The geometries of the sandstone-hosted calcite concretions of the Jurassic Valtos Sandstone Formation and Bearreraig Sandstone Formation are described and related to the processes operating during concretion growth. As concretionary bodies analogous to those studied form potential permeability barriers within some North Sea petroleum reservoirs; the relationship between the growth processes and permeability barrier formation is examined. The growth times for model spherical concretions are calculated for the complex carbonate-water system. Two growth processes are modelled, solute transport and surface reaction. Growth times for a 1m diameter concretion forming under geologically reasonable conditions are predicted to be 22.3Ma, which reduces to 8.8Ma in porewaters flowing at 1m/year. The depth of formation of the concretions is assessed, through an examination of depth dependent properties of both the host-sediment and the calcite cement, and is found to be less than 500m. Concretion formation preceeded the Paleocene igneous activity which affected the Hebrides. The majority of the concretions examined formed at burial depths which were too great to allow effective contact between the concretions and seawater. The major source of carbonate was the dissolution of aragonitic shell material from within the host sandbody. The nature of the porewaters from which the concretions formed is assessed. The majority were meteoric in origin, though some marine influence is noted. The minor element contents of the cements cannot be used to calculate porewater compositions, as disequilibrium between the porewaters and the cements can be demonstrated. A model is proposed to account for the minor element patterns. Crystal breeding can be demonstrated to have occurred during concretion growth. A hypothesis is presented to explain the petrographic features of a typical Valtos Sandstone Formation concretion.

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No description available.

Les cellules épithéliales forment un tissu de cellules étroitement juxtaposées qui assure une barrière physique et chimique entre les compartiments internes et externes du corps. L’intégrité de ces tissus est donc essentielle. Au cours du développement et de la vie adulte, le tissu doit grandir ou se régénérer, ce qui implique de nombreuses divisions cellulaires. La dernière étape de la division, la cytodiérèse, met en jeu la formation d’un anneau contractile qui, en se fermant, va séparer les cellules sœurs. Une fois complètement fermé, il donne naissance au midbody, juste sous le niveau des jonctions adhérentes, au sein des jonctions septées, chez la drosophile. L’ultime étape, l’abscission, permet la séparation physique définitive et l’isolation cytoplasmique des cellules sœurs. Si de nombreuses études ont décrit ces processus dans les cellules isolées, peu de choses sont connues quant à la cytodiérèse des cellules épithéliales. Ce travail de thèse a permis de mettre en évidence que malgré le recrutement de tous les effecteurs et régulateurs de l’abscission, celle-ci est retardée dans les cellules épithéliales. Des expériences de photo-conversion de KAEDE ont montré que l’abscission est liée à l’entrée en mitose des cellules épithéliales. La question de l’intégrité du tissu et notamment de la barrière de perméabilité a ensuite été investigué. Nous avons montré que les cellules voisines formaient des protrusions de membrane restant connectées au midbody tout au long de sa lente migration vers le pôle basal des cellules. Les expériences de FRAP menées sur les jonctions bicellulaires et tri-cellulaires des jonctions septées ont permis de montrer que celles-ci se formaient juste sous les jonctions adhérentes et toujours au-dessus du midbody, participant ainsi à la migration de ce dernier vers le pôle basal. Les contacts maintenus avec les voisines ainsi que l’assemblage polarisé des jonctions septées participent au maintien de l’intégrité du tissu au cours des divisions de cellules épithéliales. / Epithelial cells are closely juxtaposed to form a tissue playing a physical and chemical barrier between external and internal body compartments. Thus, tissue integrity is essential. During development and adult life, epithelia has to growth and regenerate meaning a lot of divisions. At the end of cell division, cytokinesis occurs, implying the formation of a contractile ring which contracts to separate daughter cells. In Drosophila, once totally closed, the contractile ring gives rise to the midbody, just below adherens junctions, in the septate junctions layer. Last step of cytokinesis, abscission, permits the final cut and the cytoplasmic isolation of daughter cells. If cytokinesis is well described in isolated cells, little is known about epithelial cells cytokinesis. This work shows that whereas all abscission regulators and effectors are recruited, abscission is delayed in epithelial cells. KAEDE photo-conversion assays show that abscission is linked to epithelial cells mitosis entry. Then we investigate how permeability barrier is maintained during cell division. We show that neighboring cells present finger-like protrusions contacting the midbody all along the midbody is moving basally across septate junctions. FRAP experiments on bicellular and tricellular septate junctions show that they form just below adherens junctions and always above the midbody, leading to its basal migration. Contacts maintained with neighbors and polarized assembly of septate junctions participate to the maintenance of tissue integrity throughout epithelial cells divisions.

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