Dislodgement force and shell morphology vary according to wave exposure in a tropical gastropod (Cittarium pica)

Wave exposure has strong influences on population density, morphology and behaviour of intertidal species in temperate zones, but little is known about how intertidal organisms in tropical regions respond to gradients in wave exposure. We tested whether dislodgement force and shell shape of a tropical gastropod, Cittarium pica, differs among shores that vary in wave exposure. After adjusting for body size, we found that C. pica from exposed shores required greater dislodgement force to remove them from the shore, had slightly larger opercula (the closure to the shell aperture), and were slightly squatter in shape (reduced in shell height relative to shell width) than C. pica from sheltered shores. These morphological adjustments are consistent with those observed in temperate gastropods, which are argued to represent adaptive responses to the risk of mortality associated with dislodgement.     

Finite element analysis of active Eustachian tube function.

The inability to open the collapsible Eustachian tube (ET) has been related to the development of chronic otitis media. Although ET dysfunction may be due to anatomic and/or mechanical abnormalities, the precise mechanisms by which these structural properties alter ET opening phenomena have not been investigated. Previous investigations could only speculate on how these structural properties influence the tissue deformation processes responsible for ET opening. We have, therefore, developed a computational technique that can quantify these structure-function relationships. Cross-sectional histological images were obtained from eight normal adult human subjects, who had no history of middle ear disease. A midcartilaginous image from each subject was used to create two-dimensional finite element models of the soft tissue structures of the ET. ET opening phenomena were simulated by applying muscle forces on soft tissue surfaces in the appropriate direction and were quantified by calculating the resistance to flow (R(v)) in the opened lumen. A sensitivity analysis was conducted to determine the relative importance of muscle forces and soft-tissue elastic properties. Muscle contraction resulted in a medial-superior rotation of the medial lamina, stretching deformation in the Ostmann's fatty tissue, and lumen dilation. Variability in baseline R(v) values correlated with tissue size, whereas the functional relationship between R(v) and a given mechanical parameter was consistent in all subjects. ET opening was found to be highly sensitive to the applied muscle forces and relatively insensitive to cartilage elastic properties. These computational models have, therefore, identified how different tissue elements alter ET opening phenomena, which elements should be targeted for treatment, and the optimal mechanical properties of these tissue constructs.     

Ultrastructural morphology of the shell and shell membrane of eggs of common snapping turtles (Chelydra serpentina)

Common snapping turtles (Chelydra serpentina) lay nearly spherical, flexible-shelled eggs having an outer mineral layer composed of calcium carbonate in the aragonite form. The mineral layer is arranged into loosely organized groups of nodular shell units, with numerous spaces (or pores) between adjacent shell units. Shell units are structurally complex, consisting of an inner tip that is morphologically distinct from the main body of the shell unit. Contained within an intact shell unit at the interface of the tip and the main part of the shell unit is the central plaque, an apparent modification of the shell membrane that may serve to nucleate calcification of shell units during shell formation. The tips of shell units are firmly attached to a single, multilayered shell membrane throughout much of incubation. The calcareous layer begins to detach from the shell membrane about half-way through incubation, and changes in shell morphology attending this detachment indicate that snapping turtles may use the shell as a source of calcium during embryogenesis. The arrangement of the mineral layer into groups of shell units, the large number of spaces between shell units, and little or no interlocking of crystallites of adjacent shell units apparently are factors contributing to the ability of these eggs to swell as they absorb water.     

