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PubMed2005

Exposure to a standard culture medium alters the response of cartilage explants to injurious unconfined compression.

Previous studies on chondral explants have not clearly described to what extent the degree and the distribution of cell death are dependent on the amount of free swelling seen during tissue equilibration in a standard culture medium. The current study hypothesized that increased fluid content inside equilibrated chondral explants, when subjected to injurious compression, would lead to greater matrix damage during unconfined compression. Equilibrated and non-equilibrated chondral explants were loaded to 30 MPa at a fast rate of loading ( approximately 600 MPa/s). Stress-strain curves were documented for each explant. Matrix damage was assessed by the length of surface fissures. Chondrocyte viability was also measured in the various layers of the explants. The stiffness of the equilibrated specimens was less than non-equilibrated specimens, and it correlated with the amount of fluid absorbed during equilibration. More matrix damage and associated cell death in the superficial zone were documented in equilibrated than non-equilibrated explants, and these correlated positively with fluid absorbed during equilibration. This study indicated that equilibration of chondral explants in a standard culture medium alters their response to mechanical loading in terms of stiffness, matrix damage and cell viability.

Animals↗
PubMed2004

Improved mechanical properties of HIPS/hydroxyapatite composites by surface modification of hydroxyapatite via in-situ polymerization of styrene.

High impact polystyrene (HIPS)/hydroxyapatite (HA) composites are potential biomaterials for bone replacements due to their good biocompatibility and adequate mechanical properties. At the present work, the surface of the micron-sized hydroxyapatite (HA) particles was modified by in situ polymerization of styrene (St), then compounded with HIPS. The effect of the modification of HA surface on morphology and mechanical properties of HIPS/HA composites were investigated. The results showed that the HA particles does not inhibit the polymerization of St. The PS segments coated on the HA surface by in situ polymerization of St enhances the compatibility between HA and HIPS, improves the dispersion of HA particles in HIPS matrix, and enhances the interfacial adhesion between HA and matrix. Thereby, the stiffness, tensile strength and notch impact strength of HIPS/HA composites are improved at the same time. And there is a critical coating thickness of PS on the HA surface for the optimum mechanical properties of HIPS/HA composites.

Biocompatible Materials↗
PubMed2005

Tissue inhibitor of metalloproteinse-1 is a marker of diastolic dysfunction using tissue doppler in patients with type 2 diabetes and hypertension.

BACKGROUND: Tissue inhibitor of metalloproteinase-1 (TIMP-1) is associated with increased fibrosis of the extracellular matrix (ECM). Myocardial stiffness is a feature of diastolic dysfunction. We assessed circulating TIMP-1 as a marker of diastolic dysfunction in patients with type 2 diabetes mellitus (DM) and hypertension, who were compared with healthy controls. METHODS: We recruited 54 patients (43 males; mean age 68 +/- 5 years) with treated type 2 DM (i.e. controlled glycaemia, hypertension, hyperlipidaemia), 35 (30 males; 69 +/- 8 years) treated nondiabetic hypertensives, and 31 healthy controls (18 males; 66 +/- 5 years). Circulating TIMP-1 was measured by ELISA. Using transthoracic echocardiography, the early (E) diastolic mitral inflow velocity was measured with pulse wave Doppler, and the early mitral annular velocity (e'), a recognized index of diastolic relaxation, was measured with tissue Doppler. The E/A ratio was also calculated and isovolumic relaxation time measured. RESULTS: Mean e' levels differed significantly between controls, diabetics and hypertensives (P < 0.0001). Circulating TIMP-1 was significantly different between patients and controls (P = 0.006), but there was no statistically significant difference between the DM and hypertension group. In both groups, only e' was negatively correlated with TIMP-1 levels, with a stronger correlation among the hypertensive patients (Spearman r = -0.544, P = 0.001) when compared with the diabetic group (r = -0.341, P = 0.011). CONCLUSION: Diastolic relaxation is impaired in diabetes and hypertensive patients. The relationship between TIMP-1 and e' may reflect increased myocardial fibrosis and consequent diastolic dysfunction, which may be more prominent in hypertension.

