A random cell motility gradient downstream of FGF controls elongation of amniote embryos

Vertebrate embryos are characterized by an elongated antero-posterior (AP) body axis, which forms by progressive cell deposition from a posterior growth zone in the embryo. Here, we used tissue ablation in the chicken embryo to demonstrate that the caudal presomitic mesoderm (PSM) has a key role in axis elongation. Using time-lapse microscopy, we analysed the movements of fluorescently labelled cells in the PSM during embryo elongation, which revealed a clear posterior-to-anterior gradient of cell motility and directionality in the PSM. We tracked the movement of the PSM extracellular matrix in parallel with the labelled cells and subtracted the extracellular matrix movement from the global motion of cells. After subtraction, cell motility remained graded but lacked directionality, indicating that the posterior cell movements associated with axis elongation in the PSM are not intrinsic but reflect tissue deformation. The gradient of cell motion along the PSM parallels the fibroblast growth factor (FGF)/mitogen-activated protein kinase (MAPK) gradient1, which has been implicated in the control of cell motility in this tissue2. Both FGF signalling gain- and loss-of-function experiments lead to disruption of the motility gradient and a slowing down of axis elongation. Furthermore, embryos treated with cell movement inhibitors (blebbistatin or RhoK inhibitor), but not cell cycle inhibitors, show a slower axis elongation rate. We propose that the gradient of random cell motility downstream of FGF signalling in the PSM controls posterior elongation in the amniote embryo. Our data indicate that tissue elongation is an emergent property that arises from the collective regulation of graded, random cell motion rather than by the regulation of directionality of individual cellular movements

The ANTARES Optical Beacon System

ANTARES is a neutrino telescope being deployed in the Mediterranean Sea. It consists of a three dimensional array of photomultiplier tubes that can detect the Cherenkov light induced by charged particles produced in the interactions of neutrinos with the surrounding medium. High angular resolution can be achieved, in particular when a muon is produced, provided that the Cherenkov photons are detected with sufficient timing precision. Considerations of the intrinsic time uncertainties stemming from the transit time spread in the photomultiplier tubes and the mechanism of transmission of light in sea water lead to the conclusion that a relative time accuracy of the order of 0.5 ns is desirable. Accordingly, different time calibration systems have been developed for the ANTARES telescope. In this article, a system based on Optical Beacons, a set of external and well-controlled pulsed light sources located throughout the detector, is described. This calibration system takes into account the optical properties of sea water, which is used as the detection volume of the ANTARES telescope. The design, tests, construction and first results of the two types of beacons, LED and laser-based, are presented.Comment: 21 pages, 18 figures, submitted to Nucl. Instr. and Meth. Phys. Res.

First results of the Instrumentation Line for the deep-sea ANTARES neutrino telescope

In 2005, the ANTARES Collaboration deployed and operated at a depth of 2500 m a so-called Mini Instrumentation Line equipped with Optical Modules (MILOM) at the ANTARES site. The various data acquired during the continuous operation from April to December 2005 of the MILOM confirm the satisfactory performance of the Optical Modules, their front-end electronics and readout system. as well as the calibration devices of the detector. The in situ measurement of the Optical Module time response yields a resolution better than 0.5 ns. The performance of the acoustic positioning system, which enables the spatial reconstruction of the ANTARES detector with a precision of about 10 cm, is verified. These results demonstrate that with the full ANTARES neutrino telescope the design angular resolution of better than 0.3 degrees can be realistically achieved

Study of large hemispherical photomultiplier tubes for the ANTARES neutrino telescope

The ANTARES neutrino telescope, to be immersed depth in the Mediterranean Sea, will consist of a 3 dimensional matrix of 900 large area photomultiplier tubes housed in pressure resistant glass spheres. The selection of the optimal photomultiplier was a critical step for the project and required an intensive phase of tests and developments carried out in close collaboration with the main manufacturers worldwide. This paper provides an overview of the tests performed by the collaboration and describes in detail the features of the PMT chosen for ANTARES

The data acquisition system for the ANTARES neutrino telescope

The ANTARES neutrino telescope is being constructed in the Mediterranean Sea. It consists of a large three-dimensional array of photo-multiplier tubes. The data acquisition system of the detector takes care of the digitisation of the photo-multiplier tube signals, data transport, data filtering, and data storage. The detector is operated using a control program interfaced with all elements. The design and the implementation of the data acquisition system are described.Comment: 20 pages, 6 figures, accepted for publication in Nucl. Instrum. Meth.

Un gradient de migration non directionnelle dans le mésoderme présomitique contribue à l’allongement de l’axe chez l’embryon de poulet

Le corps des vertébrés est allongé selon un axe antéro-postérieur. Cette forme spécifique se met en place durant l’embryogenèse par des phénomènes morphogénétiques d’élongation. Les mécanismes d’élongation qui mènent à la formation des parties antérieures du corps sont très bien décrits mais, par contre, ceux qui concernent les parties les plus postérieures ont été moins bien étudiés. Nous avons choisi l’embryon de poulet comme modèle d’étude pour aborder cette problématique. Avec des expériences d’ablation par microchirurgie, nous avons d’abord montré que la partie caudale du mésoderme présomitique (PSM) était primordiale dans le phénomène d’allongement postérieur. Grâce à des techniques de vidéo-microscopie, nous avons par la suite mis en évidence un gradient caudo-rostral de motilité directionnellement postérieure au sein du PSM. En soustrayant le mouvement du tissu grâce à un marquage de la matrice extracellulaire, nous avons démontré que ce gradient correspond à un gradient de motilité cellulaire non directionnel, indiquant que les mouvements postérieurs sont dus à la déformation tissulaire et non aux déplacements propres des cellules. Par des expériences de perte et de gain de fonction de la voie de signalisation FGF (Fibroblast Growth Factor), nous avons montré que cette voie de signalisation régule le gradient de motilité non directionnelle et l’allongement postérieur de l’embryon. Enfin, nous avons effectué des expériences suggérant que l’effet du FGF sur l’allongement de l’embryon ne passe pas par la régulation de la prolifération cellulaire mais bien par un effet sur la motilité cellulaire. Nous proposons donc un nouveau modèle d’élongation dans lequel le gradient de motilité non directionnelle présent dans le PSM contrôle l’allongement postérieur de l’axe embryonnaire

A typical relaxation of structural constraints in Hox gene clusters of squamates

Hox genes control many aspects of embryonic development in metazoans. Previous analyses of this gene family has revealed a surprising diversity in terms of gene number and organization between various animal species. In vertebrates, Hox genes are grouped into tightly organized clusters, originally claimed to be devoid of repetitive sequences. Here, we report the genomic organization of the posterior Hox loci (from Hox13 to Hox10) present in reptiles including the green anole lizard (Anolis carolinensis) and the corn snake (Pantherophis guttatus), and show that squamates have massively accumulated transposable elements, leading to gene clusters larger in size when compared to other vertebrates. In addition, we show the disappearance of highly conserved regulatory sequences within both the HoxA and HoxD clusters, as well as the rapid evolution of Hox coding regions within the squamata lineages. Finally, in parallel with these modifications in the genomic organization of squamata Hox clusters, we observe specific alterations in the expression patterns of posterior Hox genes during corn snake somitogenesis. Because transposable elements are major sources of genetic variations, we speculate that their insertion into Hox gene clusters, not reported so far for other vertebrates, may have associated with the evolution of the spectacular realm of morphological variations in the body plans of squamates