Organization and mechanical behaviour of myocyte–ligament composites in a sea-urchin lantern: the compass depressors of Stylocidaris affinis (Echinodermata, Echinoida)

 Certain components of the jaw apparatus, or lantern, of regular sea-urchins form a ’compass system’ the function of which has still to be established. This system includes ten compass depressors (CDs) which connect the compass ossicles to the inner edge of the test. Previous studies focused on the CDs of euechinoid sea-urchins. This paper provides the first detailed account of the organization and mechanical behaviour of the CDs of a cidaroid sea-urchin, Stylocidaris affinis. The outermost layer of its CDs is a coelothelium comprising apical peritoneocytes, sparse subapical myocytes and two types of granule-containing cells. The central core of the CD is dominated by longitudinally orientated collagen fibrils arranged in bundles surrounded by networks of beaded microfibrils. Myocytes are scattered throughout the core and are always surrounded by a basal lamina which usually encloses the same two types of granule-containing cells observed in the coelothelium. Each CD is attached to the outer coelomic septum by a mesentery, both the connective tissue and coelothelia of which lack myocytes and granule-containing cells. From data on the relationship between the vertical position of the lantern and the passive and active forces developed by intact CD sets, it appears that the CDs resist elevation of the lantern above its ’resting position’ and that they develop a maximal contractile force when their length corresponds to that at the resting position. The connective tissue of the CDs has the capacity to undergo nervously mediated changes in stretch resistance, although this is expressed more weakly than in the euechinoid CDs. These results are related to current views on the physiological role of the compass system. Accepted: 15 March 1998     

Purification and characterization of arginine kinase from sea-urchin eggs

In most invertebrates, creatine kinase is replaced by arginine kinase, which catalyzes reversibly the transfer of a phosphate group between adenosine triphosphate and arginine. In sea-urchin larvae, arginine kinase only is expressed whereas in adult sea-urchins both arginine kinase and creatine kinase can be found in the same tissue. In order to study their developmental regulation and properties, we have purified arginine kinase to homogeneity from the eggs of the sea-urchin Paracentrotus lividus. The purification involves ethanol and ammonium sulfate precipitations, followed by an anion-exchange chromatography, an affinity chromatography and a gel filtration. A 500-fold increase in specific activity leads to a specific activity of 360 IU/mg protein at 25 degrees C. Arginine kinase (pI = 5.7) is rapidly and irreversibly inactivated at 45 degrees C. Amino acid composition and Km values (2.08 mM for phospho-L-arginine and 1.25 mM for ADP) are also given. Determination of molecular mass by gel filtration and separation by SDS/polyacrylamide gel electrophoresis indicate that the enzyme is an 81-kDa dimer of two subunits of 42 kDa.     

Redescription of Syndesmis echinorum Fran?ois, 1886 (Turbellaria: Neorhabdocoela: Umagillidae), with comments on distinctions between Syndesmis and Syndisyrinx

Syndesmis echinorum is redescribed from specimens taken from the intestine of the sea urchin Paracentrotus lividus at the type locality, Banyuls-sur-Mer, France. This umagillid is shown to have a seminal bursa-bursal valve complex comparable to that of Syndisyrinx franciscanus. Thus, it is suggested that the genera Syndesmis and Syndisyrinx be distinguished on the basis of the form and proportions of the ejaculatory duct, male antrum, and penis stylet, rather than on the basis of the presence or absence of a seminal bursa and bursal valve. The sea urchin Sphaerechinus granularis at Banyuls harbors a worm that is, on the average, slightly more robust than S. echinorum from P. lividus; its morphology, however, agrees with that of S. echinorum. In addition to S. echinorum, P. lividus harbors at least 1 undescribed umagillid. Although 2 species found in Echinus esculentus have been referred to S. echinorum by previous workers, neither is conspecific with it.     

Non-monophyly of the avian genus Seicercus (Aves: Sylviidae) revealed by mitochondrial DNA

The phylogeny of all species and nearly all subspecies of Seicercus and representatives of all subgenera in Phylloscopus was estimated based on two mitochondrial genes. According to the gene tree, and supported by non-molecular data, Seicercus belongs in three separate clades. Two of these include only taxa currently classified as Seicercus , while the third comprises S. xanthoschistos and P. occipitalis . These results suggest that both Seicercus and Phylloscopus are paraphyletic. The gene tree suggests two more cases of non-monophyly: (1) the ' S. burkii complex' is separated into two different clades, one of which also includes S. affinis and S. poliogenys ; (2) two populations of S. affinis intermedius are more closely related to S. affinis ocularis than to a third population of intermedius . A recent proposal to split the ' S. burkii complex' into six species is corroborated, as is the recognition of the taxon cognitus as a colour morph of S. affinis intermedius . Our study also revealed unexpectedly large genetic divergences between three different populations of the monotypic S. poliogenys , indicating the presence of cryptic species. Our results underscore the importance of dense sampling at the specific and infraspecific levels in intrageneric phylogenetic studies.     

