Function-related adaptations of ultrastructure, mineral phase distribution and mechanical properties in the incisive cuticle of mandibles of Porcellio scaber Latreille, 1804

Molluscs generate preferred crystallographic orientation of biominerals by organic templates, the texture and microstructure of Caudofoveata (Aplacophora) shells

Caudofoveata are molluscs that protect their vermiform body with a scleritome, a mosaic of unconnected blade/lanceolate-shaped aragonite sclerites. For the species Falcidens gutturosus and Scutopus ventrolineatus we studied the crystallographic constitution and crystal orientation texture of the sclerites and the scleritome with electron-backscatter-diffraction (EBSD), laser-confocal-microscopy (LCM) and field-emission electron microscopy (FE-SEM) imaging. Each sclerite is an aragonite single crystal that is completely enveloped by an organic sheath. Adjacent sclerites overlap laterally and vertically are, however, not connected to each other. Sclerites are thickened in their central portion, relative to their periphery. Thickening increases also from sclerite tip towards its base. Accordingly, cross-sections through a sclerite are straight at its tip, curved and bent towards the sclerite base. Irrespective of curved sclerite morphologies, the aragonite lattice within the sclerite is coherent. Sclerite aragonite is not twinned. For each sclerite the crystallographic c-axis is parallel to the morphological long axis of the sclerite, the a-axis is perpendicular to its width and the b-axis is within the width of the sclerite. The single-crystalinity of the sclerites and their mode of organization in the scleritome is outstanding. Sclerite and aragonite arrangement in the scleritome is not given by a specific crystal growth mode, it is inherent to the secreting cells. We discuss that morphological characteristics of the sclerites and crystallographic preferred orientation (texture) of sclerite aragonite is not the result of competitive growth selection. It is generated by the templating effect of the organic substance of the secreting cells and associated extracellular biopolymers.

Structural diversity of crustacean exoskeletons and its implications for biomimetics

The crustacean cuticle is a biological composite material consisting of chitin-protein fibres in a mineralized matrix. Recent research has revealed a surprising range of fibre architectures and mineral compositions of crustacean skeletal structures adapted to various mechanical demands. It is becoming increasingly clear that the organic fibres in the cuticle may be organized in patterns differing from the standard twisted plywood model. Observed fibre architectures in protruding skeletal structures include longitudinal and circular parallel fibre arrays. Skeletal minerals often include calcium phosphates in addition to calcium carbonates. Furthermore, skeletal properties are affected by protein cross-linking, which replaces mineralization as a stiffening mechanism in some structures. Several common structural motifs, such as the stiffening of the outer skeletal layers, the incorporation of non-mineralized cuticle in exposed structures, and interchanging layers of parallel fibres and the twisted plywood structure, can be identified in skeletal elements with similar functions. These evolutionary solutions have the potential for biomimetic applications, particularly as manufacturing technologies advance. To make use of this potential, we need to understand the processes behind the formation of the crustacean exoskeleton and determine which features are truly adaptive and worth mimicking.

Biological light-weight materials: The endoskeletons of cephalopod mollusks

Structural biological hard tissues fulfill diverse tasks: protection, defence, locomotion, structural support, reinforcement, buoyancy. The cephalopod mollusk Spirula spirula has a planspiral, endogastrically coiled, chambered, endoskeleton consisting of the main elements: shell-wall, septum, adapical-ridge, siphuncular-tube. The cephalopod mollusk Sepia officinalis has an oval, flattened, layered-cellular endoskeleton, formed of the main elements: dorsal-shield, wall/pillar, septum, siphuncular-zone. Both endoskeletons are light-weight buoyancy devices that enable transit through marine environments: vertical (S. spirula), horizontal (S. officinalis). Each skeletal element of the phragmocones has a specific morphology, component structure and organization. The conjunction of the different structural and compositional characteristics renders the evolved nature of the endoskeletons and facilitates for Spirula frequent migration from deep to shallow water and for Sepia coverage over large horizontal distances, without damage of the buoyancy device. Based on Electron-Backscatter-Diffraction (EBSD) measurements and TEM, FE-SEM, laser-confocal-microscopy imaging we highlight for each skeletal element of the endoskeleton its specific mineral/biopolymer hybrid nature and constituent arrangement. We demonstrate that a variety of crystal morphologies and biopolymer assemblies are needed for enabling the endoskeleton to act as a buoyancy device. We show that all organic components of the endoskeletons have the structure of cholesteric-liquid-crystals and indicate which feature of the skeletal element yields the necessary mechanical property to enable the endoskeleton to fulfill its function. We juxtapose structural, microstructural, texture characteristics and benefits of coiled and planar endoskeletons and discuss how morphometry tunes structural biomaterial function. Both mollusks use their endoskeleton for buoyancy regulation, live and move, however, in distinct marine environments.

