The binary phase behavior of 1,3-dimyristoyl-2-stearoyl-sn-glycerol and 1,2-dimyristoyl-3-stearoyl-sn-glycerol

The binary phase behavior of pure 1,3-dimyristoyl-2-stearoyl-sn-glycerol (MSM) and 1,2-dimyristoyl-3-stearoyl-sn-glycerol (MMS) was investigated in terms of polymorphism, melting and crystallization behavior, SFC, hardness and microstructure. Samples were crystallized at cooling rates of 3.0 and 0.1 degrees C/min. The asymmetric TAG demonstrated lower melting and crystallization points at both cooling rates. All samples crystallized in the beta' polymorph when cooled at 0.1 degrees C/min and in the alpha polymorph when cooled at 3.0 degrees C/min. The experimentally determined kinetic phase diagram of MSM-MMS was monotectic for both cooling rates. This data was well described by a thermodynamic model using the Bragg-Williams approximation for non-ideality of mixing and suggested that in both the solid and liquid states, like pair interactions (MSM-MSM and MMS-MMS) were favored over MSM-MMS interaction. A strong tendency to phase separation in the solid phase was also observed. For both cooling rates, the fit of the SFC (%)-time curves to a modified form of the Avrami model indicated that crystallization occurred in two distinct kinetic steps. Depressions seen in SFC did not correspond to depressions in hardness or melting temperatures.     

Modeling the Rheological Properties of Fats: A Perspective and Recent Advances

The elastic modulus of colloidal fat crystal networks scales with the volume fraction of solids in a power–law fashion. To explain and predict how the elastic properties of these networks change with their volume fraction of solids, several physical models have been proposed. In this review, the chronology of the development of structural–mechanical models to explain the elasticity of fats is reviewed, leading to the development of the fractal model. In the fractal model, the fractal-like behavior of fat crystal networks, which can be considered fractal gels of polycrystals in oil, or colloidal crystals, is used to explain the power–law scaling behavior of the shear elastic modulus to the volume fraction of solids. Lately, however, many experimental results and simulation studies suggest that the stress distribution within networks can be dramatically heterogeneous, which means that a small part of the network carries most of the stress. This concept was introduced into a modified fractal model by deriving an expression for the effective volume fraction of stress-carrying solids. The modified fractal model fits the experimental data well and successfully explains the sometimes observed non-linear log–log behavior between the shear elastic modulus and the volume fraction of solids.     

Shell repair process in the green ormer Haliotis tuberculata: a histological and microstructural study

In the present paper, juvenile and adult shells of the green ormer Haliotis tuberculata ('Oreille de Saint-Pierre') were perforated in a zone close to the shell edge and the shell repair process was followed at two levels: (1) by observing the histology of the calcifying mantle in the repair zone and (2) by analyzing with SEM the microstructure of the shell repair zone. Histological data clearly show the presence of calcium carbonate granules into the connective tissues, but not in the epithelial cells. This suggests that calcium carbonate granules are synthesized by sub-epithelial cells and actively transported through the epithelium to the repair zone, via a process which may be similar to that described by Mount et al. [Mount, A.S., Wheeler, A.P., Paradkar, R.P., Snider, D., 2004. Hemocyte-mediated shell mineralization in the eastern oyster. Science 304, 297-300]. Furthermore, SEM observations show that the repair zone exhibits different stratified microstructures (spherulitic, thin prismatic, blocklike, sub-nacreous, nacreous, foliated-like), some of which are not continuous (i.e. lenticular) along the repair zone. This suggests a complex secreting regime of the calcifying mantle and an elaborate geometry of the epithelium involved in shell repair.     

Hyolith-type microstructure in a mollusc-like fossil from the Early Cambrian of Yunnan, China

A phosphatized bilaterally symmetrical, spirally coiled conch from the Lower Cambrian phosphates of the Meishucunian Stage of eastern Yunnan, China, is composed of two well-preserved layers. The outer one is built of longitudinal mineralized fibres not strictly parallel to each other, but may anastomose or branch. The fibres form comarginal ribs with transversal grooves between them, corresponding to growth stages. In the ribs, the fibres are arranged closely and are steeply inclined towards the apex, while in the grooves the fibres are subparallel to the conch surface and more loosely packed. Small elliptic and triangular holes are situated within and between the fibres of the outer layer. The inner layer consists of transversal fibres running around the conch. They are almost parallel to each other but partly separated by narrow discontinuous slits. Despite overall morphological similarity of the conch to shells of some early Cambrian molluscs, the observed orthogonal configuration of longitudinally oriented fibrous structures of the outer layer and transversal fibres of the inner layer is found in orthothecid hyoliths, phosphatized microstuctures of which have been described from the Lower Cambrian of the Siberian Platform. The new material from the Yangtze Platform supports a conclusion of a distinctive type of microstructure available in hyoliths different from molluscan microstructures available from the Lower Cambrian.     

The problem of Dinosaur extinction.Contribution of the study of terminal Cretaceous eggshells from Southeast France

A biostratigraphic sequence based upon dinosaureggshells during the lower and upper Rognacian (upper Cretaceous) in the Aix-en-Provence sedimentary basin (Bouches-du-Rhône, France) has been studied from a morphometric and microstructural viewpoint. Stratigraphic control of eggshell fragment sampling made it possible to follow their evolution throughout the Rognacian. Our results differ from those of certain authors who have worked in this region in that no tendancy toward abnormal eggshell thinness appears at the end of the Cretaceous which could be linked to the problem of dinosaur extinction. Furthermore, we observed no significant increase in the number of bi- or multistratified eggshells («ovum-in-ovo) in the deposits. The microstructural study shows the presence mainly of normal egg-shells belonging to at least three morphotypes.     

