191 resultados para Artefacts for fashion


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Purpose: Recently morphometric measurements of the ascending aorta have been done with ECG-gated MDCT to help the development of future endovascular therapies (TCT) [1]. However, the variability of these measurements remains unknown. It will be interesting to know the impact of CAD (computer aided diagnosis) with automated segmentation of the vessel and automatic measurements of diameter on the management of ascending aorta aneurysms. Methods and Materials: Thirty patients referred for ECG-gated CT thoracic angiography (64-row CT scanner) were evaluated. Measurements of the maximum and minimum ascending aorta diameters were obtained automatically with a commercially available CAD and semi-manually by two observers separately. The CAD algorithms segment the iv-enhanced lumen of the ascending aorta into perpendicular planes along the centreline. The CAD then determines the largest and the smallest diameters. Both observers repeated the automatic measurements and the semimanual measurements during a different session at least one month after the first measurements. The Bland and Altman method was used to study the inter/intraobserver variability. A Wilcoxon signed-rank test was also used to analyse differences between observers. Results: Interobserver variability for semi-manual measurements between the first and second observers was between 1.2 to 1.0 mm for maximal and minimal diameter, respectively. Intraobserver variability of each observer ranged from 0.8 to 1.2 mm, the lowest variability being produced by the more experienced observer. CAD variability could be as low as 0.3 mm, showing that it can perform better than human observers. However, when used in nonoptimal conditions (streak artefacts from contrast in the superior vena cava or weak lumen enhancement), CAD has a variability that can be as high as 0.9 mm, reaching variability of semi-manual measurements. Furthermore, there were significant differences between both observers for maximal and minimal diameter measurements (p<0.001). There was also a significant difference between the first observer and CAD for maximal diameter measurements with the former underestimating the diameter compared to the latter (p<0.001). As for minimal diameters, they were higher when measured by the second observer than when measured by CAD (p<0.001). Neither the difference of mean minimal diameter between the first observer and CAD nor the difference of mean maximal diameter between the second observer and CAD was significant (p=0.20 and 0.06, respectively). Conclusion: CAD algorithms can lessen the variability of diameter measurements in the follow-up of ascending aorta aneurysms. Nevertheless, in non-optimal conditions, it may be necessary to correct manually the measurements. Improvements of the algorithms will help to avoid such a situation.

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The dissertation investigates some relevant metaphysical issues arising in the context of spacetime theories. In particular, the inquiry focuses on general relativity and canonical quantum gravity. A formal definition of spacetime theory is proposed and, against this framework, an analysis of the notions of general covariance, symmetry and background independence is performed. It is argued that many conceptual issues in general relativity and canonical quantum gravity derive from putting excessive emphasis on general covariance as an ontological prin-ciple. An original metaphysical position grounded in scientific essential- ism and causal realism (weak essentialism) is developed and defended. It is argued that, in the context of general relativity, weak essentialism supports spacetime substantivalism. It is also shown that weak essentialism escapes arguments from metaphysical underdetermination by positing a particular kind of causation, dubbed geometric. The proposed interpretive framework is then applied to Bohmian mechanics, pointing out that weak essentialism nicely fits into this theory. In the end, a possible Bohmian implementation of loop quantum gravity is considered, and such a Bohmian approach is interpreted in a geometric causal fashion. Under this interpretation, Bohmian loop quantum gravity straightforwardly commits us to an ontology of elementary extensions of space whose evolution is described by a non-local law. The causal mechanism underlying this evolution clarifies many conceptual issues related to the emergence of classical spacetime from the quantum regime. Although there is as yet no fully worked out physical theory of quantum gravity, it is argued that the proposed approach sets up a standard that proposals for a serious ontology in this field should meet.

