"The effect of bacterial surface biopolymers on bacterial adhesion to surfaces was studied through experiments and modeling. Atomic Force Microscopy (AFM) provided the tool to measure the interaction forces between different bacterial cells and silicon nitride tips under different chemical conditions at a nanoscopic level. Two bacterial strains were considered: Pseudomonas putida KT2442 and Escherichia coli K-12 JM109. This study addressed the following issues: 1) the effect of solution ionic strength and solvent polarity on adhesion between Pseudomonas putida KT2442 and the silicon nitride AFM tip, 2) role of heterogeneity of bacterial surface biopolymers on bacterial adhesion, 3) role of lipopolysaccharides (LPS) on adhesion at three different scales: continuous, batch, and nanoscale, and 4) nature of interactions between E. coli JM109 and a model surface (silicon nitride tip). To address the first issue, formamide, water, and methanol were used to investigate the effect of polarity on surface characteristics of biopolymers on the bacterial surface while a range of salt concentrations between that of water to 1 M KCl were used to study the effect of ionic strength. The adhesion increased with decreasing polarity of the solvent, indicating that the polymers on the bacterial surface are hydrophilic in nature. The adhesion was slightly affected by ionic strength variations up to a concentration of 0.1 M KCl; this may have been due to the fact that the ionic concentration in the solution did not counterbalance the ionic concentration in the biopolymer brush on the bacterial surface. However, a dramatic increase in the adhesion magnitude was observed when the salt concentration increased above 0.1 M KCl. This transition in adhesion with ionic strength from a low to high value induced a transition in the elasticity of the bacterial surface biopolymers. The biopolymer brush layer did change from rigid to soft with increasing the ionic strength. The elasticity was quantified mainly by the use of the freely jointed chain (FJC) model. Our interest in investigating the role of heterogeneity on adhesion developed from the results of the first study. The bacterial surface polymers were thought to be different in their chemical and physical nature since they were found to span a range of segment lengths. Analyzing the adhesion forces for P. putida KT2442 showed that the bacterial surface is heterogeneous. The heterogeneity was evident on the same cell surface and between different cells from the same population. To resolve the third issue, approximately, 80% of the surface LPS of E. coli K-12 JM109 were removed by treating the cells with 100 mM ethylenediaminetetraacetic acid (EDTA). The effect of LPS removal on the adhesion of the cells to the silicon nitride tip was studied in water and phosphate buffered silane (PBS). The adhesion results from the AFM experiments were compared to batch retention experiments with glass as the substratum and column attachment experiments with columns packed with quartz sand. LPS controlled bacterial adhesion to the different surfaces in the study at three scales: batch, continuous, and nano-scale. Finally, the nature of interactions between E. coli JM109 and a model surface (silicon nitride tip) were investigated in solvents of varying polarity (formamide, water, and methanol). The Young’s modulus of elasticity for the bacterial surface was estimated by fitting of the Hertzian model to the force-indentation curves. Young’s modulus values increased as the solvent polarity decreased, indicating a stiffer bacterial surface in lower polarity solvents. The average adhesion force in each solvent was negatively correlated with the dielectric constant of the solvent, suggesting hydrophilic biopolymers. Specific and non-specific interaction forces between the AFM tip and the biopolymers were further characterized by applying a Poisson statistical analysis to the discrete adhesion data. The specific and non-specific interaction forces were the highest in methanol (-4 and -1.48 nN respectively). These values are in accordance with the high adhesion magnitude values measured with AFM in methanol. The results of my different studies emphasized the important role of AFM in studying biological interactions to different surfaces and in characterizing bacterial surface biopolymers."