Deformation of the distal femur: a contribution towards the pathogenesis of osteochondrosis dissecans in the knee joint.

Osteochondrosis dissecans (OD) is a process of subchondral bone necrosis occurring predominantly in young individuals at specific sites. The aetiology of this disease remains controversial with mechanical processes due to trauma and/or ischaemic factors being proposed. This study aims at explaining the aetiology of OD in the knee joint as a result of the particular deformation of the condyles. A finite element analysis of the distal third of the femur was performed. A three-dimensional model was developed based on computed tomography scans of a normal femur, consisting of cortical bone, cancellous bone and articular cartilage. This model was subjected to physiological loads at 0, 30, 60 and 90 degrees of knee flexion. A complex deformation was found within each condyle as well as between the two condyles. Both medial and lateral condyles are deformed in the medio-lateral direction and at the same time compressed between the patella and the tibia in the antero-posterior direction. This effect is highest at 60 degrees of knee flexion. In both planes, the medial condyle is distorted more than the lateral one. Strain concentration in the subchondral bone facing the patella varies with flexion, especially for angles exceeding 60 degrees. The deformation of the femur in the predominant locus of OD in the medial condyle exceeds that of the lateral condyle considerably. The analysis shows that repeated vigorous exercise including extreme knee flexion may produce rapidly changing strains which in turn could ultimately be responsible for local subchondral bone collapse.     

The cortical layer of the cytoplasm--a possible site of the action of prenervous transmitters

Specific and opposite effects of some transmitter antagonists (beta-adrenoblockers, serotonin antagonists) on mechanical properties of the surface of fertilized eggs of the sea urchin Paracentrotus lividus have been demonstrated. Possible role of endogenous transmitters in regulation of the properties of cytoplasmic cortex is discussed.     

Comparison of the biliproteins from two strains of the thermophilic cyanophyte Synechococcus lividus

C-Phycocyanins from two thermophilic strains of Synechococcus lividus that grow within different temperature ranges have been shown to be unalike. The aggregation ability of these two C-phycocyanins in sedimentation-velocity experiments varied dramatically. Surprisingly, the aggregation properties of mesophilic C-phycocyanins were found to lie between those of the two thermophilic proteins. Under identical conditions at pH7.0, one thermophilic protein (Sy I) was composed of 17S and larger aggregates, whereas the other (Sy III) was an almost homogeneous 6S aggregate. Mesophilic C-phycocyanins have a mixture of 6S, 11S and less stable 17S aggregates under these conditions. Amino acid analysis, absorption spectra, immunochemistry and fluorescence polarization all indicated differences in the composition and properties of the thermophilic proteins, which suggest that they have different modes of adaptation to very high temperatures. Allophycocyanins from the two strains of S. lividus were also purified and studied, but unlike the C-phycocyanins no major differences were found between them. Allophycocyanin was homogeneous at pH6.0, with a sedimentation coefficient of 5.54S and mol.wt. 1.03x10(5), as determined by sedimentation-equilibrium measurements.     

Phylogeny of the 5S ribosomal RNA from Synechococcus lividus II: the cyanobacterial/chloroplast 5S RNAs form a common structural class

The complete nucleotide sequence of the 5S ribosomal RNA from the cyanobacterium Synechococcus lividus II has been determined. The sequence is (sequence in text) This 5S RNA has the cyanobacterial- and chloroplast-specific nucleotide insertion between positions 30 and 31 (using the numbering system of the generalized eubacterial 5S RNA) and the chloroplast-specific nucleotide-deletion signature between positions 34 and 39. The 5S RNA of S. lividus II has 27 base differences compared with the 5S RNA of the related strain S. lividus III. This large difference may reflect an ancient divergence between these two organisms. The electrophoretic mobilities on nondenaturing polyacrylamide gels of renatured 5S RNAs from S. lividus II, S. lividus III, and spinach chloroplasts are identical, but differ considerably from that of Escherichia coli 5S RNA. This most likely reflects differences in higher-order structure between the 5S RNA of E. coli and these cyanobacterial and chloroplast 5S RNAs.     

Chromosomal localization and molecular characterization of three different 5S ribosomal DNA clusters in the sea urchin Paracentrotus lividus.

