The complex simplicity of the brittle star nervous system

Background: Brittle stars (Ophiuroidea, Echinodermata) have been increasingly used in studies of animal behavior, locomotion, regeneration, physiology, and bioluminescence. The success of these studies directly depends on good working knowledge of the ophiuroid nervous system. Results: Here, we describe the arm nervous system at different levels of organization, including the microanatomy of the radial nerve cord and peripheral nerves, ultrastructure of the neural tissue, and localization of different cell types using specific antibody markers. We standardize the nomenclature of nerves and ganglia, and provide an anatomically accurate digital 3D model of the arm nervous system as a reference for future studies. Our results helped identify several general features characteristic to the adult echinoderm nervous system, including the extensive anatomical interconnections between the ectoneural and hyponeural components, neuroepithelial organization of the central nervous system, and the supporting scaffold of the neuroepithelium formed by radial glial cells. In addition, we provide further support to the notion that the echinoderm radial glia is a complex and diverse cell population. We also tested the suitability of a range of specific cell-type markers for studies of the brittle star nervous system and established that the radial glial cells are reliably labeled with the ERG1 antibodies, whereas the best neuronal markers are acetylated tubulin, ELAV, and synaptotagmin B. The transcription factor Brn1/2/4 - a marker of neuronal progenitors - is expressed not only in neurons, but also in a subpopulation of radial glia. For the first time, we describe putative ophiuroid proprioceptors associated with the hyponeural part of the central nervous system. Conclusions: Together, our data help establish both the general principles of neural architecture common to the phylum Echinodermata and the specific ophiuroid features

The central nervous system of sea cucumbers (Echinodermata: Holothuroidea) shows positive immunostaining for a chordate glial secretion

<p>Abstract</p> <p>Background</p> <p>Echinoderms and chordates belong to the same monophyletic taxon, the Deuterostomia. In spite of significant differences in body plan organization, the two phyla may share more common traits than was thought previously. Of particular interest are the common features in the organization of the central nervous system. The present study employs two polyclonal antisera raised against bovine Reissner's substance (RS), a secretory product produced by glial cells of the subcomissural organ, to study RS-like immunoreactivity in the central nervous system of sea cucumbers.</p> <p>Results</p> <p>In the ectoneural division of the nervous system, both antisera recognize the content of secretory vacuoles in the apical cytoplasm of the radial glia-like cells of the neuroepithelium and in the flattened glial cells of the non-neural epineural roof epithelium. The secreted immunopositive material seems to form a thin layer covering the cell apices. There is no accumulation of the immunoreactive material on the apical surface of the hyponeural neuroepithelium or the hyponeural roof epithelium. Besides labelling the supporting cells and flattened glial cells of the epineural roof epithelium, both anti-RS antisera reveal a previously unknown putative glial cell type within the neural parenchyma of the holothurian nervous system.</p> <p>Conclusion</p> <p>Our results show that: a) the glial cells of the holothurian tubular nervous system produce a material similar to Reissner's substance known to be synthesized by secretory glial cells in all chordates studied so far; b) the nervous system of sea cucumbers shows a previously unrealized complexity of glial organization. Our findings also provide significant clues for interpretation of the evolution of the nervous system in the Deuterostomia. It is suggested that echinoderms and chordates might have inherited the RS-producing radial glial cell type from the central nervous system of their common ancestor, i.e., the last common ancestor of all the Deuterostomia.</p

Modelling Shear Flows with SPH and Grid Based Methods

Given the importance of shear flows for astrophysical gas dynamics, we study the evolution of the Kelvin-Helmholtz instability (KHI) analytically and numerically. We derive the dispersion relation for the two-dimensional KHI including viscous dissipation. The resulting expression for the growth rate is then used to estimate the intrinsic viscosity of four numerical schemes depending on code-specific as well as on physical parameters. Our set of numerical schemes includes the Tree-SPH code VINE, an alternative SPH formulation developed by Price (2008), and the finite-volume grid codes FLASH and PLUTO. In the first part, we explicitly demonstrate the effect of dissipation-inhibiting mechanisms such as the Balsara viscosity on the evolution of the KHI. With VINE, increasing density contrasts lead to a continuously increasing suppression of the KHI (with complete suppression from a contrast of 6:1 or higher). The alternative SPH formulation including an artificial thermal conductivity reproduces the analytically expected growth rates up to a density contrast of 10:1. The second part addresses the shear flow evolution with FLASH and PLUTO. Both codes result in a consistent non-viscous evolution (in the equal as well as in the different density case) in agreement with the analytical prediction. The viscous evolution studied with FLASH shows minor deviations from the analytical prediction.Comment: 16 pages, 17 figure

