Sperm cells are guided to the egg by chemoattractants in many species. Sperm cells are propelled in a liquid by the regular beat of their flagellum. In the presence of a concentration gradient of a chemoattractant, they can steer upwards the concentration gradient, a process called chemotaxis. Eggs release chemoattractants to guide the sperm cells to the egg. Sperm chemotaxis is best studied experimentally in the sea urchin. There, specific receptors in the flagellar membrane of the sperm cells are activated upon binding of chemoattractant molecules and trigger a signaling cascade which ultimately changes the activity of the molecular motors which drive the flagellar beat and result in a swimming response. Sea urchin sperm cells swim along circular and helical paths. Sperm cells of the sea urchin and several other species swim along helical paths far from boundary surfaces in the absence of chemoattractant. In a two-dimensional experimental geometry, sperm swimming paths are planar circles. The non-zero curvature of their swimming paths is a direct consequence of an asymmetry of their flagellar beat. In a concentration gradient of chemoattractant, sperm swimming path are drifting circles in two dimensions and bend helices in three dimensions. What is the working mechanism of sperm chemotaxis? In this thesis, we develop a theoretical description of sperm chemotaxis which can be subsumed as follows: While swimming along an approximately circular path in a concentration gradient a sperm cell traces a periodic concentration stimulus from the concentration field that has the frequency of circular swimming. The chemotactic signaling system processes this stimulus and causes a periodic modulation of the curvature of the swimming path which then gives rise to a swimming path which is a drifting circle. The relative direction of the drift with respect to the gradient direction is determined by the phase shift between the stimulus and the curvature oscillations. This picture is in perfect agreement with recent experimental findings. The mechanism is more general and also works in three dimensions for swimming along helical paths. Our results. Our theoretical description of sperm chemotaxis clarifies the concepts underlying sperm chemotaxis. In particular, we derive the role of internal timing of the chemotactic signaling system for sperm chemotaxis. We conclude that sampling a concentration field along circular and helical paths is a robust strategy for chemotaxis that does not require fine-tuning of parameters and which works reliable also in the presence of fluctuations. In a last chapter of this thesis, we discuss sperm chemotaxis in the more general context of an abstract search problem.

Made available in DSpace on 2016-08-15T20:31:13Z (GMT). No. of bitstreams: 1 PatriciaCS_DISSERT.pdf: 1847560 bytes, checksum: 7da5854056af7ddc3546a8a90de2324f (MD5) Previous issue date: 2013-08-30 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The objective of this work, cryopreserve semen from six-banded armadillos, assessing in advance the effects of centrifugation and dilution of their seminal parameters over thermoresistance test (TRT) in order to define the best diluent to be used in semen cryopreservation of this species. Semen samples (n = 12) derived from 04 adult males collected by electroejaculation were divided into four aliquots, two immediately diluted in Tris or powdered coconut water (ACP-119®), and the other centrifuged (800g/10 min) prior to dilution. The samples were incubated at 34°C for 3h and the seminal parameters evaluated at intervals of 1 h. In general, was observed greater efficiency of Tris regard to the preservation of seminal parameters along the TRT, especially up to 120 min of evaluation, but after this period the two extenders were equivalents. Still, the centrifugation caused deleterious effect on semen quality of six-banded armadillos, this method is not suitable for this species. For the cryopreservation of the semen of six banded armadillos, was then diluted in Tris-base, plus 20% egg yolk and 3% glycerol. Following the steps of dilution, the samples semen cooling and thawing was performed classical analysis simultaneously to fluorescence analysis, employing the combination of fluorophores to assess the plasma membrane integrity (Propidium iodide - PI and Hoechst - H-342), and also mitochondrial activity (CMXRos - Mito Tracker red®). Most of the changes observed in all stages of cryopreservation of semen of six-banded armadillos was secondary, but the proportions of these defects increased significantly from step cooling, and higher in step thawing, due to the impacts of heat shock and osmotic stress be more significant in this phase. The results of fluorescence analysis indicated a percentage of cells with intact plasma membrane more higher (8.4%) than that of motile cells (6.1%) on thawed semen. This may be due to the action of egg yolk, to provide greater structural integrity to some cells. In images taken by transmission electron microscopy were identified in sperm cryopreserved rupture and vesiculation of the plasma membrane, mitochondrial abnormalities, as well nuclear and acrosomal changes characterized by changes in chromatin and sperm capacitation, respectively. This study is the first report on the use of cryopreservation on semen xenarthras, and given the importance of biotech to preserve the genetic material of armadillos, the results obtained here are fundamental to promote the improvement of semen cryopreservation in six-banded armadillos, aiming its applications in other species of armadillos endangered or vulnerable to extinction / Objetivou-se com este trabalho, criopreservar o sêmen de tatus-peba, avaliando previamente os efeitos da centrifugação e da diluição sobre seus os parâmetros seminais ao longo do teste de termorresistência (TTR), a