the ability of the tissue to withstand cyclic and static compressive loads. RESULTS: Treatment with SLN resulted in a time- and dose-dependent loss of proteoglycans from cartilage explants, with significant loss seen after 3 days of exposure to 20 nM SLN: Histology indicated that initial loss of proteoglycans occurred in regions near the tissue surface and proceeded inward with increasing time of SLN exposure. SLN treatment resulted in degradation of matrix collagen types IX and II, and a concomitant increase in tissue swelling. This matrix degradation resulted in severe alterations in functional physical properties of the tissue, including compressive stiffness. The initial, focal loss of proteoglycans that resulted from SLN treatment was most accurately detected with high-frequency streaming potential measurements. CONCLUSION: Exposure of intact cartilage to SLN caused specific, molecular-level degradation of matrix molecules, which resulted in changes in the swelling behavior and marked deterioration of functional physical properties of the tissue.

Amino Acid Sequence↗
PubMed1988

Material properties of living soft tissue composites.

Collagen sponges seeded with fibroblasts have been used as a soft tissue substitute in wound healing applications. This biomaterial is a good in vitro analog of a connective tissue. Therefore, analysis of the properties of this material may be useful for theoretically modeling soft tissues. Stress-strain curves for such cell-seeded collagen sponges were measured to determine composite stiffness and ultimate tensile strength. Theoretical modeling was done by defining a particle-reinforced matrix using the composite sphere model. A system of uniaxially oriented fibers was then introduced to this equivalent homogeneous media and material properties were determined using the composite cylinder model. Geometric averaging was performed to yield the stiffness and Poissons' ratio for a composite with randomly oriented fibers. Inputs to the model were constituent material properties, cell volume fraction, and fiber volume fraction. From theoretical results, material properties of soft tissues and their substitutes depend on fiber mechanical properties and volume fraction and not cellular mechanical properties and volume fraction. Therefore, the increase in experimentally observed composite stiffness with increased cell number was due to deposition of newly synthesized stiffer collagen fibers, and not due to the physical presence of cells themselves.

Collagen↗
PubMed1997

The functional consequences of airway remodeling in asthma.

Structural changes in the airway walls involving extracellular matrix remodelling are prominent features of asthma. These changes are probably driven by mediators released as a consequence of chronic allergic inflammation. It is clear that changes in the extracellular matrix have the capacity to influence airway function in asthma. However, it is not clear how each of the many changes that occur in the airway wall contribute to altered airway function in asthma. Collagen deposition in the subepithelial matrix, and hyaluronan and versican deposition around and internal to the smooth muscle would be expected to oppose the effect of smooth muscle contraction. Conversely, geometric considerations would result in exaggerated airway narrowing for a given degree of smooth muscle shortening, as the airway wall is thickened by the deposition of these molecules internal to the smooth muscle. Elastin and cartilage reorganization and degradation in the airway walls would be expected to result in decreased airway wall stiffness and increased airway narrowing for a given amount of force generated by the smooth muscle. Degradation of matrix associated with the smooth muscle may both decrease the stiffness of the parallel elastic component and uncouple smooth muscle from the load provided by lung recoil, allowing exaggerated smooth muscle shortening. Increase in muscle mass may be associated with an increase, a decrease or no change in smooth muscle contractility. If an increase in muscle mass was associated with preservation of its contractile capacity modelling studies suggest that it could be the most important contributor to exaggerated airway narrowing. Modelling studies also suggest that the pattern of mucosal folding during smooth muscle contraction may be an important determinant of airway narrowing. The greater the number of folds, and the stiffer the subepithelial collagenous layer the more resistant the airway will be to narrowing.

Asthma↗
PubMed2000

Expression of growth factors, cytokines and matrix metalloproteinases in frozen shoulder.