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BACKGROUND: Male carriers of the FMR1 premutation are at risk of developing the fragile X-associated tremor/ataxia syndrome (FXTAS), a newly recognised and largely under-diagnosed late onset neurodegenerative disorder. Patients affected with FXTAS primarily present with cerebellar ataxia and intention tremor. Cognitive decline has also been associated with the premutation, but the lack of data on its penetrance is a growing concern for clinicians who provide genetic counselling. METHODS: The Mattis Dementia Rating Scale (MDRS) was administered in a double blind fashion to 74 men aged 50 years or more recruited from fragile X families (35 premutation carriers and 39 intrafamilial controls) regardless of their clinical manifestation. Based on previous publications, marked cognitive impairment was defined by a score <or=123 on the MDRS. RESULTS: Both logistic and survival models confirmed that in addition to age and education level, premutation size plays a significant (p<0.01 and p<0.03 for logistic and survival model, respectively) role in cognitive impairment. The estimated penetrance of marked cognitive impairment in our sample (adjusted for the mean age 63.4 years and mean education level 9.7 years) for midsize/large (70-200 CGG) and small (55-69 CGG) premutation alleles was 33.3% (relative risk (RR) 6.5; p = 0.01) and 5.9% (RR 1.15; p = 0.9) respectively. Penetrance in the control group was 5.1%. CONCLUSIONS: Male carriers of midsize to large premutation alleles had a sixfold increased risk of developing cognitive decline and the risk increases with allele size. In addition, it was observed that cognitive impairment may precede motor symptoms. These data provide guidance for genetic counselling although larger samples are required to refine these estimates.

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Taking as a starting point the seeming inconsistency of late-medieval romances notoriously 'run wild' (verwildert), this article is concerned with the description of an abstract form of narrative coherence that is based on the notion of the diagrammatic. In a first section, this concept is illustrated in a simplified manner by an analysis of Boccaccio's Decameron based on two levels of spatial structure: that of the autograph Berlin manuscript (Codex Hamilton 90) and that of the recipient's mental visualisation of the relations between the frame and the tales of the work. It is argued that the connectivity of the work as a whole depends on the perception of those two spatial representations of the plot. A second section develops this concept in a more theoretical fashion, drawing on Charles Sanders Peirce's notion of diagrammatic reasoning as a way of perceiving relations through mental and material topological representations. Correspondingly, a view of narrative is proposed that does not depend on the traditional perspective of temporal sequence but emphasizes the spatial structure of literary narrative. It is argued that these conditions form the primary ontological mode of narrative, whereas the temporal development of a story is an aesthetic illusion that has been specifically stimulated by the narrative conventions of approximately the past three centuries and must thus be considered a secondary effect. To conclude, an interpretation in miniature of an aspect of Heinrich von Neustadt's Apollonius von Tyrland that seems to have 'run wild' is undertaken from a diagrammatic perspective.

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OBJECTIVE: To compare three spin-echo sequences, transverse T1-weighted (T1WI), transverse fat-saturated (FS) T2-weighted (T2WI), and transverse gadolinium-enhanced (Gd) FS T1WI, for the visualisation of normal and abnormal finger A2 pulley with magnetic resonance (MR) imaging at 3 tesla (T). MATERIALS AND METHODS: Sixty-three fingers from 21 patients were consecutively investigated. Two musculoskeletal radiologists retrospectively compared all sequences to assess the visibility of normal and abnormal A2 pulleys and the presence of motion or ghost artefacts. RESULTS: Normal and abnormal A2 pulleys were visible in 94% (59/63) and 95% (60/63) on T1WI sequences, in 63% (40/63) and 60% (38/63) on FS T2WI sequences, and in 87% (55/63) and 73% (46/63) on Gd FS T1WI sequences when read by the first and second observer, respectively. Motion and ghost artefacts were higher on FS T2WI sequences. Seven among eight abnormal A2 pulleys were detected, and were best depicted with Gd FS T1WI sequences in 71% (5/7) and 86% (6/7) by the first and the second observer, respectively. CONCLUSION: In 3-T MRI, the comparison between transverse T1WI, FS T2WI, and Gd FS T1WI sequences shows that transverse T1WI allows excellent depiction of the A2 pulley, that FS T2WI suffers from a higher rate of motion and ghost artefacts, and transverse Gd FS T1WI is the best sequence for the depiction of abnormal A2 pulley.

