Les clusters ou districts industriels du domaine culturel et médiatique : revue du savoir économique et questionnement

Financé par le Ministère de la culture et de la communication, ce projet a pour objet de dresser un panorama des travaux à la fois théoriques et empiriques réalisés sur les clusters et districts industriels en général puis sur les clusters et districts culturels et médiatiques. Cette focalisation progressive vers le domaine culturel nous permettra de faire un point sur l’état des connaissances actuelles, leurs résultats, ainsi que sur les questions sans éléments de réponse et qui pourraient être des pistes de recherche à venir

A Young-type experiment using a single-electron source and an independent atomic-size two-center interferometer: the realization of a thought experiment

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Ion induced fragmentation of biomolecular systems at low collision energies

In this paper, we present results of different collision experiments between multiply charged ions at low collision energies (in the keV-region) and biomolecular systems. This kind of interaction allows to remove electrons form the biomolecule without transferring a large amount of vibrational excitation energy. Nevertheless, following the ionization of the target, fragmentation of biomolecular species may occur. It is the main objective of this work to study the physical processes involved in the dissociation of highly electronically excited systems. In order to elucidate the intrinsic properties of certain biomolecules (porphyrins and amino acids) we have performed experiments in the gas phase with isolated systems. The obtained results demonstrate the high stability of porphyrins after electron removal. Furthermore, a dependence of the fragmentation pattern produced by multiply charged ions on the isomeric structure of the alanine molecule has been shown. By considering the presence of other surrounding biomolecules (clusters of nucleobases), a strong influence of the environment of the biomolecule on the fragmentation channels and their modification, has been clearly proven. This result is explained, in the thymine and uracil case, by the formation of hydrogen bonds between O and H atoms, which is known to favor planar cluster geometries.</p

Spin density wave dislocation in chromium probed by coherent x-ray diffraction

We report on the study of a magnetic dislocation in pure chromium. Coherent x-ray diffraction profiles obtained on the incommensurate Spin Density Wave (SDW) reflection are consistent with the presence of a dislocation of the magnetic order, embedded at a few micrometers from the surface of the sample. Beyond the specific case of magnetic dislocations in chromium, this work may open up a new method for the study of magnetic defects embedded in the bulk.Comment: 8 pages, 7 figure

Primary processes: from atoms to diatomic molecules and clusters

International audienceThis article presents a short review of the main progresses achieved at the GANIL facilities during the last thirty years in the field of ion-atom and ion-diatomic molecule collisions. Thanks to the wide range of projectile energies and species available on the different beam lines of the facility, elementary processes such as electron capture, ionization and excitation have been extensively studied. Beside primary collision mechanisms, the relaxation processes of the collision partners after the collision have been another specific source of interest. Progresses on other fundamental processes such as Young type interferences induced by ion-molecule collisions or shake off ionization resulting from nuclear beta decay are also presented. 1. Introduction For the electronic structures of atoms and molecules, precise theoretical knowledge and high-resolution experimental data are available. But the complete understanding of dynamic processes in atomic collisions remains a challenge, due to large theoretical problems in describing time-dependent many-particle reactions, and to experimental difficulties in performing complete experiments in which all relevant quantities are accessible. Elementary collisions involving ions, atoms and molecules play an important role in many gaseous and plasma environments, where they provide both the heating and cooling mechanisms. The study of such collisions is thus not only of fundamental importance, it is also essential for the understanding of large-scale systems such as astrophysical plasmas, planetary atmospheres, gas discharge lasers, semiconductor processing plasmas, and fusion plasmas. Collisions between ions and atoms (or simple molecules) give also access to the elementary processes responsible for energy transfer in ion-matter and ion-biological molecule collisions. Complete knowledge of these elementary processes is thus of primordial importance for ion induced modification of materials as well as for radiolysis, radiotherapy and biological damages due to radiation exposure

Synthesis and accumulation of p34cdc2 and cyclin B in mouse oocytes during acquisition of competence to resume meiosis.

