Single-nucleotide polymorphisms for matrix metalloprotease-1 can affect perimysial strength and intramuscular fat content but not growth rate of cattle

Context Single-nucleotide polymorphisms (SNPs) in the gene coding for matrix metalloprotease-1 (MMP-1) are known to affect the level of intramuscular fat found in cattle. As well as a signalling molecule affecting adipogenesis, MMP-1 is a major collagenase involved in the turnover of connective tissue. Aims The aim of the work was to assess whether SNPs in the gene for MMP-1 may affect the mechanical properties of intramuscular connective tissue, and therefore meat texture. Methods Allelic frequencies of three SNPs for MMP-1 were determined in a group of black Aberdeen Angus cattle whose growth characteristics had been traced for 450 days before slaughter. Associations between the alleles of each of the three SNPs and growth rate, killing out percentage, half-carcass weight, intramuscular fat content, cooking loss, strength of perimysium in cooked M. semitendinosus and Warner–Bratzler peak force of cooked M. longissimus dorsi were studied. Key results None of the SNPs studied had any effect on growth curves, and only one SNP (ss77831914) showed differences in half-carcass weight between alleles. Carcass yield and killing out percentage showed a small difference between alleles of ss7783924. No effects were found on the Warner–Bratzler peak force of M. longissimus dorsi cooked to 70°C. Two SNPs (ss77831914 and ss77831924) showed significant differences between alleles in the raw strength of perimysium in M. semitendinosus and the amount of intramuscular fat. Conclusions Commonly occurring SNPs of the major collagenase MMP-1 can affect the strength of intramuscular connective tissue as well as intramuscular fat content. Although these differences in connective tissue strength do not influence Warner–Bratzler measures of toughness at a cooking temperature of 70°C, they may contribute to differences in toughness in low-temperature, long-time cooking. Implications Because none of the SNPs had effects on the growth curves of the cattle studied, selection of animals with the relevant alleles of SNPs ss77831914 ss77831924 could be used to produce more tender meat without affecting carcass yield.

A structural approach to understanding the interactions between colour, water-holding capacity and tenderness

The colour, water-holding capacity (WHC) and tenderness of meat are primary determinants of visual and sensory appeal. Although there are many factors which influence these quality traits, the end-results of their influence is often through key changes to the structure of muscle proteins and their spatial arrangement. Water acts as a plasticiser of muscle proteins and water is lost from the myofibrillar lattice structure as a result of protein denaturation and consequent reductions in the muscle fibre volume with increasing cooking temperature. Changes in the myofilament lattice arrangement also impact the light scattering properties and the perceived paleness of the meat. Causes of variation in the quality traits of raw meat do not generally correspond to variations in cooked meat and the differences observed between the raw muscle and cooked or further processed meat are discussed. The review will also identify the gaps in our knowledge and where further investigation would beneficial.

The effect of ageing on the water-holding capacity of pork: role of cytoskeletal proteins

The water-holding capacity (WHC) of pork decreases post-mortem but has been shown to increase during subsequent ageing. In order to test a hypothesis that water-holding capacity increases during ageing due to degradation of the cytoskeleton, WHC was followed 10 days post-mortem and related to the extent of proteolysis of cytoskeletal proteins. A fast method for measuring WHC in small meat samples was developed by the use of centrifugation. The WHC of fresh pork decreases in the first part of post-mortem storage after which it increases to the level of 1 day PM. No changes in total water content of the meat were observed which could explain changes in WHC during ageing. Vinculin and desmin degrade gradually during ageing while talin degrades rapidly. These observations are consistent with the hypothesis that degradation of the cytoskeleton slowly removes the linkage between lateral shrinkage of myofibrils and shrinkage of entire muscle fibres, so removing the force that causes flow into the extracellular space. Inflow of previously expelled water is then possible, so increasing WHC as observed in later periods of storage.