Evolution and development of gastropod larval shell morphology: experimental evidence for mechanical defense and repair

SUMMARY The structural diversity of gastropod veliger larvae offers an instructive counterpoint to the view of larval forms as conservative archetypes. Larval structure, function, and development are fine-tuned for survival in the plankton. Accordingly, the study of larval adaptation provides an important perspective for evolutionary-developmental biology as an integrated science. Patterns of breakage and repair in the field, as well as patterns of breakage in arranged encounters with zooplankton under laboratory conditions, are two powerful sources of data on the adaptive significance of morphological and microsculptural features of the gastropod larval shell. Shells of the planktonic veliger larvae of the caenogastropod Nassarius paupertus [ Gould ] preserve multiple repaired breaks, attributed to unsuccessful zooplankton predators. In culture, larvae isolated from concentrated zooplankton samples rapidly repaired broken apertural margins and restored the "ideal" apertural form, in which an elaborate projection or "beak" covers the head of the swimming veliger. When individuals with repaired apertures were reintroduced to a concentrated mixture of potential zooplankton predators, the repaired margins were rapidly chipped and broken back. The projecting beak of the larval shell is the first line of mechanical defense, covering the larval head and mouth and potentially the most vulnerable part of the shell to breakage. Patterns of mechanical failure show that spiral ridges do reinforce the beak and retard breakage. The capacity for rapid shell repair and regeneration, and the evolution of features that resist or retard mechanical damage, may play a more prominent role than previously thought in enhancing the ability of larvae to survive in the plankton.     

Ecomorphology of a generalist freshwater gastropod: complex relations of shell morphology,habitat, and fecundity

Evolutionary and ecological situations in a species’ native and invasive ranges can be drastically different. This is the case for Potamopyrgus antipodarum Gray (1843) a morphologically highly variable freshwater snail native to New Zealand, where sexual and asexual individuals coexist and experience selective pressure by sterilizing endoparasites. By contrast, only a few asexual lineages have been established in invaded regions around the globe, where parasite infection is extremely rare. We analyzed the ecomorphology of 996 native P. antipodarum in a geometric morphometric framework, using brood size as proxy for fecundity, and mtDNA and nuclear SNPs to account for relatedness and identify reproductive mode. As expected, we found genetic and morphological diversity to be higher in native than in invasive snails investigated previously, but surprisingly no higher morphological diversity in sexual versus asexual individuals. The relationships between shell morphology, habitat, and fecundity were complex. Shape variation was primarily linked to genetic relatedness but specific environmental factors including flow rate induced similar shell shapes. By contrast, shell size was largely explained by environmental factors. Fecundity was correlated with size but showed trade-offs with shape in increasingly extreme conditions. With increasing flow and toward small springs, the trend of shell shape becoming wider was reversed, i.e., snails with narrower shells were brooding more embryos. We concluded that both genetic and environmental contributions to variation in shell morphology in P. antipodarum likely play an important role in the ability of this species to adapt to a wide spectrum of habitats.     

Pollen morphology of Stachys (Lamiaceae) in Iran and its systematic implication

Pollen grains of 30 taxa of the genus Stachys (29 spp. and one subsp.), representing 9 of the currently recognized sections and 1 species of the closely related genus Sideritis (Si. montana) distributed in Iran were examined by light and scanning electron microscopy. Twenty-eight taxa are studied for the first time under aspects of pollen morphology. The basic shape of the pollen grains in most taxa studied is prolate-spheroidal, but subprolate, spheroidal and oblate-spheroidal pollen grains can also be found in few species. The grains are usually tricolpate (the amb triangular), but also tetracolpate (the amb circular to more or less square) in some species (S. iberica, S. atherocalyx and Si. montana). The surface is microreticulate (the frequent type), reticulate, perforate, foveolate-psilate or foveolate. The lumina are separated by smooth or sinuate muri which make them polygonal, more or less rounded and elongate. Major pollen morphological features of the taxa studied are compared and discussed on the basis of taxonomical concepts. In some cases, these characters are useful in delimitation of formerly introduced sections while they mostly provide further characters in separating related species from each other. For example, all members of S. sect. Aucheriana are characterized by elongated lumina. Based on the oblate-spheroidal shape of its pollen as well as tetracolpate aperture type, the results of the present study confirm sect. Pontostachys as including S. angustifolia, S. iberica, S. sparsipilosa as well as S. atherocalyx. Our results also suggest that although some species like S. fruticolosa and S. lavandulifolia are morphologically well characterized, they cannot be separated from other species of Stachys based on pollen morphology.     

