Structure,composition and properties of naturally occurring non-calcified crustacean cuticle

Crustaceans are known for their hard, calcified exoskeleton; however some regions appear different in colour and opacity. These include leg and cheliped tips in the grapsid crab, Metopograpsus frontalis. The chelipeds and leg tips contain only trace levels of calcium but a significant mass of the halogens, chlorine (Cl) and bromine (Br). In contrast, the carapace is heavily calcified and contains only a trace mass of Cl and no Br. In transverse section across the non-calcified tip regions of cheliped and leg the mass percent of halogens varies with position. As such, the exoskeleton of M. frontalis provides a useful model to examine a possible correlation of halogen concentration with the physical properties of hardness (H) and reduced elastic modulus (E_r), within a chitin-based matrix. Previously published work suggests a correlation exists between Cl concentration and hardness in similar tissues that contain a metal (e.g. zinc). However, in M. frontalis H and E_r did not vary significantly across cheliped or leg tip despite differences in halogen concentration. The non-calcified regions of M. frontalis are less hard and less stiff than the carapace but equivalent to values found for insect cuticle lacking metals. Cheliped tips showed a complex morphological layering that differed from leg tips.     

Structure and mechanical properties of hydroxypropylated starch films

Films of acid-hydrolyzed hydroxypropylated pea starch with average molecular weight M w ranging from 3.3 x 10 (4) g/mol to 1.6 x 10 (6) g/mol were prepared from 25% (w/w) solution by casting. The structure of the films was investigated by means X-ray diffraction and calorimetry, evidencing a B-type crystalline structure. In similar drying conditions, 25 degrees C and 40% of relative humidity, the crystallinity varied from 24% for the low molecular weight (A5) to almost none for the highest molecular weight (A160). The influence of the drying temperature was also investigated. A reduction of the crystallinity from 16% to almost none was found when increasing temperature from 25 to 65 degrees C. The glass transition temperature ( T g) at different water contents was determined. The difference of T g between the first and the second scan was interpreted by changes in the water distribution between phases into the B-type crystalline structure. Mechanical properties of the films determined by tensile tests and by DMTA in the glassy state showed no effect of the average molecular weight or of crystallinity. In contrast, thermomechanical experiments by DMTA showed that the average molecular weight of the sample influenced the mechanical relaxation and the moduli in the rubbery state.     

Diet composition and feeding habits of the crocodile shark,Pseudocarcharias kamoharai

Environmental Biology of Fishes - The objective of the present study was to determine the diet composition and feeding habits of the crocodile shark, Pseudocarcharias kamoharai, in Ecuadorian...     

The mechanics of cutting and the form of shark teeth (Chondrichthyes,Elasmobranchii)

Summary A rich engineering literature exists that is applicable to many aspects of vertebrate jaw mechanics and has been referred to in many studies in this sector. But mechanical engineering technology has provided few theoretical bases that are directly helpful in the study of predator teeth. Hence, analyses of puncturing and slicing functions of these teeth have lacked a firm physical technology as a background. Predator teeth have evolved to pierce and cut animal tissues that are usually compliant in that they readily undergo relatively large deformations under applied stress before they actually yield. The bulk of engineering theory is directed toward such noncompliant materials as wood and metal, the design of tools that cut them, and the mechanics involved in this. The purpose of the present paper is to scan the mechanical implications of different tooth designs, pose hypotheses that relate to primary considerations of the physics of cutting compliant substrates, and offer a preliminary approach that is intended as a useful guide to further studies on sharks and on other vertebrate groups. Thus, in this paper I have attempted to formulate some tentative and preliminary generalizations concerning the mechanics of cutting compliant materials. Then comes a survey of the teeth of a particular group of predators, three families of sharks, in terms of these preliminary formulations. The approach views the shark teeth in isolation from the complex cranial mechanism (presently under study) that functionally integrates with the teeth. Therefore, adaptive conclusions are minimal, because the evolutionary significance of tooth form cannot properly be assessed outside of an integrated study. However, certain correlations do exist between structural tooth characteristics and mechanics. Slender, smooth-edged (or nearly so) teeth can readily pierce prey, but are of less use in slicing it. Such teeth are typical of the lower jaw dentition in many sharks and, in a few species, they are present in both upper and lower jaws. Usually these slender teeth display a reversed curvature at their tips, so that although most of the tooth's crown is curved inward toward the mouth cavity, the tip is turned outward. This outward turning of the tip can enhance the probability of initial prey penetration, without much compromising the prey-retaining properties of the inward curvature of the greater, more proximal portion of the tooth. Many sharks possess upper teeth with serrations along the edges. The serrations vary from one species to another in coarseness and in distribution along tooth edges. Serrated teeth can make greater use of the available biting forces, and they have a greater cutting effect than do smooth-edged teeth. These latter depend upon friction which, because the coefficient friction is always less than 1.0 (often very much less), can make use of only a fraction of the total bite force. However, smooth tooth blades can pierce prey with less resistance and are less prone to binding (becoming immobilized) in the prey tissue. In many shark species serrations are concentrated along the proximal portions of the tooth crown, where the bases of adjacent teeth are in near contact along the jaw margin. In these regions food can be pressed during feeding, resulting in a binding of the teeth in the prey. Release of the binding must be accomplished by cutting the jammed food, to permit clearance of the prey material so it can slip past the tooth rows. The more prominent serrations in such regions may act to puncture and slice the jammed tissue. It is noted that commercial saws are typically designed in various ways to promote clearance between adjacent saw teeth. The pitch or rake of the teeth of sharks is discussed, as is the overall form of the tooth rows along the jaw margins. The relationship between the distribution of teeth along the jaw margins and surface irregularities of the prey surfaces is also considered.     