Aged↗
PubMed2004

Mechanical anisotropy of adherent cells probed by a three-dimensional magnetic twisting device.

We describe a three-dimensional magnetic twisting device that is useful in characterizing the mechanical properties of cells. With the use of three pairs of orthogonally aligned coils, oscillatory mechanical torque was applied to magnetic beads about any chosen axis. Frequencies up to 1 kHz could be attained. Cell deformation was measured in response to torque applied via an RGD-coated, surface-bound magnetic bead. In both unpatterned and micropatterned elongated cells on extracellular matrix, the mechanical stiffness transverse to the long axis of the cell was less than half that parallel to the long axis. Elongated cells on poly-L-lysine lost stress fibers and exhibited little mechanical anisotropy; disrupting the actin cytoskeleton or decreasing cytoskeletal tension substantially decreased the anisotropy. These results suggest that mechanical anisotropy originates from intrinsic cytoskeletal tension within the stress fibers. Deformation patterns of the cytoskeleton and the nucleolus were sensitive to loading direction, suggesting anisotropic mechanical signaling. This technology may be useful for elucidating the structural basis of mechanotransduction.

Anisotropy↗
PubMed1995

How do sea urchins invaginate? Using biomechanics to distinguish between mechanisms of primary invagination.

The forces that drive sea urchin primary invagination remain mysterious. To solve this mystery we have developed a set of finite element simulations that test five hypothesized mechanisms. Our models show that each of these mechanisms can generate an invagination; however, the mechanical properties of an epithelial sheet required for proper invagination are different for each mechanism. For example, we find that the gel swelling hypothesis of Lane et al. (Lane, M. C., Koehl, M. A. R., Wilt, F. and Keller, R. (1993) Development 117, 1049-1060) requires the embryo to possess a mechanically stiff apical extracellular matrix and highly deformable cells, whereas a hypothesis based on apical constriction of the epithelial cells requires a more compliant extracellular matrix. For each mechanism, we have mapped out a range of embryo designs that work. Additionally, the simulations predict specific cell shape changes accompanying each mechanism. This allows us to design experiments that can distinguish between different mechanisms, all of which can, in principle, drive primary invagination.

Animals↗
PubMed2006

Experimental composite brackets: influence of filler level on the mechanical properties.

INTRODUCTION: Despite their popularity in satisfying esthetic demands, plastic brackets still have some disadvantages because of decreased wear resistance and hardness. Our aim in this study was to investigate the mechanical properties of an alternative bracket-polymer urethane-dimethacrylate (UDMA) reinforced by silicon dioxide fillers with several filler levels. METHODS: Three bracket groups were created: (1) unfilled UDMA, (2) UDMA with a filler content of 40% by volume, and (3) UDMA with a filler content of 70% by volume. Flexural strength, fracture force, and Vickers hardness of the bracket groups were tested. In addition, a 3-medium wear test was performed. RESULTS: The filler-reinforced brackets showed increased hardness, greater stiffness, and improved wear resistance compared with unfilled brackets. Higher filler concentrations in a polymer matrix result in greater stiffness of the polymer. Despite this, the filled composite brackets were not statistically more susceptible to fractures than the unfilled plastic brackets. CONCLUSIONS: Experimental UDMA brackets reinforced with silicon dioxide fillers showed an obvious trend for improved mechanical properties compared with unfilled UDMA brackets.

Composite Resins↗
PubMed1991

Diastolic function in coronary artery disease.

