The High-Acceptance Dielectron Spectrometer HADES

HADES is a versatile magnetic spectrometer aimed at studying dielectron production in pion, proton and heavy-ion induced collisions. Its main features include a ring imaging gas Cherenkov detector for electron-hadron discrimination, a tracking system consisting of a set of 6 superconducting coils producing a toroidal field and drift chambers and a multiplicity and electron trigger array for additional electron-hadron discrimination and event characterization. A two-stage trigger system enhances events containing electrons. The physics program is focused on the investigation of hadron properties in nuclei and in the hot and dense hadronic matter. The detector system is characterized by an 85% azimuthal coverage over a polar angle interval from 18 to 85 degree, a single electron efficiency of 50% and a vector meson mass resolution of 2.5%. Identification of pions, kaons and protons is achieved combining time-of-flight and energy loss measurements over a large momentum range. This paper describes the main features and the performance of the detector system

In vivo imaging reveals transient microglia recruitment and functional recovery of photoreceptor signaling after injury.

Microglia respond to damage and microenvironmental changes within the central nervous system by morphologically transforming and migrating to the lesion, but the real-time behavior of populations of these resident immune cells and the neurons they support have seldom been observed simultaneously. Here, we have used in vivo high-resolution optical coherence tomography (OCT) and scanning laser ophthalmoscopy with and without adaptive optics to quantify the 3D distribution and dynamics of microglia in the living retina before and after local damage to photoreceptors. Following photoreceptor injury, microglia migrated both laterally and vertically through the retina over many hours, forming a tight cluster within the area of visible damage that resolved over 2 wk. In vivo OCT optophysiological assessment revealed that the photoreceptors occupying the damaged region lost all light-driven signaling during the period of microglia recruitment. Remarkably, photoreceptors recovered function to near-baseline levels after the microglia had departed the injury locus. These results demonstrate the spatiotemporal dynamics of microglia engagement and restoration of neuronal function during tissue remodeling and highlight the need for mechanistic studies that consider the temporal and structural dynamics of neuron-microglia interactions in vivo

Bridging the gap between single-cell migration and collective dynamics

Motivated by the wealth of experimental data recently available, we present a cellularautomaton-based modeling framework focussing on high-level cell functions and their concerted effect on cellular migration patterns. Specifically, we formulate a coarse-grained description of cell polarity through self-regulated actin organization and its response to mechanical cues. Furthermore, we address the impact of cell adhesion on collective migration in cell cohorts. The model faithfully reproduces typical cell shapes and movements down to the level of single cells, yet allows for the efficient simulation of confluent tissues. In confined circular geometries, we find that specific properties of individual cells (polarizability;contractility) influence the emerging collective motion of small cell cohorts. Finally, we study the properties of expanding cellular monolayers (front morphology;stress and velocity distributions) at the level of extended tissues

Functional Properties of Visual Pigments using A1 and A2 Chromophore : From Molecules to Ecology

