L'homme et l'animal dans le bassin du lac Tchad

Les Mafa, cultivateurs de mil des Monts Mandara, élèvent aussi quelques poules et du petit bétail. Le bovin, élevé dans un but essentiellement rituel, était sacrifié lors de la fête du taureau, maray. Les vaches, rares sur les pentes en terrasses du pays mafa, ne faisaient pas partie du système social : elles n'entrent pas dans la compensation matrimoniale et ne figurent pas dans les histoires qui opposent animaux domestiques et animaux sauvages. Le taureau "sacré" au contraire était sacrifié pour assurer la perpétuation des clans patrilinéaires. Cependant, de façon surprenante, en 1997 des étudiants mentionnent les vaches comme si elles avaient toujours été partie intégrante du système économique. Il s'agit soit de différences régionales, soit d'investissements nouveaux des Mafa grâce aux moyens apportés par des salaires extérieurs. La place nouvelle des vaches dans la société mafa est examinées en relation avec celle du taureau "sacré" du passé. (Résumé d'auteur

Models of self-peptide sampling by developing T cells identify candidate mechanisms of thymic selection

Conventional and regulatory T cells develop in the thymus where they are exposed to samples of self-peptide MHC (pMHC) ligands. This probabilistic process selects for cells within a range of responsiveness that allows the detection of foreign antigen without excessive responses to self. Regulatory T cells are thought to lie at the higher end of the spectrum of acceptable self-reactivity and play a crucial role in the control of autoimmunity and tolerance to innocuous antigens. While many studies have elucidated key elements influencing lineage commitment, we still lack a full understanding of how thymocytes integrate signals obtained by sampling self-peptides to make fate decisions. To address this problem, we apply stochastic models of signal integration by T cells to data from a study quantifying the development of the two lineages using controllable levels of agonist peptide in the thymus. We find two models are able to explain the observations; one in which T cells continually re-assess fate decisions on the basis of multiple summed proximal signals from TCR-pMHC interactions; and another in which TCR sensitivity is modulated over time, such that contact with the same pMHC ligand may lead to divergent outcomes at different stages of development. Neither model requires that T and T are differentially susceptible to deletion or that the two lineages need qualitatively different signals for development, as have been proposed. We find additional support for the variable-sensitivity model, which is able to explain apparently paradoxical observations regarding the effect of partial and strong agonists on T and T development

Process for the preparation of catalytically active cross-linked metal silicate

Highly active and selective hydroisomerization catalysts are prepared by heating to 300 DEG -450 DEG C. at subatmospheric pressure, a mixture of nickel synthetic mica montmorillonite (Ni-SMM) with a hydroxy aluminum polymeric solution. The resulting pillared Ni-SMM catalyst, preferably Pd-loaded, is especially useful in hydroisomerizing C4-C7 paraffins

Water-promoted ammonia oxidation by a platinum amine complex in zeolite HZSM-5 catalyst

In this study it was found that [Pt(NH3)(4)]HZSM-5 is an active catalyst for the oxidation of ammonia at low temperature that, in contrast with other catalysts, becomes more active in the presence of water. Furthermore, the selectivity to nitrogen was found to increase when water is presen

The role of carbon-hydrogen bond breaking in the epoxidation of ethylene

No abstract

Modeling the sorption dynamics of NaH using a reactive force field

We have parametrized a reactive force field for NaH, ReaxFFNaH, against a training set of ab initio derived data. To ascertain that ReaxFFNaH is properly parametrized, a comparison between ab initio heats of formation of small representative NaH clusters with ReaxFFNaH was done. The results and trend of ReaxFFNaH are found to be consistent with ab initio values. Further validation includes comparing the equations of state of condensed phases of Na and NaH as calculated from ab initio and ReaxFFNaH. There is a good match between the two results, showing that ReaxFFNaH is correctly parametrized by the ab initio training set. ReaxFFNaH has been used to study the dynamics of hydrogen desorption in NaH particles. We find that ReaxFFNaH properly describes the surface molecular hydrogen charge transfer during the abstraction process. Results on heat of desorption versus cluster size shows that there is a strong dependence on the heat of desorption on the particle size, which implies that nanostructuring enhances desorption process. To gain more insight into the structural transformations of NaH during thermal decomposition, we performed a heating run in a molecular dynamics simulation. These runs exhibit a series of drops in potential energy, associated with cluster fragmentation and desorption of molecular hydrogen. This is consistent with experimental evidence that NaH dissociates at its melting point into smaller fragments