Effect of nutritional regimen including limit feeding and breed on growth performance, carcass characteristics and meat quality in beef cattle

McGregor, E M., Campbell, C. P., Miller, S. P., Purslow, P. P. and Mandell, I. B. 2012. Effect of nutritional regimen including limit feeding and breed on growth performance, carcass characteristics and meat quality in beef cattle. Can. J. Anim. Sci. 92: 327–341. The effects of nutritional management regimen and breed on growth performance, carcass attributes, and meat quality were evaluated in 68 British (BRIT) and Continental (CONT) crossbred steers, managed on one of three nutritional regimens: (1) ad libitum access to a 77% corn diet (ALGRAIN), (2) limit feeding initially of the 77% corn diet, followed by ad libitum access to the 77% corn diet (LFGRAIN), and (3) limit feeding initially of a 90% haylage diet, followed by ad libitum access to the 77% corn diet (LFHAYL). Nutritional regimen by breed interactions (P<0.02) were present for average daily gain, dry matter intake (DMI), gain to feed, and shear force. LFHAYL regimen decreased gains to a greater extent for BRIT steers than CONT steers managed on ALGRAIN or LFGRAIN. Feed intakes were similar across nutritional regimens for CONT steers, while DMI was depressed in BRIT on LFHAYL as compared with steers started on ALGRAIN or LFGRAIN. Gain to feed was greater in BRIT cattle on LFGRAIN vs. ALGRAIN, while gain to feed was similar in CONT started on grain (ALGRAIN, LFGRAIN). Shear force was lower for beef from LFHAYL BRIT vs. beef from BRIT on ALGRAIN or LFGRAIN, while the converse was true for CONT. Limit feeding of a high-energy diet may be appropriate for cattle of British background to reduce feed input costs, while ensuring tenderness.

Bimodal collagen fibril diameter distributions direct age-related variations in tendon resilience and resistance to rupture

Scaling relationships have been formulated to investigate the influence of collagen fibril diameter (D) on age-related variations in the strain energy density of tendon. Transmission electron microscopy was used to quantify D in tail tendon from 1.7- to 35.3-mo-old (C57BL/6) male mice. Frequency histograms of D for all age groups were modeled as two normally distributed subpopulations with smaller (D(D1)) and larger (D(D2)) mean Ds, respectively. Both D(D1) and D(D2) increase from 1.6 to 4.0 mo but decrease thereafter. From tensile tests to rupture, two strain energy densities were calculated: 1) u(E) [from initial loading until the yield stress (σ(Y))], which contributes primarily to tendon resilience, and 2) u(F) [from σ(Y) through the maximum stress (σ(U)) until rupture], which relates primarily to resistance of the tendons to rupture. As measured by the normalized strain energy densities u(E)/σ(Y) and u(F)/σ(U), both the resilience and resistance to rupture increase with increasing age and peak at 23.0 and 4.0 mo, respectively, before decreasing thereafter. Multiple regression analysis reveals that increases in u(E)/σ(Y) (resilience energy) are associated with decreases in D(D1) and increases in D(D2), whereas u(F)/σ(U) (rupture energy) is associated with increases in D(D1) alone. These findings support a model where age-related variations in tendon resilience and resistance to rupture can be directed by subtle changes in the bimodal distribution of Ds.

Meat Science and Muscle Biology Symposium: manipulating meat tenderness by increasing the turnover of intramuscular connective tissue

Controlled reduction of the connective tissue contribution to cooked meat toughness is an objective that would have considerable financial impact in terms of added product value. The amount of intramuscular connective tissue in a muscle appears connected to its in vivo function, so reduction of the overall connective tissue content is not thought to be a viable target. However, manipulation of the state of maturity of the collagenous component is a biologically viable target; by increasing connective tissue turnover, less mature structures can be produced that are functional in vivo but more easily broken down on cooking at temperatures above 60°C, thus improving cooked meat tenderness. Recent work using cell culture models of fibroblasts derived from muscle and myoblasts has identified a range of factors that alter the activity of the principal enzymes responsible for connective tissue turnover, the matrix metalloproteinases (MMP). Fibroblasts cultured from 3 different skeletal muscles from the same animal show different cell proliferation and MMP activity, which may relate to the different connective tissue content and architecture in functionally different muscles. Expression of MMP by fibroblasts is increased by vitamins that can counter the negative effects of oxidative stress on new collagen synthesis. Preliminary work using in situ zymography of myotubes in culture also indicates increased MMP activity in the presence of epinephrine and reactive oxidative species. Comparison of the relative changes in MMP expression from muscle cells vs. fibroblasts shows that myoblasts are more responsive to a range of stimuli. Muscle cells are likely to produce more of the total MMP in muscle tissue as a whole, and the expression of latent forms of the enzymes (i.e., pro-MMP) may vary between oxidative and glycolytic muscle fibers within the same muscle. The implication is that the different muscle fiber composition of different muscles eaten as meat may influence the potential for manipulation of their connective tissue turnover.

