Structural

The exoskeleton of the lobster Homarus americanus is a multiphase biological composite material which consists of an organic matrix (crystalline α-chitin fibers and various types of non-crystalline proteins) and minerals (mainly calcite). In this study we discuss experimental data about the mesoscopic structure and the crystallographic texture (orientation distribution) of the α-chitin–protein fiber network in this material. The synchrotron measurements reveal very strong crystallographic textures of the α-chitin. According to these data, a large fraction of the α-chitin lattice cells is arranged with their longest axis parallel to the normal of the surface of the exoskeleton. Additionally, a smaller fraction of the α-chitin cells is oriented with their longest axis perpendicular to the cuticle surface. These structural investigations reveal the pronounced role of crystallographic orientation distributions in mineralized biological composite materials which may be of relevance for an improved understanding of biological and bio-inspired nano-composites.

This paper reports the results of the first dynamic labeling experiment with regenerating spines of sea urchins Paracentrotus lividus using the stable isotope 26Mg and NanoSIMS high-resolution isotopic imaging, which provide a direct information about the growth process. Growing spines were labeled twice (for 72 and 24 h, respectively) by increasing the abundance of 26Mg in seawater. The incorporation of 26Mg into the growing spines was subsequently imaged with the NanoSIMS ion microprobe. Stereom trabeculae initially grow as conical micro-spines, which form within less than 1 day. These micro-spines fuse together by lateral outgrowths and form a thin, open meshwork (inner stereom), which is subsequently reinforced by addition of layered thickening deposits (outer stereom). The (longitudinal) growth rate of the inner stereom is ca. 125 μm/day. A single (ca. 1 μm) thickening layer in the stereom trabeculae is deposited during 24 h. The thickening process is contemporaneous with the formation micro-spines and involves both longitudinal trabeculae and transverse bridges to a similar degree. Furthermore, the skeleton-forming cells remain active in the previously formed open stereom for at least 10 days, and do not migrate upwards until the end of the thickening process. The experimental capability presented here provides a new way to obtain detailed information about the skeleton formation of a multitude of marine, calcite producing organisms.

The centrosome plays a major role in the spatial organization of the microtubular network and has a controlled cycle of duplication, the two duplicated centrosomes functioning as mitotic poles during subsequent cell division. However, a comprehensive description of the overall organization of the centrosome in animal cells is lacking. In order to integrate the various pieces contributing to the centrosome structure and to optimize the quality of the data, we have undertaken an extensive ultrastructural study of centrosomes isolated from human lymphoblasts, which involved (i) orientation of centrosomes by sedimentation before embedding and sectioning, (ii) ultrathin serial sectioning, (iii) digitalization of micrographs to obtain quantitative data, and finally, (iv) comparison between two methods of isolation, which differ by the presence or absence of EDTA. Using this strategy, we have unambiguously described the pericentriolar organization of two distinct sets of appendages (distal and subdistal) about the so-called parental centriole. New structures have been also observed in association with the microtubule sets in this study: (i) external columns, which are dense structures localized at the basis of the subdistal appendages and (ii) internal columns, which are made of globular subunits integrated in a more luminal and probably helical structure. We have also observed that removal of divalent cations by the EDTA during the isolation procedure could affect the centrosomal structure at different levels (subdistal appendages, internal and external columns, pericentriolar matrix), including a significant variation in centriole diameter. A scheme of the overall organization of the centrosome from animal cells and of its modulation by divalent cations can be drawn from this study. Our data gives a view of the centrosome as an organelle displaying a complex and possibly dynamic structural organization.

This study investigated the effect of multilingual linguistic experience on the perception of Japanese durational contrast by learners of Japanese as a foreign language at the basic level. Five monolingual and five multilingual learners listened to recorded sentences c)ntaining the utterances /iken / or /ikken / and were asked to identify which of the two utterances was employed in each sentence. Amongst the multilingual subjects, some spoke languages that utilize segmental durations phonemically, whilst some spoke languages that did not. Although the results indicated superior perceptual performance by multilingual learners over monolingual learners, there was no significant difference between the two types of multilingual subjects. The results support the hypothesis that not only specific but also non-specific, broadened linguistic experience can result in increased perceptual performance. Other results and their implications are discussed. (Contains 29 references.) (MDM) Reproduc...

The Pinto shear zone is one of several Late Cretaceous shear zones within the eastern fringe of the Mesozoic magmatic arc of the southwest Cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening. We demonstrate an extensional origin for the shear zone by describing the shear-zone geometry and kinematics, hanging wall deformation style, progressive changes in deformation

Purpose – Construction labour productivity is of great interest to practitioners and researchers because it affects project cost and time overrun. This paper evaluates and ranks the importance, frequency and severity of project delay factors that affect the construction ...