This study tests the hypothesis 033 that growing murine oocytes, which are incompetent to resume meiosis, are deficient in their content of p34cdc2 and/or cyclin B, the two subunits of maturation promoting factor (MPF). Accumulation of the two MPF components occurred in an asynchronous manner in growing oocytes. Cyclin B content reached maximal levels in oocytes that were not yet competent to undergo germinal vesicle breakdown (GVB), the first obvious morphological manifestation of the resumption of meiosis. Thus, the amount of cyclin B is not the limiting factor rendering these growing oocytes incompetent to undergo GVB. In contrast, synthesis and accumulation of p34cdc2 increased during the period of oocyte growth in vivo when they became competent to undergo GVB. A similar increase in the amount of p34cdc2 also occurred in cultured granulosa cell-free oocytes despite the lack of oocyte growth, but these cultured oocytes did not become GVB competent. Thus, the accumulation of p34cdc2 is probably necessary, but not sufficient, for mouse oocytes to become competent to undergo GVB. This accumulation occurs autonomously in oocytes independently of growth or of the participation of follicular somatic cells

Induction of precocious germinal vesicle breakdown (GVB) by GVB-incompetent mouse oocytes: possible role of mitogen-activated protein kinases rather than p34cdc2 kinase.

In contrast to fully grown mouse oocytes, growing oocytes released from their preantral follicles are not capable of resuming meiosis spontaneously. However, when these denuded growing oocytes are incubated for 2-3 days in control medium and then treated with okadaic acid, 95% undergo precocious germinal vesicle breakdown (GVB) and chromosome condensation within 24 h. This study shows that both events apparently occur through a p34cdc2 kinase-independent pathway, since activity of this kinase was detected only after the oocytes had undergone okadaic acid-stimulated GVB. In contrast, microtubule-associated or mitogen-activated protein (MAP) kinases 1 and 2 were activated before GVB and their level of activity was correlated with the percentage of oocytes undergoing GVB, suggesting that these kinases may promote GVB and chromosome condensation under these experimental conditions. In addition, it is shown that MAP kinases accumulate during normal oocyte growth in vivo, but not in cultured denuded oocytes in vitro even when these oocytes become competent to undergo okadaic acid-stimulated GVB. Unlike p34cdc2, the accumulation of MAP kinases requires close physical interactions between the oocytes and the granulosa cells but is not necessary for the oocyte to become GVB-competent. Therefore, it MAP kinases are actually involved in the induction of GVB and chromosome condensation during normal oocyte maturation, acquisition of GVB competence requires oocyte-autonomous accumulation of MAP kinase activator(s) rather than the MAP kinases themselves

Secretion of cumulus expansion enabling factor by mouse oocytes: relationship to oocyte growth and competence to resume meiosis.

Fully grown mouse oocytes secrete a factor that enables the surrounding cumulus cells to undergo cumulus expansion upon stimulation of the cumulus cells with follicle-stimulating hormone in vitro. The secretion of the enabling factor by the oocytes is developmentally regulated; it is normally secreted in significant amounts only by oocytes that are competent of undergoing spontaneous germinal vesicle breakdown (GVB) and resuming the first meiotic division. The objective of this study was to determine the relationship, if any, among secretion of this factor, oocyte growth, and the competence to undergo GVB. Three experimental systems were used to assess these relationships. First, 20 to 25% of the oocytes isolated from the small antral follicles of mutant hypogonadal mice were GVB-incompetent even though they had achieved a size typical of GVB-competent oocytes. These large GVB-incompetent oocytes secreted almost as much cumulus expansion enabling factor as the GVB-competent oocytes. Second, GVB-incompetent oocytes cultured under conditions that promote the acquisition of GVB competence but not oocyte growth acquired the ability to secrete the cumulus expansion enabling factor. Third, the precocious activation of maturation promoting factor (MPF) in GVB-incompetent oocytes by okadaic acid did not stimulate the secretion of cumulus expansion enabling factor. It is concluded that the secretion of cumulus expansion enabling factor by oocytes is (1) independent of oocyte growth, (2) independent of competence to undergo GVB, and (3) not induced by the precocious activation of MPF. Thus, although the secretion of the enabling factor normally coincides with the time of acquisition of competence to undergo GVB, the ability to secrete the enabling factor is independent of oocyte maturation. Nevertheless, factors that promote the oocyte\u27s program that leads to competence to undergo GVB probably also promote the ability to secrete the enabling factor