The elastic fibre system in the veins of the human lower oesophagus

The elastic, elaunin and oxytalan fibres in the tunica mucosa and tela submucosa of veins of the human lower oesophagus were studied in 30 necropsy and biopsy specimens using appropriate histological and ultrastructural methodologies. Elaunin fibres predominate in the veins endowed with muscle cells. Scattered oxytalan and elastic fibres were also observed, the latter being more numerous at the vein periphery. No noteworthy fibre arrangement was encountered in veins lacking muscle cells. Those fibres disposed around the veins were considered to belong to the connective tissue of the tunica mucosa and tela submucosa in which they are embedded. The veins of the lower oesophagus seem to be of low elasticity, which may be related to a blockage mechanism described for the gastro-oesophageal blood stream in cases of portal hypertension.     

Generating fibre orientation maps in human heart models using Poisson interpolation

Smoothly varying muscle fibre orientations in the heart are critical to its electrical and mechanical function. From detailed histological studies and diffusion tensor imaging, we now know that fibre orientations in humans vary gradually from approximately ? 70° in the outer wall to +80° in the inner wall. However, the creation of fibre orientation maps for computational analyses remains one of the most challenging problems in cardiac electrophysiology and cardiac mechanics. Here, we show that Poisson interpolation generates smoothly varying vector fields that satisfy a set of user-defined constraints in arbitrary domains. Specifically, we enforce the Poisson interpolation in the weak sense using a standard linear finite element algorithm for scalar-valued second-order boundary value problems and introduce the feature to be interpolated as a global unknown. User-defined constraints are then simply enforced in the strong sense as Dirichlet boundary conditions. We demonstrate that the proposed concept is capable of generating smoothly varying fibre orientations, quickly, efficiently and robustly, both in a generic bi-ventricular model and in a patient-specific human heart. Sensitivity analyses demonstrate that the underlying algorithm is conceptually able to handle both arbitrarily and uniformly distributed user-defined constraints; however, the quality of the interpolation is best for uniformly distributed constraints. We anticipate our algorithm to be immediately transformative to experimental and clinical settings, in which it will allow us to quickly and reliably create smooth interpolations of arbitrary fields from data-sets, which are sparse but uniformly distributed.     

Collagen fibre orientation in the annulus fibrosus of intervertebral disc during bending and torsion measured by x-ray diffraction

X-ray diffraction has been used to measure the orientation of the collagen fibres in the ventral annulus fibrosus of intact L1/2 rabbit intervertebral disc during in vitro bending and torsion. Fibres are tilted with respect to the axis of the spine. As predicted by theory, fibre tilt decreases in those regions of the annulus which are stretched by bending but increases in the slackened regions. Good agreement with the quantitative predictions of bending theory was obtained in three of the six series of experiments, the predicted trend being found in all six. Tilt direction alternates in successive lamellae of the annulus. When discs were subjected to both clockwise and anticlockwise torsion of 5°, the two families of titled fibres reoriented in the expected directions.     

Modelling skeletal muscle fibre orientation arrangement

Skeletal muscle tissues have complex geometries. In addition, the complex fibre orientation arrangement makes it quite difficult to create an accurate finite element muscle model. There are many possible ways to specify the complex fibre orientations in a finite element model, for example defining a local element coordinate system. In this paper, an alternative method using ABAQUS, which is combination of the finite element method and the non-uniform rational B-spline solid representation, is proposed to calculate the initial fibre orientations. The initial direction of each muscle fibre is specified as the tangent direction of the NURBS curve which the fibre lies on, and the directions of the deformed fibres are calculated from the initial fibre directions, the deformation gradients and the fibre stretch ratios. Several examples are presented to demonstrate the ability of the proposed method. Results show that the proposed method is able to characterise both the muscle complex fibre orientation arrangement and its complex mechanical response.     

