Current perspectives of the signaling pathways directing neural crest induction.

The neural crest is a migratory population of embryonic cells with a tremendous potential to differentiate and contribute to nearly every organ system in the adult body. Over the past two decades, an incredible amount of research has given us a reasonable understanding of how these cells are generated. Neural crest induction involves the combinatorial input of multiple signaling pathways and transcription factors, and is thought to occur in two phases from gastrulation to neurulation. In the first phase, FGF and Wnt signaling induce NC progenitors at the border of the neural plate, activating the expression of members of the Msx, Pax, and Zic families, among others. In the second phase, BMP, Wnt, and Notch signaling maintain these progenitors and bring about the expression of definitive NC markers including Snail2, FoxD3, and Sox9/10. In recent years, additional signaling molecules and modulators of these pathways have been uncovered, creating an increasingly complex regulatory network. In this work, we provide a comprehensive review of the major signaling pathways that participate in neural crest induction, with a focus on recent developments and current perspectives. We provide a simplified model of early neural crest development and stress similarities and differences between four major model organisms: Xenopus, chick, zebrafish, and mouse

Photodissociation chemistry footprints in the Starburst galaxy NGC 253

We report the first detection of PDR molecular tracers, namely HOC+, and CO+, and confirm the detection of the also PDR tracer HCO towards the starburst galaxy NGC 253, claimed to be mainly dominated by shock heating and in an earlier stage of evolution than M 82, the prototypical extragalactic PDR. Our CO+ detection suffers from significant blending to a group of transitions of 13CH3OH, tentatively detected for the first time in the extragalactic interstellar medium. These species are efficiently formed in the highly UV irradiated outer layers of molecular clouds, as observed in the late stage nuclear starburst in M 82. The molecular abundance ratios we derive for these molecules are very similar to those found in M 82. This strongly supports the idea that these molecules are tracing the PDR component associated with the starburst in the nuclear region of NGC 253. A comparison with the predictions of chemical models for PDRs shows that the observed molecular ratios are tracing the outer layers of UV illuminated clouds up to two magnitudes of visual extinction. Chemical models, which include grain formation and photodissociation of HNCO, support the scenario of a photo-dominated chemistry as an explanation to the abundances of the observed species. From this comparison we conclude that the molecular clouds in NGC 253 are more massive and with larger column densities than those in M 82, as expected from the evolutionary stage of the starbursts in both galaxies.Comment: 32 pages, 4 figures, Published in Ap

Retirement incentives, individual heterogeneity and labour transitions of employed and unemployed workers

In this paper we analyze the sensitivity of the labour market decisions of workers close to retirement with respect to the incentives created by public regulations. We improve upon the extensive prior literature on the effect of pension incentives on retirement in two ways. First, by modeling the transitions between employment, unemployment and retirement in a simultaneous manner, paying special attention to the transition from unemployment to retirement (which is particularly important in Spain). Second, by considering the influence of unobserved heterogeneity in the estimation of the effect of our (carefully constructed) incentive variables. Using administrative data, we find that, when properly defined, economic incentives have a strong impact on labour market decisions in Spain. Unemployment regulations are shown to be particularly influential for retirement behaviour, along with the more traditional determinants linked to the pension system. Pension variables also have a major bearing on both workers’ reemployment decisions and on the strategic actions of employers. The quantitative impact of the incentives, however, is greatly affected by the existence of unobserved heterogeneity among workers. Its omission leads to sizable biases in the assessment of the sensitivity to economic incentives, a finding that has clear consequences for the credibility of any model-based policy analysis. We confirm the importance of this potential problem in one especially interesting instance: the reform of early retirement provisions undertaken in Spain in 2002. We use a difference-in-difference approach to measure the behavioural reaction to this change, finding a large overestimation when unobserved heterogeneity is not taken into account.Retirement, unemployment, incentives, Pension system, Unobserved, heterogeneity, Spain.

