贝壳珍珠层不同取向弹性模量的研究

研究天然生物材料的组织结构特征与其性能之间的关系对于材料的仿生有重要意义．在自制的激光测试设备上用三点弯曲法对贝壳珍珠层不同取向的弹性模量进行了研究,报道了不同取向和加载方式条件下弹性模量的变化规律。结果表明,在平行和倾斜于生长纹路方向上弹性模量的平均值分别为60．3GPa和56．7GPa,而垂直于纹路方向的为48GPa,呈现出各向异性。弹性模量的各向异性主要来自于珍珠层微观组织结构和贝壳生长纹结构的特点．     

Dynamics of sheet nacre formation in bivalves

Formation of nacre (mother-of-pearl) is a biomineralization process of fundamental scientific as well as industrial importance. However, the dynamics of the formation process is still not understood. Here, we use scanning electron microscopy and high spatial resolution ion microprobe depth-profiling to image the full three-dimensional distribution of organic materials around individual tablets in the top-most layer of forming nacre in bivalves. Nacre formation proceeds by lateral, symmetric growth of individual tablets mediated by a growth-ring rich in organics, in which aragonite crystallizes from amorphous precursors. The pivotal role in nacre formation played by the growth-ring structure documented in this study adds further complexity to a highly dynamical biomineralization process.     

Identification of genes associated with shell color in the black-lipped pearl oyster,Pinctada margaritifera

Background

Color polymorphism in the nacre of pteriomorphian bivalves is of great interest for the pearl culture industry. The nacreous layer of the Polynesian black-lipped pearl oyster Pinctada margaritifera exhibits a large array of color variation among individuals including reflections of blue, green, yellow and pink in all possible gradients. Although the heritability of nacre color variation patterns has been demonstrated by experimental crossing, little is known about the genes involved in these patterns. In this study, we identify a set of genes differentially expressed among extreme color phenotypes of P. margaritifera using a suppressive and subtractive hybridization (SSH) method comparing black phenotypes with full and half albino individuals.

Results

Out of the 358 and 346 expressed sequence tags (ESTs) obtained by conducting two SSH libraries respectively, the expression patterns of 37 genes were tested with a real-time quantitative PCR (RT-qPCR) approach by pooling five individuals of each phenotype. The expression of 11 genes was subsequently estimated for each individual in order to detect inter-individual variation. Our results suggest that the color of the nacre is partially under the influence of genes involved in the biomineralization of the calcitic layer. A few genes involved in the formation of the aragonite tablets of the nacre layer and in the biosynthesis chain of melanin also showed differential expression patterns. Finally, high variability in gene expression levels were observed within the black phenotypes.

Conclusions

Our results revealed that three main genetic processes were involved in color polymorphisms: the biomineralization of the nacreous and calcitic layers and the synthesis of pigments such as melanin, suggesting that color polymorphism takes place at different levels in the shell structure. The high variability of gene expression found within black phenotypes suggests that the present work should serve as a basis for future studies exploring more thoroughly the expression patterns of candidate genes within black phenotypes with different dominant iridescent colors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1776-x) contains supplementary material, which is available to authorized users.     

Tissue inhibitor of metalloproteinase gene from pearl oyster Pinctada martensii participates in nacre formation

Tissue inhibitors of metalloproteinases (TIMPs) are nature inhibitors of matrix metalloproteinases and play a vital role in the regulation of extracellular matrix turnover, tissue remodeling and bone formation. In this study, the molecular characterization of TIMP and its potential function in nacre formation was described in pearl oyster Pinctada martensii. The cDNA of TIMP gene in P. martensii (Pm-TIMP) was 901 bp long, containing a 5′ untranslated region (UTR) of 51 bp, a 3′ UTR of 169 bp, and an open reading fragment (ORF) of 681 bp encoding 226 amino acids with an estimated molecular mass of 23.37 kDa and a theoretical isoelectric point of 5.42; The predicted amino acid sequence had a signal peptide, 13 cysteine residues, a N-terminal domain and a C-terminal domain, similar to that from other species. Amino acid multiple alignment showed Pm-TIMP had the highest (41%) identity to that from Crassostrea gigas. Tissue expression analysis indicated Pm-TIMP was highly expressed in nacre formation related-tissues, including mantle and pearl sac. After decreasing Pm-TIMP gene expression by RNA interference (RNAi) technology in the mantle pallium, the inner nacreous layer of the shells showed a disordered growth. These results indicated that the obtained Pm-TIMP in this study participated in nacre formation.     