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The large spatial inhomogeneity in transmit B, field (B-1(+)) observable in human MR images at hi h static magnetic fields (B-0) severely impairs image quality. To overcome this effect in brain T-1-weighted images the, MPRAGE sequence was modified to generate two different images at different inversion times MP2RAGE By combining the two images in a novel fashion, it was possible to create T-1-weigthed images where the result image was free of proton density contrast, T-2* contrast, reception bias field, and, to first order transmit field inhomogeneity. MP2RAGE sequence parameters were optimized using Bloch equations to maximize contrast-to-noise ratio per unit of time between brain tissues and minimize the effect of B-1(+) variations through space. Images of high anatomical quality and excellent brain tissue differentiation suitable for applications such as segmentation and voxel-based morphometry were obtained at 3 and 7 T. From such T-1-weighted images, acquired within 12 min, high-resolution 3D T-1 maps were routinely calculated at 7 T with sub-millimeter voxel resolution (0.65-0.85 mm isotropic). T-1 maps were validated in phantom experiments. In humans, the T, values obtained at 7 T were 1.15 +/- 0.06 s for white matter (WM) and 1.92 +/- 0.16 s for grey matter (GM), in good agreement with literature values obtained at lower spatial resolution. At 3 T, where whole-brain acquisitions with 1 mm isotropic voxels were acquired in 8 min the T-1 values obtained (0.81 +/- 0.03 S for WM and 1.35 +/- 0.05 for GM) were once again found to be in very good agreement with values in the literature. (C) 2009 Elsevier Inc. All rights reserved.

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The aim of the present work was to find out whether NPY synthesized in human adrenal chromaffin cells controls in an autocrine/paracrine fashion the release of catecholamines by these cells. Accordingly, the constitutive and regulated release of both NPY and catecholamines was measured simultaneously in cultured human chromaffin cells. In addition, by using both RT-PCR and a combination of specific agonists and antagonists, we characterized the expression of NPY receptors on these cells as well as their pharmacology. Our results were as follows. 1) Human chromaffin cells constitutively secrete NPY. 2) Nicotine elicits a rapid increase in the release of both catecholamines and NPY; this release of NPY is more sustained than that of catecholamines. 3) RT-PCR shows expression of Y1, Y2, Y4, and Y5 receptor mRNA by chromaffin cells; these receptors are functional, as various receptor specific agonists elicit an increase in intracellular calcium. 4) Peptide YY, in contrast to NPY, is not able to stimulate the release of catecholamines. This finding was corroborated by the observation that no receptor-specific antagonists were able to reduce constitutive catecholamine release, whereas an NPY-immunoneutralizing antibody markedly attenuated the secretion. Taken together, these data suggest that NPY originating from the adrenal medulla locally enhances the secretion of catecholamines, presumably by acting via the putative y3 receptor.

