6,617 research outputs found
Cellular pattern formation during Dictyostelium aggregation
The development of multicellularity in the life cycle of Dictyostelium discoideum provides a paradigm model system for biological pattern formation. Previously, mathematical models have shown how a collective pattern of cell communication by waves of the messenger molecule cyclic adenosine 3′5′-monophosphate (cAMP) arises from excitable local cAMP kinetics and cAMP diffusion. Here we derive a model of the actual cell aggregation process by considering the chemotactic cell response to cAMP and its interplay with the cAMP dynamics. Cell density, which previously has been treated as a spatially homogeneous parameter, is a crucial variable of the aggregation model. We find that the coupled dynamics of cell chemotaxis and cAMP reaction-diffusion lead to the break-up of the initially uniform cell layer and to the formation of the striking cell stream morphology which characterizes the aggregation process in situ. By a combination of stability analysis and two-dimensional simulations of the model equations, we show cell streaming to be the consequence of the growth of a small-amplitude pattern in cell density forced by the large-amplitude cAMP waves, thus representing a novel scenario of spatial patterning in a cell chemotaxis system. The instability mechanism is further analysed by means of an analytic caricature of the model, and the condition for chemotaxis-driven instability is found to be very similar to the one obtained for the standard (non-oscillatory) Keller-Segel system. The growing cell stream pattern feeds back into the cAMP dynamics, which can explain in some detail experimental observations on the time evolution of the cAMP wave pattern, and suggests the characterization of the Dictyostelium aggregation field as a self-organized excitable medium
Cavity optomechanics with ultra-high Q crystalline micro-resonators
We present the first observation of optomechanical coupling in ultra-high Q
crystalline whispering-gallery-mode (WGM) resonators. The high purity of the
crystalline material enables optical quality factors in excess of 10^{10} and
finesse exceeding 10^{6}. Simultaneously, mechanical quality factors greater
than 10^{5} are obtained, still limited by clamping losses. Compared to
previously demonstrated cylindrical resonators, the effective mass of the
mechanical modes can be dramatically reduced by the fabrication of CaF2
microdisc resonators. Optical displacement monitoring at the 10^{-18}
m/sqrt{Hz}-level reveals mechanical radial modes at frequencies up to 20 MHz,
corresponding to unprecedented sideband factors (>100). Together with the weak
intrinsic mechanical damping in crystalline materials, such high sindeband
factors render crystalline WGM micro-resonators promising for backaction
evading measurements, resolved sideband cooling or optomechanical normal mode
splitting. Moreover, these resonators can operate in a regime where
optomechanical Brillouin lasing can become accessible
Interplay of cell-cell signalling and multicellular morphogenesis during Dictyostelium aggregation
The cellular slime mould Dictyostelium discoideum provides a paradin model system for the study of multicellular pattern formation. Its life cycle involves a route to primitive multicellular organization which has independently evolved in terrestrial species of at least four groups of microorganisms (myxobacteria, acrasiomycota-the cellular slime moulds, myxomycota, and ciliata [1]). In these species a large number of single cells (nuclei in myxomycota) form through a process of aggregation and differentiation, a fruiting body
Global surfaces of section in the planar restricted 3-body problem
The restricted planar three-body problem has a rich history, yet many
unanswered questions still remain. In the present paper we prove the existence
of a global surface of section near the smaller body in a new range of energies
and mass ratios for which the Hill's region still has three connected
components. The approach relies on recent global methods in symplectic geometry
and contrasts sharply with the perturbative methods used until now.Comment: 11 pages, 1 figur
Complement factor h-antibody-associated hemolytic uremic syndrome: pathogenesis, clinical presentation, and treatment.