Mechanical properties, degree of sclerotisation and elemental composition of the gastric mill in the red swamp crayfish Procambarus clarkii (Decapoda, Crustacea)

The gastric mill of Decapoda is a unique feature, which comprises teeth, stabilizing ossicles, and particle sorting setae. Involved in the fragmentation and sorting of the food, this structure serves as interface between the organism and its environment. As material properties complement morphology and hold information about function and trophic preferences, we here provide a basis for more comparative research on gastric mills. For gastric mill components of the adult red swamp crayfish Procambarus clarkii, we studied (a) the micro-structure via scanning electron microscopy, (b) the elemental composition by energy-dispersive X-ray spectroscopy, (c) the heterogeneities in material properties and degree of tanning (autofluorescence) by confocal laser scanning microscopy, and (d) the mechanical properties hardness and elasticity by nanoindentation technique. The morphology and micro-structure were previously described for this species, but the mechanical properties and the autofluorescence were not studied before. As epicuticle and exocuticle could be analyzed individually, material property gradients, with values decreasing from the interacting surface towards interior, could be determined. Finally, we were able to relate the mechanical property data with the elemental composition and the degree of tanning. We found that the epicuticle of the teeth is among the hardest and stiffest biological materials in invertebrates due to the incorporations of high proportions of silicon.

Effects of Titanium Implant Combined with Nano-Indentation in the Vertical Control of Physiological Anchorage Spee’s Wire System Corrective Technology

In the current research, there was, in the vertical control of physiological anchorage spee’s wire system (PASS) technology, an investigation on the role of titanium implant technology in combination with nano-indentation experiment the present research. The human jaw was selected as the sample to be cut vertically, by doing do, to obtain a test slice of about 2 mm through a cutting machine following the nano-indentation test. The slice was frozen and preserved in normal saline and taken out during the test. 40 outpatients who underwent orthodontic therapy were chosen as the research objects and rolled in a random manner into a control category and an category of observation. Then, mechanical biological therapy (MBT) correction was conducted for the outpatients from the control category and PASS correction therapy based on the nano-indentation experiment was for outpatients from the category of observation. Following therapy, the therapy conditions of these two categories were compared, showing that the load was a fixed value and the depth of the indentation was increasing. Under the action of external force, the periodontal ligament might undergo elastic deformation, changing with duration. Dentition alignment duration, ligation duration, and deligation duration were less for participants in the observation group compared to those in the control category (P less than 0.05). The inclination between the longitudinal plane of the top central incisor and the sella juncture root point plane (UI-SN angle), the spacing between both the top mandibular teeth margin and the nasal root juncture molar seat point (UI-NA distance), and the angle between the upper and lower central incisor tooth axis and the NA connection (UI-NA angle) were all significantly less pronounced in the category of observation than in the control category (P less than 0.05). However, the angle of upper and lower central incisor long axis (UI-LI angle) from the category of observation was hugely greater than that of the control category (P less than 0.05). The value of dental arch width in the category of observation was smaller significantly than the value of the control category (P less than 0.05), and the changes in dental arch width were larger than those in the control category (P less than 0.05). Besides, the measured values of X-ray projections of outpatients from the category of observation were higher at of the control category (P less than 0.05). In conclusion, nano-indentation experiment can accurately match the equipment required in the therapy. The application of titanium materials combined with the vertical control of PASS correction technology can effectively alleviate the clinical symptoms of outpatients so as to improve the therapy effect.