Les machines électriques à aimants, appréciées pour leurs densités énergétiques volumique et massique, équipent la majorité des véhicules électrifiés. Par contre, dans la zone à puissance constante d’un système de propulsion, les commander à pertes maîtrisées, cela en démagnétisant les aimants mais de façon réversible, reste une gageure, particulièrement sous environnement thermique changeant. Les solutions simples pour se prémunir d’une démagnétisation irréversible sont coûteuses: surdimensionner ou ajouter une terre très très rare (Dysprosium).Cette thèse propose d’ajouter à l’approche de la démagnétisation réversible universellement utilisée, celle d’une boite de vitesse électromagnétique. Pour cela on conçoit une machine pentaphasée à aimants.Passer de trois à cinq phases permet alors d’augmenter les paramètres de réglage de l’alimentation électrique et, moyennant approximation, de disposer de deux machines fictives à p et 3p paires de pôles, chacune pouvant contribuer de façon équivalente à la production du couple. Pratiquement, l’utilisation optimale de ces deux machines permet de reconstituer la fonction de boite de vitesse. / Permanent Magnet electrical machines , appreciated for their high power density, equip the majority of the electrified vehicles. However, controlling these machines, in the constant power range of the propulsion system while mastering the losses, with PM reversible demagnetization remains a challenge especially under varying thermal environnement. Proposed solutions aim for protecting PM from irreversible demagnetization are costly: oversizing or using very rare earth PM (Dysprosium).In this thesis, we proposed to add on the reversible demagnetization approach universally used , that of a electromagnetic gearbox. Thus, we design a five phase PM machine.Increasing the phase number from three to five, increases the adjustment parameter of the electrical supply, and allows to have two fictitious machines with p and 3 p poles. Each fictitious machine contributes equivalently in producing torque. Practically, the optimal use of these two machines leads to reconstructing the gearbox function.

Devido à demanda de aumento de produtividade na indústria, a utilização de materiais mais leves, busca por processos de reparos com melhor custo, em conjunto com o avanço das tecnologias, evolução dos processos de soldagem, e esses fatores aliados aos benefícios de uma maior taxa de fusão, melhor controle da penetração, faz com que o processo MIG/MAG com polaridade variável apresente consideráveis avanços tecnológicos nos dias de hoje. Porém, a seleção dos parâmetros da curva típica de corrente do processo (composta por corrente de pico, corrente de base positiva e corrente negativa), é ainda um desafio. Buscando um melhor entendimento da influência da polaridade negativa no processo de soldagem, em especial para aplicações de revestimento, este trabalho tem como objetivo analisar os parâmetros da polaridade negativa individualmente em relação a geometria do cordão de solda, utilizando aço ao carbono como metal base e aço inoxidável como metal de adição. O planejamento e execução do experimento foi realizado com base na metodologia de projeto de experimentos Box-Behnken, variando três fatores: tempo de corrente negativa, intensidade de corrente negativa e tempo de corrente de base positiva, em três níveis cada um. Primeiramente alcançou-se a minimização da diluição, onde o tempo de corrente negativa apresentou efeito significativo no resultado final, sendo seu nível máximo (15 ms) o responsável pelos menores valores de diluição, aproximadamente 12%, recomendado em soldagem para revestimento. Posteriormente, foi avaliado o efeito dos parâmetros em relação à penetração, altura e largura do cordão de solda, sendo o tempo de corrente negativa em 15 ms e corrente negativa em -150 A os níveis que apresentaram melhores resultados. Após, os parâmetros da polaridade negativa utilizados foram comparados com os termos proporcionais sugeridos pela literatura (%EN). E, por fim, avaliou-se uma forma de curva de corrente sem a corrente de base positiva, onde foi concluído que não há significância das diferentes formas de curva de corrente sobre os resultados na penetração do cordão de solda, assim como a necessidade da corrente de base na estabilidade do arco elétrico do processo. / Due to the demand to increase productivity in the industry, use lighter materials, search for repair processes with better cost, in combination with advancement of technologies, evolution of welding processes, connected with the benefits of a higher melting rate and better control of the process penetration, it was possible to verify advances in the MIG / MAG Variable Polarity welding technique. However, the selection of the parameters of the typical process current curve (composed by peak current, positive current and negative current), is still a challenge today. The aim of this work is to analyze the negative polarity parameters individually in relation to the weld bead geometry, using carbon steel as base metal and stainless steel as addition metal. The planning and execution of the experiment was performed based on the Box-Behnken design methodology, varying three factors: negative current time, negative current intensity and positive base current time, at three levels each one. Firstly, dilution minimization was achieved, which showed that the negative current time had a strong influence on the final result, and the maximum level (15 ms) was responsible for the lowest dilution values, nearby 12%, ideal for welding cladding. Then, the parameters were evaluated in relation to penetration, height and width of the weld bead, which for welding cladding, the negative current time 15 ms and negative current -150 A presented better results. Next, the negative polarity parameters were compared with the proportional terms suggested by the literature (%EN). Finally, a form of current curve without positive base current was evaluated, where it can be concluded that the current curve form does not have significance on the penetration results, as well as the necessity of the base current in the arc electric stability.

