bluuebunny/crossref_metadata_2025 huggingface dataset:
Convergence and extension at gastrulation require a myosin IIB-dependent cortical actin network
Authors: Skoglund Paul , Rolo Ana , Chen Xuejun , Gumbiner Barry M. , Keller Ray
Published: June 2008

Force-producing convergence (narrowing) and extension (lengthening) of tissues by active intercalation of cells along the axis of convergence play a major role in axial morphogenesis during embryo development in both vertebrates and invertebrates, and failure of these processes in human embryos leads to defects including spina bifida and anencephaly. Here we use Xenopus laevis, a system in which the polarized cell motility that drives this active cell intercalation has been related to the development of forces that close the blastopore and elongate the body axis, to examine the role of myosin IIB in convergence and extension. We find that myosin IIB is localized in the cortex of intercalating cells, and show by morpholino knockdown that this myosin isoform is essential for the maintenance of a stereotypical, cortical actin cytoskeleton as visualized with time-lapse fluorescent confocal microscopy. We show that this actin network consists of foci or nodes connected by cables and is polarized relative to the embryonic axis, preferentially cyclically shortening and lengthening parallel to the axis of cell polarization, elongation and intercalation, and also parallel to the axis of convergence forces during gastrulation. Depletion of MHC-B results in disruption of this polarized cytoskeleton, loss of the polarized protrusive activity characteristic of intercalating cells, eventual loss of cell-cell and cell-matrix adhesion, and dose-dependent failure of blastopore closure,arguably because of failure to develop convergence forces parallel to the myosin IIB-dependent dynamics of the actin cytoskeleton. These findings bridge the gap between a molecular-scale motor protein and tissue-scale embryonic morphogenesis.
BMP antagonism is required in both the node and lateral plate mesoderm for mammalian left-right axis establishment
Authors: Mine Naoki , Anderson Ryan M. , Klingensmith John
Published: June 2008

In mouse, left-right (L-R) patterning depends on asymmetric expression of Nodal around the node, leading to Nodal expression specifically in the left lateral plate mesoderm (LPM). Bone morphogenetic protein (BMP) signaling is also involved, but the mechanistic relationship with Nodal expression remains unclear. We find that BMP signal transduction is higher in the right LPM, although Bmp4, which is required for L-R patterning, is expressed symmetrically. By contrast, the BMP antagonists noggin (Nog) and chordin (Chrd) are expressed at higher levels in the left LPM. In Chrd;Nog double mutants, BMP signaling is elevated on both sides, whereas Nodal expression is absent. Ectopic expression of Nog in the left LPM of double mutants restores Nodalexpression. Ectopic Bmp4 expression in the left LPM of wild-type embryos represses Nodal transcription, whereas ectopic Nogin the right LPM leads to inappropriate Nodal expression. These data indicate that chordin and noggin function to limit BMP signaling in the left LPM, thereby derepressing Nodal expression. In the node, they promote peripheral Nodal expression and proper node morphology, potentially in concert with Notch signaling. These results indicate that BMP antagonism is required in both the node and LPM to facilitate L-R axis establishment in the mammalian embryo.
Patterns of cell signaling pathway activation that characterize mammary development
Authors: Andrechek Eran R. , Mori Seiichi , Rempel Rachel E. , Chang Jeffrey T. , Nevins Joseph R.
Published: June 2008

Previous work has detailed the histological and biochemical changes associated with mammary development and remodeling. We have now made use of gene expression profiling, and in particular of the previously described signatures of cell signaling pathway activation, to explore the events associated with mammary gland development. We find that there is elevated E2F-specific pathway activity prior to lactation and relatively low levels of other important signaling pathways, such as RAS, MYC and SRC. Upon lactation and continuing into the involution phase, these patterns reverse with a dramatic increase in RAS, SRC and MYC pathway activity and a decline in E2F activity. At the end of involution, these patterns return to that of the adult non-lactating mammary gland. The importance of the changes in E2F pathway activity, particularly during the proliferative phase of mammary development,was confirmed through the analysis of mice deficient for various E2F proteins. Taken together, these results reveal a complex pattern of pathway activity in relation to the various phases of mammary gland development.
Association of trxG and PcG proteins with thebxdmaintenance element depends on transcriptional activity
Authors: Petruk Svetlana , Smith Sheryl T. , Sedkov Yurii , Mazo Alexander
Published: June 2008

Polycomb group (PcG) and trithorax group (trxG) proteins act in an epigenetic fashion to maintain active and repressive states of expression of the Hox and other target genes by altering their chromatin structure. Genetically, mutations in trxG and PcG genes can antagonize each other's function, whereas mutations of genes within each group have synergistic effects. Here, we show in Drosophila that multiple trxG and PcG proteins act through the same or juxtaposed sequences in the maintenance element (ME) of the homeotic gene Ultrabithorax. Surprisingly, trxG or PcG proteins, but not both, associate in vivo in any one cell in a salivary gland with the ME of an activated or repressed Ultrabithoraxtransgene, respectively. Among several trxG and PcG proteins, only Ash1 and Asx require Trithorax in order to bind to their target genes. Together, our data argue that at the single-cell level, association of repressors and activators correlates with gene silencing and activation, respectively. There is, however, no overall synergism or antagonism between and within the trxG and PcG proteins and, instead, only subsets of trxG proteins act synergistically.
Association of tetraspanin CD9 with transmembrane TGFα confers alterations in cell-surface presentation of TGFα and cytoskeletal organization
Authors: Imhof Isabella , Gasper Warren J. , Derynck Rik
Published: June 2008