The epidermis cells of mandible teeth in the terrestrial isopod Porcellio scaber: Differentiations for mineralisation with calcium phosphate and carbonate

Generally, the mineralisation of the crustacean cuticle occurs when the cuticle has expanded after moulting. However, in the partes incisivae of Porcellio scaber, cuticle mineralisation with calcium phosphate already occurs before the moult. We investigated the ultrastructure and distribution of organelles within the epidermis cells and searched for calcium-containing organelles using EDX and EFTEM analysis. We found two different cell types. Calcium carbonate-secreting C-cells, which resemble the epithelial cells of the general integument, and the P-cells, which, as an unusual feature, have cell extensions up to 400 μm long. During secretion of the partes incisivae, these extensions end at the unmineralised tip and the phosphate-containing middle region. Their cell bodies contain most of the mitochondria located in basal folds and a high amount of endoplasmic reticulum. The cell extensions contain many microtubules, endoplasmic reticulum, large and small vesicles and densely stained rod-shaped cisternae. The rod-shaped cisternae and the endoplasmic reticulum contain calcium. During cuticle mineralisation, vesicles, which probably belong to the endo-lysosomal system, contain calcium and phosphorus. They occur at some distance and close to the cuticle. The mineral in these vesicles has a similar composition to that within the cuticle, suggesting that they play a role in cuticle mineralisation.

The dorsal tergite cuticle of Helleria brevicornis: Ultrastructure, mineral distribution, calcite microstructure and texture

Among the terrestrial Crustacea, isopods have most successfully established themselves in a large variety of terrestrial habitats. As in most Crustacea, their cuticle consists of a hierarchically organised organic phase of chitin-protein fibrils, containing calcium carbonate and some calcium phosphate. In previous studies, we examined the tergite cuticle of Tylos europaeus, which lives on seashores and burrows into moist sand. In this study, we investigate the closely related species Helleria brevicornis, which is completely terrestrial and lives in leaf litter and humus and burrows into the soil. To get deeper insights in relation between the structure of the organic and mineral phase in species living in diverse habitats, we have investigated the structure, and the chemical and crystallographic properties of the tergite cuticle using various preparation techniques, and microscopic and analytical methods. The results reveal long and short epicuticular sensilla with brushed tips on the tergite surface that do not occur in T. europaeus. As in T. europaeus a distal exocuticle, which contains a low number of organic fibres, contains calcite while the subjacent layers of the exo- and endocuticle contain amorphous calcium carbonate. The distal exocuticle contains a polygonal pattern of mineral initiation sites that correspond to interprismatic septa described for decapod crabs. The shape and position of calcite units do not follow the polygonal pattern of the septa. The results indicate that the calcite units form by crystallisation from an amorphous phase that progresses from both margins of the septa to the centres of the polygons.

Functional adaptations in the tergite cuticle of the desert isopod Hemilepistus reaumuri (Milne-Edwards, 1840)

To survive in its extreme habitat, the cuticle of the burrowing desert isopod Hemilepistus reaumuri requires properties distinct from isopods living in moist or mesic habitats. In particular, the anterior tergites are exposed to high mechanical loads and temperatures when individuals guard the entrance of their burrow. We have, therefore, investigated the architecture, composition, calcite texture and local mechanical properties of the tergite cuticle, with particular emphasis on large anterior cuticle tubercles and differences between the anterior and posterior tergite. Unexpectedly, structure and thickness of the epicuticle resemble those in mesic isopod species. The anterior tergite has a thicker endocuticle and a higher local stiffness than the posterior tergite. Calcite distribution in the cuticle is unusual, because in addition to the exocuticle the endocuticle distally also contains calcite. The calcite consists of a distal layer of dense and highly co-oriented crystal-units, followed proximally by irregularly distributed and, with respect to each other, misoriented calcite crystallites. The calcite layer at the tip of the tubercle is thicker relative to the tubercle slopes, and its crystallites are more misoriented to each other. A steep decrease of local stiffness and hardness is observed within a distal region of the cuticle, likely caused by a successive increase in the ACC/calcite ratio rather than changes in the degree of mineralisation. Comparison of the results with other isopods reveals a much lower ACC/calcite ratio in H. reaumuri and a correlation between the degree of terrestriality of isopod species and the magnesium content of the cuticle.