Diastolic function in coronary artery disease is modified to a variable extent. There are distinct abnormalities produced during acute ischemia, and following myocardial infarction. The pathophysiology of diastolic abnormalities in these two syndromes is reviewed. During acute ischemia filling pressures of the left ventricle are increased. Pulmonary edema may be produced. Silent ischemia causes less of an increase in filling pressures. The diastolic pressure-volume relation is shifted in an upward manner with a variable contribution from altered myocardial relaxation, increased muscle stiffness, acute pericardial restriction, ventricular interaction, and acute chamber dilatation. The impairment of myocardial relaxation plays a central role and has been quantified in multiple clinical and experimental studies. Filling of the left ventricle during ischemia is altered due to the factors which shift the pressure-volume relation. The acute increase in left atrial pressure may increase filling rates somewhat surprisingly, given the reduced left ventricular compliance. Myocardial fibrosis following infarction may elevate filling pressures, but the degree of elevation is closely tied to the intravascular volume status. Shifts in the diastolic pressure-volume relation reflect a loss of chamber compliance due to an increase in muscle stiffness. Increased amounts of extracellular matrix, specifically collagen, produce this permanent increase in muscle stiffness which is central to the diastolic abnormalities in chronic coronary artery disease.

Angina Pectoris↗
PubMed2004

Adrenomedullin induces matrix metalloproteinase-2 activity in rat aortic adventitial fibroblasts.

BACKGROUND: The delicate balance of the extracellular matrix (ECM) determines the stiffness of the vascular wall, and adventitial fibroblasts are involved in ECM formation by synthesizing and degrading matrix proteins. In the present study, we examined the effect of the bioactive peptide adrenomedullin (AM) on activity and expression of matrix metalloproteinases (MMPs) in cultured aortic adventitial fibroblasts. METHODS AND RESULTS: In cultured adventitial fibroblasts isolated from aorta of adult Wistar rats, 10(-6)mol/L angiotensin II (Ang II) significantly (p<0.05) down-regulated MMP-2 activity as determined by in vitro gelatin zymography. In contrast, 10(-7)mol/L synthetic rat AM significantly (p<0.05) stimulated zymographic MMP-2 activity by 23%, increasing intracellular cAMP, and AM abolished the action of Ang II, augmenting the MMP-2 activity. Similarly, Ang II down-regulated MMP-2 protein expression assessed by Western blotting, whereas AM increased it. Furthermore, 8-bromo-cAMP, an analogue of cAMP, mimicked the effect of AM, and H-89, an inhibitor for protein kinase A (PKA), significantly decreased the basal and AM-induced MMP-2 activity. CONCLUSION: This study provides a new insight into the biological action of AM and its intracellular signaling system of cAMP/PKA stimulating the matrix degrading enzyme MMP-2, suggesting an important role for this molecule in modulating ECM deposition in the adventitial layer.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1988

A biomechanical profile across the patellar groove articular cartilage: implications for defining matrix health.

Macroscopically normal articular cartilage across all bovine patellar grooves studied exhibited a smooth gradation in mechanical properties. Rigorous standardisation of microtensile and microcompressive testing showed that stiffness in tension of the deep matrix, its tendency to rupture and compressive stiffness all dropped progressively across the medial margin and trough of the groove, and reached their lowest values at approximately one quarter of the distance up the lateral margin. The changes in mechanical properties were correlated with ultrastructural differences. The deep matrix of very stiff tissue from the medial margin showed a dense arrangement of fibril segments orientated with varying degrees of obliquity about a radial mean. The more compliant tissue had a markedly less dense fibrillar array with a pronounced radial orientation. It is suggested that the gradation in mechanical properties results from differential loading of the joint surface. From the available evidence it seems likely that the compliance of the cartilage increases proportionately with the reduction in load. The results are discussed with reference to Broom's (1986b) model of the fibrillar architecture of cartilage. It is proposed that increasing compliance is related to a graduated reduction across the joint surface in the number and/or strength of the interfibrillar bonds, resulting from differential loading. A proportionate number of fibrils would have a reduced number of short-period lateral deflections and thus an increasingly overall radial orientation. This would result in a concomitant graded reduction in the degree of constraint exerted by the three dimensional fibrillar network on the hydrated proteoglycans.