La polarité apico-basale des cellules épithéliales est maintenue au niveau cellulaire par l’implication de plusieurs complexes protéiques, PAR, SCRIBBLE et CRUMBS, et par l’intégrité des jonctions serrées et adhérentes. La polarité apico-basale est essentielle au bon déroulement des processus de division, d’apoptose et de migration cellulaire, et de nombreuses études montrent que les perturbations de la polarité interviennent dans la progression tumorale. Notre travail a porté sur la recherche d’anomalies biologiques modifiant la polarité apico basale et l’adhésion cellulaire dans les carcinomes mammaires. Nous avons choisi d’étudier deux types spéciaux de carcinomes mammaires infiltrant, dont le phénotype suggère l’existence d’altérations spécifiques de ces processus biologiques: le type lobulaire (ILC) et le type micropapillaire (IMPC). Ces études ont été menées par des analyses combinées phénotypiques, transcriptomiques et génomiques comparatives, par rapport à un groupe de carcinomes mammaires sans autre spécificité (IC-NST).Les carcinomes lobulaires sont caractérisés par les altérations du complexe E cadhérine/ caténine et leur capacité de dissémination métastatique. Nous avons montré qu’ils présentent également une sous-expression de la protéine du complexe PAR, PAR-3, associée à des altérations des gènes impliqués dans l’adhésion cellulaire (ADAM12, LOXL2), l’interaction cellule-matrice extracellulaire (MMP11, COL11A1, etc…) et l’invasion (ACTR2, PAK1). Des défauts quantitatifs des constituants de la matrice extracellulaire ont également été mis en évidence. Nous avons, ainsi, pu établir une signature transcriptomique spécifique de cette entité tumorale, en accord avec les caractéristiques morphologiques observées. Les carcinomes micropapillaires présentent une polarité anormale caractérisée par des marqueurs apicaux positionnés vers la matrice extracellulaire ou absents, la perte de l’orientation de la protéine golgienne GM130, des anomalies des jonctions serrées (occludine) et des protéines du complexe PAR (CDC42 et aPKC). Au niveau génomique, nous avons mis en évidence des mutations somatiques de gènes impliqués dans la régulation de la polarité (DNAH9, FOXO3) et de la ciliogenèse (BBS9, BBS12, SEC63), l’organisation du cytosquelette (HSP90B1, UBR4, ZFYVE26) et la motilité (FMN2). Au niveau transcriptomique, une nette perturbation des gènes impliqués dans l’adhésion cellule cellule, cellule-matrice extracellulaire et l’angiogenèse est observée. Les IMPC présentent également une surexpression spécifique d’une protéine du complexe CRUMBS, LIN7A. Nous avons établi un modèle in vitro et avons montré que LIN7A est un puissant perturbateur de la polarité apico basale. Sa surexpression dans la lignée MCF10A cultivée en 3D induit la formation d’acini multilobés, à forte capacité proliférative et sans lumière centrale. Cette absence de lumière centrale est due à une inhibition de l’apoptose. Les cellules MCF10A-LIN7A présentent également une capacité accrue de croissance en suspension, témoignant d’une résistance à l’anoïkis. Cette résistance est due, entre autre, à une diminution de la phosphorylation de la protéine p38. Cette description approfondie des altérations biologiques de types