北方山溪鲵精子发生不同阶段的显微与超微结构

用光镜和电镜观察了北方山溪鲵(Batrachuperus tibetanus)精子发生过程中各种类型生精细胞的显微与超微结构变化。结果显示,北方山溪鲵在4~8月时处于精子发生期,精子形成在7~8月。成熟精子的结构具有小鲵科精子的一些共同特征,如顶体前端呈三叶草状,尾部由轴纤维、波动膜、轴丝及轴丝旁纤维构成,轴纤维粗大呈圆柱形,尾部无线粒体等。比较分析认为,在两栖类的系统发育中,轴纤维、波动膜和轴丝旁纤维的消失为近裔性状。     

Nanoporous Structures Similar to Those Reported from Squid Sucker Teeth are also Present in Egg Shells of a Terrestrial Flatworm (Platyhelminthes; Rhabditophora; Geoplanidae) from Hachijojima (Izu Islands,Japan)

Shells of the egg cocoon of a terrestrial planarian (Diversibipalium sp.) from Hachijojima were found to be composed of a lattice of parallel nanotubes of ca. 120 nm diameter oriented perpendicular to the shell's surface. The arrangement of the porous proteinaceous tubes closely resembles that has recently been reported from the sucker teeth of squid and to date is the only other example of this kind of structure. Although the array of nanotubes undoubtedly contributes to the stiffness of the shell and helps protecting the embryo, questions such as to how the planary worm produces the array of nanotubes and what exactly their chemical and physical properties are versus those of the squid sucker tooth still remain to be answered.     

Microstructural organization of the central nervous system in the harvestman Leiobunum japonicum (Arachnida: Opiliones)

Although the order Opiliones constitutes the third‐largest group of arachnids, this creature is still mysterious and has a rich unexplored field compared to what is known about insects and crustaceans. The order Opiliones is traditionally regarded as a close relative of mites, mainly because of morphological similarities in external body structure; however microstructural organization of the ganglionic neurons and nerves in the harvestman Leiobunum japonicum is quite similar to the central nervous system (CNS) in all extant arachnids. The CNS consists of a large neural cluster with paired appendicular nerves. The esophagus passes through the neural cluster and divides it into the upper supraesophageal ganglion (SpG) and the lower subesophageal ganglion (SbG). The dorsal part of the SpG has a quite condensed cell body compared with other parts of the CNS and has two main components, the protocerebrum and the cheliceral ganglion. The protocerebrum receives the optic nerves and has four main groups of neuropiles from the optic lobes, the superior central body, the lateral neuropils (corpora pedunculata) and the inferior neuropil. However, a pair of pedipalpal and four pairs of appendage nerves including several pairs of abdominal nerves arise from the nerve masses of the SbG.     

扬子鳄视觉器官组织学研究

本文用光镜和电镜研究了扬子鳄(Alligator sinensis)的组织学,同时测量了其眼球的一些光学参数.扬子鳄眼球呈扁圆球形,角膜径与球径的比值为1:1.44;晶状体与角膜的比值为1:1.40。角膜内具鲍氏膜;虹膜内的括约肌、睫状体内的睫状肌均属横纹肌,视细胞椭圆体内线粒体嵴突与线粒体长轴相平行,这与报道的其它鳄类不同。虹膜内未见扩瞳肌纤维,角膜缘缺巩膜小骨片,晶状体环垫薄,因而其视觉调节能力仍然很弱。视网膜中视细胞由视杆细胞、单锥细胞、双锥细胞组成,其中以视杆细胞占多数。视细胞与神经节细胞核比值平均为2.5:1,表明扬子鳄的组织结构与其弱光视觉相适应。     

Age-related properties at the microscale affect crack propagation in cortical bone

The increased risk for fracture with age is associated not only with reduced bone mass but also with impaired bone quality. At the microscale, bone quality is related to porosity, microstructural organization, accumulated microdamage and intrinsic material properties. However, the link between these characteristics and fracture behavior is still missing. Bone tissue has a complex structure and as age-related compositional and structural changes occur at all hierarchical length scales it is difficult to experimentally identify and discriminate the effect of each mechanism. The aim of this study was therefore to use computational models to analyze how microscale characteristics in terms of porosity, intrinsic toughness properties and microstructural organization affect the mechanical behavior of cortical bone. Tensile tests were simulated using realistic microstructural geometries based on microscopy images of human cortical bone. Crack propagation was modelled using the extended finite element method where cement lines surrounding osteons were modelled with an interface damage law to capture crack deflections along osteon boundaries. Both increased porosity and impaired material integrity resulted in straighter crack paths with cracks penetrating osteons, similar to what is seen experimentally for old cortical bone. However, only the latter predicted a more brittle failure behavior. Furthermore, the local porosity influenced the crack path more than the macroscopic porosity. In conclusion, age-related changes in cortical bone affect the crack path and the mechanical response. However, increased porosity alone was not driving damage in old bone, but instead impaired tissue integrity was required to capture brittle failure in aging bone.