Matrix metalloproteinases are less essential for the in-situ gelatinolytic activity in heart muscle than in skeletal muscle

In order to determine whether the contribution of matrix metalloproteinases (MMPs) to the tissue gelatinolytic activity is similar between myocardium and skeletal muscle tissue, in-situ zymography was applied to myoblasts originated from myocardium or skeletal muscle of rodents as well as tissue sections of heart and soleus muscles of rats. Gelatinolyic activity was observed in cytoplasm and nucleus of both heart and skeletal myoblasts. The chelating agent EDTA blocked much of the gelatinolytic activity and the organomercurial activator of MMPs increased the activity in cells of both muscle origins. However, the inhibition of gelatinolytic activity by a broad spectrum MMP inhibitor was less profound in heart myoblasts than that in skeletal myoblasts. Gelatinolytic activity was also expressed in the endomysium and perimysium of tissue sections of heart and soleus muscles. Similar with findings in the cell studies, the gelatinase activity was increased by the MMP activator, mostly blocked by EDTA and partially inhibited by the MMP inhibitor. In the presence of the MMP inhibitor, the remaining gelatinolytic activity in the tissue sections was again higher in myocardium than that in soleus muscle. This observation was further supported by the gelatinolytic activity examined in tissue homogenates. Our findings suggest that other proteinases, in addition to MMPs, are more responsive for the tissue gelatinolytic activity in heart muscle as compared with that in skeletal muscle.

Ageing changes in the tensile properties of tendons: influence of collagen fibril volume fraction

Connective tissues are biological composites comprising of collagen fibrils embedded in (and reinforcing) the hydrated proteoglycan-rich (PG) gel within the extracellular matrices (ECMs). Age-related changes to the mechanical properties of tissues are often associated with changes to the structure of the ECM, namely, fibril diameter. However, quantitative attempts to correlate fibril diameter to mechanical properties have yielded inconclusive evidence. Here, we described a novel approach that was based on the rule of mixtures for fiber composites to evaluate the dependence of age-related changes in tendon tensile strength (sigma) and stiffness (E) on the collagen fibril cross-sectional area fraction (rho), which is related to the fibril volume fraction. Tail tendons from C57BL6 mice from age groups 1.6-35.3 months old were stretched to failure to determine sigma and E. Parallel measurements of rho as a function of age were made using transmission electron microscopy. Mathematical models (rule of mixtures) of fibrils reinforcing a PG gel in tendons were used to investigate the influence of rho on ageing changes in sigma and E. The magnitudes of sigma, E, and rho increased rapidly from 1.6 months to 4.0 months (P-values <0.05) before reaching a constant (age independent) from 4.0 months to 29.0 months (P-values >0.05); this trend continued for E and rho (P-values >0.05) from 29.0 months to 35.3 months, but not for sigma, which decreased gradually (P-values <0.05). Linear regression analysis revealed that age-related changes in sigma and E correlated positively to rho (P-values <0.05). Collagen fibril cross-sectional area fraction rho is a significant predictor of ageing changes in sigma and E in the tail tendons of C57BL6 mice.

