Different Modes Of Migration Research Articles

Investigations on the migration mode method (MMM) for reactor calculations

Current calculation codes for reactor analysis are based on the multi-group method to evaluate energy distribution of neutron flux. Usually a two energy group diffusion equation is adopted. This choice is adequate for PWRs associated to cross sections libraries tabulated versus fuel exposure and other state parameters as moderator density, fuel temperature, boron concentration. An improvement of this approach is represented by the migration mode method (MMM) by which the neutron spectrum is expanded in terms of base functions corresponding to the different modes of migration of the neutrons in the energy dimension. For a thermal reactor, three such functions may be easily identified: the Maxwellian distribution of the neutrons at thermal equilibrium with the moderator, the 1/ E slowing down distribution (corrected to take into account resonance absorption effects) and the fission neutron spectrum. The (space-dependent) coefficients of the expansion are calculated by solving a differential equation which results having a structure similar to the one relevant to multi-group theory. The method can therefore be easily implemented adopting existing diffusion theory codes. With the present work, some investigations on the MMM are described relevant to UO 2 fuelled PWR systems. Demonstrative results are given to validate the potentiality of the method.

Tumbling, an Interactive Way to Move Forward

The migration of Drosophila border cells has become a powerful model with which to genetically identify guidance cues that control the directed migration of a group of interconnected cells. During oogenesis, border cells delaminate from an epithelial layer and move collectively toward the oocyte. In vivo observation has been added to the impressive experimental toolkit available to study border cell migration. These studies reveal two previously unknown migratory behaviors: one in which cells within the border cell cluster constantly change their position, and another called "tumbling," by which the entire border cell cluster rotates forward. Unexpectedly, the same receptor tyrosine kinases control these different modes of migration through separate downstream pathways. An early mode is mediated by the actin regulatory proteins ELMO and Mbc and resembles cellular polarization during individual cell migration; whereas during a later phase, communication between cells, facilitated by mitogen-activated protein kinase and phospholipase C-gamma, organizes the polarity of the entire cluster.

ROCK signaling mediates the adoption of different modes of migration and invasion in human mammary epithelial tumor cells

For the invasive migration of tumor cells, at least two mechanisms are currently discussed: (1) the mesenchymal mode depending on extracellular proteolysis and (2) the proteolysis-independent amoeboid mode depending on the activity of the Rho kinase ROCK. The ability of tumor cells to switch between different modes of motility has been shown to limit the efficiency of agents aimed to reduce invasion. Here we show by combining 2D and 3D migration assays that human mammary tumor cells exhibited a strongly reduced migration velocity as compared to their normal counterparts indicating that high invasiveness is not necessarily correlated with high migratory capacity in 2D assays. This reduced migration was apparently due to significant differences in actin organization, decreased persistence of lamellipodia by 50% and increased cell substrate adhesion. These differences resulted from a 2.5-fold higher activity of ROCK and were mediated by its downstream effectors myosin light chain kinase and cofilin. Thus, inhibition of ROCK activity caused a marked increase in 2D migration efficiency by 40%, without, however, affecting 3D invasion. A massive reduction of invasion by 60% was achieved by the simultaneous inhibition of the ROCK-dependent amoeboid and the extracellular proteolysis-dependent mesenchymal mode. These results may point to a new efficient strategy for blocking tumor cell invasion in vivo.

Time‐lapse analysis reveals different modes of primordial germ cell migration in the medaka Oryzias latipes

Previous studies have shown that medaka primordial germ cells (PGC) are first distinguishable by olvas expression during late gastrulation, and that they migrate to the gonadal region through the lateral plate mesoderm. Here, we demonstrate that medaka nanos expression marks the germ line at early gastrulation stage. By marking the germ line with green fluorescent protein (GFP) fused to the nanos 3' untranslated region, we were able to visualize the behavior of PGC using time-lapse imaging. We show that there are three distinct modes of PGC migration that function at different stages of development. At early gastrulation stage, PGC actively migrate towards the marginal zone, a process that requires the function of a chemokine receptor, CXCR4. However, at late gastrulation stage, PGC change the mode and direction of their movement, as they are carried towards the midline along with somatic cells undergoing convergent movements. After aligning bilaterally, PGC actively migrate to the posterior end of the lateral plate mesoderm. This posterior movement depends on the activity of both HMGCoAR and a ligand of CXCR4, SDF-1a. These results demonstrate that PGC undergo different modes of migration to reach the prospective gonadal region of the embryo.