Ameloblastic secretion and calcification of the enamel layer in shark teeth

Tooth primordia at early stages of mineralization in the sharks Negaprion brevirostris and Triaenodon obesus were examined electron microscopically for evidence of ameloblastic secretion and its relation to calcification of the enamel (enameloid) layer. Ameloblasts are polarized with most of the mitochondria and all of the Golgi dictyosomes localized in the infranuclear end of the cell toward the squamous outer cells of the enamel organ. Endoplasmic reticular membranes and ribosomes are also abundant in this region. Ameloblastic vesicles bud from the Golgi membranes and evidently move through perinuclear and supranuclear zones to accumulate at the apical end of the cell. The vesicles secrete their contents through the apical cell membrane in merocrine fashion and appear to contribute precursor material both for the basal lamina and the enameline matrix. The enamel layer consists of four zones: a juxta-laminar zone containing newly polymerized mineralizing fibrils (tubules); a pre-enamel zone of assembly of matrix constituents; palisadal zones of mineralizing fibrils (tubules); and interpalisadal zones containing granular amorphous matrix, fine unit fibrils, and giant cross-banded fibers with a periodicity of 17.9 nm. It seems probable that amorphous, non-mineralizing fibrillar and mineralizing fibrillar constituents of the matrix are all products of ameloblastic secretion. Odontoblastic processes are tightly embedded in the matrix of the palisadal zones and do not appear to be secretory at the stages investigated. The shark tooth enamel layer is considered homologous with that of other vertebrates with respect to origin of its mineralizing fibrils from the innerental epithelium. The term enameloid is appropriate to connote the histological distinction that the enamel layer contains odontoblastic processes but should not signify that shark tooth enamel is a modified type of dentine. How amelogenins and/or enamelins secreted by amelo- blasts in the shark and other vertebrates are related to nucleation and growth of enamel crystallites is still not known.     

Structure and mechanical properties of the soft zone separating bulk dentin and enamel in crowns of human teeth: insight into tooth function

The 200-300 microm soft zone of dentin, found beneath enamel in crowns of human teeth, is thought to fulfill important roles in tooth function, but little is known about its structure-mechanical relations. Scanning electron microscopy images of fracture surfaces showed that near the dentino-enamel junction (DEJ), a porous reticulate matrix of intertubular-dentin contains tubules with no peritubular lining. Peritubular-dentin however is found at some distance from the DEJ, and it gradually thickens with increasing depth into the bulk dentin. Concurrently, tighter packing of the collagen fibers is observed with a gradual increase in mineral deposits on and between the fibers. This structurally graded zone is known to be softer when tested for micro-hardness. It undergoes greater strain compared to bulk dentin, when measured using Moiré interferometry. We investigated the deformation and stiffness of this zone by means of non-contact laser-speckle interferometry (ESPI), and nanometer-scale deformations were tracked during compression-testing performed in water. We report a significantly reduced stiffness of this zone compared to bulk dentin, with mid-buccal regions of teeth averaging 3.5 GPa compared with 9.7 GPa in mid-lingual regions. Our results support and expand upon the hypothesis that the durability of the whole tooth relies upon a bucco-lingual asymmetric matching of stiffness by means of an interphase: a cushioning soft layer between enamel and bulk dentin.     