Dietary-induced changes of muscle growth rate in pigs: effects on in vivo and postmortem muscle proteolysis and meat quality

The effects of various growth rates in pigs induced by four different feeding strategies on the activity of the calpain system and on postmortem (PM) muscle proteolysis and tenderness development were studied. An increased growth rate may be caused by an increased protein turnover, which results in up-regulated levels of proteolytic enzymes in vivo that, in turn, possibly will affect PM tenderness development. It can be hypothesized that increased proteolytic activity pre-slaughter will increase the PM tenderization rate. From postnatal d 28 to d 90 (phase 1) the pigs were divided into two groups, given either ad libitum (A) or restricted (R, 60% of ad libitum) access to feed. The two groups were then divided into two subgroups, given either restricted or ad libitum access to feed from d 91 to slaughter at d 165 (phase 2). Measurements of the activity of mu-calpain, m-calpain, and calpastatin; concentrations of total collagen and the percent of soluble collagen; and RNA, DNA, and elongation factor-2 where made at slaugther. Myofibrillar fragmentation index (MFI) was determined at slaughter and 24 h PM. Warner-Braztler shear force was determined 1 d and 4 d PM. Pigs fed restricted diets in phase 1 and fed ad libitum in phase 2 (RA pigs) had increased growth rates in the last phase compared to pigs fed ad libitum during both phase 1 and phase 2 (AA pigs). The increased growth rate (compensatory growth) was followed by an increased proteolytic potential (mu-calpain:calpastatin ratio), increased MFI values, and higher tenderization rates. There was a positive correlation between the activities of m-calpain and growth rates (r = 0.35, P = 0.03), and between RNA levels and growth rates (r = 0.43, P = 0.006). The proposed hypothesis is largely supported by the results. The activities of both mu- and m-calpain at slaughter were highest in fast-growing pigs. The calpain activity was highest in RA pigs, which in turn also had the fastest growth rates prior tslaughter among the four groups. This implies that the synthesis of these enzymes was up-regulated during the second feeding period to a larger extent in RA pigs. The proteolytic potential and the MFI values indicate that the up-regulated in vivo calpain activity had an effect on PM protein degradation, which also is supported by the higher tenderization rate in RA pigs.

Role of Ca(2+) for the mechanical properties of fibrillin

Fibrillin-rich microfibrils are important structural elements widespread throughout connective tissues. Genetic defects identified in the Ca(2+) binding sites of fibrillin have severe effects and in addition Ca(2+) has a marked effect on the microfibrillar structure. We have studied the role of Ca(2+) on the mechanical behavior of fibrillin-rich microfibrils using the micro-needle technique. We find that Ca(2+)-depletion results in a 50% decrease in rest length and reduces the stiffness of fibrillin-rich microfibrils. At high strain, irreversible damage occurs. This behavior is consistent with Ca(2+) stabilization of interactions between consecutive EGF-like domains and breakdown in the quaternary structure upon over-extension.

Development of components of the extracellular matrix, basal lamina and sarcomere in chick quadriceps and pectoralis muscles

1. The purpose of this study was to investigate differences in the development of components of the cell/matrix linkage in two functionally different muscle types: the pectoralis muscle, a major locomotory muscle in birds but not particularly functional in chickens, and the quadriceps muscle, a smaller and more functionally active muscle in the chicken. 2. The development of the extracellular matrix, basal lamina and sarcomere in the pectoralis and quadriceps muscles in chick embryos was examined biochemically to determine differences in the rate of development between these two muscles. Samples of these muscle types were dissected out from chick embryos from embryonic day 10 until 8 weeks post hatch. 3. Using SDS-PAGE electrophoresis and western blotting with antibodies against sarcomeric actin, laminin and collagens I, III and IV, it was apparent that muscle development begins earlier in the quadriceps muscle than in the pectoralis, and that late in the developmental process (d 18) both muscle types were well differentiated. The final concentration of collagens in the mature muscle remained higher in the quadriceps than in the pectoralis muscle. 4. The onset of development of the extracellular matrix, basal lamina and sarcomere was earlier in the quadriceps than the pectoralis, which could have functional implications for these muscles as a whole.

Fibrillin-rich microfibrils of the extracellular matrix: ultrastructure and assembly

Fibrillin-rich microfibrils are a unique class of extensible connective tissue macromolecules. Their critical contribution to the establishment and maintenance of diverse extracellular matrices was underlined by the linkage of their principal structural component fibrillin to Marfan syndrome, a heritable connective tissue disorder with pleiotropic manifestations. Microscopy and preparative techniques have contributed substantially to the understanding of microfibril structure and function. The supramolecular organisation of microfibrillar assemblies in tissues has been examined by tissue sectioning and X-ray diffraction methods. Published findings are discussed and new information reported on the organisation of microfibrils in the ciliary zonular fibrils by environmental scanning electron microscopy. This review summarises microscopy and X-ray diffraction studies that are informing current understanding of the ultrastructure of fibrillin-rich microfibrils.