In this paper the chromosomal localization and molecular cloning and characterization of three 5S rDNA clusters of 700 bp (base pairs), 900 bp, and 950 bp in the sea urchin Paracentrotus lividus are reported. Southern blot hybridization demonstrated the existence of three 5S rDNA repeats of differing length in the P. lividus genome. Fluorescence in situ hybridization analysis, performed in parallel on both haploid and diploid metaphases and interphase nuclei using different 5S rDNA units as probes, localized these 5S rDNA clusters in 3 different pairs of P. lividus chromosomes. This is the first complete gene mapping not only in a sea urchin but also in the phylum of echinoderms as a whole.     

Nature of sterols affects plasma membrane behavior and yeast survival during dehydration

The plasma membrane (PM) is a main site of injury during osmotic perturbation. Sterols, major lipids of the PM structure in eukaryotes, are thought to play a role in ensuring the stability of the lipid bilayer during physicochemical perturbations. Here, we investigated the relationship between the nature of PM sterols and resistance of the yeast Saccharomyces cerevisiae to hyperosmotic treatment. We compared the responses to osmotic dehydration (viability, sterol quantification, ultrastructure, cell volume, and membrane permeability) in the wild-type (WT) strain and the ergosterol mutant erg6Δ strain. Our main results suggest that the nature of membrane sterols governs the mechanical behavior of the PM during hyperosmotic perturbation. The mutant strain, which accumulates ergosterol precursors, was more sensitive to osmotic fluctuations than the WT, which accumulates ergosterol. The hypersensitivity of erg6Δ was linked to modifications of the membrane properties, such as stretching resistance and deformation, which led to PM permeabilization during the volume variation during the dehydration-rehydration cycles. Anaerobic growth of erg6Δ strain with ergosterol supplementation restored resistance to osmotic treatment. These results suggest a relationship between hydric stress resistance and the nature of PM sterols. We discuss this relationship in the context of the evolution of the ergosterol biosynthetic pathway.     

Epizoic bacteria on the foot of the Antarctic bivalve, Lissarca notorcadensis Melville and Standen, 1907 (Filibranchia: Philobryidae)

Specimens of the epizoic bivalve Lissarca notorcadensis Melville and Standen, 1907 (Philobryidae) were collected from the spines of cidaroid sea-urchins in the Weddell Sea during austral summer 1991. Soft tissues were investigated by means of transmission electron microscopy. Epizoic bacteria were discovered on the foot in 14 out of 15 specimens of the bivalve, and in juveniles as well as in adults. The presence of bacteria was restricted to a certain area of the foot's surface, where they were populating the brushborder of the epidermal epithelium. This is the first time that epizoic bacteria have been described, either from an Antarctic bivalve or from the foot epithelium of any bivalve species. Aspects of the fine structure were studied with regard to future work on this symbiosis. The bacteria undergo a full life-cycle in the brushborder. Whereas young or dividing bacteria are found more distally between the microvilli, senescent stages seem to disintegrate and be absorbed between their bases. The bacteria could contribute to the nutrition of their host by breaking down macromolecular or particulate organic matter, which would facilitate parenteral absorption by the bivalve, as well as eventual digestion. The ultrastructural findings suggest that the bacteria are neither sulphidotrophic nor methylotrophic. Based on their appearance, they are classified as sub-cuticular bacteria, which have been recently described from the surface epithelia in various marine invertebrate species. The phenomenon is discussed in respect to seasonal food limitation for Antarctic suspension feeders and the brooding behaviour of the host species and its ecological success. Received: 26 April 1996 / Accepted: 8 September 1996     

Nonlinear gross response analysis of a lumbar motion segment in combined sagittal loadings

A 3-D nonlinear mathematical model is used to analyze the mechanical response of a lumbar L2-3 motion segment including the posterior elements when subjected to combined sagittal plane loads. The loadings consist of axial compression force, anterior and posterior shear forces, and flexion and extension moments. The facet articulation is modelled as a general moving contact problem and the ligaments as a collection of uniaxial elements. The disk nucleus is considered as an inviscid fluid and the annulus as a composite of collagenous fibers embedded in a matrix of ground substance. The presence of axial compression force reduces the segmental stiffness in flexion whereas a reverse trend is predicted in extension. In the presence of axial compression with and without sagittal shear force, flexion considerably increases the intradiscal pressure while extension reduces it. In other words, under an identical compression force, disk pressure is predicted to be noticeably larger in flexion than in extension. The segmental mechanical response in extension loadings is markedly influenced by the changes in the relative geometry of the articular surfaces at the lower regions. Finally, the deformation of the bony structures plays a significant role in the segmental mechanics under relatively large loads.     