Frozen shoulder is a chronic fibrosing condition of the capsule of the joint. The predominant cells involved are fibroblasts and myofibroblasts which lay down a dense matrix of type-I and type-III collagen within the capsule. This subsequently contracts leading to the typical features of pain and stiffness. Cytokines and growth factors regulate the growth and function of the fibroblasts of connective tissue and remodelling of the matrix is controlled by the matrix metalloproteinases (MMPs) and their inhibitors. Our aim was to determine whether there was an abnormal expression or secretion of cytokines, growth factors and MMPs in tissue samples from 14 patients with frozen shoulder using the reverse transcription/polymerase chain reaction (RT/PCR) technique and to compare the findings with those in tissue from four normal control shoulders and from five patients with Dupuytren's contracture. Tissue from frozen shoulders demonstrated the presence of mRNA for a large number of cytokines and growth factors although the frequency was only slightly higher than in the control tissue. The frequency for a positive signal for the proinflammatory cytokines Il-beta and TNF-alpha and TNF-beta, was not as great as in the Dupuytren's tissue. The presence of mRNA for fibrogenic growth factors was, however, more similar to that obtained in the control and Dupuytren's tissue. This correlated with the histological findings which in most specimens showed a dense fibrous tissue response with few cells other than mature fibroblasts and with very little evidence of any active inflammatory cell process. Positive expressions of the mRNA for the MMPs were also increased, together with their natural inhibitor TIMP. The notable exception compared with control and Dupuytren's tissue was the absence of MMP-14, which is known to be a membrane-type MMP required for the activation of MMP-2 (gelatinase A). Understanding the control mechanisms which play a part in the pathogenesis of frozen shoulder may lead to the development of new regimes of treatment for this common, protracted and painful chronic fibrosing condition.

Cytokines↗
PubMed2005

[Cardiovascular system and aging].

Aging is one of the most important cardiovascular risk factors. Age-related morphologic changes in large resistance vessels include an intima-media-thickening and increased deposition of matrix substance, ultimately leading to a reduced compliance and an increased stiffness of the vessels. Aging of the heart is mainly characterized by an increase of the left ventricular mass in relation to the chamber volume and a decrease of diastolic function. There is some controversy in regard to the question if these changes in the vessel wall are the consequence of aging or if a decrease in physical activity is a major contributor of this process. With age the cardiovascular profile is changing. Whereas smoking is less prominent, arterial hypertension and diabetes mellitus are more often encountered. Primary and secondary prevention through cardiovascular risk factor management is also very important in the aging population due to the increased risk of acute vascular complications with age. Preventive measures have to include life style factor interventions as well as optimized drug therapy. There is no scientific evidence that vascular aging can be prevented by administration of supplements such as antioxidant vitamins. Aspirin is effective for cardiovascular prevention up to a higher age. Betablockers and ACE-inhibitors are generally underused in older patients after myocardial infarctions. Statins are effective in reducing cardiovascular complications up to an age of 80 years. Myocardial infarction in elderly patients is often characterized by atypical symptoms and may be even silent. Interventional therapy in elderly patients is as successful as in younger patients but has an increased complication rate. Ambulatory cardiac rehabilitation in elderly patients leads to significant improvements of physical capacity, well-being and quality of life and may help to prevent social isolation.

Age Distribution↗
PubMed2003

AGEing and osteoarthritis: a different perspective.

PURPOSE OF REVIEW: Across the world, osteoarthritis is the most commonly occurring musculoskeletal disease of the elderly, affecting more than 25% of the population older than 60 years of age. By far the single greatest risk factor for the development of osteoarthritis is age, but a mechanism to explain this relation has not yet been identified. If such a mechanism is identified, this potentially also provides a novel target for osteoarthritis therapy. The identification of new therapeutic targets is of utmost importance, because a disease-modifying treatment for osteoarthritis is not available and, because of the graying of the population, the number of patients with osteoarthritis will continue to increase, which will pose an enormous social and economic burden on society. RECENT FINDINGS: Advanced glycation end products accumulate in human articular cartilage with increasing age, and affect biomechanical, biochemical, and cellular characteristics of the tissue. As an illustration, accumulation of advanced glycation end products increase cartilage stiffness and brittleness while decreasing the synthesis and degradation of cartilage matrix constituents. Articular cartilage becomes more prone to damage, and thus osteoarthritis, at elevated concentrations of advanced glycation end products. SUMMARY: The reviewed literature demonstrates that the age-related accumulation of advanced glycation end products in articular cartilage may provide a molecular mechanism capable of (at least in part) explaining the age-related increase in the incidence of osteoarthritis. This conclusion paves the way for new strategies to prevent or treat osteoarthritis via inhibition and/or reversal of this process.