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La majorité des organelles d'une cellule adaptent leur nombre et leur taille pendant les processus de division cellulaire, de trafic vésiculaire ou suite à des changements environnementaux par des processus de fusion et de fragmentation membranaires. Ceci est valable notamment pour le golgi, les mitochondries, les péroxisomes et les lysosomes. La vacuole est le compartiment terminal de la voie endocytaire dans la levure Saccharomyces cerevisiae\ elle correspond aux lysosomes des cellules mammifères. Suite à un choc hyperosmotique, la vacuole se fragmente en plusieurs petites vésicules. Durant ce projet, cette fragmentation a été étudiée en utilisant la technique de microscopie confocale in vivo. J'ai observé que la division de la vacuole se produit d'une façon asymétrique. La première minute après le choc osmotique, les vacuoles rétrécissent et forment des longues invaginations tubulaires. Cette phase est dépendante de la protéine Vps1, un membre de la famille des protéines apparentées à la dynamine, ainsi que d'un gradient transmembranaire de protons. Pendant les 10-15 minutes qui suivent, des vésicules se détachent dans les régions où l'on observe les invaginations pendant la phase initiale. Cette deuxième phase qui mène à la fission des nouveaux compartiments vacuolaires dépend de la production du lipide PI(3,5)P2 par la protéine Fab1. J'ai établi la suite des événements du processus de fragmentation des vacuoles et propose la possibilité d'un rôle régulateur de la protéine kinase cycline-dépendante Pho85.¦En outre, j'ai tenté d'éclaircir plus spécifiquement le rôle de Vps1 pendant la fusion et fission des vacuoles. J'ai trouvé que tous les deux processus sont dépendants de l'activité GTPase de cette protéine. De plus l'association avec la membrane vacuolaire paraît régulée par le cycle d'hydrolyse du GTP. Vps1 peut lier la membrane sans la présence d'un autre facteur protéinique, ce qui permet de conclure à une interaction directe avec des lipides de la membrane. Cette interaction est au moins partiellement effectuée par le domaine GTPase, ce qui est une nouveauté pour un membre de cette famille de protéines. Une deuxième partie de Vps1, nommée insert B, est impliquée dans la liaison à la vacuole, soit par interaction directe avec la membrane, soit par régulation du domaine GTPase. En assumant que Vps1 détienne deux régions capables de liaison aux membranes, je conclus qu'elle pourrait fonctionner comme facteur de « tethering » lors de la fusion des vacuoles.¦-¦La cellule contient plusieurs sous-unités, appelées organelles, possédant chacune une fonction spécifique. Dépendant des processus qui s'y déroulent à l'intérieur, un environnement chimique spécifique est requis. Pour maintenir ces différentes conditions, les organelles sont séparées par des membranes. Lors de la division cellulaire ou en adaptation à des changements de milieu, les organelles doivent être capables de modifier leur morphologie. Cette adaptation a souvent lieu par fusion ou division des organelles. Le même principe est valable pour la vacuole dans la levure. La vacuole est une organelle qui sert principalement au stockage des aliments et à la dégradation des différents composants cellulaires. Alors que la fusion des vacuoles est un processus déjà bien décrit, la fragmentation des vacuoles a jusqu'ici été peu étudiée. Elle peut être induit par un choc osmotique: à cause de la concentration de sel élevé dans le milieu, le cytosol de la levure perd de l'eau. Par un flux d'eau de la vacuole au cytosol, la cellule est capable d'équilibrer celui-ci. Quand la vacuole perd du volume, elle doit réadapter le rapport entre surface membranaire et volume, ce qui se fait efficacement par une fragmentation d'une grande vacuole en plusieurs petites vésicules. Comment ce processus se déroule d'un point de vue morphologique n'a pas été décrit jusqu'à présent. En analysant la fragmentation vacuolaire par microscopie, j'ai trouvé que celle-ci se déroule en deux phases. Pendant la première minute suivant le choc osmotique, les vacuoles rétrécissent et forment des longues invaginations tubulaires. Cette phase dépend de la protéine Vps1, un membre de la famille des protéines apparentées à la dynamine, ainsi que du gradient transmembranaire de protons. Ce gradient s'établit par une pompe membranaire, la V-ATPase, qui transporte des protons dans la vacuole en utilisant l'énergie libérée par hydrolyse d'ATP. Après cette phase initiale, la formation de nouvelles vésicules vacuolaires dépend de la synthèse du lipide PI(3,5)P2.¦Dans la deuxième partie de l'étude, j'ai tenté de décrire comment Vps1 lie la membrane pour effectuer un remodelage de la vacuole. Vps1 est nécessaire pour la fusion et la fragmentation des vacuoles. J'ai découvert que tous les deux processus dépendent de sa capacité d'hydrolyser du GTP. Ainsi l'association avec la membrane est couplée au cycle d'hydrolyse du GTP. Vps1 peut lier la membrane sans la présence d'une autre protéine, et interagit donc très probablement avec les lipides de la membrane. Deux parties différentes de la protéine sont impliquées dans la liaison, dont une, inattendue, le domaine GTPase.¦-¦Numerous organelles undergo membrane fission and fusion events during cell division, vesicular traffic, or in response to changes in environmental conditions. Examples include Golgi (Acharya et al., 1998) mitochondria (Bleazard et al., 1999) peroxisomes (Kuravi et al., 2006) and lysosomes (Ward et al., 1997). In the yeast Saccharomyces cerevisiae the vacuole is the terminal component of the endocytic pathway and corresponds to lysosomes in mammalian cells. Yeast vacuoles fragment into multiple small vesicles in response to a hypertonic shock. This rapid and homogeneous reaction can serve as a model to study the requirements of the fragmentation process. Here, I investigated osmotically induced fragmentation by time-lapse microscopy. I observe that the small fragmentation products originate directly from the large central vacuole by asymmetric scission rather than by consecutive equal divisions and that fragmentation occurs in two distinct phases. During the first minute, vacuoles shrink and generate deep invaginations, leaving behind tubular structures. This phase requires the dynamin-like GTPase Vps1 and the vacuolar proton gradient. In the subsequent 10-15 minutes, vesicles pinch off from the tubular structures in a polarized fashion, directly generating fragmentation products of the final size. This phase depends on the production of phosphatidylinositol- 3,5-bisphosphate by the Fab1 complex. I suggest a possible regulation of vacuole fragmentation by the CDK Pho85. Based on my microscopy study I established a sequential involvement of the different fission factors.¦In addition to the morphological description of vacuole fragmentation I more specifically aimed to shed some light on the role of Vps1 in vacuole fragmentation and fusion. I find that both functions are dependent on the GTPase activity of the protein and that also the membrane association of the dynamin-like protein is coupled to the GTPase cycle. I found that Vps1 has the capacity for direct lipid binding on the vacuole and that this lipid binding is at least partially mediated through residues in the GTPase domain, a complete novelty for a dynamin family member. A second stretch located in the region of insert Β has also membrane-binding activity or regulates the association with the vacuole through the GTPase domain. Under the assumption of two membrane-binding regions I speculate on Vps1 as a possible tethering factor for vacuole fusion.