近亲游仆虫Euplotes affinis Dujardin的毛基体水平的扫描电镜研究

应用生物化学去纤毛、去膜和扫描电镜观察相结合的方法,详细地观察了近亲游仆虫的皮层纤毛器毛基体和非纤电器的结构、模式,揭示和发现了其他游仆虫上尚未明了和未报道过的结构特征。     

Mechanical analysis of the sea-urchin lantern: the overall system in Paracentrotus lividus

The dental apparatus or Aristotle's lantern of sea-urchins is a complex system of interacting skeletal ossicles, joints, muscles and ligaments arranged in a rigorous geometry and involved in a variety of activities. In this paper we study the movement of the whole lantern system modelled as a rigid body. The model lantern is constrained at its apex by the peristomial membrane and its movement is controlled by five pairs of antagonistic forces (retractor and protractor muscles). The other main forces applied to the lantern are the elastic reactions of both muscles and ligamental structures (compass depressors and peristomial membrane). The lantern is allowed to perform vertical movements and lateral inclinations but cannot rotate around its main axis. The equilibrium conditions of the system have been found by means of a numerical iterative procedure for solving non-linear equations. The results of the present analysis allow simulation of the overall mechanical activity of the lantern taking into account the experimental data regarding active and passive muscular forces and the tensile constraints due to ligaments.     

Three-dimensional response of a lumped parameter head-neck model due to impact and impulsive loading

A numerical procedure developed previously for predicting sagittal-plane motion of the human head-neck system due to impact and impulsive loading has been extended to three dimensions. In both situations, a lumped parameter approach is employed, but the current model lumps the mechanical response of each intervertebral joint into a single force-deformation relation evaluated from mechanical properties assembled by various investigators. Computations were performed to obtain the response of the model to a two-dimensional case of flexion whiplash, to one three-dimensional case of side impact to the skull and to another involving base acceleration normal to the sagittal plane. Agreement of the kinematic variables with the results of both the previous two-dimensional analysis and experimental data from a volunteer run is satisfactory, but somewhat poorer correspondence was found for the three-dimensional predictions upon comparison with data obtained from a physical model and from a volunteer when subjected to the prescribed loading. The differences in response are attributed to higher stiffness of facet separation of the model relative both to the structure and the volunteer, to insufficient damping, as well as to substantial differences in the mechanical deformation characteristic of the components of the prototypes and the numerical model.     

Mechanical Heterogeneity Favors Fragmentation of Strained Actin Filaments

We present a general model of actin filament deformation and fragmentation in response to compressive forces. The elastic free energy density along filaments is determined by their shape and mechanical properties, which were modeled in terms of bending, twisting, and twist-bend coupling elasticities. The elastic energy stored in filament deformation (i.e., strain) tilts the fragmentation-annealing reaction free-energy profile to favor fragmentation. The energy gradient introduces a local shear force that accelerates filament intersubunit bond rupture. The severing protein, cofilin, renders filaments more compliant in bending and twisting. As a result, filaments that are partially decorated with cofilin are mechanically heterogeneous (i.e., nonuniform) and display asymmetric shape deformations and energy profiles distinct from mechanically homogenous (i.e., uniform), bare actin, or saturated cofilactin filaments. The local buckling strain depends on the relative size of the compliant segment as well as the bending and twisting rigidities of flanking regions. Filaments with a single bare/cofilin-decorated boundary localize energy and force adjacent to the boundary, within the compliant cofilactin segment. Filaments with small cofilin clusters were predicted to fragment within the compliant cofilactin rather than at boundaries. Neglecting contributions from twist-bend coupling elasticity underestimates the energy density and gradients along filaments, and thus the net effects of filament strain to fragmentation. Spatial confinement causes compliant cofilactin segments and filaments to adopt higher deformation modes and store more elastic energy, thereby promoting fragmentation. The theory and simulations presented here establish a quantitative relationship between actin filament fragmentation thermodynamics and elasticity, and reveal how local discontinuities in filament mechanical properties introduced by regulatory proteins can modulate both the severing efficiency and location along filaments. The emergent behavior of mechanically heterogeneous filaments, particularly under confinement, emphasizes that severing in cells is likely to be influenced by multiple physical and chemical factors.     