The presence of circulating autoantibodies, primarily to complement factor H antibodies (CFH-Abs) in plasma characterizes the autoimmune form of atypical hemolytic uremic syndrome (aHUS). This acquired form of aHUS defines a distinct subgroup of aHUS patients, which requires diagnostic and treatment approaches in part different from those of the genetically defined forms. The mechanisms leading to CFH-Ab production and disease onset are not completely understood, but CFH-Ab HUS seems to be secondary to a combination of genetic predisposition and environmental factors. Early diagnosis of this specific aHUS entity is important, as prompt induction of plasma exchange and concomitant immunosuppression leads to a favorable outcome. Nevertheless, information on clinical features and outcome in children is limited. Here, we review the literature on the biological and clinical features of CFH-Ab HUS and discuss therapeutic options
Torque magnetometry on single-crystal high temperature superconductors near the critical temperature: a scaling approach
Angular-dependent magnetic torque measurements performed near the critical
temperature on single crystals of HgBa_{2}CuO_{4+y}, La_{2-x}Sr{x}CuO_{4}, and
YBa_{2}Cu_{3}O_{6.93} are scaled, following the 3D XY model, in order to
determine the scaling function dG^{\pm}(z)/dz which describes the universal
critical properties near T_{c}. A systematic shift of the scaling function with
increasing effective mass anisotropy \gamma = (m_{ab}*/m_{c}*)^{1/2} is
observed, which may be understood in terms of a 3D-2D crossover. Further
evidence for a 3D-2D crossover is found from temperature-dependent torque
measurements carried out in different magnetic fields at different field
orientations \delta, which show a quasi 2D "crossing region'' (M*,T*). The
occurrence of this "crossing phenomenon'' is explained in a phenomenological
way from the weak z dependence of the scaling function around a value z = z*.
The "crossing'' temperature T* is found to be angular-dependent. Torque
measurements above T_{c} reveal that fluctuations are strongly enhanced in the
underdoped regime where the anisotropy is large, whereas they are less
important in the overdoped regime.Comment: 9 pages, 10 figures, submitted to PR
Isotope effects in underdoped cuprate superconductors: a quantum phenomenon
We show that the unusual doping dependence of the isotope effects on
transition temperature and zero temperature in - plane penetration depth
naturally follows from the doping driven 3D-2D crossover, the 2D quantum
superconductor to insulator transition (QSI) in the underdoped limit and the
change of the relative doping concentration upon isotope substitution. Close to
the QSI transition both, the isotope coefficient of transition temperature and
penetration depth approach the coefficient of the relative dopant
concentration, and its divergence sets the scale. These predictions are fully
consistent with the experimental data and imply that close to the underdoped
limit the unusual isotope effect on transition temperature and penetration
depth uncovers critical phenomena associated with the quantum superconductor to
insulator transition in two dimensions.Comment: 6 pages, 3 figure
Probing superconductivity in MgB2 confined to magnetic field tuned cylinders by means of critical fluctuations
We report and analyze reversible magnetization measurements on a high quality
MgB2 single crystal in the vicinity of the zero field transition temperature,
T_c=38.83 K, at several magnetic fields up to 300 Oe, applied along the c-axis.
Though MgB2 is a two gap superconductor our scaling analysis uncovers
remarkable consistency with 3D-xy critical behavior, revealing that close to
criticality the order parameter is a single complex scalar as in 4He. This
opens up the window onto the exploration of the magnetic field induced finite
size effect, whereupon the correlation length transverse to the applied
magnetic field H_i applied along the i-axis cannot grow beyond the limiting
magnetic length L_Hi, related to the average distance between vortex lines. We
find unambiguous evidence for this finite size effect. It implies that in type
II superconductors, such as MgB2, there is the 3D to 1D crossover line H_pi and
xi denotes the critical amplitudes of the correlation lengths above and below
T_c along the respective axis. Consequently, above H_pi(T) and T<T_c
superconductivity is confined to cylinders with diameter L_Hi (1D). In
contrast, above T_c the uncondensed pairs are confined to cylinders.
Accordingly, there is no continuous phase transition in the (H,T)-plane along
the H_c2-lines as predicted by the mean-field treatment
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