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This article stems from the statement that dialogical approaches to a study of the self face a double challenge: that of developing a conception of the self that both avoids social reductionism and accounts for the stability of the self. In discussing this double challenge, we identify three much debated issues: (a) To what does the notion of "Alter" exactly refer? (b) How could we conceptualize the fact that Subject-Alter interactions are not only interpersonal but entail larger social entities, in particular institutions? (c)What importance should we attach to the materiality of objects? We discuss these three questions from two standpoints - that of linguistics and that of psychology - and illustrate our theoretical proposals with an analysis of an excerpt taken from a focus-group discussion. In conclusion, we argue that the dialogism of discourse provides us with some clues about the dialogicality of the mind, whereas the latter invites us to develop a theory showing the importance of interactions in the construction of the self, to pay more attention to the transpersonal dimension of the social, and to consider that the material world contributes to the construction of the self.

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A number of recent studies revealed that epigenetic modifications play a central role in the regulation of lipid and of other metabolic pathways such as cholesterol homeostasis, bile acid synthesis, glucose and energy metabolism. Epigenetics refers to aspects of genome functions regulated in a DNA sequence-independent fashion. Chromatin structure is controlled by epigenetic mechanisms through DNA methylation and histone modifications. The main modifications are histone acetylation and deacetylation on specific lysine residues operated by two different classes of enzymes: Histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively. The interaction between these enzymes and histones can activate or repress gene transcription: Histone acetylation opens and activates chromatin, while deacetylation of histones and DNA methylation compact chromatin making it transcriptionally silent. The new evidences on the importance of HDACs in the regulation of lipid and other metabolic pathways will open new perspectives in the comprehension of the pathophysiology of metabolic disorders.

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OBJECTIVE: Data about the consequences of laparoscopic adjustable gastric banding (LAGB) on phospho-calcic and bone metabolism remain scarce. SUBJECTS: We studied a group of 37 obese premenopausal women (age: 24-52 y; mean BMI = 43.7 kg/m2) who underwent LAGB. METHODS: Serum calcium, phosphate, alkaline phosphatase, parathormone (PTH), vitamin D3, serum C-telopeptides, IGFBP-3 and IGF-1 were measured at baseline, 6, 12, 18 and 24 months after surgery. Body composition, bone mineral content (BMC) and density (BMD) were measured using dual-X-ray absorptiometry (DXA) at baseline, 6, 12 and 24 months after surgery. RESULTS: There was no clinically significant decrease of calcemia; PTH remained stable. Serum telopeptides increased by 100% (P < 0.001) and serum IGFBP-3 decreased by 16% (P < 0.001) during the first 6 months, and then stabilized, whereas IGF-1 remained stable over the 2 y. BMC and BMD decreased, especially at the femoral neck; this decrease was significantly correlated with the decrease of waist and hip circumference. CONCLUSIONS: We concluded that there was no evidence of secondary hyperparathyroidism 24 months after LAGB. The observed bone resorption could be linked to the decrease of IGFBP-3, although this decrease could be attributable to other confounding factors. Serum telopeptides seem to be a reliable marker of bone metabolism after gastric banding. DXA must be interpreted cautiously during major weight loss, because of the artefacts caused by the important variation of fat tissue after LAGB.

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The blood pressure (BP), heart rate (HR), and humoral effects of single intravenous (i.v.) doses of the angiotensin-converting enzyme (ACE) inhibitor captopril was investigated in five normotensive healthy volunteers. Each subject received at 1-week intervals a bolus dose of either captopril (1, 5, and 25 mg) or its vehicle. The study was conducted in a single-blind fashion, and the order of treatment phases was randomized. The different doses of captopril had no acute effect on BP and HR. They induced a dose-dependent decrease in plasma ACE activity and plasma angiotensin II levels. The angiotensin-(1-8) octapeptide was isolated by solid-phase extraction and high-performance liquid chromatography (HPLC) prior to radioimmunoassay (RIA). All three doses of captopril reduced circulating angiotensin II levels within 15 min of drug administration. Only with the 25-mg dose was the angiotensin II concentration below the detection limit at 15 min and still significantly reduced 90 min after drug administration. Simultaneous and progressive decreases in plasma aldosterone levels were observed both with ACE inhibition and during vehicle injection, but the relative fall was more pronounced after captopril administration. No adverse reaction was noticed. These results demonstrate that captopril given parenterally blocks the renin-angiotensin system in a dose-dependent manner. Only with the dose of 25 mg was the inhibition of plasma-converting enzyme activity and the reduction of plasma angiotensin II sustained for at least 1 1/2 h.

