118,052 research outputs found
Signal Processing during Developmental Multicellular Patterning
Developing design strategies for tissue engineering and regenerative medicine is limited by our nascent understanding of how cell populations self-organize into multicellular structures on synthetic scaffolds. Mechanistic insights can be gleaned from the quantitative analysis of biomolecular signals that drive multicellular patterning during the natural processes of embryonic and adult development. This review describes three critical layers of signal processing that govern multicellular patterning: spatiotemporal presentation of extracellular cues, intracellular signaling networks that mediate crosstalk among extracellular cues, and finally, intranuclear signal integration at the level of transcriptional regulation. At every level in this hierarchy, the quantitative attributes of signals have a profound impact on patterning. We discuss how experiments and mathematical models are being used to uncover these quantitative features and their impact on multicellular phenotype
The influence of receptor-mediated interactions on reaction-diffusion mechanisms of cellular self-organisation
Understanding the mechanisms governing and regulating self-organisation in the developing embryo is a key challenge that has puzzled and fascinated scientists for decades. Since its conception in 1952 the Turing model has been a paradigm for pattern formation, motivating numerous theoretical and experimental studies, though its verification at the molecular level in biological systems has remained elusive. In this work, we consider the influence of receptor-mediated dynamics within the framework of Turing models, showing how non-diffusing species impact the conditions for the emergence of self-organisation. We illustrate our results within the framework of hair follicle pre-patterning, showing how receptor interaction structures can be constrained by the requirement for patterning, without the need for detailed knowledge of the network dynamics. Finally, in the light of our results, we discuss the ability of such systems to pattern outside the classical limits of the Turing model, and the inherent dangers involved in model reduction
L-selectin mediated leukocyte tethering in shear flow is controlled by multiple contacts and cytoskeletal anchorage facilitating fast rebinding events
L-selectin mediated tethers result in leukocyte rolling only above a
threshold in shear. Here we present biophysical modeling based on recently
published data from flow chamber experiments (Dwir et al., J. Cell Biol. 163:
649-659, 2003) which supports the interpretation that L-selectin mediated
tethers below the shear threshold correspond to single L-selectin carbohydrate
bonds dissociating on the time scale of milliseconds, whereas L-selectin
mediated tethers above the shear threshold are stabilized by multiple bonds and
fast rebinding of broken bonds, resulting in tether lifetimes on the timescale
of seconds. Our calculations for cluster dissociation suggest that
the single molecule rebinding rate is of the order of Hz. A similar
estimate results if increased tether dissociation for tail-truncated L-selectin
mutants above the shear threshold is modeled as diffusive escape of single
receptors from the rebinding region due to increased mobility. Using computer
simulations, we show that our model yields first order dissociation kinetics
and exponential dependence of tether dissociation rates on shear stress. Our
results suggest that multiple contacts, cytoskeletal anchorage of L-selectin
and local rebinding of ligand play important roles in L-selectin tether
stabilization and progression of tethers into persistent rolling on endothelial
surfaces.Comment: 9 pages, Revtex, 4 Postscript figures include
Long-Range Exciton Diffusion in Two-Dimensional Assemblies of Cesium Lead Bromide Perovskite Nanocrystals
F\"orster Resonant Energy Transfer (FRET)-mediated exciton diffusion through
artificial nanoscale building block assemblies could be used as a new
optoelectronic design element to transport energy. However, so far nanocrystal
(NC) systems supported only diffusion length of 30 nm, which are too small to
be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion
length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional
assembly of cesium lead bromide perovskite nanocrystals (PNC). Exciton
diffusion was directly measured via steady-state and time-resolved
photoluminescence (PL) microscopy, with physical modeling providing deeper
insight into the transport process. This exceptionally efficient exciton
transport is facilitated by PNCs high PL quantum yield, large absorption
cross-section, and high polarizability, together with minimal energetic and
geometric disorder of the assembly. This FRET-mediated exciton diffusion length
matches perovskites optical absorption depth, opening the possibility to design
new optoelectronic device architectures with improved performances, and
providing insight into the high conversion efficiencies of PNC-based
optoelectronic devices
Correlated Exciton Transport in Rydberg-Dressed-Atom Spin Chains
We investigate the transport of excitations through a chain of atoms with
non-local dissipation introduced through coupling to additional short-lived
states. The system is described by an effective spin-1/2 model where the ratio
of the exchange interaction strength to the reservoir coupling strength
determines the type of transport, including coherent exciton motion, incoherent
hopping and a regime in which an emergent length scale leads to a preferred
hopping distance far beyond nearest neighbors. For multiple impurities, the
dissipation gives rise to strong nearest-neighbor correlations and
entanglement. These results highlight the importance of non-trivial
dissipation, correlations and many-body effects in recent experiments on the
dipole-mediated transport of Rydberg excitations.Comment: 5 page
The effects of polydispersity and metastability on crystal growth kinetics
We investigate the effect of metastable gas-liquid (G-L) separation on
crystal growth in a system of either monodisperse or slightly size-polydisperse
square well particles, using a simulation setup that allows us to focus on the
growth of a single crystal. Our system parameters are such that, inside the
metastable G-L binodal, a macroscopic layer of the gas phase "coats" the
crystal as it grows, consistent with experiment and theoretical free energy
considerations. Crucially, the effect of this metastable G-L separation on the
crystal growth rate depends qualitatively on whether the system is
polydisperse. We measure reduced polydispersity and qualitatively different
local size ordering in the crystal relative to the fluid, proposing that the
required fractionation is dynamically facilitated by the gas layer. Our results
show that polydispersity and metastability, both ubiquitous in soft matter,
must be considered in tandem if their dynamical effects are to be understood.Comment: Published in Soft Matter. DOI: 10.1039/C3SM27627
Simulation of nanostructure-based high-efficiency solar cells: challenges, existing approaches and future directions
Many advanced concepts for high-efficiency photovoltaic devices exploit the
peculiar optoelectronic properties of semiconductor nanostructures such as
quantum wells, wires and dots. While the optics of such devices is only
modestly affected due to the small size of the structures, the optical
transitions and electronic transport can strongly deviate from the simple bulk
picture known from conventional solar cell devices. This review article
discusses the challenges for an adequate theoretical description of the
photovoltaic device operation arising from the introduction of nanostructure
absorber and/or conductor components and gives an overview of existing device
simulation approaches.Comment: Invited paper, accepted for publication in IEEE Journal of Selected
Topics in Quantum Electronic
The porin and the permeating antibiotic: A selective diffusion barrier in gram-negative bacteria
Gram-negative bacteria are responsible for a large proportion of antibiotic resistant bacterial diseases. These bacteria have a complex cell envelope that comprises an outer membrane and an inner membrane that delimit the periplasm. The outer membrane contains various protein channels, called porins, which are involved in the influx of various compounds, including several classes of antibiotics. Bacterial adaptation to reduce influx through porins is an increasing problem worldwide that contributes, together with efflux systems, to the emergence and dissemination of antibiotic resistance. An exciting challenge is to decipher the genetic and molecular basis of membrane impermeability as a bacterial resistance mechanism. This Review outlines the bacterial response towards antibiotic stress on altered membrane permeability and discusses recent advances in molecular approaches that are improving our knowledge of the physico-chemical parameters that govern the translocation of antibiotics through porin channel
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