Ligand presentation is a major determinant of receptor activation. The epidermal growth factor receptor (EGFR), a tyrosine kinase receptor, is activated by growth factors of the transforming growth factor α (TGFα) family. The tetraspanin CD9 interacts with transmembrane TGFα and decreases its ectodomain shedding to release soluble TGFα. Here we report that CD9 has a role in the maturation of transmembrane TGFα and its stabilization at the cell surface, and in the cell-surface distribution in polarized epithelial cells. Furthermore, coexpression of CD9 and TGFα confers changes in cytoskeletal organization with a decrease in actin stress fibers and focal adhesions, and changes in RhoA and Rac1 GTPase activity. These alterations are reversed by blocking EGFR signaling. Finally, we demonstrate changes in cell adhesion and migration resulting from coexpression of TGFα with CD9. These results provide insight into the role of CD9 in the presentation of TGFα in epithelial and carcinoma cells, whose physiology is driven by ligand-induced EGFR activation.
Differential growth factor regulation of N-cadherin expression and motility in normal and malignant oral epithelium
Authors: Diamond Michelle E. , Sun Limin , Ottaviano Adam J. , Joseph Mathew J. , Munshi Hidayatullah G.
Published: June 2008

Aberrant expression of N-cadherin is associated with tumor progression in squamous cell carcinomas (SCCs). Consequently, we examined the regulation of N-cadherin by TGFβ1, an important mediator of keratinocyte and SCC function. N-cadherin expression was increased in oral SCC (OSCC) cell lines, regulating motility and correlating with TGFβ1 production. Moreover, in normal keratinocytes TGFβ1 increased expression of N-cadherin to regulate motility. TGFβ1-mediated N-cadherin expression in the oral keratinocytes was blocked using siRNA targeting Smads. Unexpectedly, we found that EGF blocked TGFβ1-mediated N-cadherin expression in oral keratinocytes and not in OSCC cells. Mechanistically, EGF enhanced Smad phosphorylation in the linker region, and attenuated TGFβ1-mediated phosphorylation of Smad at the C-terminus, localization of Smad to the nucleus as well as Smad-driven promoter activity exclusively in oral keratinocytes but not in OSCC cells. The effect of EGF on TGFβ1-mediated Smad-driven promoter activity and N-cadherin expression was reversed when activation of ERK1/2 was blocked. Although EGF and TGFβ1 independently promoted migration of both oral keratinocytes and OSCC cells, EGF decreased TGFβ1-mediated migration of oral keratinocytes but enhanced migration of OSCC cells. Together, these data support a model wherein EGF signaling has an important negative regulatory role on TGFβ1-mediated N-cadherin expression and motility in normal oral keratinocytes, and in which loss of this regulatory mechanism accompanies malignant transformation of the oral epithelium.
A model of autoregulated blood flow in the cerebral vasculature
Authors: Moore S , David T
Published: June 2008

The circle of Willis is a ring-like arterial structure located in the base of the brain, and is responsible for the distribution of oxygenated blood throughout the cerebral mass. Among the general population, approximately 50 per cent have a complete circle of Willis; for the others a multitude of possible anatomical variations are present, with absent or hypoplastic vessels being common. Certain conditions such as a build-up of atherosclerotic plaque on the arterial wall can result in ischaemic damage and stroke-like symptoms. A three-dimensional computer model has been developed based on the results of magnetic resonance imaging data, incorporating a numerical algorithm to simulate the body's autoregulation mechanism, such that oxygen delivery to the cerebral territories can be predicted in response events, leading to a reduction in cerebral blood flow. Sample results are presented for combinations of occlusion and stenosis.
Development and application of a one-dimensional blood flow model for microvascular networks
Authors: Lee J , Smith N
Published: June 2008

Although there are many studies available in the literature on time domain modelling of one-dimensional network blood flow, the rheological properties of blood in this modelling framework have often been disregarded through the inviscid assumption. While such a simplification may be suitable for studying the larger vessels of the circulatory system, this approach cannot be taken when modelling flow in the vessels of microcirculation in which viscous effects are much more significant. In this study, a one-dimensional network blood flow model was extended to incorporate experimentally characterized properties of the non-Newtonian blood rheology, namely the reduction in the effective viscosity in thin vessels (the Fahraeus—Lindqvist effect) and the non-proportional splitting of red blood cells at junctions (phase separation). The numerical implementation was based on the high-order finite element technique with implicit time integration, necessitated by the decreasing length of vessels on this scale. Investigations in small networks indicated that the diameter dependence of viscosity has a major impact on the overall flow and should be included in small vessel networks. However, since phase separation effects are significant only in small arterioles of diameter less than 40 μm and require significant additional complexities in computation, they may be neglected for networks that do not contain these vessels without causing a large error. The proposed technique was applied to a coronary vascular subnetwork extracted from micro computed tomography scans of a rat heart consisting of approximately 2500 segments and proved to be computationally efficient, thus providing a potential foundation for applying this model to future physiological investigations.
Subject-specific computational simulation of left ventricular flow based on magnetic resonance imaging
Authors: Long Q , Merrifield R , Xu X Y , Kilner P , Firmin D N , Yang G-Z
Published: June 2008