Differentiation of myoblasts in serum-free media: effects of modified media are cell line-specific

Myoblast cell lines are grown and differentiated readily in cell culture. Two cell lines typically used for investigating the growth and differentiation of muscle are the mouse cell line C2C12 and the rat cell line L6. The differentiation of these cells in vitro requires a switch from a serum-rich medium to a less rich medium after the cells have reached confluence. Since the components present in serum are not well characterized, the use of a better defined medium for these studies was investigated. C2C12 and L6 myoblasts were differentiated in both serum-containing and serum-free media. The differentiation state of these cultures was then tested both microscopically and biochemically. Cultures were checked for myotube formation, the activity of creatine phosphokinase and the presence of sarcomeric actin. In C2C12 cells, the extent of differentiation was greater in the serum-free than in the serum-containing system. In both media types, the C2C12 cells produced sarcomeric actin, showing the presence of sarcomere structure in the myotubes. In L6 cells, however, myotubes were readily formed in medium containing 2% horse serum, but not in the serum-free system. In addition, the ability of C2C12 cells to differentiate on substrates coated with extracellular matrix proteins was shown to be media-dependent. The presence of extracellular matrix proteins did not enable L6 cells to form myotubes when cultured in serum-free media. Primary cultures of chick myoblasts were able to differentiate in both media tested, with Dulbecco's modified Eagle medium containing horse serum being a more efficient medium for cell fusion. This study shows a divergence in muscle cell line responses in three cell lines, two of which are typically used as 'model systems' for understanding muscle growth and development.

Epinephrine upregulates calpain activity in cultured C2C12 muscle cells

C2C12 cells were grown to confluence at 37 degrees C under a continuous 5% CO(2) stream and myotube formation was stimulated. The cultures were then incubated with or without 2 microg/mL epinephrine for 18 h prior to harvesting and calpain extraction. Epinephrine treatment resulted in a three-fold increase in extractable mu-calpain activity (P < 0.05), a three-fold increase in extractable m-calpain activity (P < 0.05), a 36% increase in calpastatin activity (P < 0.001), and a 16% decrease (P < 0.05) in the total protein content in the C2C12 cell homogenate. These results suggest that calpains may play a role in protein metabolism and that the hormone epinephrine may be directly involved in the regulation of their cellular expression.

Age related compliance of the lamina cribrosa in human eyes

AIMS

To investigate changes in the mechanical compliance of ex vivo human lamina cribrosa with age.

METHODS

A laser scanning confocal microscope was used to image the surface of the fluorescently labelled lamina cribrosa in cadaver eyes. A method was developed to determine changes in the volume and strain of the lamina cribrosa created by increases in pressure. The ability of the lamina cribrosa to reverse its deformation on removal of pressure was also measured.

RESULTS

Volume and strain measurements both demonstrated that the lamina cribrosa increased in stiffness with age and the level of pressure applied. The ability of the lamina cribrosa to regain its original shape and size on removal of pressure appeared to decrease with age, demonstrating an age related decrease in resilience of the lamina cribrosa.

CONCLUSIONS

The mechanical compliance of the human lamina cribrosa decreased with age. Misalignment of compliant cribriform plates in a young eye may exert a lesser stress on nerve axons, than that exerted by the rigid plates of an elderly lamina cribrosa. The resilience of the lamina cribrosa also decreased with age, suggesting an increased susceptibility to plastic flow and permanent deformation. Such changes may be of importance in the explanation of age related optic neuropathy in primary open angle glaucoma.