Social Security and the search behaviour of workers approaching retirement

This paper explores the links between unemployment, retirement and their associated public insurance programs. It is a contribution to a growing body of literature focused on a better understanding of the labor behavior of advanced-age workers, which has gained importance as the pension crisis looms. It also contributes to the literature of optimal unemployment insurance by exploring the interaction of unemployment benefits and retirement pensions. The analysis combines the development of a new theoretical model and a detailed exploration of the empirical regularities using the Spanish Muestra Continua de Vidas Laborales (MCVL) dataset. The model is an extension of the standard search model, designed to reproduce the non-stationary environment faced by workers of advanced ages (in the age range 50/65). Via calibrated simulations we show that the basic empirical re-employment and retirement patterns can be considered as rational responses to both the labor market conditions and the institutional incentives. Generous Unemployment Benefits (for durations of up to two years) together with very significant early retirement penalties, make optimal to stay unemployed without searching for large groups of unemployed workers. This moral hazard problem can be substantially alleviated through institutional reform. We explore several potential reforms and find that changing the details of early retirement pensions seems more promising than changing the Unemployment Benefit system.Unemployment, Retirement, Search models

X-ray spectral variability of Seyfert 2 galaxies

Variability across the electromagnetic spectrum is a property of AGN that can help constraining the physical properties of these galaxies. This is the third of a serie of papers with the aim of studying the X-ray variability of different families of AGN. The main purpose of this work is to investigate the variability pattern in a sample of optically selected type 2 Seyfert galaxies. We use the 26 Seyferts in the Veron-Cetty and Veron catalogue with data available from Chandra and/or XMM-Newton public archives at different epochs, with timescales ranging from a few hours to years. All the spectra of the same source are simultaneously fitted and we let different parameters to vary in the model. Whenever possible, short-term variations and/or long-term UV flux variations are studied. We divide the sample in Compton-thick, Compton-thin, and changing-look candidates. Short-term variability at X-rays is not found. From the 25 analyzed sources, 11 show long-term variations; eight (out of 11) are Compton-thin, one (out of 12) is Compton-thick, and the two changing-look candidates are also variable. The main driver for the X-ray changes is related to the nuclear power (nine cases), while variations at soft energies or related with absorbers at hard X-rays are less common, and in many cases these variations are accompained with variations of the nuclear continuum. At UV frequencies nuclear variations are nor found. We report for the first time two changing-look candidates, MARK273 and NGC7319. A constant reflection component located far away from the nucleus plus a variable nuclear continuum are able to explain most of our results; the Compton-thick candidates are dominated by reflection, which supresses their continuum making them seem fainter, and not showing variations, while the Compton-thin and changing-look candidates show variations.Comment: Accepted for publication in A&

X-ray spectral variability of seven LINER nuclei with XMM-Newton and Chandra data

One of the most important features in active galactic nuclei (AGN) is the variability of their emission. Variability has been discovered at X-ray, UV, and radio frequencies on time scales from hours to years. Among the AGN family and according to theoretical studies, Low-Ionization Nuclear Emission Line Region (LINER) nuclei would be variable objects on long time scales. Our purpose is to investigate spectral X-ray variability in LINERs and to understand the nature of these kinds of objects, as well as their accretion mechanism. Chandra and XMM-Newton public archives were used to compile X-ray spectra of seven LINER nuclei at different epochs with time scales of years. To search for variability we fit all the spectra from the same object with a set of models, in order to identify the parameters responsible for the variability pattern. We also analyzed the light curves in order to search for short time scale (from hours to days) variability. Whenever possible, UV variability was also studied. We found spectral variability in four objects, with variations mostly related to hard energies (2-10 keV). These variations are due to changes in the soft excess, and/or changes in the absorber, and/or intrinsic variations of the source. Another two galaxies seem not to vary. Short time scale variations during individual observations were not found. Our analysis confirms the previously reported anticorrelation between the X-ray spectral index and the Eddington ratio, and also the correlation between the X-ray to UV flux ratio and the Eddington ratio. These results support an Advection Dominated Accretion Flow (ADAF) as the accretion mechanism in LINERs.Comment: 35 pages, 53 figures, recently accepted pape