spéciaux de carcinomes mammaires doit permettre à moyen terme de proposer une prise en charge plus spécifique des patientes, grâce à l’identification de nouvelles possibilités thérapeutiques ou à une stratégie de désescalade thérapeutique. / Apicobasal polarity is maintained by the combined action of several protein complexes – PAR, SCRIBBLE and CRUMBS – together with the structural organisation of adherent and tight junctions. Apicobasal polarity is important for the regulation of cell division, apoptosis and cell migration, and several studies show that disruption of cell polarity is involved in tumour progression. Our work focused on the biological mechanisms responsible for the altered apicobasal polarity and cell adhesion observed in breast cancers. To do so, we studied two types of breast carcinomas – invasive lobular carcinoma (ILC) and invasive micropapillary carcinoma (IMPC) – whose morphology suggests specific alterations of these cell processes. Our approach combined genomic, transcriptomic and phenotypical comparative analyses, using a group of invasive carcinomas not special type (IC-NST) as control.Lobular carcinomas are generally characterized by alterations of the E cadherin/ catenin complex and their ability to disseminate. We have shown that they also present a downregulation of PAR-3, a protein of the PAR complex, associated with deregulation of genes involved in cell adhesion (ADAM12, LOXL2), cell-extracellular matrix interactions (MMP11, COL11A1, etc…) and invasion (ACTR2, PAK1). Quantitative defects in components of the extracellular matrix were also observed. We have thus been able to establish a transcriptomic signature for this tumour entity, in agreement with the phenotypical observations.Micropapillary carcinomas show an abnormal polarity characterized by the absence of apical markers or their localization at the inverted apical pole, the loss of Golgi protein GM130 correct orientation and abnormal expression or localization of occludin (tight junctions), CDC42 and aPKC(PAR complex proteins). At the genomic level, we have identified somatic mutations in genes involved in polarity (DNAH9, FOXO3) and ciliogenesis regulation (BBS9, BBS12, SEC63), cytoskeleton organisation (HSP90B1, UBR4, ZFYVE26) and motility (FMN2). At the transcriptomic level, we observed deregulation of genes involved in cell-cell, cell-extracellular matrix adhesion and angiogenesis. IMPC also demonstrate the specific overexpression of a protein of the CRUMBS complex, LIN7A. We established an in vitro model and showed that LIN7A is an efficiently modifier of MCF10A’s polarity. Its overexpression in MCF10A cells cultured in 3-D conditions induces the formation of proliferating multi-lobar acini with no central lumen. This absence of a central lumen is due to an inhibition of apoptosis. MCF10A-LIN7A cells also show an increased ability to grow in suspension, indicating resistance to anoikis. This resistance seems to be linked to a decrease of p38protein’s phosphorylation.This detailed description of biological alterations in special types of breast carcinomas will contribute to more specific treatments provided to the patients, through the identification of new therapeutic targets or therapeutic strategies.