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We tested the hypothesis that elevation in heart rate (HR) during submaximal exercise in the heat is related, in part, to increased percentage of maximal O(2) uptake (%Vo(2 max)) utilized due to reduced maximal O(2) uptake (Vo(2 max)) measured after exercise under the same thermal conditions. Peak O(2) uptake (Vo(2 peak)), O(2) uptake, and HR during submaximal exercise were measured in 22 male and female runners under four environmental conditions designed to manipulate HR during submaximal exercise and Vo(2 peak). The conditions involved walking for 20 min at approximately 33% of control Vo(2 max) in 25, 35, 40, and 45 degrees C followed immediately by measurement of Vo(2 peak) in the same thermal environment. Vo(2 peak) decreased progressively (3.77 +/- 0.19, 3.61 +/- 0.18, 3.44 +/- 0.17, and 3.13 +/- 0.16 l/min) and HR at the end of the submaximal exercise increased progressively (107 +/- 2, 112 +/- 2, 120 +/- 2, and 137 +/- 2 beats/min) with increasing ambient temperature (T(a)). HR and %Vo(2 peak) increased in an identical fashion with increasing T(a). We conclude that elevation in HR during submaximal exercise in the heat is related, in part, to the increase in %Vo(2 peak) utilized, which is caused by reduced Vo(2 peak) measured during exercise in the heat. At high T(a), the dissociation of HR from %Vo(2 peak) measured after sustained submaximal exercise is less than if Vo(2 max) is assumed to be unchanged during exercise in the heat.