Aging↗
PubMed2004

Absence of a tough-brittle transition in the statistical fracture of unidirectional composite tapes under local load sharing.

We show analytically that a planar, unidirectional fibrous composite, which is an idealized random heterogeneous material consisting of stiff fibers of random strength embedded in parallel in a compliant matrix, fractures in a brittle manner when the fibers engage in idealized, local load sharing. Both the fibers and matrix are assumed time independent. This brittle behavior occurs irrespective of the disorder (variability) in fiber strengths, which we represent by a power distribution function. Our result goes far toward settling a long-standing question, not resolvable by computer simulations, regarding whether or not the brittle failure regime gives way to a tough, ductile-like failure regime as the variability in fiber strengths is increased past some threshold. We establish this result by calculating upper and lower bounding distributions for composite strength using the Chen-Stein theorem of extreme value statistical theory when failure events are dependent. These bounds both have weakest link character and, by comparing them with empirical strength distributions generated by Monte Carlo simulations, we find that the upper bound is a good approximation to the actual failure probability when the fiber strength variability is large. This regime is where previous models have broken down, raising speculation about a brittle-ductile transition.

Journal Article↗
PubMed1988

Nonlinear incompressible finite element for simulating loading of cardiac tissue--Part II: Three dimensional formulation for thick ventricular wall segments.

A three dimensional incompressible and geometrically as well as materially nonlinear finite element is formulated for future implementation in models of cardiac mechanics. The stress-strain relations in the finite element are derived from a recently proposed constitutive law which is based on the histological composition of the myocardium. The finite element is formulated for large deformations and considers incompressibility by introducing the hydrostatic pressure as an additional variable. The results of passive loading cases simulated by this element allow to analyze the mechanical properties of ventricular wall segments, the main of which are that the circumferential direction is stiffer than the longitudinal one, that its shear stiffness is considerably lower than its tensile and compressive stiffness and that, due to its mechanically prominent role, the collagenous matrix may affect the myocardial perfusion.

Heart↗
PubMed2006

Bone strength: current concepts.

Bones serve several mechanical functions, including acoustic amplification in the middle ear, shielding vital organs from trauma, and serving as levers for muscles to contract against. Bone is a multiphase material made up of a tough collagenous matrix intermingled with rigid mineral crystals. The mineral gives bone its stiffness. Without sufficient mineralization, bones will plastically deform under load. Collagen provides toughness to bone making it less brittle so that it better resists fracture. Bone adapts to mechanical stresses largely by changing its size and shape, which are major determinants of its resistance to fracture. Tissue is added in regions of high mechanical stress providing an efficient means for improving bone strength. Experiments have shown that small additions of bone mineral density (BMD) (5-8%) caused by mechanical loading can improve bone strength by over 60% and extend bone fatigue life by 100-fold. Consequently, it is clear that bone tissue possesses a mechanosensing apparatus that directs osteogenesis to where it is most needed for improving bone strength. The biological processes involved in bone mechanotransduction are poorly understood and further investigation of the molecular mechanisms involved might uncover drug targets for osteoporosis. Several pathways are emerging from current research, including membrane ion channels, ATP signaling, second messengers, such as prostaglandins and nitric oxide, insulin-like growth factors, and Wnt signaling.

Biomechanical Phenomena↗
PubMed2005

The influence of various substances on the biomechanical behavior of lamina cribrosa and peripapillary sclera.