On the fracture mechanisms of nacre: Effects of structural orientation

The nacre of mollusk shells is distinguished by an exceptional mechanical efficiency which is derived essentially from its lamellar structure and frequently acts as a source of inspiration for the development of biomimetic materials. The structure and mechanical properties of nacre have been intensively investigated with a special focus on its toughening strategies; nevertheless, the fracture mechanisms, more specifically the critical stress/strain conditions for the failure of nacre, and the effects of structural orientation and hydration state remain largely unexplored. Here uniaxial compression tests were performed on nacre of both dry and hydrated states with different off-axis angles, i.e., the inclination of loading axis with respect to the lamellar structure, ranging from 0° to 90°. The mechanical properties and fracture characteristics of nacre and their dependences on the structural orientation and hydration state were elucidated in terms of mechanics behind failure. Quantitative relationships were established between the mechanical properties and off-axis angle based on different failure criteria. The competition between the fracture modes of fragmentation and shearing was quantified by comparing their respective driving force and resistance on the interfacial plane. This study may aid the understanding on the mechanical behavior of nacre and nacre-inspired synthetic materials and promote a better replication of the underlying design principles of nacre in man-made systems.     

In vitro culture of mantle tissue of the abalone Haliotis varia Linnaeus

The study is aimed at developing a technology for the production of in vitro pearl through tissue culture of mantle of the abalone, Haliotis varia Linnaeus, as the production of free and spherical pearls in vivo is rather difficult in abalones. In the basic study, the cell yield was intensified from the explant after 24h incubation. Among the cells liberated, the granulocytes were dominant over hyalinocytes. The size of granulocytes ranged from 3 to 16 microm and of hyalinocytes from 13 to 18 microm. Fibroblast-like cells appeared in cultures after day 2. Both granulocytes and hyalinocytes developed pseudopodial-like extensions in all directions and formed organic matrix. Granulocytes contained granules in the cytoplasm. Specific granules were responsible for nucleation of crystals. Some crystals exhibited green colour resembling mother of pearl of abalone. scanning electron microscope (SEM) study revealed the oolitic amorphous state and rhombohedral state of crystals. Its analysis through energy dispersive X-ray microanalyzer (EDS) indicated the presence of calcium. The rhombohedral crystals under polarized light showed its high birefringence (0.18) and uniaxial optically negative calcite nature with high content of calcium. A mean survival of cells was found to be 102 days in T 25 flasks and 32 days in petri dishes. Growth of cells was studied. Thirty percent of cultures were found to have contaminated during the study. The study provides basic knowledge in the development of a technology for in vitro pearl production.     

淡水育珠蚌体外培养外套膜细胞分泌的珍珠质的性质研究

对淡水育珠蚌分泌珍珠质的外表皮进行组织培养,取培养不同时间的培养基,用氨基酸分析仪对其氨基酸测定的结果与空白培养基作比较,其中珍珠所含的各种氨基酸均增加,尤其是珍珠中含量最高的丙氨酸,甘氨酸和谷氨酸增加最多,珍珠药用有效成分之一的牛磺酸,随组织培养时间的延长而逐渐增加,用等离子光谱测定培养组织匀浆液与未培养比较,钙的含量也大为增加,结果表明,离体组织培养分泌的珍珠质的化学成分和性质与活体基本相同。     

Unlocalized crack initiation and propagation in staggered biomaterials

Many types of tissues in living organisms exhibit a combination of different properties to fulfil their mechanical functions in complex environments. Nacre with more than 90% brittle and hard phase and a little protein matrix, exhibits high strength and toughness, which is difficult to achieve in artificial materials. Researchers have shown that the toughness of nacre is related to the cracking process. Most of them, however, assume an obvious pre-existing crack on the model and the initiation of the microscopical pre-existing crack is not considered yet. Based on fracture mechanics with the cohesive zone model, we reveal the mechanism of the crack initiation and propagation pattern in staggered biomaterials without any pre-existing crack. The simulation result shows that there are two crack propagation modes: localized mode and unlocalized mode. A crack initiates and propagates in a small area in the localized mode, while cracks initiate at different points and propagate in various paths in the unlocalized mode. The crack initiation mechanism from the intrinsic properties of the material is clarified using energy based stability analysis. The result shows that the shear interfacial mechanism significantly delays the crack initiation.