Animals↗
PubMed2005

Altered cartilage mechanics and histology in knee osteoarthritis: relation to clinical assessment (ICRS Grade).

OBJECTIVE: Substantial changes in articular cartilage composition and mechanical properties occur during the development of osteoarthritis (OA). While softening in the initial stage is reported and sometimes used as an indicator of early OA, there is a lack of data relating the macroscopic appearance of cartilage to its mechanical and histological properties in all stages of degeneration. Knowledge about the mechanical quality of the tissue is important for diagnostic reasons and the understanding of the development of OA. DESIGN: The cartilage areas of 21 osteoarthritic human cadaver tibia plateaus were classified using the International Cartilage Repair Society (ICRS) system. A material testing device determined the Young's modulus of the cartilage by unconfined compression. Histological analysis used haematoxylin and eosin staining and Safranin-O staining for the evaluation of the Mankin score. RESULTS: A correlation between increasing ICRS Grade and stiffness reduction was found (R2=0.69). Stiffness values were for ICRS Grades 1, 2 and 3: E1=0.50+/-0.14 MPa, E2=0.37+/-0.13 MPa and E3=0.28+/-0.12 MPa, respectively. The histological evaluation confirmed the ICRS classification (R2=0.74). A moderate correlation between Mankin score and cartilage stiffness was observed (R2=0.47). CONCLUSIONS: The results indicate a relation between structural, mechanical and histological changes in all stages of the degeneration. With increasing ICRS Grade the cartilage stiffness, which is primarily influenced by the integrity of the extracellular matrix, decreases. Therefore, methods of stiffness determination such as indentation may be used to characterize cartilage in all stages of OA. However, the data suggest that differentiating between healthy cartilage and ICRS Grade 1 may be difficult using mechanical testing alone.

Aged↗
PubMed2005

Increased aortic stiffness in the insulin-resistant Zucker fa/fa rat.

Accumulating clinical evidence indicates increased aortic stiffness, an independent risk factor for cardiovascular and all-cause mortality, in type 2 diabetic and glucose-intolerant individuals. The present study sought to determine whether increased mechanical stiffness, an altered extracellular matrix, and a profibrotic gene expression profile could be observed in the aorta of the insulin-resistant Zucker fa/fa rat. Mechanical testing of Zucker fa/fa aortas showed increased vascular stiffness in longitudinal and circumferential directions compared with Zucker lean controls. Unequal elevations in developed strain favoring the longitudinal direction resulted in a loss of anisotropy. Real-time quantitative PCR and immunohistochemistry revealed increased expression of fibronectin and collagen IV alpha 3 in the Zucker fa/fa aorta. In addition, expression of transforming growth factor-beta and several Smad proteins was increased in vessels from insulin-resistant animals. In rat vascular smooth muscle cells, 12-18 h of exposure to insulin (100 nmol/l) enhanced transforming growth factor-beta1 mRNA expression, implicating a role for hyperinsulinemia in vascular stiffness. Thus there is mechanical, structural, and molecular evidence of arteriosclerosis in the Zucker fa/fa rat at the glucose-intolerant, hyperinsulinemic stage.

Animals↗
PubMed2000

Regulation of matrix proteins and impact on vascular structure.

The vascular extracellular matrix is responsible for the mechanical properties of the vessel wall and is also involved in biologic processes such as cellular adhesion, regulation, and proliferation. Thus, an adequate balance of its components is necessary for the normal functioning of the vasculature. Vascular disorders affect this balance, and this plays a key role in their pathophysiology. Atherogenesis is accompanied by an increase in matrix deposition in response to low-density lipoprotein accumulation. However, this matrix, mainly collagen, also has a protective role by forming a fibrous cap around the lipid core, avoiding contact with blood. A decrease in the amount of collagen will weaken the cap and make it prone to rupture, leading to thrombosis and acute coronary syndromes. In hypertension, the increase in matrix deposition results in vascular stiffness and cardiac dysfunction. In this paper, we discuss the relevance of matrix regulation in these conditions.