Modelling skeletal muscle fibre orientation arrangement

Skeletal muscle tissues have complex geometries. In addition, the complex fibre orientation arrangement makes it quite difficult to create an accurate finite element muscle model. There are many possible ways to specify the complex fibre orientations in a finite element model, for example defining a local element coordinate system. In this paper, an alternative method using ABAQUS, which is combination of the finite element method and the non-uniform rational B-spline solid representation, is proposed to calculate the initial fibre orientations. The initial direction of each muscle fibre is specified as the tangent direction of the NURBS curve which the fibre lies on, and the directions of the deformed fibres are calculated from the initial fibre directions, the deformation gradients and the fibre stretch ratios. Several examples are presented to demonstrate the ability of the proposed method. Results show that the proposed method is able to characterise both the muscle complex fibre orientation arrangement and its complex mechanical response.     

Collagen orientation patterns in human secondary osteons, quantified in the radial direction by confocal microscopy

The composite structure of secondary osteon lamellae, key micro-mechanical components of human bone, has intrigued researchers for the last 300 years. Scanning confocal microscopy here for the first time systematically quantifies collagen orientations by location within the lamellar thickness. Fully calcified lamellar specimens, extinct or bright in cross-section under circularly polarized light, were gently flattened, and then examined along their thickness direction, the radial direction in the previously embedding osteon. Collagen orientation was measured from confocal image stacks. So-called extinct lamellae and so-called bright lamellae are found to display distinct, characteristic patterns of collagen orientation distribution. Orientations longitudinal to the osteon axis in extinct lamellae, transverse to the osteon axis in bright lamellae, and oblique to the osteon axis in both lamellar types, show parabolic distribution through specimen thickness. Longitudinal collagen in extinct lamellae, and transverse collagen in bright lamellae, peaks at middle third of lamellar thickness, while oblique collagen peaks at outer thirds of both types. Throughout the thickness, longitudinal collagen orientations characterize extinct lamellar specimens, while orientations oblique to the original osteon axis characterize bright lamellar specimens. Measured patterns complement previous indirect results by different methods and reinforce previously hypothesized differences in lamellar mechanical functions.     

Collagen and mature elastic fibre organisation as a function of depth in the human cornea and limbus

A network of circumferentially oriented collagen fibrils exists in the periphery of the human cornea, and is thought to be pivotal in maintaining corneal biomechanical stability and curvature. However, it is unknown whether or not this key structural arrangement predominates throughout the entire corneal thickness or exists as a discrete feature at a particular tissue depth; or if it incorporates any elastic fibres and how, with respect to tissue depth, the circumcorneal annulus integrates with the orthogonally arranged collagen of the central cornea. To address these issues we performed a three-dimensional investigation of fibrous collagen and elastin architecture in the peripheral and central human cornea using synchrotron X-ray scattering and non-linear microscopy. This showed that the network of collagen fibrils circumscribing the human cornea is located in the posterior one-third of the tissue and is interlaced with significant numbers of mature elastic fibres which mirror the alignment of the collagen. The orthogonal arrangement of collagen in the central cornea is also mainly restricted to the posterior stromal layers. This information will aid the development of corneal biomechanical models aimed at explaining how normal corneal curvature is sustained and further predicting the outcome of surgical procedures.     

Insensitivity of tensile failure properties of human bridging veins to strain rate: implications in biomechanics of subdural hematoma

The effects of strain rate on tensile failure properties of human parasagittal bridging veins were studied in eight unembalmed cadavers. While bathed in physiological saline at 37 degrees C, the intact vessel was stretched axially by a servo-controlled hydraulic testing machine at either a low strain rate of 0.1-2.5 s-1 or a high rate of 100-250 s-1. The mean ultimate stretch ratios for low and high strain rates, respectively, were 1.51 +/- 0.24 (S.D. n = 29) and 1.55 +/- 0.15 (n = 34), and the ultimate stresses were 3.24 +/- 1.65 (n = 17) and 3.42 +/- 1.38 MPa (n = 20). Neither difference between strain rates was significant (p greater than 0.45). Thus, our results do not support the hypothesis that sensitivity of the ultimate strain of bridging veins to strain rate explains the acceleration tolerance data for subdural hematoma in primates [Gennarelli, R. A. and Thibault, L. E. (1982) Biomechanics of acute subdural hematoma. J. Trauma 22, 680-686].     