Finite element analysis of a three-dimensional open-celled model for trabecular bone

Based on a regular array of cubic unit cells, each containing a body-centered spherical void, we created an idealized three-dimensional model for both subchondral trabecular bone and a class of porous foams. By considering only face-to-face stacking of unit cells, the inherent symmetry was such that, except at the surface, the displacements and stresses within any one unit cell were representative of the entire porous structure. Using prescribed displacements the model was loaded in both uniaxial compressive strain and uniaxial shear strain. Based on the response to these loads, we found the tensor of elastic constants for an equivalent homogeneous elastic solid with cubic symmetry. We then compared the predicted modulus with our experimental values for bovine trabecular bone and literature values for an open-celled latex rubber foam.     

Finite element analysis of biological soft tissue surrounded by a deformable membrane that controls transmembrane flow

Background

Many biological soft tissues are hydrated porous hyperelastic materials, which consist of a complex solid skeleton with fine voids and fluid filling these voids. Mechanical interactions between the solid and the fluid in hydrated porous tissues have been analyzed by finite element methods (FEMs) in which the mixture theory was introduced in various ways. Although most of the tissues are surrounded by deformable membranes that control transmembrane flows, the boundaries of the tissues have been treated as rigid and/or freely permeable in these studies. The purpose of this study was to develop a method for the analysis of hydrated porous hyperelastic tissues surrounded by deformable membranes that control transmembrane flows.

Results

For this, we developed a new nonlinear finite element formulation of the mixture theory, where the nodal unknowns were the pore water pressure and solid displacement. This method allows the control of the fluid flow rate across the membrane using Neumann boundary condition. Using the method, we conducted a compression test of the hydrated porous hyperelastic tissue, which was surrounded by a flaccid impermeable membrane, and a part of the top surface of this tissue was pushed by a platen. The simulation results showed a stress relaxation phenomenon, resulting from the interaction between the elastic deformation of the tissue, pore water pressure gradient, and the movement of fluid. The results also showed that the fluid trapped by the impermeable membrane led to the swelling of the tissue around the platen.

Conclusions

These facts suggest that our new method can be effectively used for the analysis of a large deformation of hydrated porous hyperelastic material surrounded by a deformable membrane that controls transmembrane flow, and further investigations may allow more realistic analyses of the biological soft tissues, such as brain edema, brain trauma, the flow of blood and lymph in capillaries and pitting edema.

     

Involvement of Hox genes in shell morphogenesis in the encapsulated development of a top shell gastropod (Gibbula varia L.)

Regulatory gene expression during the patterning of molluscan shells has only recently drawn the attention of scientists. We show that several Hox genes are expressed in association with the shell gland and the mantle in the marine vetigastropod Gibbula varia (L.). The expression of Gva-Hox1, Gva-Post2, and Gva-Post1 is initially detected in the trochophore larval stage in the area of the shell field during formation of embryonic shell. Later, during development, these genes are expressed in the mantle demonstrating their continuous role in larval shell formation and differentiation of mantle edge that secretes the adult shell. Gva-Hox4 is expressed only late during the development of the veliger-like larva and may also be involved in the adult shell morphogenesis. Additionally, this gene also seems to be associated with secretion of another extracellular structure, the operculum. Our data provide further support for association of Hox genes with shell formation which suggest that the molecular mechanisms underlying shell synthesis may consist of numerous conserved pattern-formation genes. In cephalopods, the only other molluscan class in which Hox gene expression has been studied, no involvement of Hox genes in shell formation has been reported. Thus, our results suggest that Hox genes are coopted to various functions in molluscs.     