The supramolecular organisation of fibrillin-rich microfibrils determines the mechanical properties of bovine zonular filaments

The zonular filaments from the eyes of cows are rich in microfibrils containing fibrillin. Tensile tests, stress-relaxation tests and X-ray diffraction studies were used to study the relationship between the mechanical behaviour of zonular filaments and the molecular packing and structure of the fibrillin-rich microfibrils. Zonular filaments show a non-linear (J-shaped) stress-strain curve and appreciable stress-relaxation. It is proposed that the non-linear properties are due to local variations in waviness in the microfibrils or assemblies of microfibrils, which straighten out and become more regularly aligned with strain. Previous and current X-ray diffraction results consistently show a partial ordering of microfibrils in zonular filaments into staggered aggregates which become more ordered and laterally aligned on stretching. Although the removal and re-addition of Ca(2+) is known to change the molecular structure of fibrillin, no effect was observed on the tensile properties of the zonular filaments. It is hypothesised that strain-induced deformation in the supramolecular aggregate packing may not be Ca(2+)-sensitive but could dominate the mechanical behaviour of microfibrillar arrays in zonular filaments.

X-Ray diffraction studies of fibrillin-rich microfibrils: effects of tissue extension on axial and lateral packing

X-ray diffraction of hydrated fibrillin rich microfibrils, in the form of zonular filaments from bovine eyes, demonstrated meridional diffraction peaks indexing on a fundamental periodicity of approximately 56 nm in the relaxed state. The effect of sample extensions of up to 50% in length produced an increase in the axial periodicity of only 4% as judged by alteration of the diffraction peak position of the third meridional order. This effect was shown to be reversible. Further extension to 100% of the tissue rest length caused extensive deterioration in the quality of the diffraction and resulted in a more complex meridional diffraction series, where the fundamental axial periodicity also changed to a length of approximately 80 nm. The fibrillin diffraction image also contains an equatorial diffraction peak that is enhanced upon tissue extension. The measurement of the molecular spacing from the equatorial diffraction profile indicated that the closest approach of molecules gave a broad interference peak of spacing 28 nm, this is nearly twice the molecular diameter as estimated from electron microscopy of dehydrated samples.

Calcium determines the supramolecular organization of fibrillin-rich microfibrils

Microfibrils are ubiquitous fibrillin-rich polymers that are thought to provide long-range elasticity to extracellular matrices, including the zonular filaments of mammalian eyes. X-ray diffraction of hydrated bovine zonular filaments demonstrated meridional diffraction peaks indexing on a fundamental axial periodicity (D) of approximately 56 nm. A Ca2+-induced reversible change in the intensities of the meridional Bragg peaks indicated that supramolecular rearrangements occurred in response to altered concentrations of free Ca2+. In the presence of Ca2+, the dominant diffracting subspecies were microfibrils aligned in an axial 0.33-D stagger. The removal of Ca2+ caused an enhanced regularity in molecular spacing of individual microfibrils, and the contribution from microfibrils not involved in staggered arrays became more dominant. Scanning transmission electron microscopy of isolated microfibrils revealed that Ca2+ removal or addition caused significant, reversible changes in microfibril mass distribution and periodicity. These results were consistent with evidence from x-ray diffraction. Simulated meridional x-ray diffraction profiles and analyses of isolated Ca2+-containing, staggered microfibrillar arrays were used to interpret the effects of Ca2+. These observations highlight the importance of Ca2+ to microfibrils and microfibrillar arrays in vivo.

Collagen orientation and molecular spacing during creep and stress-relaxation in soft connective tissues

Collagen fibres form cross-helical, cross-ply or quasi-random feltworks in extensible connective tissues; strain-induced reorientation of these networks gives rise to the non-linear mechanical properties of connective tissue at finite strains. Such tissues are also generally viscoelastic (i.e. display time-dependent properties). The hypothesis that time-dependent reorientation of collagen fibres is responsible for the viscoelasticity of such tissues is examined here using time-resolved X-ray diffraction measurements during stress-relaxation and creep transients applied to rat skin and bovine intramuscular connective tissue. Differences in the intensity and angular orientation of the third and fifth orders of the 67 nm meridional D-spacing of collagen molecules were shown before and after the application of loads or displacements. However, no changes in the D-spacing or angular orientation of collagen occurred during the time course of either stress-relaxation or creep in both tissues. This indicates that collagen fibre reorientation is not a primary source of their viscoelastic properties. The non-linear (strain-dependent) nature of the stress-relaxation response in these tissues suggests that relaxation processes within the collagen fibres or at the fibre-matrix interface may be responsible for their viscoelastic nature.