Collagen fibre reorientation around a crack in biaxially stretched aortic media

The distribution and orientation of collagen fibres in unstretched and biaxially stretched pig aortic media have been determined by an X-ray diffraction technique in order to analyse the reorientation of collagen fibres in response to the presence of a notch in biaxially stretched samples. The results show that collagen fibres align themselves following the force trajectories. This results in preferential fibre orientation perpendicular to the advancing crack tip, in agreement with the theoretical predictions of stress concentration effect due to the presence of an elliptical notch in an elastic plate.     

Collagen cross-linking compounds in human urine

We report the isolation and chemical characterization of collagen cross-linking compounds, 3-hydroxypyridinium and dihydroxylysinonorleucine, from human urine.     

Myoendothelial junctions in human saphenous veins

The frequency and morphology of myoendothelial junctions occurring in human saphenous veins have been investigated. The junctions are mostly established via slender membrane projections. The junction slit is filled with a electron-dense material, sometimes forming shark-tooth-like configurations. The morphology of these heterocellular junctions is not compatible in all cases with gap junctions, tight junctions or desmosomes. The frequency of heterocellular junctions in endothelial cells of saphenous veins is estimated to be approximately 10 per cell, but their significance is not finally determined.     

beta-tubulin affects cellulose microfibril orientation in plant secondary fibre cell walls

Cellulose microfibrils are the major structural component of plant secondary cell walls. Their arrangement in plant primary cell walls, and its consequent influence on cell expansion and cellular morphology, is directed by cortical microtubules; cylindrical protein filaments composed of heterodimers of alpha- and beta-tubulin. In secondary cell walls of woody plant stems the orientation of cellulose microfibrils influences the strength and flexibility of wood, providing the physical support that has been instrumental in vascular plant colonization of the troposphere. Here we show that a Eucalyptus grandisbeta-tubulin gene (EgrTUB1) is involved in determining the orientation of cellulose microfibrils in plant secondary fibre cell walls. This finding is based on RNA expression studies in mature trees, where we identified and isolated EgrTUB1 as a candidate for association with wood-fibre formation, and on the analysis of somatically derived transgenic wood sectors in Eucalyptus. We show that cellulose microfibril angle (MFA) is correlated with EgrTUB1 expression, and that MFA was significantly altered as a consequence of stable transformation with EgrTUB1. Our findings present an important step towards the production of fibres with altered tensile strength, stiffness and elastic properties, and shed light on one of the molecular mechanisms that has enabled trees to dominate terrestrial ecosystems.     

Influence of rapidly successive head impacts on brain strain in the vicinity of bridging veins

Acute subdural hematoma due to a bridging vein rupture is a devastating but rare injury. There has to date been no satisfactory biomechanical explanation for this infrequent but costly injury. We surmise that it may be associated with multiple head impacts. Though numerical models have been used to estimate vein strains in single impact events, none to date have examined the influence on localized brain strain of rapidly consecutive impacts. Using the Simulated Injury Monitor, we investigated the hypothesis that such double impacts can increase strain beyond that created by any single impact. Input to our parametric study comprised hypothetical biphasic rotational head accelerations producing a maximum angular velocity of 40 rad./s. In each of 19 simulations, two identical angular inputs are applied at right angles to each other but with time separations varying from 0 to 40 ms. For these double impacts, it has been generally found that strain in the region of the bridging veins is different, than what would be associated with any corresponding single impact. In some cases, the effect is to actually reduce the tissue strain. In others, the strain in the region of the bridging veins is increased markedly. The mechanistic explanation for the strain increase is that the tissue strain from the first impact has not diminished fully when strain from the second impact is initiated. Rapidly consecutive impacts could be a potential mechanism leading to vein rupture that warrants further investigation.