Les épithéliums jouent le rôle de barrière physique et chimique chez les Métazoires. Les épithéliums subissent des remodelages pendant l’embryogénèse. La morphogénèse des tissus est dirigée par des déformations cellulaires coordonnées fonctionnant grâce à des réseaux contractiles intracellulaires constitués d’actine et de myosine. Ce réseau d’actomyosine peut être soit pulsatile, soit stable. Un exemple est l’élongation de l’ectoderme ventro-latéral par intercalation cellulaire, le long de l’axe antéro-postérieur (AP) de l’embryon de la Drosophile. Les jonctions parallèles à l’axe dorso-ventral (DV) rétrécissent et forment de manière irréversible de nouvelles jonctions parallèles à l’axe AP. Des pulsations de myosine-II (Myo-II) médio-apicale se déplacent de manière anisotrope vers les jonctions parallèles à l’axe DV. Ceci provoque le rétrécissement graduel des jonctions, rétrécissement stabilisé par une population de Myo-II polarisée dans le plan du tissu et enrichie au niveau de ces jonctions. Les mécanismes cellulaires qui régulent la pulsatilité, la stabilité et la polarité de la Myo-II restent à élucider. Lors de ma thèse, j’ai identifié de nouveaux effecteurs régulant l’activation et la polarité planaire de la voie Rho1-Rok-Myo-II aux niveaux des jonctions. J'ai d'abord caractérisé le rôle de la kinase Misshapen dans l’activation polarisée de la voie Rho1 au niveau des jonctions. Misshapen agit en aval de la signalisation GPCR afin de favoriser l’activation de Rho1 et contrôle la polarisation de cette activation en transmettant l’information des récepteurs Toll. Puis j'ai identifié Pebble comme la RhoGEF régulant Rho1 et l'accumulation de Myo-II aux jonctions. / Epithelial build up strong mechanical and chemical barriers in Metazoans. Epithelia can be dramatically remodeled during embryogenesis. Tissue morphogenesis is driven by coordinated cellular deformations which are powered by intracellular contractile networks constituting actin and Myosin. Actomyosin networks can either be pulsatile or stable. One example is the elongation of the ventral-lateral ectoderm by cell intercalation, along antero-posterior (AP) axis of Drosophila embryo. Junctions parallel to the dorso-ventral (DV) axis shrink and form new junctions along AP axis. Medial apical Myosin-II (Myo-II) pulses flow anisotropically towards junctions aligned in DV axis, resulting in steps of junction shrinkage which are stabilized by a planar-polarized pool of Myo-II enriched at these junctions. Sequential deformation and stabilization drive irreversible tissue deformations akin to a ratchet. The cellular mechanisms that regulate Myo-II pulsatility, stability and polarity remained to be unfurled. During my PhD, I identified new regulators for Rho1-Rok-Myo-II pathway at junctions, and Myo-II planar polarity. On the one hand, I characterized the function of Misshapen kinase in polarized activation of Rho1 pathway at junctions. Misshapen acts downstream GPCR signaling to enhance Rho1 activation, and controls the polarization of this activation by transducing information from Toll receptors. Also, I identified Pebble as RhoGEF regulating Rho1 at junctions and Myo-II accumulation.

Although the mechanisms underlying the divergence of the first cell types in the mouse, the trophectoderm (TE) and the inner cell mass (ICM) have received considerable attention, the upstream signals stimulating their divergence are not well understood. The work presented here examines the roles that morphogenetic factors such as cell adhesion and polarization play in the development of these cell types. I show here that in embryos completely lacking both maternal and zygotic E-cadherin, the normal epithelial morphology of outer cells is disrupted but individual cells still initiate TE and ICM-like fates. A larger proportion of cells than normal expressed TE markers like Cdx2 (a homeodomain containing transcription factor), suggesting that formation of an organized epithelium is not necessary for TE-specific gene expression. Individual cells in such embryos still generate an apical-like domain that correlates with elevated Cdx2 expression. I also show that repolarization can occur in isolated early ICMs from both wild type and Cdx2 mutant embryos, indicating that Cdx2 is not required to initiate polarity. Importantly, I demonstrate a critical role for the Rho-associated kinase ROCK in apical-basal polarization of preimplantation blastomeres. Loss of apical-basal polarization leads to a reduction of Cdx2 expression in outer blastomeres due to activation of Lats1/2 kinase and reduced nuclear Yap1. The influence of polarization upon Lats1/2 kinase is stage-dependent however, as apolar 8-cell blastomeres retain nuclear Yap1. Cell position appears to serve as an additional cue for nuclear localization of Yap and Cdx2 expression from the 8-cell stage to E3.5. Cell polarization plays an additional role in the embryo of maintaining cells in consistently outer or inner positions, thus ensuring that Cdx2 is expressed exclusively in the developing TE. The results of this work demonstrate important links between morphogenesis, cell fate and patterning in the preimplantation embryo. Both cell polarization and cell position act as critical cues to determine gene expression and to pattern this expression within the embryo.

