Topological defects: A problem for cyclic universes?

We study the behaviour of cosmic string networks in contracting universes, and discuss some of their possible consequences. We note that there is a fundamental time asymmetry between defect network evolution for an expanding universe and a contracting universe. A string network with negligible loop production and small-scale structure will asymptotically behave during the collapse phase as a radiation fluid. In realistic networks these two effects are important, making this solution only approximate. We derive new scaling solutions describing this effect, and test them against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation background it has generated, can significantly affect the dynamics of the universe both locally and globally. The network can be an important source of radiation, entropy and inhomogeneity. We discuss the possible implications of these findings for bouncing and cyclic cosmological models.Comment: 11 RevTeX 4 pages, 6 figures; version to appear in Phys. Rev.

Cardiovascular development: towards biomedical applicability: Regulation of cardiomyocyte differentiation of embryonic stem cells by extracellular signalling

Investigating the signalling pathways that regulate heart development is essential if stem cells are to become an effective source of cardiomyocytes that can be used for studying cardiac physiology and pharmacology and eventually developing cell-based therapies for heart repair. Here, we briefly describe current understanding of heart development in vertebrates and review the signalling pathways thought to be involved in cardiomyogenesis in multiple species. We discuss how this might be applied to stem cells currently thought to have cardiomyogenic potential by considering the factors relevant for each differentiation step from the undifferentiated cell to nascent mesoderm, cardiac progenitors and finally a fully determined cardiomyocyte. We focus particularly on how this is being applied to human embryonic stem cells and provide recent examples from both our own work and that of others

VE-Cadherin Regulates Endothelial Actin Activating Rac and Increasing Membrane Association of Tiam

Previously published reports support the concept that, besides promoting homotypic intercellular adhesion, cadherins may transfer intracellular signals. However, the signaling pathways triggered by cadherin clustering and their biological significance are still poorly understood. We report herein that transfection of VE-cadherin (VEC) cDNA in VEC null endothelial cells induces actin rearrangement and increases the number of vinculin positive adhesion plaques. VEC expression augments the level of active Rac but decreases active Rho. Microinjection of a dominant negative Rac mutant altered stress fiber organization, whereas inhibition of Rho was ineffective. VEC expression increased protein and mRNA levels of the Rac-specific guanosine exchange factor Tiam-1 and induced its localization at intercellular junctions. In addition, in the presence of VEC, the amounts of Tiam, Rac, and the Rac effector PAK as well as the level of PAK phosphorylation were found increased in the membrane/cytoskeletal fraction. These observations are consistent with a role of VEC in localizing Rac and its signaling partners in the same membrane compartment, facilitating their reciprocal interaction. Through this mechanism VEC may influence the constitutive organization of the actin cytoskeleton