A detailed investigation of left ventricle (LV) flow patterns could improve our understanding of the function of the heart and provide further insight into the mechanisms of heart failure. This study presents patient-specific modelling with magnetic resonance imaging (MRI) to investigate LV blood flow patterns in normal subjects. In the study, the prescribed LV wall movements based on the MRI measurements drove the blood flow in and out of the LV in computational fluid dynamics simulation. For the six subjects studied, the simulated LV flow swirls towards the aortic valve and is ejected into the ascending aorta with a vertical flow pattern that follows the left-hand rule. In diastole, the inflow adopts a reasonably straight route (with no significant secondary flow) towards the apex in the rapid filling phase with slight variations in the jet direction between different cases. When the jet reaches about two thirds of the distance from the inflow plane to the apex, the blood flow starts to change direction and swirls towards the apex. In the more slowly filling phase, a centrally located jet is evident with vortices located on both sides of the jet on an anterior—posterior plane that passes through the mitral and aortic valves. In the inferior—superior plane, a main vortex appears for most of the cases in which an anticlockwise vortex appears for three cases and a clockwise vortex occurs for one case. The simulated flow patterns agree well qualitatively with MRI-measured flow fields.
Ureteric stents: Investigating flow and encrustation
Authors: Waters S L , Heaton K , Siggers J H , Bayston R , Bishop M , Cummings L J , Grant D M , Oliver J M , Wattis J A D
Published: June 2008

Blockages of the ureter, e.g. due to calculi (kidney stones), can result in an increase in renal pelvic pressure. This may be relieved by inserting a stent (essentially a permeable hollow tube). However, a number of complications are associated with stent use. Stents can result in reflux (backflow of urine along the ureter), which will promote recurrent urinary infection and possible renal parenchymal damage. Furthermore, long-term stent use is associated with infection and precipitation of salts from the urine, which can lead to a build-up of crystalline deposits on the stent surface, making stent removal difficult and painful. This paper examines factors governing urine flow in a stented ureter, the implications for reflux, and the processes by which the stent surface encrusts, in particular focusing on the influence of bacterial infection. An interdisciplinary approach is adopted, involving a combination of theoretical investigations and novel experiments.
Experimental investigation of nasal airflow
Authors: Doorly D , Taylor D J , Franke P , Schroter R C
Published: June 2008

The airway geometry of the nasal cavity is manifestly complex, and the manner in which it controls the airflow to accomplish its various physiological functions is not fully understood. Since the complex morphology and inaccessibility of the nasal passageways precludes detailed in-vivo measurements, either computational simulation or in-vitro experiments are needed to determine how anatomical form and function are related. The fabrication of a replica model of the nasal cavity, of a high optical clarity and derived from in-vivo scan data is described here, together with characteristics of the flow field investigated using particle image velocimetry (PIV) and flow visualization.

Flow visualization is shown to be a capable and convenient technique for identifying key phenomena. Specifically the emergence of the jet from the internal nasal valve into the main cavity, how it impacts on the middle turbinate, and the large enhancement of dispersion that accompanies the initial appearance of flow instability are revealed as particularly significant features. The findings from the visualization experiments are complemented by PIV imaging, which provides quantitative detail on the variations in velocity in different regions of the nasal cavity. These results demonstrate the effectiveness of the cavity geometry in partitioning the flow into high shear zones, which facilitate rapid heat transfer and humidification from the nasal mucosa, and slower zones affording greater residence times to facilitate olfactory sensing.

The experimental results not only provide a basis for comparison with other computational modelling but also demonstrate an alternative and flexible means to investigate complex flows, relevant to studies in different parts of the respiratory or cardiovascular systems.
Microhaemodynamics within the blade tip clearance of a centrifugal turbodynamic blood pump
Authors: Antaki J F , Diao C-G , Shu F-J , Wu J-C , Zhao R , Kameneva M V
Published: June 2008

A persistent challenge facing the quantitative design of turbodynamic blood pumps is the great disparity of spatial scales between the primary and auxiliary flow paths. Fluid passages within journals and adjacent to the blade tips are often on the scale of several blood cells, confounding the application of macroscopic continuum models. Yet, precisely in these regions there exists the highest shear stress, which is most likely to cause cellular trauma. This disparity has motivated these microscopic studies to visualize the kinematics of the blood cells within the small clearances of a miniature turbodynamic blood pump.

