Variations of microstructure, mineral density and tissue elasticity in B6/C3H mice

200-MHz scanning acoustic microscopy (SAM) and synchrotron radiation μCT (SR-μCT) were used to assess microstructural parameters, acoustic impedance Z and tissue degree of mineralization of bone (DMB) in site-matched regions of interest in femoral bone of two inbred strains. Transverse femoral sections taken from 5 C57BL/6J@Ico (B6) and 5 C3H/HeJ@Ico (C3H) mice (5.5 months old) were explored. Mass density ρ, elastic coefficient c₁₁ and Young's modulus E₁ were locally derived in the distal epiphysis, distal metaphysis for trabecular bone and mid-diaphysis for cortical bone using a rule-of-mixture model. Structural parameter estimations obtained from X-ray tomographic and acoustic images were almost identical. Both strains had the same bone diameter, but the C3H mice had greater cortical thickness and smaller cancellous diameter than did B6 mice. The average DMB and impedance values were in the range between 1.13 and 1.33 g cm^− 3 and 5.8 and 7.8 Mrayl, respectively. All tissue parameters were lower in B6 mice than in C3H mice. However, interstrain differences of DMB were much less (up to 3.8%) than differences of Z (up to 13.2%). SAM and SR-μCT fulfill the requirement for a simultaneous evaluation of cortical bone microstructure and material properties at the tissue level. However, SAM provides a quantitative estimate of elastic properties at the tissue level that cannot be captured by SR-μCT. The strong differences in the measured acoustic impedances among the two inbred strains indicate that the impedance is a good parameter to detect genetic variations of the skeletal phenotype in small animal models.     

Incremental changes in anisotropy induce incremental changes in the material properties of electrospun scaffolds

Electrospinning can be used to selectively process a variety of natural and synthetic polymers into highly porous scaffolds composed of nano-to-m diameter fibers. This process shows great potential as a gateway to the development of physiologically relevant tissue engineering scaffolds. In this study, we examine how incremental changes in fiber alignment modulate the material properties of a model scaffold. We prepared electrospun scaffolds of gelatin composed of varying fiber diameters and degrees of anisotropy. The scaffolds were cut into a series of "dog-bone" shaped samples in the longitudinal, perpendicular and transverse orientations and the relative degree of fiber alignment, as measured by the fast Fourier transform (FFT) method, was determined for each sample. We measured peak stress, peak strain and the modulus of elasticity as a function of fiber diameter and scaffold anisotropy. Fiber alignment was the variable most closely associated with the regulation of peak stress, peak strain and modulus of elasticity. Incremental changes, as judged by the FFT method, in the proportion of fibers that were aligned along a specific axis induced incremental changes in peak stress in the model scaffolds. These results underscore the critical role that scaffold anisotropy plays in establishing the material properties of an electrospun tissue engineering scaffold and the native extracellular matrix.     

轻度Alzheimer病人与正常老年人全脑白质各向异性的比较

目的利用扩散张量成像研究轻度Alzheimer患者全脑范围内的白质异常.方法对16例正常志愿者和13例轻度Alzheimer患者进行了扩散张量成像扫描,经过后处理生成的FA图在SPM99下进行头颅标准化后,对AD组和对照组的全脑白质FA图进行基于体素的统计学比较.结果对照组和AD组的双侧内颞叶、右颞中回、左侧顶叶白质,左侧外囊,右上纵束和穹窿的各向异性FA值差异具有显著性意义.结论基于体素的全脑白质FA图比较能够发现AD患者中大脑白质的异常,从而更加全面地评价AD患者的结构改变.     

健康成年人脑白质弥散各向异性与年龄的关系的初步研究

目的通过使用磁共振弥散张量成像(DTI)对健康成年人的脑白质纤维束进行检测,探讨DTI及部分各向异性值(FA)对评价年龄引起的脑白质变化的价值。方法对30例健康志愿者(青年组20~30岁,老年组60~83岁,各15例)进行脑部MR横轴位检查,扫描序列包括T1Flair、T2WI、FLAIR、DWI、DTI。在大脑胼胝体膝、压部、内囊膝部及后肢、视辐射、额叶白质及丘脑选定兴趣区(ROI),测量其FA值并做统计学分析。结果青年组胼胝体膝部及压部、内囊膝部、视辐射、额叶白质的FA值分别为:(0.70±0.05)、(0.70±0.05)、(0.63±0.04)、(0.53±0.05)、(0.43±0.06),老年组相应ROI的FA值分别为:(0.62±0.09)(、0.66±0.07)、(0.57±0.07)(、0.38±0.06)(、0.38±0.07),两组差异有统计学意义(P<0.05);内囊后肢及丘脑FA值差异无统计学意义。结论DTI对评价脑白质纤维束的密度及有序性有价值,FA值能反映两组不同年龄人群脑白质的生理状态。     

Anisotropy of nickel release and corrosion in austenitic stainless steels

The study of 316L-type stainless steel reveals a significant anisotropy of nickel release that is dependent on the orientation of the test surface with respect to the casting and rolling direction. Cross-sectional specimens (transversal cuts with respect to the rolling direction) show a substantially higher sensitivity to corrosion phenomena compared with longitudinal cuts and they release nickel ions at rates 10–100 times higher. These findings indicate that orientation needs to be taken into account when interpreting test results, in particular when comparing different grades of austenitic stainless steel, as well as in product and production design.     