fim de definir o melhor diluente a ser utilizado no processo de criopreservação seminal desta espécie. Amostras de sêmen (n=12) oriundas de 04 machos adultos coletados por eletroejaculação foram divididas em quatro alíquotas, sendo duas imediatamente diluídas em Tris ou água de coco em pó (ACP-119®), e as demais centrifugadas (800g/10 min) previamente à diluição. As amostras foram incubadas a 34°C por 3h, e os parâmetros seminais avaliados em intervalos de 1h. Em termos gerais, observou-se uma melhor eficiência do Tris quanto à preservação dos parâmetros seminais ao longo do TTR, principalmente até os 120 min de avaliação, mas após este período, os dois diluentes se equipararam. Ainda, verificou-se efeito deletério da centrifugação sobre a qualidade do sêmen de tatus-peba, não sendo esse método indicado para esta espécie. Para a criopreservação, o sêmen de tatus-peba foi então diluído em solução a base de Tris,acrescidade 20% de gema e 3% de glicerol.Após as etapas de diluição, refrigeração e descongelação, foi realizada a análise clássica das amostras simultânea à análise por fluorescência, empregando a associação de fluoróforos para avaliar a integridade da membrana plasmática (Iodeto de Propídio - PI e Hoechst - H-342), e também a atividade mitocondrial (CMXRos - Mito Tracker red®).A maioria das alterações observadas em todos os estágios da criopreservação do sêmen de tatus-peba foi secundária, mas as proporções desses defeitos aumentaram significativamente a partir da etapa de refrigeração, sendo maior após a descongelação, devido aos impactos do choque térmico e do estresse osmótico serem mais expressivos nessa fase. Os resultados da análise de fluorescência indicaram um percentual de células com a membrana plasmática intacta maior (8,4%) do que o de células móveis (6,1%), em sêmen descongelado.Isto pode ser devido à ação da gema de ovo em permitir maior integridade estrutural a algumas células. Em imagens captadas por eletromicroscopia de transmissão foram identificados, em espermatozoides criopreservados, ruptura e vesiculação da membrana plasmática, anormalidades mitocondriais, como também modificações nucleares e acrossomal caracterizadas por alterações da cromatina e capacitação espermática, respectivamente. Este estudo é o primeiro relato sobre a utilização da criopreservação em sêmen de xenartras, e dada a importância desta biotécnica para a preservação do material genético de tatus, os resultados aqui obtidos são fundamentais para promover o aperfeiçoamento da criopreservação seminal em tatus-peba, visando sua aplicação em outras espécies de tatus ameaçadas ou vulneráveis à extinção

<p>The present dissertation focuses on two themes: (i) elastic instability of flow and (ii) elastic instability of microscopic filaments.</p> <p><br></p> <p>(i) The presence of macromolecules often leads to the viscoelastic nature of industrial and biological fluids. The flow of viscoelastic fluids in porous media is important in many industrial, geophysical, and biological applications such as enhanced oil recovery, groundwater remediation, biofilm formation, and drug delivery. The stretching of polymeric chains as the viscoelastic fluid passes through the microstructure of the porous media induces large elastic stresses, which leads to viscoelastic instability at the Weissenberg number greater than a critical value, where the Weissenberg number quantifies the ratio of elastic to viscous forces. Viscoelastic instability can lead to a time-dependent chaotic flow even at negligible inertia, which is sometimes also known as elastic turbulence due to its analogous features to traditional inertial turbulence. In the present thesis, we investigate the pore-scale viscoelastic instabilities and the flow states induced by the instabilities in symmetric and asymmetric geometries. We found that the topology of the polymeric stress field regulates the formation of different flow states during viscoelastic instabilities. Viscoelastic instability-induced flow states exhibit hysteresis due to the requirement of a finite time for the transformation of polymeric stress topology. Further, we study viscoelastic flows through ordered and disordered porous geometries and explore the effect of viscoelastic instability on sample-scale transport properties. Viscoelastic instability enhances transverse transport in ordered porous media and longitudinal transport in disordered porous media. We also derive a relationship between the polymeric stress field and the Lagrangian stretching field. The Lagrangian stretching field helps to predict the feature of flow states and transport in complex flows. The experimental measurement of the polymeric stress field is extremely challenging. The framework established here can be used to obtain the topology of the polymeric stress field directly from the easily measured velocity field.  </p> <p><br></p> <p><br></p> <p>(ii) The interaction between flow and elastic filaments plays an important role in sperm and bacterial motility and cell division. The sperm cells of many organisms use long elastic flagellum to propel themselves and also face complex flows and boundaries during their search for egg cells. Strong flows have the potential to mechanically inhibit flagellar motility through elastohydrodynamic interactions. We explore the effects of an extensional flow on the buckling dynamics of sperm flagella through detailed numerical simulations and microfluidic experiments. Compressional fluid forces lead to rich buckling dynamics of the sperm flagellum beyond a critical dimensionless sperm number, which represents the ratio of viscous force to elastic force. Shear flows navigate the sperm cells in complex geometries and flows. We have also studied the effect of flow strength and flagellar elastic deformation on the sperm trajectory in simple shear and Poiseuille flows.</p>