A transparent model of a miniature centrifugal pump having an adjustable tip clearance (50–200μm) was prepared for direct optical visualization of the region between the impeller blade tip and the stationary housing. Synchronized images of the blood cells were obtained by a microscopic visualization system, consisting of an inverted microscope fitted with long-working-distance objective lens (40×), mercury lamp, and high-resolution charge-coupled device camera electronically triggered by the rotation of the impeller.

Experiments with 7μm fluorescent particles revealed the influence of the gap dimension on the trajectory across the blade thickness. The lateral component of velocity (perpendicular to the blade) was dramatically enhanced in the 50μm gap compared with the 200μm gap, thereby reducing the exposure time. Studies with diluted bovine blood (Ht=0.5 per cent) showed that the concentration of cells traversing the gap is also reduced dramatically (30 per cent) as the blade tip clearance is reduced from 200μm to 50μm. These results motivate further investigation into the microfluidic phenomena responsible for cellular trauma within turbodynamic blood pumps.
Particle image velocimetry of a flow at a vaulted wall
Authors: Kertzscher U , Berthe A , Goubergrits L , Affeld K
Published: June 2008

The assessment of flow along a vaulted wall (with two main finite radii of curvature) is of general interest; in biofluid mechanics, it is of special interest. Unlike the geometry of flows in engineering, flow geometry in nature is often determined by vaulted walls. Specifically the flow adjacent to the wall of blood vessels is particularly interesting since this is where either thrombi are formed or atherosclerosis develops. Current measurement methods have problems assessing the flow along vaulted walls. In contrast with conventional particle image velocimetry (PIV), this new method, called wall PIV, allows the investigation of a flow adjacent to transparent flexible surfaces with two finite radii of curvature. Using an optical method which allows the observation of particles up to a predefined depth enables the visualization solely of the boundary layer flow. This is accomplished by adding a specific dye to the fluid which absorbs the monochromatic light used to illuminate the region of observation. The obtained images can be analysed with the methods of conventional PIV and result in a vector field of the velocities along the wall. With wall PIV, the steady flow adjacent to the vaulted wall of a blood pump was investigated and the resulting velocity field as well as the velocity fluctuations were assessed.
Rotational moulding – a simplified theory
Authors: Pethrick R. A. , Hudson N E.
Published: June 2008

This paper considers a number of theoretical approaches to the description of the various and complex issues, which are involved in rotational moulding. Experimental rheological measurements are presented to illustrate the changes, which can occur in typical rotational moulding grade polymers when subjected to shear and also when held at an elevated temperature for a sustained period of time. The influence of temperature on the viscosity is indicated to be an important factor in bubble release and maintenance of wall thickness in thick-walled mouldings. Although it is desirable to have a low viscosity in the melt to aid bubble release and hence reduce the time that the polymer spends in the melt phase, the problem of maintaining contact with the mould wall can arise in thick moulded sections.
Quasi-static axial cutting of AA6061 T4 and T6 round extrusions
Authors: Majumder A. , Altenhof W. , Vijayan V. , Jin S Y.
Published: June 2008

A cutting deformation mode of AA6061 T4 and T6 round extrusions with two different wall thicknesses of 1.587 and 3.175 mm was experimentally investigated under quasi-static loading conditions. A heat treated 4140 steel alloy cutter was developed and used to initiate cutting deformation in the extrusions to investigate the load—displacement and energy absorption characteristics under uniaxial compressive loading. This research extended the observations associated with cutting deformation of lightweight extrusions already completed by the authors on AA6061 T6 tubes with a wall thickness of 3.175 mm. Clean cutting behaviour was observed for the extrusions with a T6 temper and wall thickness of 1.587 mm and for extrusions with a T4 temper and wall thickness of 3.175 mm. Extrusions with a T4 temper and wall thickness of 1.587 mm exhibited characteristics of braided cutting. An almost constant cutting force was observed for the cutting deformation mode, however, variations in the cutting force were observed for the AA6061 T4 extrusions with a wall thickness of 1.587 mm after approximately 50 mm crosshead displacement. The average total energy absorption (TEA), as a result of the cutting deformation, was observed to be 2.0 and 2.7 kJ for the AA6061 T4 and T6 extrusions with a wall thickness of 1.587 mm, respectively. Average TEA for the AA6061 T4 extrusions with a wall thickness of 3.175 mm was observed to be 4.2 kJ under the cutting deformation.
Polyethylene-carbon material for polymer electrolyte membrane fuel cell bipolar plates
Authors: Greenwood P. , Chen R. , Thring R. H.
Published: June 2008