Fibrillin-rich microfibrils: an X-ray diffraction study of the fundamental axial periodicity

Microfibrils are ubiquitous matrix polymers which are thought to provide elastic properties in all extracellular matrix structures. The major component of the elastic microfibrils is the protein fibrillin; its molecular structure is unknown. In electron microscopy, microfibrils appear as beaded structures exhibiting a variable periodicity, indicating that they may be elastomeric. The X-ray diffraction of fibrillin-rich microfibrils in the form of zonular filaments from bovine eyes exhibits meridional diffraction peaks indexing on a fundamental periodicity of 55 nm in the relaxed state. The application of a 40% extension produced a lengthening of the periodicity by 3% as judged by alteration of the D spacing of the principal peaks. This effect was shown to be reversible. Changes in the periodicity of the meridional reflections indicate changes in the fundamental structure of the microfilaments, but cannot account for all long range elastomeric properties of fibrillin-containing microfibrils.

Biomechanical and biochemical study of a standardized wound healing model

Standardized protocols were developed for use in a detailed investigation into the biomechanical and biochemical properties of a dermal wound healing model in the rat. The use of a rapid freezing method at -80 degrees C minimized the detrimental effects of freezing on the biomechanical properties of the tissue and also allowed for convenient inter-laboratory collaboration to be performed. The methodology described allowed for the simultaneous and reproducible measurement of tensile strength, collagen cross-linking and proteolytic enzyme activity. Increases in the tensile properties of the tissue with time were consistent with an active process of remodelling process as indicated by changes in the cross-link and enzyme profiles. Initially the granulation tissue was comparatively rich in the keto-imine cross-link hydroxylysino-keto-norleucine, which was later replaced by the aldimine cross-link dehydro-hydroxy-lysinonorleucine. The mature cross-link histidino-hydroxy-lysinonorleucine was not observed within the granulation tissue at any stage and was also absent in aged control skin. A peak of matrix metalloproteinase-9 activity was observed at early timepoints (48 hr) and then decreased rapidly to normal levels and is consistent with an acute inflammatory response. In contrast matrix metalloproteinase-2 activity peaked later (3 days) and then decreased gradually, consistent with its role as one of the predominant enzymes involved in the remodelling process. The results described validate the animal model used and emphasize its potential for use in combined biomechanical and biochemical studies of acute wound healing.

Ocular elasticity. Is engineering stiffness a more useful characterization parameter than ocular rigidity?

PURPOSE

The purpose of this study is to present an analysis of the pressure-volume relation of the eye in basic engineering terms, so as to characterize the deformability of the ocular shell based on its intrinsic stiffness (Young modulus) and morphology, as opposed to the empirical measure of ocular rigidity.

METHODS

Starting from the structural mechanics equations describing the stress of spherical thin-walled vessels, the differential equation governing the eye pressure-volume relation is derived. This analysis, which is more rigorous than previously published derivations, assumes that the ocular shell has a Poisson ratio of 0.5. This assumption is experimentally confirmed by ultrasonic measurements of changes in bovine corneal thickness with intraocular pressure.

RESULTS

Even with a number of simplifying assumptions, this basic analysis yields a complex result, showing that the Young modulus of the ocular shell material increases rapidly with distension of the eye, and is approximately proportional to the fourth power of the ocular shell radius.

CONCLUSION

Due to the complexity of the phenomenon, engineering analysis does not lead to a simple picture of pressure-volume relation of the eye. However, it does explicitly separate the material properties of the ocular shell from morphologic contributions to pressure-volume relation of the eye. This approach allows pathologic changes in the pressure-volume relation of the eye to be related more easily to the fundamental structural mechanisms governing the nonlinear mechanical properties of ocular shell materials.