Structure, composition, and peptide binding properties of detergent soluble bilayers and detergent resistant rafts

Lipid bilayers composed of unsaturated phosphatidylcholine (PC), sphingomyelin (SM), and cholesterol are thought to contain microdomains that have similar detergent insolubility characteristics as rafts isolated from cell plasma membranes. We chemically characterized the fractions corresponding to detergent soluble membranes (DSMs) and detergent resistant membranes (DRMs) from 1:1:1 PC:SM:cholesterol, compared the binding properties of selected peptides to bilayers with the compositions of DSMs and DRMs, used differential scanning calorimetry to identify phase transitions, and determined the structure of DRMs with x-ray diffraction. Compared with the equimolar starting material, DRMs were enriched in both SM and cholesterol. Both transmembrane and interfacial peptides bound to a greater extent to DSM bilayers than to DRM bilayers, likely because of differences in the mechanical properties of the two bilayers. Thermograms from 1:1:1 PC:SM:cholesterol from 3 to 70 degrees C showed no evidence for a liquid-ordered to liquid-disordered phase transition. Over a wide range of osmotic stresses, each x-ray pattern from equimolar PC:SM:cholesterol or DRMs contained a broad wide-angle band at 4.5 A, indicating that the bilayers were in a liquid-crystalline phase, and several sharp low-angle reflections that indexed as orders of a single lamellar repeat period. Electron density profiles showed that the total bilayer thickness was 57 A for DRMs, which was approximately 5 A greater than that of 1:1:1 PC:SM:cholesterol and 10 A greater than the thickness of bilayers with the composition of DSMs. These x-ray data provide accurate values for the widths of raft and nonraft bilayers that should be important in understanding mechanisms of protein sorting by rafts.     

Influence of solvent composition on the mechanical properties of arterial elastin

We have investigated the effects of changes in solution composition on the mechanical properties of rings of arterial elastin. The time course of force equilibration at constant strain following a change in the composition of the bathing solution was measured. Both the force developed during slow extension and force relaxation following rapid straining were also measured in each of the test solutions. The results are difficult to summarize because all of the primitive quantities measured--sample dimension, slope of the force-extension curve, force overshoot and time of relaxation--as well as the derived quantities such as elastic modulus changed in different and apparently uncorrelated ways. Changes in pH and ionic composition of the bathing solution had small effects consistent with the low fixed charge density of elastin. Solutions of glucose, sucrose, and ethylene glycol had larger effects consistent with changes in hydrophobic interactions. The viscosity of the solution that penetrated the intrafibrillar space of the elastin appeared to be a major determinant of the dynamic response.     

Correlation between mechanical properties and wall composition of the canine superior vena cava

The mechanical properties (modulus of elasticity and stress-relaxation) of different venous segments of the canine superior vena cava were determined as well as the composition of the vessel wall by means of physical, biochemical and histological methods. It was found that the wall of the vena cava was structurally and mechanically a function of the metric position with respect to the right heart: the modulus of elasticity increased, the stress-relaxation decreased, the concentration of hydroxyproline, collagen and elastin increased and the amount of muscle fibres decreased with increasing distal distance from the right heart. A significant linear correlation coefficient was observed between the modulus of elasticity and the structural wall components. The data presented show the axial heterogeneity and the dependency of the mechanical properties upon the venous vessel wall composition.     