Finite element analysis of the impact of bone nanostructure on its piezoelectric response

Biomechanics and Modeling in Mechanobiology - The piezoelectric response of bone at the submicron scale is analyzed under mechanical loadings using the finite element (FE) method. A new algorithm...     

Correspondence analysis of shell morphology in the African freshwater snail Biomphalaria pfeifferi (Kraus 1848) (Pulmonata: Gastropoda)

A correspondence analysis of shell measurements taken from 521 widely dispersed specimens of the African aquatic pulmonate snail Biomphalaria pfeifferi suggests the existence of eight morphological groups. These groups appear to relate to either ecophysiological factors or to factors associated with the stability of the freshwater system rather than to aspects of geographic distribution and genetic isolation.     

Structure of shell muscles in the Cambrian gastropod genus Bemella (Gastropoda: Archaeobranchia: Helcionellidae)

Zones with peculiar microornamentation interpreted as muscle scars were found on the internal molds of the Early Cambrian gastropod genus Bemella Missarzhevsky, 1969 (family Helcionellidae). Shell muscles of helcionellids are reconstructed based on the topographic pattern of muscle scars, i.e., the pedal, cephalic, and mantle retractors are recognized. The reconstruction proposed here of the shell musculature corroborates affinity between ancient gastropod and helcionelloid mollusks.     

Finite element thermal analysis of bone cement for joint replacements

A finite element technique was developed to investigate the thermal behavior of bone cement in joint replacement procedures. Thermal tests were designed and performed to provide the parameters in a kinetic model of bone cement exothermic polymerization. The kinetic model was then coupled with an energy balance equation using a finite element formulation to predict the temperature history and polymerization development in the bone-cement-prosthesis system. Based on the temperature history, the possibility of the thermal bone necrosis was then evaluated. As a demonstration, the effect of cement mantle thickness on the thermal behavior of the system was investigated. The temperature profiles in the bone-cement-prosthesis system have shown that the thicker the cement, the higher the peak temperature in the bone. In the 7 mm thick cement case, a peak temperature of over 55 degrees C was predicted. These high temperatures occurred in a small region near the bone/cement interface. No damage was predicted in the 3 mm and 5 mm cement mantle thickness cases. Although thermal damage was predicted in the bone for the 7 mm mantle thickness case, the amount of thermal necrosis predicted was minimal. If more cement is used in the surgical procedure, more heat will be generated and the potential for thermal bone damage may rise. The systems should be carefully selected to reduce thermal tissue damage when more cement is used. The methodology developed in this paper provides a numerical tool for the quantitative simulation of the thermal behavior of bone-cement-prosthesis designs.     

Variation in shell morphology of Calliostoma zizyphinum (Gastropoda: Trochidae)

Differences in shell morphology in the intertidal prosobranchmollusc Calliostoma zizyphinum were studied from a number ofsites within four geographical regions of the British Isleswith varying exposures to wave action and crab predation. Meanvalues of damage scarring were highest in shells sampled fromStrangford Lough, Northern Ireland, and lowest in individualsfrom the Atlantic Coast. Shells collected from the Isle of Manwere smaller than those from either the Atlantic coast of Irelandor Strangford Lough. Shells from Strangford Lough had tallershells (higher aspect ratio) than shells from the County Downand Atlantic Coast and shells from the County Down coast weremore squat (lower aspect ratio) than those from all other areas.Shells from Strangford Lough were significantly thinner thanshells from all other geographical areas. The relationshipsbetween shell damage scarring and shell size and tallness werenot consistent among areas. Shells from Strangford Lough arenotable in showing a steep, positive relationship between shellsize and scarring and a steep, negative relationship betweenscarring and tallness. The unusual shell phenotypes observedin shells from Strangford Lough may be explained by rapid shellgrowth, which would not only allow Calliostoma to attain a sizerefuge from crab predators but also to recover successfullyfrom repeated crab attacks on the shell lip. Such a strategywould result in larger, thinner shells with a high number ofdamage scars. (Received 6 June 2006; accepted 20 December 2006)     

The morphology and anatomy of the Antarctic gastropod Bathyberthella antarctica (Opisthobranchia, Notaspidea, Pleurobranchidae)

The external morphology, anatomy and histology of the Antarctic notaspidean Bathyberthella antarctica Willan & Bertsch, 1987 is described and the intraspecific variability presented. The species is compared with the only other known pleurobranch from the high Antarctic zone, Tomthompsonia antarctica (Thiele, 1912).     