Arteriosclerosis↗
PubMed2004

Rheology of airway smooth muscle cells is associated with cytoskeletal contractile stress.

Recently reported data from mechanical measurements of cultured airway smooth muscle cells show that stiffness of the cytoskeletal matrix is determined by the extent of static contractile stress borne by the cytoskeleton (Wang N, Tolić-Nørrelykke IM, Chen J, Mijailovich SM, Butler JP, Fredberg JJ, and Stamenović D. Am J Physiol Cell Physiol 282, C606-C616, 2002). On the other hand, rheological measurements on these cells show that cytoskeletal stiffness changes with frequency of imposed mechanical loading according to a power law (Fabry B, Maksym GN, Butler JP, Glogauer M, Navajas DF, and Fredberg JJ. Phys Rev Lett 87: 148102, 2001). In this study, we examine the possibility that these two empirical observations might be interrelated. We combine previously reported data for contractile stress of human airway smooth muscle cells with new data describing rheological properties of these cells and derive quantitative, mathematically tractable, and experimentally verifiable empirical relationships between contractile stress and indexes of cell rheology. These findings reveal an intriguing role of the contractile stress: although it maintains structural stability of the cell under applied mechanical loads, it may also regulate rheological properties of the cytoskeleton, which are essential for other cell functions.

Cytoskeleton↗
PubMed2003

Partitioning of cortical and deep cytoskeleton responses from transient magnetic bead twisting.

We attempted to estimate in living adherent epithelial alveolar cells, the degree of structural and mechanical heterogeneity by considering two individualized cytoskeleton components, i.e., a submembranous "cortical" cytoskeleton and a "deep" cytoskeleton (CSK). F-actin structure characterizing each CSK component was visualized from spatial reconstructions at low and high density, respectively, especially in a 10-microm-cubic neighborhood including the bead. Specific mechanical properties (Young elastic and viscous modulus E and n) were revealed after partitioning the magnetic twisting cytometry response using a double viscoelastic "solid" model with asymmetric plastic relaxation. Results show that the cortical CSK response is a faster (tau1 < or = 0.7 s), softer (E1: 63-109 Pa), moderately viscous (n1: 7- 18 Pas), slightly tensed, and easily damaged structure compared to the deep CSK structure which appears slower (tau2 approximately 1/2 min), stiffer (E2: 95-204 Pa), highly viscous (n2: 760-1967 Pa s), more tensed, and fully elastic, while exhibiting a larger stress hardening behavior. Adding drug depolymerizing actin filaments decreased predominantly the deep CSK stiffness. By contrast, an agent altering cell-matrix interactions affected essentially the cortical CSK stiffness. We concluded that partitioning the CSK within cortical and deep structures is largely consistent with their respective functional activities.

Actins↗
PubMed1993

Renin-angiotensin system and myocardial collagen matrix remodeling in hypertensive heart disease: in vivo and in vitro studies on collagen matrix regulation.

The interstitial space of the myocardium is composed of nonmyocyte cells and a highly organized collagen network which serves to maintain the architecture and mechanical behavior of the myocardial walls. It is the myocardial collagen matrix that determines myocardial stiffness in the normal and structurally remodeled myocardium. In hypertensive heart disease, the heterogeneity in myocardial structure, created by the altered behavior of nonmyocyte cells, particularly cardiac fibroblasts which are responsible for collagen synthesis and degradation, explains the appearance of diastolic and/or systolic dysfunction of the left ventricle that leads to symptomatic heart failure. Several lines of evidence suggest that circulating and myocardial renin-angiotensin systems (RAS) are involved in the regulation of the structural remodeling of the nonmyocyte compartment, including the cardioprotective effects of angiotensin converting enzyme (ACE) inhibition that was found to prevent myocardial fibrosis in the rat with renovascular hypertension. In cultured adult rat cardiac fibroblasts angiotensin II was shown to directly stimulate collagen synthesis and to inhibit collagenase activity, which is the key enzyme for collagen degradation, that would lead to collagen accumulation. In the spontaneously hypertensive rat, an appropriate experimental model for primary hypertension in man, left ventricular hypertrophy could be regressed and abnormal myocardial diastolic stiffness due to interstitial fibrosis could be restored to normal by inhibition of the myocardial RAS. These antifibrotic or cardioreparative effects of ACE inhibition that occurred irrespective of blood pressure normalization may be valuable in reversing left ventricular diastolic dysfunction in hypertensive heart disease.