Structure and permeability of the egg capsule of the bonnethead shark, Sphyrna tiburo

In the viviparous bonnethead shark, Sphyrna tiburo, a fluid-filled, acellular egg capsule surrounds fertilized eggs and developing embryos throughout gestation. Like other placental shark species, the capsule remains intact even at the placental implantation site. Although its intervention between the uterine and embryonic tissues of the placenta has long been thought to mediate physiological exchange, little information is available concerning even its basic structure or permeability to solutes. The 1 mum thick capsule wall consists of an inner layer of gelatinous material and an outer layer consisting of at least three laminae of orthogonally arranged fibrous material. These fibers are irregular and often branched. Permeability experiments showed that solutes less than 1,355 Da diffuse across the egg capsule whereas those greater than 6,000 Da do not pass through the membrane. Solute movement across the capsule is a concentration-dependent phenomenon indicating diffusion rather than active transport. Experimental data also suggest that there is an increase in the permeability of the egg capsule to low molecular weight materials during mid- and late gestation. These observations are discussed in relation to the function of the egg capsule as a mediator of maternal-embryonic interactions in matrotrophic sharks.     

Structure, composition, and assembly of basement membrane

Basement membranes are thin layers of matrix separating parenchymal cells from connective tissue. Their ultrastructure consists of a three-dimensional network of irregular, fuzzy strands referred to as "cords"; the cord thickness averages 3-4 nm. Immunostaining reveals that the cords are composed of at least five substances: collagen IV, laminin, heparan sulfate proteoglycan, entactin, and fibronectin. Collagen IV has been identified as a filament of variable thickness persisting after the other components have been removed by plasmin digestion or salt extraction. Heparan sulfate proteoglycan appears as sets of two parallel lines, referred to as "double tracks," which run at the surface of the cords. Laminin is detected in the cords as diffuse material within which thin wavy lines may be distinguished. The entactin and fibronectin present within the cords have not been identified as visible structures. The ability of laminin, heparan sulfate proteoglycan, fibronectin, and entactin to bind to collagen IV has been demonstrated by visualization with rotary shadowing and/or biochemical studies. Incubation of three of these substances-collagen IV, laminin (with small entactin contamination), and proteoglycan-at 35 degrees C for 1 hr resulted in a precipitate that was sectioned for electron microscopic examination and processed for gold immunolabeling for each of the three incubated substances. Three structures are present in the precipitate: 1) a lacework, exclusively composed of heparan sulfate proteoglycan in the form of two parallel lines, similar to double tracks; 2) semi-solid, irregular accumulations, composed of the three initial substances distributed on a cord network; and 3) convoluted sheets, which are also composed of the three initial substances distributed on a cord network but which, in addition, have the uniform appearance and thickness of the lamina densa of basement membrane. Hence these sheets are closely similar to the main component of authentic basement membranes.     

Structure and mechanical properties of the ribosomal L1 stalk three-way junction

The L1 stalk is a key mobile element of the large ribosomal subunit which interacts with tRNA during translocation. Here, we investigate the structure and mechanical properties of the rRNA H76/H75/H79 three-way junction at the base of the L1 stalk from four different prokaryotic organisms. We propose a coarse-grained elastic model and parameterize it using large-scale atomistic molecular dynamics simulations. Global properties of the junction are well described by a model in which the H76 helix is represented by a straight, isotropically flexible elastic rod, while the junction core is represented by an isotropically flexible spherical hinge. Both the core and the helix contribute substantially to the overall H76 bending fluctuations. The presence of wobble pairs in H76 does not induce any increased flexibility or anisotropy to the helix. The half-closed conformation of the L1 stalk seems to be accessible by thermal fluctuations of the junction itself, without any long-range allosteric effects. Bending fluctuations of H76 with a bulge introduced in it suggest a rationale for the precise position of the bulge in eukaryotes. Our elastic model can be generalized to other RNA junctions found in biological systems or in nanotechnology.     

Structure and mechanical properties of wet-spun fibers made from natural cellulose nanofibers

Cellulose nanofibers were prepared by TEMPO-mediated oxidation of wood pulp and tunicate cellulose. The cellulose nanofiber suspension in water was spun into an acetone coagulation bath. The spinning rate was varied from 0.1 to 100 m/min to align the nanofibers to the spun fibers. The fibers spun from the wood nanofibers had a hollow structure at spinning rates of >10 m/min, whereas the fibers spun from tunicate nanofibers were porous. Wide-angle X-ray diffraction analysis revealed that the wood and tunicate nanofibers were aligned to the fiber direction of the spun fibers at higher spinning rates. The wood spun fibers at 100 m/min had a Young's modulus of 23.6 GPa, tensile strength of 321 MPa, and elongation at break of 2.2%. The Young's modulus of the wood spun fibers increased with an increase in the spinning rate because of the nanofiber orientation effect.     