Biomineral Reactivity: The Kinetics of the Replacement Reaction of Biological Aragonite to Apatite

We present results of bioaragonite to apatite conversion in bivalve, coral and cuttlebone skeletons, biological hard materials distinguished by specific microstructures, skeletal densities, original porosities and biopolymer contents. The most profound conversion occurs in the cuttlebone of the cephalopod Sepia officinalis, the least effect is observed for the nacreous shell portion of the bivalve Hyriopsis cumingii. The shell of the bivalve Arctica islandica consists of cross-lamellar aragonite, is dense at its innermost and porous at the seaward pointing shell layers. Increased porosity facilitates infiltration of the reaction fluid and renders large surface areas for the dissolution of aragonite and conversion to apatite. Skeletal microstructures of the coral Porites sp. and prismatic H. cumingii allow considerable conversion to apatite. Even though the surface area in Porites sp. is significantly larger in comparison to that of prismatic H. cumingii, the coral skeleton consists of clusters of dense, acicular aragonite. Conversion in the latter is sluggish at first as most apatite precipitates only onto its surface area. However, the process is accelerated when, in addition, fluids enter the hard tissue at centers of calcification. The prismatic shell portion of H. cumingii is readily transformed to apatite as we find here an increased porosity between prisms as well as within the membranes encasing the prisms. In conclusion, we observe distinct differences in bioaragonite to apatite conversion rates and kinetics depending on the feasibility of the reaction fluid to access aragonite crystallites. The latter is dependent on the content of biopolymers within the hard tissue, their feasibility to be decomposed, the extent of newly formed mineral surface area and the specific biogenic ultra- and microstructures.

Mineral in skeletal elements of the terrestrial crustacean Porcellio scaber: SRμCT of function related distribution and changes during the moult cycle

Terrestrial isopods moult first the posterior and then the anterior half of the body, allowing for storage and recycling of CaCO₃. We used synchrotron-radiation microtomography to estimate mineral content within skeletal segments in sequential moulting stages of Porcellio scaber. The results suggest that all examined cuticular segments contribute to storage and recycling, however, to varying extents. The mineral within the hepatopancreas after moult suggests an uptake of mineral from the ingested exuviae. The total maximum loss of mineral was 46% for the anterior and 43% for the posterior cuticle. The time course of resorption of mineral and mineralisation of the new cuticle suggests storage and recycling of mineral in the posterior and anterior cuticle. The mineral in the anterior pereiopods decreases by 25% only. P. scaber has long legs and can run fast; therefore, a less mineralised and thus lightweight cuticle in pereiopods likely serves to lower energy consumption during escape behaviour. Differential demineralisation occurs in the head cuticle, in which the cornea of the complex eyes remains completely mineralised. The partes incisivae of the mandibles are mineralised before the old cuticle is demineralised and shed. Probably, this enables the animal to ingest the old exuviae after each half moult.