PURPOSE: Changes in the biomechanical properties of the lamina cribrosa (LC) and of the peripapillary sclera (ppSc) may play a role in the pathogenesis of glaucoma. The purpose of this study was to assess the influence of glyceraldehyde, methylglyoxal, and collagenase A on the mechanical properties of the LC and ppSc. METHODS: Two strips of 1-mm width were cut from each of 80 porcine eyes and 24 pairs of enucleated human eyes. One strip contained the LC and the other the adjacent superior ppSc. One half of the strips was divided into groups and treated with 0.5 M glyceraldehyde, 0.5 M methylglyoxal, and 0.1% collagenase A. The other strips served as the control. The stress strain relation was measured in the stress range of 0.02 to 6.0 MPa by a biomaterial tester. RESULTS: Stress values at 20% strain of the human LC changed from 1.97 +/- 1.48 to 3.40 +/- 1.60 MPa after incubation with glyceraldehyde (P = 0.029), from 2.42 +/- 2.22 to 5.46 +/- 1.91 MPa (P = 0.014) after incubation with methylglyoxal, and from 2.43 +/- 1.3 to 1.35 +/- 0.19 MPa after incubation with collagenase A. The stress values of human ppSc without glyceraldehyde were 3.40 +/- 2.59 and 7.45 +/- 4.46 MPa after incubation with glyceraldehyde (P = 0.047), 4.80 +/- 3.05 MPa without methylglyoxal and 16.10 +/- 5.53 MPa (P = 0.001) after incubation with methylglyoxal, 4.14 +/- 2.56 MPa without collagenase A, and 1.97 +/- 0.55 MPa after incubation with collagenase A. At a 20% strain, Young's moduli of the untreated LC were in the range of E = 11.8 to 15.6 MPa and E = 28.5 to 36.0 MPa of the untreated ppSc. CONCLUSIONS: Glyceraldehyde and methylglyoxal increase the stiffness of the LC and of the ppSc in human and in porcine eyes. These substances induce changes in the extracellular matrix according to the Maillard reaction as it occurs during the ageing process or in case of high blood glucose levels. Collagenase reduces the stiffness of the tissues.

Animals↗
PubMed2006

The stiffness of collagen fibrils influences vascular smooth muscle cell phenotype.

Cells receive signals from the extracellular matrix through receptor-dependent interactions, but they are also influenced by the mechanical properties of the matrix. Although bulk properties of substrates have been shown to affect cell behavior, we show here that nanoscale properties of collagen fibrils also play a significant role in determining cell phenotype. Type I collagen fibrils assembled into thin films provide excellent viewing of cells interacting with individual fibrils. Cells can be observed to extensively manipulate the fibrils, and this behavior seems to result in an incompletely spread stellate morphology and a nonproliferative phenotype that is typical of these cells in collagen gels. We show here that thin films of collagen fibrils can be dehydrated, and when seeded on these dehydrated fibrils, smooth muscle cells spread and proliferate extensively. The dehydrated collagen fibrils appear to be similar to the fully hydrated collagen fibrils in topology and in presentation of beta(1) integrin ligation sites, but they are mechanically stiffer. This decrease in compliance of dehydrated fibrils is seen by a failure of cell movement of dehydrated fibrils compared to their ability to rearrange fully hydrated fibrils and from direct measurements by nanoindentation and quantitative atomic force measurements. We suggest that increase in the nanoscale rigidity of collagen fibrils can cause these cells to assume a proliferative phenotype.

Animals↗
PubMed1992

Chondrocytes in agarose culture synthesize a mechanically functional extracellular matrix.

The ability of chondrocytes from calf articular cartilage to synthesize and assemble a mechanically functional cartilage-like extracellular matrix was quantified in high cell density (approximately 10(7) cells/ml) agarose gel culture. The time evolution of chondrocyte proliferation, proteoglycan synthesis and loss to the media, and total deposition of glycosaminoglycan (GAG)-containing matrix within agarose gels was characterized during 10 weeks in culture. To assess whether the matrix deposited within the agarose gel was mechanically and electromechanically functional, we measured in parallel cultures the time evolution of dynamic mechanical stiffness and oscillatory streaming potential in uniaxial confined compression, and determined the intrinsic equilibrium modulus, hydraulic permeability, and electrokinetic coupling coefficient of the developing cultures. Biosynthetic rates were initially high, but by 1 month had fallen to a level similar to that found in the parent calf articular cartilage from which the cells were extracted. The majority of the newly synthesized proteoglycans remained in the gel. Histological sections showed matrix rich in proteoglycans and collagen fibrils developing around individual cells. The equilibrium modulus, dynamic stiffness, and oscillatory streaming potential rose to many times (>5x) their initial values at the start of the culture; the hydraulic permeability decreased to a fraction (approximately 1/10) that of the cell-laden porous agarose at the beginning of the culture. By day 35 of culture, DNA concentration (cell density), GAG concentration, stiffness, and streaming potential were all approximately 25% that of calf articular cartilage. The frequency dependence of the dynamic stiffness and potential was similar to that of calf articular cartilage. Together, these results suggested the formation of a mechanically functional matrix.