Enhancing organizational performance with social media use: the catalysing effect of corporate entrepreneurship

Social Media use has become pervasive and firms are increasingly relying on it, not only to relate to customers, but also to leverage internal processes like innovation. The strategic use of these tools can facilitate also the entrepreneurial orientation of the firm, as it provides useful knowledge which can make the firm more entrepreneurial, stimulating it to find new opportunities or innovative ideas where other companies do not recognize them. However, despite the relevance of the phenomenon in current hyper-competitive environments, empirical research on the topic remains scarce. To shed some light on the issue, the main purpose of the paper is to examine how Social Media use impacts the different dimensions of corporate entrepreneurship (new business venturing, innovativeness, proactiveness and self-renewal), enhancing also organizational performance. The study is intended to extend knowledge on this topic, by providing understanding of the path firms should take to benefit from Social Media use to become more entrepreneurial and achieve higher organizational performance, developing and nurturing competitive advantages. The paper analyses data obtained from a sample of 201 technological firms located in Spain. The methodology used is Structural equation modelling with LISREL analysis. Findings confirms how the use of Social Media tools positively impacted all the different dimensions of corporate entrepreneurship, translating also in enhanced performance. This paper contributes to the literature by empirically confirming in a structural model how Social Media use helps to create business value, by enhancing proactive behaviours, promoting strategic renewal inside the firm and increasing innovativeness and new business venturing and displaying the internal and sequential relationships among these dimensions.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

On the resonances and polarizabilities of split ring resonators

In this paper, the behavior at resonance of split ring resonators SRRs and other related topologies, such as the nonbianisotropic SRR and the broadside-coupled SRR, are studied. It is shown that these structures exhibit a fundamental resonant mode the quasistatic resonance and other higher-order modes which are related to dynamic processes. The excitation of these modes by means of a properly polarized time varying magnetic and/or electric fields is discussed on the basis of resonator symmetries. To verify the electromagnetic properties of these resonators, simulations based on resonance excitation by nonuniform and uniform external fields have been performed. Inspection of the currents at resonances, inferred from particle symmetries and full-wave electromagnetic simulations, allows us to predict the first-order dipolar moments induced at the different resonators and to develop a classification of the resonances based on this concept. The experimental data, obtained in SRR-loaded waveguides, are in agreement with the theory and point out the rich phenomenology associated with these planar resonant structures.MEC (España)-TEC2004-04249-C02-01 y TEC2004-04249-C02-02Comunidad Europea (programa Eureka)-2895 TELEMACAgencia de Subvenciones de la República Checa-102/03/044

X-ray spectral variability of LINERs selected from the Palomar sample

Variability is a general property of active galactic nuclei (AGN). At X-rays, the way in which these changes occur is not yet clear. In the particular case of low ionisation nuclear emission line region (LINER) nuclei, variations on months/years timescales have been found for some objects, but the main driver of these changes is still an open question. The main purpose of this work is to investigate the X-ray variability in LINERs, including the main driver of such variations, and to search for eventual differences between type 1 and 2 objects. We use the 18 LINERs in the Palomar sample with data retrieved from Chandra and/or XMM-Newton archives corresponding to observations gathered at different epochs. All the spectra for the same object are simultaneously fitted in order to study long term variations. The nature of the variability patterns are studied allowing different parameters to vary during the spectral fit. Whenever possible, short term variations from the analysis of the light curves and UV variability are studied.Comment: 49 pages, accepted. arXiv admin note: text overlap with arXiv:1305.222

Crisis del franquismo, conflictividad social y cambio democrático en España. Un análisis desde las fuentes diplomáticas británicas

13 páginas.Peer reviewe