Bipolar plates are the interconnects between cells within a fuel cell stack. They must be highly electrically conductive in order to maximize the voltage across the stack and must be highly thermally conductive to aid cooling of the stack. Traditionally, bipolar plates are made from graphite or stainless steel; both have drawbacks such as low mechanical strength and corrosion problems, respectively. In order to overcome the problems associated with these common materials and to improve on the electrical and thermal conductivity aspects as well as decrease costs and improve manufacturability, a mixture of polyethylene and carbon was investigated. Composites of polyethylene and carbon were mixed using a two-roll mill and injection moulded. Micrographs of the polyethylene-carbon (PE-C) blends show how the microstructure of the polyethylene network and carbon particles provide increased mechanical properties but further addition of carbon leads to the degradation of these properties. Increasing carbon content led to an ever-increasing electrical conductivity. The material was tested for tensile and flexural properties, resulting in a maximum tensile and flexural strength of ∼24 MPa (at 26 wt% carbon in polyethylene) and ∼35 MPa (at 40 wt% carbon in polyethylene), respectively. The samples displayed electrical conductivity, with a maximum of 1.19 S/cm in-plane and 1.05 S/cm through-plane being achieved at a carbon loading of 65 wt%. The PE-C composite displayed low densities of 1.56 g/cm3 and desirable mechanical strength close to the US Department of Energy target level of 44.26 MPa for flexural strength. It was concluded that PE-C composites with different types of carbon and carbon fibres should be tested in order to reach the electrical conductivity target levels of 100 S/cm.
Adhesively bonded T-joints in polyvinyl chloride windows
Authors: Chaves F J P. , da Silva L F M. , de Castro P M S T.
Published: June 2008

The main objective of this study was to assess the applicability of adhesives to connect a transom to a polyvinyl chloride window frame. A modified methacrylate adhesive was selected and tested for its mechanical properties, environmental ageing, and glass transition temperature. Adhesively bonded T-joints were manufactured and tested, and compared with the conventional screw-fastened T-joints in terms of joint strength. The initial geometry of the adhesively bonded T-joints was optimized with a finite element analysis (FEA). Finally, the various manufacturing processes (welding, screw fastening, and adhesive, bonding) were compared in economical terms, which is a critical parameter for the industry.

The joint strength results show that adhesively bonded joints perform as well or better than the conventional screw-fastened joints. The FEA indicates that further improvements are possible with little changes in the geometry. The adhesive bonding process is also attractive in economical terms.
Application of impression creep data in life assessment of power plant materials at high temperatures
Authors: Sun W , Hyde T H , Brett S J
Published: June 2008

This paper contains a description of some recent progress on impression creep testing and its applications. Several fundamental aspects, involving the conversion relationships, typical test types, and validity of the test technique, are briefly described. Applications of the test data to assist with the life assessment of components, particularly those with service-exposed materials, using data obtained from scoop samples, are outlined. Correlations of impression data with the corresponding uniaxial data, where available, and with corresponding hardness results, are presented. These results indicate that the impression creep test method is a potential, simple, and economic life assessment technique for power plant life assessment applications. Proposals for future exploitation and for improvement of the technique are addressed.
Deformation behaviour of aluminium during machining: Modelling by Eulerian and smoothed-particle hydrodynamics methods
Authors: Akarca S. S. , Song X. , Altenhof W. J , Alpas A. T
Published: June 2008

Large-strain deformation behaviour of aluminium that accompanies continuous chip formation during machining (1100 Al) has been studied using experimental and numerical techniques. In experimental studies, local values of plastic strains were determined in the primary and the secondary deformation zones of machined 1100 Al. This was completed through a careful examination of metallographic sections taken from the material ahead of the tool tip, in which orientation changes in the flow lines in the material and shear angles were used to calculate plastic strains. Variations in local flow stresses were estimated from microhardness measurements. The examination of the stresses and strains at each measurement location generated a stress—strain relationship for the 1100 Al material. An important observation from the experimental portion of this research indicated that the material stress—strain response was independent of the feed rates considered in this study. Additionally, the response was observed to obey an exponential relationship with stress saturation occurring at approximately 300 MPa. Parameters associated with the Johnson—Cook constitutive equation were also determined from the experimental work. An Eulerian finite-element method and a relatively new so-called mesh-free method [smoothed-particle hydrodynamics (SPH) method] have been applied to the simulation of machining. The application of these methods permits simulation of the machining process without use of any mesh separation criterion. Appropriate values of the coefficients of friction, for numerical studies, were determined in parametric studies by correlating the experimentally measured chip thicknesses with the numerically predicted values. The effectiveness of the Eulerian and SPH methods in predicting the response of 1100 Al during orthogonal machining has been assessed through a rigorous comparison of the stress—strain distribution within formed chips during steady-state cutting. Both the Eulerian and SPH models showed good overall correlation with the experimentally measured stress—strain distribution when the exponential type material behaviour was assumed in modelling. A maximum stress of 300 MPa at the tool tip was obtained from the numerical simulations using the assumed exponential material behaviour. The location of maximum stress corresponded to the position of maximum strain (8.0). The application of the Johnson—Cook type constitutive equation resulted in predicting a lower maximum equivalent strain (4.5) and higher maximum stresses (325 MPa).
Historical Property Rights, Sociality, and the Emergence of Impersonal Exchange in Long-Distance Trade
Authors: Kimbrough Erik O , Smith Vernon L , Wilson Bart J
Published: June 2008