The first event in vision is the absorption of a photon by a visual pigment molecule in a retinal photoreceptor cell. Activation of the molecule triggers a chemical amplification cascade, which finally leads to a change in the membrane potential of the cell. However, a visual pigment molecule may also be spontaneously activated by thermal energy. The resulting electrical response is identical to that caused by a photon. Such false light signals form a background noise limiting the detection of dim light. The absorption spectrum of a visual pigment (its ability to use different wavelengths of light) and its propensity for thermal activation both depend on the minimum amount of energy required for activation (the activation energy Ea). These properties of the pigment can be tuned on an evolutionary time scale by changes in the amino acid sequence of the protein part (the opsin) or on a physiological time scale by changing the light-sensitive cofactor bound to the opsin, the chromophore. The latter option is accessible only to poikilothermic vertebrates having two alternative chromophores (retinal A1 and A2). In this thesis, functional consequences of the A1-A2 exchange were investigated. In the first part, the relation between the changes of the absorption spectrum and the activation energy was quantitatively measured in several species of amphibians and fishes using both chromophores. The A2-induced shift of the absorption spectrum towards longer wavelengths was found always to correlate with a decrease in Ea. Later investigations have confirmed that decreasing Ea increases the rate of thermal activations. Thus the switch from A1 to A2 in the same opsin gives a more red-sensitive but noisier pigment. Against this background, the second part of the thesis investigates chromophore usage in eight populations of nine-spined sticklebacks (Pungitius pungitius) from different light environments. The amino acid sequence of the rods was found to be identical in all populations, implying that variations in spectral sensitivity depended only on the A1:A2 ratios. The cone absorption spectra also suggested that the variation within each cone class was due to varying chromophore proportions alone. The differences between populations could not be consistently explained as adaptations to the different light environments. However, an important and quite unexpected result was that the same individual could have quite different chromophore proportions in rods and cones (more A2 in cones). This shows that there are mechanisms by which chromophore proportions in different photoreceptors can be regulated much more selectively than previously thought. Since pigment noise is sensitivity-limiting mainly in dim light, it may be suggested that cones (working mainly in brighter light) can better afford using the noisy A2 chromophore to shift their spectral sensitivities for a better match to a long-wavelength photic environment.Näkötapahtuma alkaa, kun verkkokalvon fotoreseptorisoluissa sijaitseva näköpigmenttimolekyyli absorboi fotonin. Molekyylin aktivoituminen käynnistää kemiallisen vahvistusketjun, jonka lopputuloksena solun kalvojännite muuttuu. Näköpigmenttimolekyyli voi kuitenkin aktivoitua myös spontaanisti lämpöenergian vaikutuksesta (termisesti), synnyttäen sähköisen vasteen joka on täysin samanlainen kuin fotonin aiheuttama. Tällaiset väärät valosignaalit muodostavat taustakohinan, joka rajoittaa heikkojen valojen havaitsemista. Näköpigmentin absorptiospektri (sen kyky käyttää valon eri aallonpituuksia) ja sen taipumus aktivoitua termisesti riippuvat molemmat aktivaation vaatimasta minimienergiamäärästä (ns. aktivaatioenergiasta Ea). Pigmentin ominaisuuksia voidaan säätää joko evolutiivisella aikaskaalalla proteiiniosan (opsiinin) aminohapposekvenssiä muuttamalla tai fysiologisella aikaskaalalla opsiiniin sidotun valoherkän kofaktorin, ns. kromoforin, vaihdolla. Jälkimmäinen optio on vain vaihtolämpöisillä selkärankaisilla, joilla on käytössään kaksi vaihtoehtoista kromoforia (retinaali A1 ja A2). Tässä väitöskirjassa tutkittiin A1-A2-vaihdon funktionaalisia seurauksia. Ensimmäisessä osassa mitattiin kvantitatiivisesti absorptiospektrin ja aktivaatioenergian muutosten suhdetta useilla sammakko- ja kalalajeilla. Todettiin että A2:een liittyvä absorptiospektrin siirtyminen pitempiin aallonpituuksiin korreloi aina Ea:n laskun kanssa. Myöhemmät tutkimukset ovat vahvistaneet, että Ea:n alentaminen lisää termisten aktivaatioiden määrää. A1-kromoforin vaihtaminen A2:een samassa opsiinissa antaa siis punaherkemmän mutta kohinaisemman pigmentin. Tätä taustaa vasten väitöskirjan toisessa osassa tutkittiin kromoforin käyttöä kahdeksassa, eri valoympäristöissä elävässä kymmenpiikkipopulaatiossa (Pungitius pungitius). Sauvasolujen opsiinien aminohapposekvenssi todettiin identtiseksi kaikissa populaatioissa, joten spektraaliherkkyyden vaihtelu johtui yksinomaan vaihtelevista A1:A2 suhteista. Myös tappisolujen absorptiospektrit viittasivat siihen, että kunkin tappiluokan sisäinen vaihtelu johtui vain kromoforisuhteista. Populaatioiden välisiä eroja ei pystytty johdonmukaisesti selittämään adaptaatioina eri valoympäristöihin. Sen sijaan tärkeä ja täysin odottamaton tulos oli, että saman yksilön sauvoissa ja tapeissa saattoi olla aivan eri kromoforisuhteet (tapeissa enemmän A2). Tämä osoittaa, että on mekanismeja joilla eri reseptoreiden kromoforisuhteita voidaan säätää paljon yksilöidymmin kuin on tiedetty. Koska pigmenttikohina rajoittaa näön herkkyyttä lähinnä heikossa valossa, voidaan ajatella, että nimenomaan tappien spektraaliherkkyyksiä on varaa siirtää A2:lla paremmin vastaamaan keltaisen järven valospektriä, ilman että kohinasta johtuva hinta on liian korkea

Zebrafish differentially process colour across visual space to match natural scenes

Animal eyes have evolved to process behaviourally important visual information, but how retinas deal with statistical asymmetries in visual space remains poorly understood. Using hyperspectral imaging in the field, in-vivo 2-photon imaging of retinal neurons and anatomy, here we show that larval zebrafish use a highly anisotropic retina to asymmetrically survey their natural visual world. First, different neurons dominate different parts of the eye, and are linked to a systematic shift in inner retinal function: Above the animal, there is little colour in nature and retinal circuits are largely achromatic. Conversely, the lower visual field and horizon are colour-rich and are predominately surveyed by chromatic and colour-opponent circuits that are spectrally matched to the dominant chromatic axes in nature. Second, in the horizontal and lower visual field bipolar cell terminals encoding achromatic and colour opponent visual features are systematically arranged into distinct layers of the inner retina. Third, above the frontal horizon, a high-gain ultraviolet-system piggy-backs onto retinal circuits, likely to support prey-capture