EMERGENCE OF REGULATORY NETWORKS IN SIMULATED EVOLUTIONARY PROCESSES

Despite spectacular progress in biophysics, molecular biology and biochemistry our ability to predict the dynamic behavior of multicellular systems under different conditions is very limited. An important reason for this is that still not enough is known about how cells change their physical and biological properties by genetic or metabolic regulation, and which of these changes affect the cell behavior. For this reason, it is difficult to predict the system behavior of multicellular systems in case the cell behavior changes, for example, as a consequence of regulation or differentiation. The rules that underlie the regulation processes have been determined on the time scale of evolution, by selection on the phenotypic level of cells or cell populations. We illustrate by detailed computer simulations in a multi-scale approach how cell behavior controlled by regulatory networks may emerge as a consequence of an evolutionary process, if either the cells, or populations of cells are subject to selection on particular features. We consider two examples, migration strategies of single cells searching a signal source, or aggregation of two or more cells within minimal multiscale models of biological evolution. Both can be found for example in the life cycle of the slime mold Dictyostelium discoideum. However, phenotypic changes that can lead to completely different modes of migration have also been observed in cells of multi-cellular organisms, for example, as a consequence of a specialization in stem cells or the de-differentiation in tumor cells. The regulatory networks are represented by Boolean networks and encoded by binary strings. The latter may be considered as encoding the genetic information (the genotype) and are subject to mutations and crossovers. The cell behavior reflects the phenotype. We find that cells adopt naturally observed migration strategies, controlled by networks that show robustness and redundancy. The model simplicity allow us to unambiguously analyze the regulatory networks and the resulting phenotypes by different measures and by knockouts of regulatory elements. We illustrate that in order to maintain a cells' phenotype in case of a knockout, the cell may have to be able to deal with contradictory information. In summary, both the cell phenotype as well as the emerged regulatory network behave as their biological counterparts observed in nature.

Introducing the migration mode method for the solution of the space and energy dependent diffusion equation

The most usual method to take into account energy dependence in whole core spatial neutronics calculation is the multigroup method. In thermal spectrum reactors as PWR, two-group theory is sufficient to describe accurately the neutron spectrum variation among spatial regions. When the spectrum hardens the precision of two-group theory decreases and more groups are necessary to keep a good accuracy. The aim of the computation method presented here is to represent with a good accuracy the spectral transitions which appear in these situations, without increasing the number of unknowns (i.e. the number of energy groups). The neutron spectrum is considered as a combination of base shapes corresponding to the different modes of migration of the neutrons in the energy dimension. The resulting energy flux distribution is a continuous function that fits the real one. The spatial discretization leads to matrices having the same structure of the ones obtained with multi-group theory. Then the method can be easily applied to existing codes solving the diffusion equation on the whole core in 3D. A methodological comparison between the migration mode method and the multigroup (few-group) method as well as a numerical comparison is presented.

Cellular and molecular mechanisms of neuronal migration in neocortical development.

The complicated mammalian brain structure arises from accurate movements of neurons from their birthplace to their final locations. Detailed observation of this migration process by various methods revealed that neuronal migration is highly motile and that there are different modes of migration. Moreover, mouse mutants or human disorders that disrupt normal migration have provided significant insights into molecular pathways that control the neuronal migration. Although our knowledge is still fragmentary, it is becoming clear that various molecules are participating in this process. In this review, we outline about the cellular and molecular mechanisms of neuronal migration in the cerebral cortex.

Different kinds of diadromy: Different kinds of conservation problems

Conservation issues of diadromous fishes are discussed in relation to the IUCN list of threatened species. There is still uncertainty in respect of the accurate categorization of some species and a better understanding is required of their migration strategies. Apart from the enhanced vulnerability to fisheries and environmental impacts in the river mouths and estuaries, the conservation threats appear to be largely identical to those affecting other migrating riverine species. Attention is drawn to different modes of migration in diadromous fish.

Electrical mobilities of 6Li and 7Li ions in molten nitrates; Relation between the isotope effect and the ionic interactions

Abstract The electrical mobilities of 6Li and 7Li ions in molten nitrates have been studied by electrophoresis. The isotope effects at temperatures from 250°C to 400°C have been measured in the low melting point mixture NaNO3KNO3, and a large increase in the isotope effect is observed with increasing temperature. These results are correlated with the transport properties of lithium and of the constitutive elements of the melt. It is shown that ionic interactions strongly influence the isotope effect; by describing these interactions as inducers of a multi-order process it is possible to show that the isotope effect is mainly due to the isotope distribution between these different modes of migration.