The morphology and mechanical properties of endomysium in series-fibred muscles: variations with muscle length

In the series-fibred muscle architecture commonly found in large muscles of mammals and birds, the intrafasciculary-terminating muscle fibres have no direct tendinous attachments. Contractile force produced in these fibres must be transmitted between adjacent muscle fibres via the endomysial connective tissue which separates them. The endomysium is thus an essential mechanical component in such muscles. Studies of motor end-plate banding patterns and the frequent occurrence of tapering ends of fibres within the fascicles of the bovine sternomandibularis muscle show it to be a series-fibred muscle. Sodium hydroxide digestion of fixed samples of this muscle to remove the myofibrillar apparatus revealed the endomysium to be a disordered planar network of mainly curvilinear collagen fibrils. The orientation distribution of the collagen fibrils in the endomysial network was measured by image analysis of scanning electron micrographs. Analysis of endomysial preparations from muscle fixed at sarcomere lengths between 1-4 microns showed that the orientation distribution of collagen fibrils is quantitatively related to muscle length. At rest sarcomere length the collagen fibril network is not completely random, but has a slight circumferential bias. The orientation distribution shows a progressive shift towards the circumferential direction at short sarcomere lengths and towards the longitudinal direction at long sarcomere lengths. The relationship between the number-weighted mean collagen orientation and sarcomere length was compared to two geometric models of network behaviour, the isoareal and constant shape models. Both fitted the data reasonably, although the constant shape model described the rate of change of mean orientation more closely. From fibrous composites theory, the reinforcement efficiency factor, eta, was calculated from the measured collagen fibril orientation distributions. These calculations predict a non-linearly increasing longitudinal tensile modulus for the endomysium with increasing sarcomere length, in agreement with its known non-linear properties, but confirm that the tensile properties of the endomysium are unsuitable for transmission of tensile force from muscle fibres contracting near rest length. This reinforces a previous interpretation that contractile force is transmitted between neighbouring muscle fibres by trans-laminar shear through the endomysium rather than by in-plane tension.

Functional morphology of the endomysium in series fibered muscles

Many skeletal muscles, including the feline biceps femoris, are composed of short, tapered myofibers arranged in an overlapping longitudinal series. The endomysium of such muscles transfers tension between overlapping myofibers, and is thus an elastic element in series with them. The endomysium of the cat biceps femoris contains curvilinear collagen fibrils in an approximately isotropic (random) array. The collagen fibrils undergo only a modest reorientation as the myofibers shorten or lengthen within the physiological range. A geometrical model predicts no change in the thickness of the endomysium on changing muscle fiber length and quantifies the expected collagen fibril reorientation in the endomysium as a function of muscle extension. It is also demonstrated that a high proportion of the collagen fibrils will be curvilinear at all sarcomere lengths. The organization of endomysial collagen is appropriate for the transfer of loads between myofibers by means of shear.

Strain-induced reorientation of an intramuscular connective tissue network: implications for passive muscle elasticity

The most abundant intramuscular connective tissue component, the perimysium, of bovine M. sternomandibularis muscle was shown to be a crossed-ply arrangement of crimped collagen fibres which reorientate and decrimp on changing muscle fibre sarcomere length. Reorientation of perimysial strands was observed by light microscopy and identification of these strands as collagen fibres was confirmed by high-angle X-ray diffraction. Mean collagen fibre direction with respect to the muscle fibres ranged from approximately 80 degrees at sarcomere length = 1.1 micron to approximately 20 degrees at 3.9 microns. This behaviour was well described by a model of a crimped planar network surrounding a muscle fibre bundle of constant volume but varying length. Modelling of the mechanical properties of the perimysium at different sarcomere lengths produced a load-sarcomere length curve which was in good agreement with the passive elastic properties of the muscle, especially at long sarcomere lengths. It is concluded that the role of the perimysial collagen network is to prevent over-stretching of the muscle fibre bundles.

Positional variations in fracture toughness, stiffness and strength of descending thoracic pig aorta

The tunica media and intima of descending thoracic aortas from commercially killed pigs showed variations in mechanical properties along their length in a region extending from just in front of the first intercostal artery to the sixth intercostal artery. Along this region circumferential toughness, measured as work of fracture from a tear test, increased, longitudinal and circumferential ultimate tensile strength increased and longitudinal and circumferential stress-strain gradients of excised strips increased further away from the heart. These increasing mechanical properties are positively correlated with an increase in collagen mass fraction and an increase of radius/thickness ratio away from the heart over this region.