Animals↗
PubMed2000

Quantitative MR microscopy of enzymatically degraded articular cartilage.

Structural changes in bovine patellar articular cartilage, induced by component selective enzymatic treatments, were investigated by measuring tissue T(2) relaxation at 9.4 T. This MRI parameter was compared with Young's modulus, a measure of elastic stiffness and loadbearing ability of cartilage tissue. Collagenase was used to digest the collagen network and chondroitinase ABC to remove proteoglycans. Polarized light microscopy and digital densitometry were used to assess enzyme penetration after 44 hr of enzymatic digestion. T(2) relaxation in superficial cartilage increased significantly only in samples treated with collagenase. A statistically significant decrease in Young's modulus was observed in both enzymatically treated sample groups. These results confirm that T(2) of articular cartilage is sensitive to the integrity of collagen in the extracellular matrix. Nonetheless, it does not appear to be an unambiguous indicator of cartilage stiffness, which is significantly impaired in osteoarthrosis.

Animals↗
PubMed2003

Proteoglycans production by aortic vascular smooth muscle cells from hypertensive rats.

Remodeling of large and small arteries contributes to the development and complications of hypertension. Artery structural changes in chronic sustained hypertension include vascular smooth muscle cells (VSMC) proliferation and extracellular matrix (ECM) modifications. Extracellular constituents such as proteoglycans (PGs), may modulate vascular stiffness and VSMC growth and differentiation. We examined the effect of growth factors on secreted and membrane-bound PGs synthesis by cultured aortic smooth muscle cells (SMC) from 12- to 14- week-old spontaneously hypertensive rats (SHR) and age-matched Wistar rats. After stimulation with platelet-derived growth factor (PDGF-BB), 10% fetal calf serum (FCS) or 0.1% FCS as control, PGs synthesis (dpm/ng DNA) was evaluated in the medium (M-ECM) and in the cell layer (P-ECM) by a double-isotopic label method using both [3H]-glucosamine and [35S]-sodium sulfate which are incorporated into all complex carbohydrates or only into sulfated dysaccharides, respectively. Data are presented as percent of the control (0.1% FCS). SHR VSMC displayed a significantly greater synthesis of M-ECM [3H]-PGs than Wistar rat cells, with both treatments, but no differences in M-ECM [35S] uptake were found in any case. In the P-ECM, both PDGF-BB and 10% FCS produced a greater effect on [3H]-PGs and sulfated PGs synthesis in VSMC from SHR. An important change seen in SHR cells was a significant decreased sulfation, assessed by [35S]/[3H] ratio, in basal and stimulation conditions. Present results indicate the existence of changes in PGS synthesis and modulation in VSMC from a conduit-artery of SHR and support the pathophysiological role proposed for matrix proteoglycans in the vascular wall changes associated to hypertension and related vascular diseases as atherosclerosis.

Animals↗
PubMed2005

Interfacial stress transfer and property mismatch in discontinuous nanofiber/nanotube composite materials.

Novel nanotubes/nanofibers with high strength and stiffness did not lead to high failure strengths/strains of nanocomposite materials. Therefore, the interfacial stress transfer and possible stress singularities, arising at the interfacial ends of discontinuous nanofibers embedded in a matrix, subjected to tensile and shear loading, were investigated by finite element analysis. The effects of Young's moduli and volume fractions on interfacial stress distributions were studied. Round-ended nanofibers were proposed to remove the interfacial singular stresses, which were caused by high stiffness mismatch of the nanoscale reinforcement and the matrix. However, the normal stress induced in the nanofiber through interfacial stress transfer was still less than 2 times that in the matrix. This stress value is far below the high strength of the nanofiber. Therefore, the load transfer efficiency of discontinuous nanofibers or nanotube composites is very low. Hence, nanofibers or nanotubes in continuous forms, which also preclude the formation of singular interfacial stress zones, are recommended over discontinuous nanofibers to achieve high strengths in nanocomposite materials.

Finite Element Analysis↗