Collagen↗
PubMedDate not supplied

Use of a rotating bioreactor toward tissue engineering the temporomandibular joint disc.

This objective of this study was to determine the effects of a rotating bioreactor in temporomandibular joint (TMJ) disc tissue engineering. Porcine TMJ disc cells were seeded at a density of 20 million cells/mL onto nonwoven poly(glycolic acid) (PGA) scaffolds in spinner flasks for 1 week and then cultured either under static conditions or in a rotating bioreactor for a period of 6 weeks. A series of analyses was performed, including mechanical testing, measurement of cellularity, quantification of matrix biosynthesis with a hydroxyproline assay and enzyme-linked immunosorbent assays, and observation of matrix distribution with immunohistochemistry. Between the bioreactor and static cultures, there were marked differences in gross appearance, histological structure, and distribution of collagen types I and II. Engineered constructs from the bioreactor contracted earlier and to a greater extent, resulting in a denser matrix and cell composition. In addition, immunostaining intensity was generally uniform in static constructs, in contrast to higher intensity around the periphery of bioreactor constructs. Moreover, bioreactor constructs had higher amounts of collagen II than did static constructs. However, differences in total matrix content and compressive stiffness were generally not significant. On the basis of the results of this study there is no clear benefit from use of the rotating bioreactor, although a sequence of static culture followed by rotating bioreactor culture may prove in the future to be more beneficial than either alone.

Animals↗
PubMed1993

Viscoelastic shear properties of articular cartilage and the effects of glycosidase treatments.

The objectives of this study were to determine the viscoelastic shear properties of articular cartilage and to investigate the effects of the alteration of proteoglycan structure on these shear properties. Glycosidase treatments (chondroitinase ABC and Streptomyces hyaluronidase) were used to alter the proteoglycan structure and content of the tissue. The dynamic viscoelastic shear properties of control and treated tissues were measured and statistically compared. Specifically, cylindrical bovine cartilage specimens were subjected to oscillatory shear deformation of small amplitude (gamma degrees = 0.001 radian) over a physiological range of frequencies (0.01-20 Hz) and at various compressive strains (5, 9, 12, and 16%). The dynamic complex shear modulus was calculated from the measurements. The experimental results show that the solid matrix of normal articular cartilage exhibits intrinsic viscoelastic properties in shear over the range of frequencies tested. These viscoelastic shear properties were found to be dependent on compressive strains. Our data also provide significant insights into the structure-function relationships for articular cartilage. Significant correlations were found between the material properties (the magnitude of dynamic shear modulus, the phase shift angle, and the equilibrium compressive modulus), and the biochemical compositions of the cartilage (collagen, proteoglycan, and water contents). The shear modulus was greatly reduced when the proteoglycans were degraded by either chondroitinase ABC or Streptomyces hyaluronidase. The results suggest that the ability of collagen to resist tension elastically provides the stiffness of the cartilage matrix in shear and its elastic energy storage capability. Proteoglycans enmeshed in the collagen matrix inflate the collagen network and induce a tensile prestress in the collagen fibrils. This interaction of the collagen and proteoglycan within the cartilage matrix provides the complex mechanism that allows the tissue to resist shear deformation.

Animals↗