This laboratory experiment explores the extent to which impersonal exchange emerges from personal exchange with opportunities for long-distance trade. We design a three-commodity production and exchange economy in which agents in three geographically separated villages must develop multilateral exchange networks to import a good only available abroad. For treatments, we induce two distinct institutional histories to investigate how past experience with property rights affects the evolution of specialization and exchange. We find that a history of unenforced property rights hinders our subjects' ability to develop the requisite personal social arrangements to support specialization and effectively exploit impersonal long-distance trade. (JEL C90, D23, D51, P14)
Reference-Dependent Preferences and Labor Supply: The Case of New York City Taxi Drivers
Authors: Farber Henry S
Published: June 2008

I develop a model of daily labor supply where preferences are dependent on a reference daily income level, and I apply this model to data on the labor supply of New York City taxi drivers. I find that there may be a reference level of income on a given day that affects labor supply. However, there is substantial day-to-day variation in a given driver's reference level, and most shifts end before reaching the reference income level. This pattern is inconsistent with an important role for reference-dependent preferences. (JEL J22, L92)
Estimates of the Impact of Crime Risk on Property Values from Megan's Laws
Authors: Linden Leigh , Rockoff Jonah E
Published: June 2008

We estimate the willingness to pay for reductions in crime risk using the location and move-in dates of sex offenders. We find significant effects of sex offenders' locations that are geographically localized. House prices within 0.1 miles of a sex offender fall by 4 percent on average. We then use this finding to estimate the costs to victims of sexual offenses, and find costs of over $1 million per victim—far greater than previous estimates. However, we cannot reject the alternative hypotheses that individuals overestimate risks posed by offenders or that living near an offender poses significant costs exclusive of crime risk. (JEL K42, R23, R31)
Ordering the Extraction of Polluting Nonrenewable Resources
Authors: Chakravorty Ujjayant , Moreaux Michel , Tidball Mabel
Published: June 2008

A well-known theorem by Herfindahl states that the low-cost nonrenewable resource must be exploited first. Consider resources that are differentiated only by their pollution content. For instance, both coal and natural gas are used to generate electricity, yet coal is more polluting. We show that the ordering of extraction need not be driven by whether a resource is clean or dirty. Coal may be used first, followed by natural gas, and again by coal. Such “vacillation” does not occur under cost heterogeneity. A perverse policy implication is that regulating pollution may accelerate use of the polluting resource. (JEL Q32, Q38, Q53, Q58)
When Does Coordination Require Centralization? Corrigendum
Authors: Alonso Ricardo , Dessein Wouter , Matouschek Niko
Published: June 2008

The equations on page 153 of “When Does Coordination Require Centralization?,” which appeared in the March 2008 issue of the American Economic Review, failed to print in their entirety.

The complete text of Section III, Decision Making, appears below.
The Time-Varying Volatility of Macroeconomic Fluctuations
Authors: Justiniano Alejandro , Primiceri Giorgio E
Published: June 2008

We investigate the sources of the important shifts in the volatility of US macroeconomic variables in the postwar period. To this end, we propose the estimation of DSGE models allowing for time variation in the volatility of the structural innovations. We apply our estimation strategy to a large-scale model of the business cycle and find that shocks specific to the equilibrium condition of investment account for most of the sharp decline in volatility of the last two decades. (JEL C51, E32)
What's the Matter with Tie-Breaking? Improving Efficiency in School Choice
Authors: Erdil Aytek , Ergin Haluk
Published: June 2008

In several school choice districts in the United States, the student proposing deferred acceptance algorithm is applied after indifferences in priority orders are broken in some exogenous way. Although such a tie-breaking procedure preserves stability, it adversely affects the welfare of the students since it introduces artificial stability constraints. Our main finding is a polynomial-time algorithm for the computation of a student-optimal stable matching when priorities are weak. The idea behind our construction relies on a new notion which we call a stable improvement cycle. We also investigate the strategic properties of the student-optimal stable mechanism. (JEL C78, D82, I21)
Income and Democracy
Authors: Acemoglu Daron , Johnson Simon , Robinson James A , Yared Pierre
Published: June 2008

Existing studies establish a strong cross-country correlation between income and democracy but do not control for factors that simultaneously affect both variables. We show that controlling for such factors by including country fixed effects removes the statistical association between income per capita and various measures of democracy. We present instrumental-variables estimates that also show no causal effect of income on democracy. The cross-country correlation between income and democracy reflects a positive correlation between changes in income and democracy over the past 500 years. This pattern is consistent with the idea that societies embarked on divergent political-economic development paths at certain critical junctures. (JEL D72, E21)
Information Aggregation in Polls
Authors: Morgan John , Stocken Phillip C
Published: June 2008

We study information transmission via polling. A policymaker polls constituents, who differ in their information and ideology, to determine policy. Full revelation is an equilibrium in a poll with a small sample, but not with a large one. In large polls, full information aggregation can arise in an equilibrium where constituents endogenously sort themselves into centrists, who respond truthfully, and extremists, who do not. We find polling statistics that ignore strategic behavior yield biased estimators and mischaracterize the poll's margin of error. We construct estimators that account for strategic behavior. Finally, we compare polls and elections. (JEL C42, D83)
Stability in Supply Chain Networks
Authors: Ostrovsky Michael
Published: June 2008

