Numerical Methods for Two-Dimensional Stem Cell Tissue Growth.

Growth of developing and regenerative biological tissues of different cell types is usually driven by stem cells and their local environment. Here, we present a computational framework for continuum tissue growth models consisting of stem cells, cell lineages, and diffusive molecules that regulate proliferation and differentiation through feedback. To deal with the moving boundaries of the models in both open geometries and closed geometries (through polar coordinates) in two dimensions, we transform the dynamic domains and governing equations to fixed domains, followed by solving for the transformation functions to track the interface explicitly. Clustering grid points in local regions for better efficiency and accuracy can be achieved by appropriate choices of the transformation. The equations resulting from the incompressibility of the tissue is approximated by high-order finite difference schemes and is solved using the multigrid algorithms. The numerical tests demonstrate an overall spatiotemporal second-order accuracy of the methods and their capability in capturing large deformations of the tissue boundaries. The methods are applied to two biological systems: stratified epithelia for studying the effects of two different types of stem cell niches and the scaling of a morphogen gradient with the size of the Drosophila imaginal wing disc during growth. Direct simulations of both systems suggest that that the computational framework is robust and accurate, and it can incorporate various biological processes critical to stem cell dynamics and tissue growth

Logistical constraints on international trade in the Maghreb

Without a competitive transport industry, the Maghreb countries will not truly benefit from reform aimed at increasing the region's share of international trade. A study of barriers to the region's trade, especially with countries of the European Union, identified more than 30 barriers, in four categories: barriers to imports, to exports, of infrastructure and equipment, and of intra-Maghreb trade. These include: 1) direct barriers including: (a) from traditional distortions (price, discriminatory access to markets); (b) nontariff barriers (administrative, regulatory and tax-related restrictions); (c) traffic agreements (protecting national flags); and (d) lack of infrastructure and equipment; and 2) indirect barriers deriving from: (a) trade harmonization (simplified customs procedures and tariffs structures, elimination of quotas, reduction of customs tariffs on transport equipment); and (b) technology lags (telecommunications and handling). The authors quantify barriers in terms of"tariff equivalents,"expressed as a nominal rate of protection based on the freeon board value of the merchandise. But the nominal rate of protection measures only the direct costs of distortions. The effective rate of protection measures both direct and indirect effects, and effective rates are generally twice as high as nominal rates. To reconcile macroeconomic and microeconomic approaches to measuring effective rates, the authors use a partial equilibrium model (SMART model) to estimate the impact on the balance of payments of eliminating excess costs. Most of the corrective policies they recommend concern multimodal transport in the trade between Europe and the Arab Maghreb Union. The challenges are considerable: not only does such a system pave the way for cost and time savings ("just-in-time"transport), but it also adopts the logistics management that the most advanced European enterprises use to orchestrate their raw material purchasing, production and marketing functions. A multimodal transport system allow them to reduce inventories significantly and to respond better to volatile demand. Essential for just-in-time multimodal transport and logistics management include efficient modern transport techniques, efficient communications systems, efficient modern merchandise handling, and appropriate regulations. These conditions are still not fully in place in the Maghreb countries, except partially in some parts of the clothing and textile industry.Economic Theory&Research,Transport and Trade Logistics,Common Carriers Industry,Environmental Economics&Policies,Payment Systems&Infrastructure,Economic Theory&Research,Transport and Trade Logistics,Common Carriers Industry,TF054105-DONOR FUNDED OPERATION ADMINISTRATION FEE INCOME AND EXPENSE ACCOUNT,Environmental Economics&Policies

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