Although the mechanisms underlying the divergence of the first cell types in the mouse, the trophectoderm (TE) and the inner cell mass (ICM) have received considerable attention, the upstream signals stimulating their divergence are not well understood. The work presented here examines the roles that morphogenetic factors such as cell adhesion and polarization play in the development of these cell types. I show here that in embryos completely lacking both maternal and zygotic E-cadherin, the normal epithelial morphology of outer cells is disrupted but individual cells still initiate TE and ICM-like fates. A larger proportion of cells than normal expressed TE markers like Cdx2 (a homeodomain containing transcription factor), suggesting that formation of an organized epithelium is not necessary for TE-specific gene expression. Individual cells in such embryos still generate an apical-like domain that correlates with elevated Cdx2 expression. I also show that repolarization can occur in isolated early ICMs from both wild type and Cdx2 mutant embryos, indicating that Cdx2 is not required to initiate polarity. Importantly, I demonstrate a critical role for the Rho-associated kinase ROCK in apical-basal polarization of preimplantation blastomeres. Loss of apical-basal polarization leads to a reduction of Cdx2 expression in outer blastomeres due to activation of Lats1/2 kinase and reduced nuclear Yap1. The influence of polarization upon Lats1/2 kinase is stage-dependent however, as apolar 8-cell blastomeres retain nuclear Yap1. Cell position appears to serve as an additional cue for nuclear localization of Yap and Cdx2 expression from the 8-cell stage to E3.5. Cell polarization plays an additional role in the embryo of maintaining cells in consistently outer or inner positions, thus ensuring that Cdx2 is expressed exclusively in the developing TE. The results of this work demonstrate important links between morphogenesis, cell fate and patterning in the preimplantation embryo. Both cell polarization and cell position act as critical cues to determine gene expression and to pattern this expression within the embryo.

My dissertation research consists of two themes: (a) the analysis of Middle Jurassic - Early Cretaceous marine magnetic anomalies (M-anomalies) in order to construct a comprehensive geomagnetic polarity timescale and (b) the investigation of the upper oceanic crustal architecture using downhole geophysical logs. These themes were chosen to better understand how remotely-sensed geophysical signals elucidate the formation and evolution of oceanic crust. This revised Pacific-wide MGPTS model shows significant improvement in its reliability, exhibits global applicability, and highlights changes in the paleo-Pacific spreading regime. By integrating Atlantic Manomaly analyses with the new MGPTS model and reviewing previous seismic studies, we shed new light on the causes of a ubiquitously distributed ?Atlantic anomaly smooth zone? where little coherency among M5-M15 anomaly sequence is observed. For the second theme, I analyzed the architecture of 15 m.y. old superfast spreading East Pacific Rise crust drilled at Ocean Drilling Program Hole 1256D in the eastern Pacific. An intact upper oceanic crustal section was penetrated at this site to a depth of 1507 mbsf. In situ crustal architecture was mapped from resistivity imagery (electrofacies by Formation MicroScanner) combined with recovered cores and other logs. Highlights of this research are: (1) most of the extrusive section consists of massive flows and fragmented formations including breccias, which has important implications for the magnetic source layer and pathways of hydrothermal alteration; (2) the dike complex is composed of sheeted-dikes dipping away from the paleo-spreading axis consistent with submersible observations at other sites in the eastern Pacific; (3) the crustal construction processess from ridge axis to abyssal plain during 0-50 kyr time are consistent with previous seismic reflection studies based on the integration of our stratigraphy model with lava flow observations from the southern East Pacific Rise.