This paper studies matching in vertical networks, generalizing the theory of matching in two-sided markets. It gives sufficient conditions for the existence of stable networks and presents an algorithm for finding two of them. One is the best stable network for the agents on the “upstream” end of an industry. The other is best for the agents on the “downstream” end. The paper describes several properties of the set of stable networks and discusses applications of the theory to the design of matching markets with more than two types of agents and to the empirical analysis of supply chains. (JEL C78, D85, L14)

The woollen coat, for example, which covers the day-labourer, as coarse and rough as it may appear, is the produce of the joint labour of a great multitude of workmen. The shepherd, the sorter of the wool, the wool-comber or carder, the dyer, the scribbler, the spinner, the weaver, the fuller, the dresser, with many others, must all join their different arts in order to complete even this homely production. —Adam Smith (1776)
Stationary Concepts for Experimental 2x2-Games
Authors: Selten Reinhard , Chmura Thorsten
Published: June 2008

Five stationary concepts for completely mixed 2x2-games are experimentally compared: Nash equilibrium, quantal response equilibrium, action-sampling equilibrium, payoff-sampling equilibrium (Martin J. Osborne and Ariel Rubinstein 1998), and impulse balance equilibrium. Experiments on 12 games, 6 constant sum games, and 6 nonconstant sum games were run with 12 independent subject groups for each constant sum game and 6 independent subject groups for each nonconstant sum game. Each independent subject group consisted of four players 1 and four players 2, interacting anonymously over 200 periods with random matching. The comparison of the five theories shows that the order of performance from best to worst is as follows: impulse balance equilibrium, payoff-sampling equilibrium, action-sampling equilibrium, quantal response equilibrium, Nash equilibrium. (JEL C70, C91)
Contracts, Hold-Up, and Exports: Textiles and Opium in Colonial India
Authors: Kranton Rachel , Swamy Anand V
Published: June 2008

Trade and export, it is argued, spur economic growth. This paper studies the microeconomics of exporting. We build a heuristic model of transactions between exporters and producers and relate it to East India Company (EIC) operations in colonial Bengal. Our model and the historical record stress two difficulties: the exporter and its agents might not uphold payment agreements, and producers might not honor sales contracts. The model shows when procurement succeeds or fails, highlighting the tension between these two hold-up problems. We analyze several cases, including the EIC's cotton textile venture, the famous Opium Monopoly, and present-day contract farming. (JEL D86, F14, N55, N75)
Translation initiation by the c-myc mRNA internal ribosome entry sequence and the poly(A) tail
Authors: Thoma Christian , Fraterman Sven , Gentzel Marc , Wilm Matthias , Hentze Matthias W.
Published: June 2008

Eukaryotic mRNAs possess a poly(A) tail that enhances translation via the 7mGpppN cap structure or internal ribosome entry sequences (IRESs). Here we address the question of how cellular IRESs recruit the ribosome and how recruitment is augmented by the poly(A) tail. We show that the poly(A) tail enhances 48S complex assembly by the c-myc IRES. Remarkably, this process is independent of the poly(A) binding protein (PABP). Purification of native 48S initiation complexes assembled on c-myc IRES mRNAs and quantitative label-free analysis by liquid chromatography and mass spectrometry directly identify eIFs 2, 3, 4A, 4B, 4GI, and 5 as components of the c-myc IRES 48S initiation complex. Our results demonstrate for the first time that the poly(A) tail augments the initiation step of cellular IRES-driven translation and implicate a distinct subset of translation initiation factors in this process. The mechanistic distinctions from cap-dependent translation may allow specific translational control of the c-myc mRNA and possibly other cellular mRNAs that initiate translation via IRESs.
Approximation to the Transient Solution of theM/Ek/1 Queue
Authors: Griffiths J. D. , Leonenko G. M. , Williams J. E.
Published: June 2008

This paper considers the Erlang queueing system M/Ek/1, where customers arrive at random at mean rate λ and the service times have an Erlang distribution with parameter k and mean service rate μ. It is difficult to obtain a transient solution in explicit form to the queue equations because of their complex structure. We propose a simple method of computing Wq(t)—the mean waiting time of a customer arriving in the queue at time t, based on an exponential function approximation.
A Stochastic Approximation Method to Compute Bid Prices in Network Revenue Management Problems
Authors: Topaloglu Huseyin
Published: June 2008

We present a stochastic approximation method to compute bid prices in network revenue management problems. The key idea is to visualize the total expected revenue as a function of the bid prices and to use sample path-based derivatives to search for a good set of bid prices. We deal with the discrete nature of the network revenue management setting by formulating a smoothed version of the problem, which assumes that it is possible to accept a fraction of an itinerary request. We show that the iterates of our method converge to a stationary point of the total expected revenue function of the smoothed version. Computational experiments demonstrate that the bid prices obtained by our method outperform the ones obtained by standard benchmark methods, and our method is especially advantageous when the bid prices are not recomputed frequently.
How Near-Misses Influence Decision Making Under Risk: A Missed Opportunity for Learning
Authors: Dillon Robin L. , Tinsley Catherine H.
Published: June 2008