Elevated carbon dioxide alters the plasma composition and behaviour of a shark

Increased carbon emissions from fossil fuels are increasing the pCO₂ of the ocean surface waters in a process called ocean acidification. Elevated water pCO₂ can induce physiological and behavioural effects in teleost fishes, although there appear to be large differences in sensitivity between species. There is currently no information available on the possible responses to future ocean acidification in elasmobranch fishes. We exposed small-spotted catsharks (Scyliorhinus canicula) to either control conditions or a year 2100 scenario of 990 μatm pCO₂ for four weeks. We did not detect treatment effects on growth, resting metabolic rate, aerobic scope, skin denticle ultrastructure or skin denticle morphology. However, we found that the elevated pCO₂ group buffered internal acidosis via accumulation with an associated increase in Na⁺, indicating that the blood chemistry remained altered despite the long acclimation period. The elevated pCO₂ group also exhibited a shift in their nocturnal swimming pattern from a pattern of many starts and stops to more continuous swimming. Although CO₂-exposed teleost fishes can display reduced behavioural asymmetry (lateralization), the CO₂-exposed sharks showed increased lateralization. These behavioural effects may suggest that elasmobranch neurophysiology is affected by CO₂, as in some teleosts, or that the sharks detect CO₂ as a constant stressor, which leads to altered behaviour. The potential direct effects of ocean acidification should henceforth be considered when assessing future anthropogenic effects on sharks.     

Effect of mcl-PHA synthesis in flax on plant mechanical properties and cell wall composition

Transgenic Research - The high demand for new biomaterials makes synthesis of polyhydroxyalkanoates (PHA) in plants an interesting and desirable achievement. Production of polymers in plants is an...     

Structure,formation, mechanical properties,and disposal of the embryo attachment system of an estuarine crab,Sesarma haematocheir

In most decapod crustaceans, fertilized eggs extruded from the gonopore attach to ovigerous hairs within the incubation chamber of the female. The attachment is effected by an "embryo attachment system." The three continuous components of this system are the egg envelope, the funiculus, and the investment coat, which wraps around an ovigerous hair. Transmission electron microscopy (TEM) revealed that the embryo of Sesarma haematocheir is enfolded by three distinct envelopes (E1, E2, and E3), whereas the embryo attachment system is composed of only the outermost, single envelope (E1) with two sublayers (E1a and E1b). This envelope (E1) originates from the outer layer of the vitelline membrane (envelope of the ovum) with two sublayers (E1a' and E1b'). The sequence and timing of events in the formation of the embryo attachment system was determined on the basis of observations of female behavior, ultrastructure, and mechanical properties of the membranes. The egg envelope (E1a' + E1b') is not adhesive immediately after extrusion from the gonopore; but 5 min after egg-laying, it becomes adhesive-a change associated with "fusion" of the two sublayers (E1)-and attaches the eggs to the ovigerous hairs from 5 to 30 min after egg-laying. The layer E1a' always binds to an ovigerous hair at specific, electron-dense attachment sites that are distributed longitudinally on the surface of each hair. Plasticity of the egg envelope changes, and the female kneads her eggs by the movement of ovigerous setae; this movement forms the investment coat on the ovigerous hair (10-40 min after egg-laying). Thirty minutes after egg-laying, the egg envelope again divides into two sublayers (E1a and E1b), and the adhesiveness rapidly decreases. The plasticity of the envelope remains, and the funiculus is formed, accompanied by kneading of the eggs (40-90 min after egg-laying). The embryos hatch one month after incubation, and the attachment systems all slip off their ovigerous hairs by the actions of the ovigerous-hair slipping substance (OHSS). This substance appears to act specifically at the attachment sites on the hair, lysing the bond with layer E1a, and thereby disposing of the embryonic attachment system and preparing the hairs for the next clutch of embryos.     

Thermodynamics, mechanical and electrical properties of biomembranes

A set of thermodynamic fundamental equations has been derived, including chemical potentials of immobile components of a biomembrane. The results obtained are compatible with those known in the theory of shells, but the approach developed is more general. The influence of an outer electric field on the isotropic tension of a biomembrane has been discussed. The thermodynamic relations derived are used for the treatment of intracellular pressure changes recently observed by Terakawa (1985, J. Physiol. 369, 229) in the squid giant axon at potential variations.