High-resolution structural and elemental analyses of calcium storage structures synthesized by the noble crayfish Astacus astacus

During premolt, crayfish develop deposits of calcium ions, called gastroliths, in their stomach wall. The stored calcium is used for the calcification of parts of the skeleton regularly renewed for allowing growth. Structural and molecular analyses of gastroliths have been primarily performed on three crayfish species, Orconectes virilis, Procambarus clarkii, and more recently, Cherax quadricarinatus. We have performed high-resolution analyses of gastroliths from the native noble crayfish, Astacus astacus, focusing on the microstructure, the mineralogical and elemental composition and distribution in a comparative perspective. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) observations showed a classical layered microstructure composed of 200-nm diameter granules aligned along fibers. These granules are themselves composed of agglomerated nanogranules of 50nm-mean diameters. Denser regions of bigger fused granules are also present. Micro-Raman spectroscopy show that if A. astacus gastroliths, similarly to the other analyzed gastroliths, are mainly composed of amorphous calcium carbonate (ACC), they are also rich in amorphous calcium phosphate (ACP). The presence of a carotenoid pigment is also observed in A. astacus gastrolith contrary to C. quadricarinatus. Energy-dispersive X-ray spectroscopy (EDX) analyses demonstrate the presence of minor elements such as Mg, Sr, Si and P. The distribution of this last element is particularly heterogeneous. X-ray absorption near edge structure spectroscopy (XANES) reveals an alternation of layers more or less rich in phosphorus evidenced in the mineral phase as well as in the organic matrix in different molecular forms. Putative functions of the different P-comprising molecules are discussed.

Functional adaptation of crustacean exoskeletal elements through structural and compositional diversity: a combined experimental and theoretical study

The crustacean cuticle is a composite material that covers the whole animal and forms the continuous exoskeleton. Nano-fibers composed of chitin and protein molecules form most of the organic matrix of the cuticle that, at the macroscale, is organized in up to eight hierarchical levels. At least two of them, the exo- and endocuticle, contain a mineral phase of mainly Mg-calcite, amorphous calcium carbonate and phosphate. The high number of hierarchical levels and the compositional diversity provide a high degree of freedom for varying the physical, in particular mechanical, properties of the material. This makes the cuticle a versatile material ideally suited to form a variety of skeletal elements that are adapted to different functions and the eco-physiological strains of individual species. This review presents our recent analytical, experimental and theoretical studies on the cuticle, summarising at which hierarchical levels structure and composition are modified to achieve the required physical properties. We describe our multi-scale hierarchical modeling approach based on the results from these studies, aiming at systematically predicting the structure-composition-property relations of cuticle composites from the molecular level to the macro-scale. This modeling approach provides a tool to facilitate the development of optimized biomimetic materials within a knowledge-based design approach.

A crayfish molar tooth protein with putative mineralized exoskeletal chitinous matrix properties

Some crustaceans possess exoskeletons that are reinforced with calcium carbonate. In the crayfish Cherax quadricarinatus, the molar tooth, which is part of the mandibular exoskeleton, contains an unusual crystalline enamel-like apatite layer. As this layer resembles vertebrate enamel in composition and function, it offers an interesting example of convergent evolution. Unlike other parts of the crayfish exoskeleton, which is periodically shed and regenerated during the molt cycle, molar mineral deposition takes place during the pre-molt stage. The molar mineral composition transforms continuously from fluorapatite through amorphous calcium phosphate to amorphous calcium carbonate and is mounted on chitin. The process of crayfish molar formation is entirely extracellular and presumably controlled by proteins, lipids, polysaccharides, low-molecular weight molecules and calcium salts. We have identified a novel molar protein termed Cq-M15 from C. quadricarinatus and cloned its transcript from the molar-forming epithelium. Its transcript and differential expression were confirmed by a next-generation sequencing library. The predicted acidic pI of Cq-M15 suggests its possible involvement in mineral arrangement. Cq-M15 is expressed in several exoskeletal tissues at pre-molt and its silencing is lethal. Like other arthropod cuticular proteins, Cq-M15 possesses a chitin-binding Rebers-Riddiford domain, with a recombinant version of the protein found to bind chitin. Cq-M15 was also found to interact with calcium ions in a concentration-dependent manner. This latter property might make Cq-M15 useful for bone and dental regenerative efforts. We suggest that, in the molar tooth, this protein might be involved in calcium phosphate and/or carbonate precipitation.