Although organizations appear to learn from obvious failures, we argue that it is harder for them to learn from “near-misses”—events in which chance played a role in averting failure. In this paper, we formalize the concept of near-misses and hypothesize that organizations and managers fail to learn from near-misses because they evaluate such events as successes and thus feel safer about the situation. We distinguish perceived (“felt”) risk from calculated statistical risk and propose that lower levels of perceived risk encourage people with near-miss information to make riskier subsequent decisions compared to people without near-miss information. In our first study, we confirm the tendency to evaluate near-misses as successes by having participants rate a project manager whose decisions result in either (a) mission success, (b) near-miss, or (c) failure. Participants (both students and NASA employees and contractors) give similar ratings to managers whose decisions produced near-misses and to managers whose decisions resulted in successes, and both ratings are significantly different from ratings of managers who experienced failures. We suggest that the failure to hold managers accountable for near-misses is a foregone learning opportunity for both the manager and the organization. In our second set of studies, we confirm that near-miss information leads people to choose a riskier alternative because of a lower perceived risk following near-miss events. We explore several alternative explanations for these findings, including the role of Bayesian updating in processing near-miss data. Ultimately, the analysis suggests that managers and organizations are reducing their perception of the risk, although not necessarily updating (lowering) the statistical probability of the failure event. We speculate that this divergence arises because perceived risk is the product of associative processing, whereas statistical risk arises from rule-based processing.
A Branch-and-Price-and-Cut Method for Ship Scheduling with Limited Risk
Authors: Hwang Hee-Su , Visoldilokpun Siriwat , Rosenberger Jay M.
Published: June 2008

Maritime logistics operations are full of uncertainty such as severe weather, mechanical problems, strikes, and fluctuating freight rates. Traditional ship-scheduling models ignore uncertainty, even in highly volatile markets. Although ship operators can increase revenue by delivering many spot cargoes, they have to embrace the risk of the fluctuation of spot rates. We present a set-packing model that limits risk using a quadratic variance constraint. We use a traditional Kelley's cutting plane algorithm and a delayed column-and-cut generation (branch-and-price-and-cut) algorithm on medium-sized ship-scheduling problems with restricted variance. We develop a second set of cuts that are more restrictive under certain conditions. Computational results show that the variance of profit can be significantly reduced with a reasonable increase in cost.
FactSage and ChemApp: Two tools for the prediction of multiphase chemical equilibria in solutions
Authors: Eriksson Gunnar , Königsberger Erich
Published: June 2008

The integrated thermochemical databank system FactSage enables the user to calculate equilibria and thermodynamic properties for a wide variety of multicomponent, multiphase systems and reactions. The Gibbs energy models and databases for condensed, gaseous, and aqueous solutions presently implemented in FactSage are of interest to chemical and physical metallurgy, chemical and corrosion engineering, inorganic and solution chemistry, geochemistry, environmental science, etc. Models for the aqueous phase include the Pitzer and Helgeson formalisms, but data and subroutines for concentrated aqueous solutions have also been provided by OLI Systems. ChemApp is a programmer's library for thermochemistry that also incorporates the Gibbs energy minimizer of FactSage. Applications of ChemApp include, for example, the handling of repetitive complex equilibrium calculations in application-specific programs and its linking to third-party process simulation packages. In this work, user-defined aqueous solution models in FactSage and ChemApp have been applied to the hydrometallurgical processing of aluminum ores in both caustic and acidic leach solutions.
Chemical speciation of polynuclear complexes containing [Ln2M3L6] units
Authors: Torres Julia , Kremer Carlos , Domínguez Sixto
Published: June 2008

Polynuclear complexes are widely regarded as potential materials in separation, catalysis, gas storage, and molecular recognition. Those complexes including lanthanide (Ln) ions in the heterobimetallic assembly enhance the structural flexibility and increase the thermodynamic stability. Up to now, the studies on these heteropolynuclear complexes have been mostly focused on lanthanide-copper compounds in the solid state. We have expanded the studies to solution chemistry, to know if such polynuclear species can also exist in aqueous solution. In this work, potentiometric titrations (25.0 °C, and I = 0.50 M Me4NCl) were carried out for systems containing a Ln ion (La, Ce, Sm), a bridging ligand (oda or ida), and a second metal ion. We found that polynuclear units can be formed with MII (Mn, Fe, Co, Ni, Cu, Zn, Cd, and VO2+) and MIII (Fe, Al) ions, and can be predominant in aqueous solution. Species like [Ln2M3L6] or [Ln2M3L6(OH)
n
]
n-
(n = 3, 6) can exist in solution, representing the same 2:3:6 stoichiometry found in the solid state. The presence of an amino group in the ida ligand makes the mononuclear complexes with 3d metal ions more stable, preventing to some extent the formation of polynuclear complexes.