The mechanical properties of the rabbit carpal tunnel subsynovial connective tissue

The rabbit model is commonly used to study carpal tunnel syndrome (CTS). It has been proposed that the subsynovial connective tissue (SSCT) in the carpal tunnel may play a role in the etiology of CTS, but the material properties of the rabbit SSCT are unknown. The purpose of this study was to develop a method to measure the shear properties of the rabbit SSCT. In six rabbit cadaver forepaws, the excursion of the third digit flexor digitorum superficialis (FDS) and load to failure of the SSCT were measured in a custom device. The mean excursion to full flexion in this model was 7.08 mm (S.D. 0.77). The mean shearing force at full flexion was 317 mN (S.D. 166). At full flexion percentage of maximum shear force in the SSCT was 54.5% (S.D. 19.4). The mean energy absorbed at full flexion was 0.29 mJ (S.D. 0.31). The mean excursion needed to reach 5% of the maximum shear force was 3.04 mm (S.D. 0.99). The testing model presented in this study demonstrates structural parameters to evaluate the shear properties of the SSCT in a rabbit model. The data presented could be used for estimating sample sizes in a more comprehensive study of the effect of CTS on the SSCT properties.     

Distribution patterns and bioerosion of the sea urchin Centrostephanus coronatus (Diadematoida: Diadematidae), at the reef of Playa Blanca, Colombian Pacific

Regular sea-urchins are one of the main bioeroding organisms affecting coral reefs around the world. The abundance, distribution and bioerosion rate of the sea-urchin Centrostephanus coronatus, were determined in different reef zones of Playa Blanca fringing reef (Gorgona Island, Colombian pacific coast) during 1997 and 1998. The erosion rates were determined calcinating the gut content of the sea-urchins to eliminate all organic components and preserve the inorganic portion of calcium carbonate. C. coronatus showed the highest densities towards the central zones of the reef (plain-crest and front) (12.4 ind/m2; range 0-48 ind/m2). The highest mean bioerosion rate was 0.103 kgCaCO3/m2/yr in the reef plain-crest (0-0.69 kgCaCO3/m2/yr). In the other zones, (back reef and reef front) the mean bioerosion rates were 0.071 (range 0-0.39) and 0.052 (range 0-0.31) kgCaCO3/m2/yr respectively. According to the present data, it can be seen that the destruction of coralline skeletons, produced in this reef by sea-urchins is rather low, compared with the abrasion caused by these organisms in other places of the world. However, the combined action of C. coronatus and other bioeroding organisms (borers and grazers). along with some adverse environmental factors to corals, can be causing a negative balance between normal processes of reef accretion-destruction in Gorgona Island reefs.     

The mechanical role of the endodermis in Equisetum plant stems

The endodermis of different species of the genus Equisetum has different configurations, two or one continuous layers or a sheath only around the vascular bundles. The question whether the endodermis contributes to the mechanical stability of the aerial shoots is investigated in two ways: In a direct approach, the endodermis of segments of E. hyemale was dissected longitudinally and the mechanical stability against ovalization measured as a function of the orientation of the cuts with respect to the forces applied. A comparative approach tested the mechanical stability of eight different species of Equisetum against ovalization of the cross-section for samples, which were either fully turgescent or had reduced turgor pressure. The double-layer endodermis substantially contributed to the mechanical stability of E. affinis and E. hyemale. Equisetum arvense, E. pratense, E. sylvaticum, and E. telmateja are mechanically stabilized by their single layer of endodermis at least under conditions of low turgor pressure.     

New insights into mutable collagenous tissue: correlations between the microstructure and mechanical state of a sea-urchin ligament

The mutable collagenous tissue (MCT) of echinoderms has the ability to undergo rapid and reversible changes in passive mechanical properties that are initiated and modulated by the nervous system. Since the mechanism of MCT mutability is poorly understood, the aim of this work was to provide a detailed morphological analysis of a typical mutable collagenous structure in its different mechanical states. The model studied was the compass depressor ligament (CDL) of a sea urchin (Paracentrotus lividus), which was characterized in different functional states mimicking MCT mutability. Transmission electron microscopy, histochemistry, cryo-scanning electron microscopy, focused ion beam/scanning electron microscopy, and field emission gun-environmental scanning electron microscopy were used to visualize CDLs at the micro- and nano-scales. This investigation has revealed previously unreported differences in both extracellular and cellular constituents, expanding the current knowledge of the relationship between the organization of the CDL and its mechanical state. Scanning electron microscopies in particular provided a three-dimensional overview of CDL architecture at the micro- and nano-scales, and clarified the micro-organization of the ECM components that are involved in mutability. Further evidence that the juxtaligamental cells are the effectors of these changes in mechanical properties was provided by a correlation between their cytology and the tensile state of the CDLs.