Micromechanics-Based Conversion of CT Data into Anisotropic Elasticity Tensors, Applied to FE Simulations of a Mandible

Computer Tomographic (CT) image data have become a standard basis for structural analyses of bony organs. In this context, regression functions between stiffness components and Hounsfields units (HU) from CT, related to X-ray attenuation coefficients, are widely used for the definition of the (actually inhomogeneous and anisotropic) material behavior inside the organ. Herein, we suggest to derive the functional dependence of the fully orthotropic stiffness tensors on the Hounsfield units from the physical information contained in the X-ray attenuation coefficients: (i) Based on voxel average rules for the X-ray attenuation coefficients, we assign to each voxel the volume fraction occupied by water (marrow) and that occupied by solid bone matrix. (ii) By means of a continuum micromechanics representation for bone, which is based on voxel-invariant (species and whole bone-specific) stiffness properties of solid bone matrix and of water, we convert the aforementioned volume fractions into voxel-specific orthotropic stiffness tensor components. The micromechanics model, in combination with the average rule for X-ray attenuation coefficients, predicts a quasi-linear relationship between axial Young’s modulus and HU, and highly nonlinear relationships for both circumferential and radial Young’s moduli as well as for the shear moduli in all principal material directions. Corresponding whole-organ Finite Element (FE) analyses of a partially edentulous human mandible characterized by atrophy of the alveolar ridge show that volumetric strain concentrations/peaks within the organ are decreased when considering material anisotropy, and increased when considering material inhomogeneity.

Altered lipid composition in cortical lipid rafts occurs at early stages of sporadic Alzheimer's disease and facilitates APP/BACE1 interactions

The presence of lipid alterations in lipid rafts from the frontal cortex in late stages of Alzheimer's disease (AD) has been recently demonstrated. Here, we have isolated and analyzed the lipid composition of lipid rafts from different brain areas from control and AD subjects at initial neuropathologic stages. We have observed that frontal cortex lipid rafts are profoundly altered in AD brains from the earliest stages of AD, namely AD I/II. These changes in the lipid matrix of lipid rafts affected both lipid classes and fatty acids and were also detected in the entorhinal cortex, but not in the cerebellum from the same subjects. Paralleling these changes, lipid rafts from AD frontal and entorhinal cortices displayed higher anisotropy for environment-sensitive probes, indicating that lipid changes in AD lipid rafts increased membrane order and viscosity in these domains. The pathophysiological consequences of these alterations in the development and progression of AD were strengthened by the significant, and specific, accumulation of β-secretase within the lipid rafts of AD subjects even at the earliest stages. Our results provide a mechanistic connection between lipid alterations in these microdomains and amyloidogenic processing of amyloid precursor protein.     

Anisotropy of Young's modulus of human tibial cortical bone

The anisotropy of Young's modulus in human cortical bone was determined for all spatial directions by performing coordinate rotations of a 6 by 6 elastic stiffness matrix. The elastic stiffness coefficients were determined experimentally from ultrasonic velocity measurements on 96 samples of normal cortical bone removed from the right tibia of eight human cadavers. The following measured values were used for our analysis: c ₁₁ =19.5 GPa, c ₂₂ =20.1 GPa, c ₃₃ =30.9 GPa, c ₄₄ =5.72 GPa, c ₅₅ =5.17 GPa, c ₆₆ =4.05 GPa, c ₂₃ =12.5 GPa. The remaining coefficients were determined by assuming that the specimens possessed at least an orthorhombic elastic symmetry, and further assuming that c ₁₃ =c ₂₃ , c ₁₂ =c ₁₁ –2c ₆₆ . Our analysis revealed a substantial anisotropy in Young's modulus in the plane containing the long axis of the tibia, with maxima of 20.9 GPa parallel to the long axis, and minima of 11.8 GPa perpendicular to this axis. A less pronounced anisotropy was observed in the plane perpendicular to the long axis of the tibia. To display our results for the full three-dimensional anisotropy of cortical bone, a closed surface was used to represent Young's modulus in all spatial directions.