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RESUME L'architecture nucléaire ainsi que l'ultrastructure des microtubules ont été abondamment étudiées par des méthodes cytochimiques utilisant des échantillons fixés chimiquement, enrobés dans des résines ou fixés à basse température. Les échantillons fixés à basse température pouvant aussi avoir été substitués, déshydratés et enrobés dans des résines pour la plupart hydrophiles. Ici, nous avons étendu ces études en utilisant la microscopie électronique effectuée sur des sections hydratées (CEMOVIS) permettant d'observer les échantillons dans un état le plus proche de leur état natif. De plus, nous avons effectué de la tomographie électronique sur des sections hydratées (TOVIS) afin d'obtenir une vision tridimensionnelle de : 1) la périphérie du noyau et de la région périchromatinienne et 2) de la lumière des microtubules. Concernant l'architecture nucléaire Nos observations montrent que le nucléole et la chromatine condensée sont facilement visualisés grâce à la texture spécifique qu'ils arborent. Au contraire, la visualisation de domaines nucléaires importants et spécialement ceux qui contiennent des ribonucléoprotéines, est rendue difficile, à cause du faible contraste qui caractérise l'espace interchromatinien. Ceci est essentiellement dû à la quantité d'information présente dans le volume de la section qui semble être superposée, lorsque observée sur des micrographies en deux dimensions. La tomographie nous a permis de mieux visualiser les différentes régions du noyau. Les mottes de chromatine condensée sont décorées à leur périphérie (région périchromatinienne), par nombre de fibrilles et granules. Des tunnels d'espace interchromatinien sont occasionnellement observés en train de traverser des régions de chromatine condensée favorisant l'accès aux pores nucléaires. Enfin, nous avons pu, au niveau d'un pore unique, observer la plupart des structures caractéristiques du complexe de pore nucléaire. Concernant l'ultrastructure des microtubules: Nous avons démontré que la polarité d'un microtubule observé in situ en section transversale, par CEMOVIS, est directement déduite de l'observation de la chiralité de ses protofilaments. Cette chiralité, a été établie précédemment comme étant liée à la morphologie des sous unités de tubuline. La tomographie électronique effectuée sur des sections hydratées, nous a permis d'observer les microtubules dans leur contexte cellulaire avec une résolution suffisante pour visualiser des détails moléculaires, comme les monomères de tubuline. Ainsi, des molécules n'ayant pas encore été caractérisées, ont été observées dans la lumière des microtubules. Ces observations ont été effectuées autant sur des cellules observées en coupe par CEMOVIS que sur des cellules congelées dans leur totalité par immersion dans un bain d'éthane liquide. Enfin, nous avons montré que les microtubules étaient aussi de formidables objets, permettant une meilleure compréhension des artéfacts de coupe occasionnés lors de la préparation des échantillons par CEMOVIS. Les buts des études qui seront menées â la suite de ce travail seront de 1) essayer de localiser des domaines nucléaires spécifiques par des approches cytochimiques avant la congélation des cellules. 2) Appliquer des méthodes de moyennage afin d'obtenir un modèle tridimensionnel de la structure du complexe de pore nucléaire dans son contexte cellulaire. 3) Utiliser des approches biochimiques afin de déterminer la nature exacte des particules qui se trouvent dans la lumière des microtubules. ABSTRACT Nuclear architecture as well as microtubule ultrastructure have been extensively investigated by means of different methods of ultrastructural cytochemistry using chemically fixed and resin embedded samples or following cryofixation, cryosubstitution and embedding into various, especially partially hydrophilic resins. Here, we extend these studies using cryoelectron microscopy of vitreous sections (CEMOVIS) which allows one to observe the specimen as close as possible to its native state. Furthermore, we applied cryoelectron tomography of vitreous sections (TOVIS) in order to obtain athree-dimensional view of: 1) the nuclear periphery, and of the perichromatin region, and 2) the microtubule lumen. Concerning the nuclear architecture: Our observations show that nucleoli and condensed chromatin are well recognisable due to their specific texture. Conversely, the visualisation of other important nuclear domains, especially those containing ribonucleoproteins, is seriously hampered by a generally low contrast of the interchromatin region. This is mainly due to the plethora of information superposed in the volume of the section observed on two-dimensional micrographs. Cryoelectron tomography allowed us to better visualise nuclear regions. Condensed chromatin clumps are decorated on their periphery, the perichromatin region, by numerous fibrils and granules. Tunnels of interchromatin space can occasionally be found as crossing condensed chromatin regions, thus, allowing the access to nuclear pores. Finally, we were able to use TOVIS to directly distinguish most of the nuclear pore complex structures, at the level of a single pore. Concerning the microtubule ultrastructure: We have demonstrated that the polarity of across-sectioned microtubule observed in situ by CEMOVIS wás directly deducible from the visualisation of the tubulin protofiíaments' chirality. This chirality has been established before as related to the shape. of the tubulin subunits. Cryoelectron tomography allowed us to observe microtubules in their cellular context at a resolution sufficient to resolve molecular details such as their tubulin monomers. In this way, uncharacterized molecules were visualised in the microtubule lumen. These observations were made either on samples prepared by CEMOVIS or plunge freezing of whole cells. Finally, we have shown that microtubules are also relevant objects for the understanding of cutting artefacts, when performing CEMOVIS. The goals of our further studies will be to: 1) try to speciifically target different nuclear domains by cytochemical approaches in situ, prior to cryofixation. 2) Apply averaging methods in order to obtain a three-dimensional model of the nuclear pore complex at work, in its cellular context. 3) Use biochemical analysis combined in a second time to immunocytochemical approaches, to determine the exact nature of the microtubule's luminal particles.

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We build a theoretical framework that allows for endogenous conflict behaviour (i.e., fighting efforts) and for endogenous natural resource exploitation (i.e., speed, ownership, and investments). While depletion is spread in a balanced Hotelling fashion during peace, the presence of conflict creates incentives for rapacious extraction, as this lowers the stakes of future contest. This voracious extraction depresses total oil revenue, especially if world oil demand is relatively elastic and the government's weapon advantage is weak. Some of these political distortions can be overcome by bribing rebels or by government investment in weapons. The shadow of conflict can also make less efficient nationalized oil extraction more attractive than private extraction, as insecure property rights create a holdup problem for the private firm and lead to a lower license fee. Furthermore, the government fights less intensely than the rebels under private exploitation, which leads to more government turnover. Without credible commitment to future fighting efforts, private oil depletion is only lucrative if the government's non-oil office rents are large and weaponry powerful, which guarantees the government a stronger grip on office and makes the holdup problem less severe.