Prospects of observing a quasar HII region during the Epoch of Reionization with redshifted 21cm

We present a study of the impact of a bright quasar on the redshifted 21cm signal during the Epoch of Reionization (EoR). Using three different cosmological radiative transfer simulations, we investigate if quasars are capable of substantially changing the size and morphology of the H II regions they are born in. We choose stellar and quasar luminosities in a way that is favourable to seeing such an effect. We find that even the most luminous of our quasar models is not able to increase the size of its native H II region substantially beyond those of large H II regions produced by clustered stellar sources alone. However, the quasar H II region is found to be more spherical. We next investigate the prospects of detecting such H II regions in the redshifted 21cm data from the Low Frequency Array (LOFAR) by means of a matched filter technique. We find that H II regions with radii ~ 25 comoving Mpc or larger should have a sufficiently high detection probability for 1200 hours of integration time. Although the matched filter can in principle distinguish between more and less spherical regions, we find that when including realistic system noise this distinction can no longer be made. The strong foregrounds are found not to pose a problem for the matched filter technique. We also demonstrate that when the quasar position is known, the redshifted 21cm data can still be used to set upper limits on the ionizing photon rate of the quasar. If both the quasar position and its luminosity are known, the redshifted 21 cm data can set new constrains on quasar lifetimes.Comment: 17 pages, 12 figures, 3 tables, accepted for publication in MNRAS; changes in introduction and figure

Explicit convection and scale-aware cumulus parameterizations: high-resolution simulations over areas of different topography in Germany

An increase in the spatial resolution of regional climate model simulations improves the representation of land surface characteristics and may allow the explicit calculation of important physical processes such as convection. The present study investigates further potential benefits with respect to precipitation, based on a small ensemble of high-resolution simulations with WRF with grid spacings up to 1 km. The skill of each experiment is evaluated regarding the temporal and spatial performance of the simulation of precipitation of one year over both a mountainous region in southwestern Germany and a mainly flat region in northern Germany. This study allows us to differentiate between the impact of grid spacing, topography, and convection parameterization. Furthermore, the performance of a state-of-the-art convection parameterization scheme in the gray zone of convection is evaluated against an explicit calculation of convection only. Our evaluation demonstrates the following: high-resolution simulations (5 and 1 km) are generally able to represent the diurnal cycle, structure, and intensity distribution of precipitation, when compared to observational datasets such as radar data and interpolated station data. The influence of the improved representation of the topography at higher resolution (1 km) becomes apparent in complex terrain, where the localization of precipitation maxima is more accurate, although these maxima are overestimated. In flat areas, differences in spatial evaluations arise between simulations with parameterized and explicitly calculated convection, whereas smaller grid spacings (1 km vs 5km) show hardly any impact on precipitation results

Cyathidium plantei sp. n., an extant cyrtocrinid (Echinodermata, Crinoidea)-morphologically identical to the fossil Cyathidium depressum (Cretaceous, Cenomanian)

A new species of the holopodid genus Cyathidium was found on rocks off Grande Comore in a depth of around 200 m. Based on external morphology of resting animals, the new species Cyathidium plantei sp. n is described, with emphasis on comparison to the only other extant species (C. foresti) as well as to the four extinct representatives of the genus. Concerning morphological characters, the new species is almost identical to the Cretaceous C. depressum. A cladistic analysis of the entire family, including the genus Holopus, shares a peculiar pattern of bending of the arms, which in principle is an apomorphic character of the family and in detail shows variations within the family. In addition, stratigraphic data are used for the determination of the evolutionary direction. This analysis reveals that the two recent species are closely related to each other, and to the fossil C. depressum. from which the entire family is probably derived

Le Peuple : organe quotidien du syndicalisme

01 mars 19331933/03/01 (A13,N4428)-1933/03/01

Additional file 9 of The complex simplicity of the brittle star nervous system

Mixed nerves. Interactive 3D model. See caption to Additional file 6 for instructions. (HTML 5079 kb

Additional file 3 of The complex simplicity of the brittle star nervous system

Ectoneural system. 3D animation. (AVI 3840 kb

Additional file 8 of The complex simplicity of the brittle star nervous system

Hyponeural system. Interactive 3D model. See caption to Additional file 6 for instructions. (HTML 4680 kb