Finite element analysis of a new customized composite post system for endodontically treated teeth

This paper investigated the mechanical behavior of a new customized post system built up with a composite framework presently utilized for crowns, bridges, veneers and inlay/onlay dental restorations. The material has been shaped so to follow perfectly the profile of the root canal in order to take advantage of the better mechanical properties of composites with respect to metallic alloys commonly used for cast posts.

The analysis has been carried out with 3D finite element models previously validated on the basis of experimental work. The new post system has been compared to a variety of restorations using either prefabricated or cast posts. The structural efficiency of the new restoration has been evaluated for an upper incisor under different loading conditions (mastication, bruxism, impact).

Results prove that maximum stress values in restored teeth are rather insensitive to post types and materials. However, the new customized composite restoration allows to reduce significantly the stresses inside the dentinal regions where conservative clinical interventions are not possible.     

Finite element analysis of defibrillation fields in a human torso model for ventricular defibrillation

In order to optimize defibrillation electrode systems for ventricular defibrillation thresholds (DFTs), a Finite Element Torso model was built from fast CT scans of a patient who had large cardiac dimensions (upper bound of normal) but no heart disease. Clinically used defibrillation electrode configurations, i.e. Superior Vena Cava (SVC) to Right Ventricle (RV) (SVC-RV), left pectoral Can to RV (Can-RV) and Can + SVC-RV, were analyzed. The DFTs were calculated based on 95% ventricular mass having voltage gradient> 5 V/cm and these results were also compared with clinical data. The low voltage gradient regions with voltage gradient < 5 V/cm were identified and the effect of electrode dimension and location on DFTs were also investigated for each system. A good correlation between the model results and the clinical data supports the use of Finite Element Analysis of a human torso model for optimization of defibrillation electrode systems. This correlation also indicates that the critical mass hypothesis is the primary mechanism of defibrillation. Both the FEA results and the clinical data show that Can + SVC-RV system offers the lowest voltage DFTs when compared with SVC-RV and Can-RV systems. Analysis of the effect of RV, SVC and Can electrode dimensions and locations can have an important impact on defibrillation lead designs.     

Pollen morphology of Campanula (Campanulaceae) and allied genera in Iran with special focus on its systematic implication

Pollen grains of 47 taxa of Campanulaceae including 35 taxa of Campanula that represent its five subgenera and nine sections are investigated. Moreover, five species and three subspecies representing three sections in Asyneuma and one species of each genera Legousia, Michauxia, Zeugandra and Theodorovia were studied by light and scanning electron microscopy. The basic shape of the pollen grains was spheroidal. The apertures vary from tri- to hexa-porate. The sculpturing pattern of exine was rugulate-echinate, rugulate-microechinate or in few species rugulate-microreticulate and microechinate. The most valuable characters for subgeneric classification were the length and density of echini. The length of echini were significantly long (> 2 μm) in C. sclerotricha, Legousia falcata and Michauxia laevigata. Pollen grains show low variation in different species of subgen. Rapunculus, but were variable among different species of some groups, such as sect. Rupestres, probably indicating their non-monophyly despite homogeneity with respect to other morphological characters. Pollen morphology does not support recognition of Asyneuma, Legousia, Michauxia, Symphyandra, Theodorovia, and Zeugandra as separated from Campanula, since none of them exhibit any unique feature.