The purpose of this study was to establish whether curcumin protects renal proximal tubule cells against ischemic injury, determine whether this postulated cytoprotective effect is mediated through the upregulation of HSP70, and investigate whether the mechanism by which curcumin induces HSP70 expression and confers its protective effect is through activation of the Unfolded Protein Response. LLC-PK1 cells were cultured on collagen-coated filters to mimic conditions of in vivo renal proximal tubule cells and induce cell polarization. Injury with and without curcumin treatment was studied by using chemically-induced ATP-depletion which mimics renal ischemic injury. Cell injury was assessed using a TUNEL assay in order to evaluate DNA cleavage associated with ischemia-induced apoptosis and actin staining used to assess cytoskeletal disruption. Renal ischemic damage was further investigated by determining detachment of the Na-K ATPase from the basolateral membrane, which represents loss of cell polarity. Cells were incubated with curcumin in a dose- and time-response fashion and subsequent levels of HSP70 expression were assessed. Cells were then incubated with AEBSF, an inhibitor of the Unfolded Protein Response (UPR) and HSP70 and BiP/GRP78 (an ER resident chaperone that is upregulated by the UPR) expression levels were evaluated. Results demonstrated that treatment with curcumin during two hours of injury results in significantly less injury-related apoptosis and cytoskeletal disruption compared to control injured cells. It was demonstrated that curcumin induces HSP70 in both a dose- and time-response fashion. Moreover, curcumin treatment resulted in profound stabilization of Na-K ATPase on the basolateral membranes as there was significantly less Na-K ATPase detachment in cells treated with curcumin during two hours of injury compared to control injured cells. Finally, treatment with AEBSF inhibited HSP70 upregulation in curcumin-treated cells as well as inhibiting the GRP78 over-expression otherwise demonstrated in curcumin-treated cells. Protection of proximal tubule cells against renal ischemic injury by curcumin was therefore indicated to be mediated by the activation of the UPR through which HSP70 is upregulated. Curcumins activation of the UPR and induction of HSP70 explains the stabilization of Na-K ATPase on the cytoskeleton and also provides a potential mechanism explaining many of curcumins therapeutic and protective qualities.

The role that signaling molecules play during morphogenesis and their interactions is a field of intense study and the sea urchin represents a facile system to study these aspects of development in the early embryo. In many instances, the S. purpuratus genome contains relatively simple receptor-ligand signaling systems compared to vertebrate counterparts and this provides interesting opportunities to study their diversity of function during the morphogenetic events that shape the embryo. The Eph-Ephrin signaling components are an excellent example of this and they are represented by dozens of members in the vertebrate system with developmental functions that include axon guidance, cell migration and tissue segregation. In contrast, the sea urchin genome contains a single Eph receptor and a single Ephrin ligand and by interacting with different effectors of signal transduction, this simple, bipartite system can fulfill a variety of functional roles during morphogenesis. Studying the function of Eph-Ephrin signaling in the sea urchin embryo, I have revealed two distinct morphogenetic movements in which Eph-Ephrin signaling is necessary; apical constriction of ciliary band cells and pigment cell migration. In both examples, a functionally relevant Ephrin gradient establishes spatial information in the developing tissues, producing a reaction from cells expressing the Eph receptor. In the case of pigment cells, the distribution of migrating cells is affected and in the case of ciliary band cells, apical constriction occurs. The different outcomes of Eph-Ephrin signaling in these two tissues exemplifies signaling components communicating spatial information and initiating morphogenetic programs with outcomes dependent on cellular context. Furthermore, I have identified downstream components of Eph-Ephrin signaling that have necessary functions in both models, illustrating how different cellular programs can be induced by the same signaling iii iv components. My research contributes to understanding fundamental aspects of how complex 3 dimensional tissues arise from the genes and regulatory elements encoded in metazoan genomes. / Graduate