Self-organized Vortex State in Two-dimensional Dictyostelium Dynamics

We present results of experiments on the dynamics of Dictyostelium discoideum in a novel set-up which constraints cell motion to a plane. After aggregation, the amoebae collect into round ''pancake" structures in which the cells rotate around the center of the pancake. This vortex state persists for many hours and we have explicitly verified that the motion is not due to rotating waves of cAMP. To provide an alternative mechanism for the self-organization of the Dictyostelium cells, we have developed a new model of the dynamics of self-propelled deformable objects. In this model, we show that cohesive energy between the cells, together with a coupling between the self-generated propulsive force and the cell's configuration produces a self-organized vortex state. The angular velocity profiles of the experiment and of the model are qualitatively similar. The mechanism for self-organization reported here can possibly explain similar vortex states in other biological systems.Comment: submitted to PRL; revised version dated 3/8/9

Geometric origin of mechanical properties of granular materials

Some remarkable generic properties, related to isostaticity and potential energy minimization, of equilibrium configurations of assemblies of rigid, frictionless grains are studied. Isostaticity -the uniqueness of the forces, once the list of contacts is known- is established in a quite general context, and the important distinction between isostatic problems under given external loads and isostatic (rigid) structures is presented. Complete rigidity is only guaranteed, on stability grounds, in the case of spherical cohesionless grains. Otherwise, the network of contacts might deform elastically in response to load increments, even though grains are rigid. This sets an uuper bound on the contact coordination number. The approximation of small displacements (ASD) allows to draw analogies with other model systems studied in statistical mechanics, such as minimum paths on a lattice. It also entails the uniqueness of the equilibrium state (the list of contacts itself is geometrically determined) for cohesionless grains, and thus the absence of plastic dissipation. Plasticity and hysteresis are due to the lack of such uniqueness and may stem, apart from intergranular friction, from small, but finite, rearrangements, in which the system jumps between two distinct potential energy minima, or from bounded tensile contact forces. The response to load increments is discussed. On the basis of past numerical studies, we argue that, if the ASD is valid, the macroscopic displacement field is the solution to an elliptic boundary value problem (akin to the Stokes problem).Comment: RevTex, 40 pages, 26 figures. Close to published paper. Misprints and minor errors correcte

A 13C solid-state NMR study of ion-exchange resins derived from natural polysaccharides

Several ion-exchange resins and resin precursor materials formed from natural polysaccharides have been examined by solid-state NMR spectroscopy. NMR spectra of the dry resin contained broad, featureless bands due to the polysaccharide backbone of the resin and additional broad bands due to attached groups. On addition of water to the resin, the resonances for the polysaccharide backbone remained broad, but those due to the attached groups were considerably sharpened. Many individual resonances of the attached groups could thus be assigned, so that changes brought about by chemical modification of the polysaccharide during the production of an ion-exchange resin could be followed

Discriminatory effect of anti-Pr-β-hCG-TT antibodies on the neutralization of the biological activity of placental and pituitary gonadotropins

Serum from monkeys and human subjects immunized with an anti-hCG vaccine, Pr-β-hCG-TT, have been analyzed for their capacity to neutralize hCG/LH induced biological effects in two test systems. The monkey antiserum blocked completely the hCG induced ovulation in mice, but the same amount of the antiserum and even two-fold higher concentrations, did not reduce the number of ovulating animals on priming with ovine LH. The immunized monkey and the human sera were competent to neutralise the hCG induced testosterone production in Leydig cells. The monkey and seven out of twelve human sera did not interfere in the hLH action on these cells. Five human sera, however, showed varying degree of inhibition of hLH induced steroidogenesis in this sensitive bioassay. These subjects nevertheless maintained regular menstrual cycles and serum progesterone levels in the luteal phase were consonant with ovulation. In a subject whose serum had shown a fairly high degree of cross-reaction with hLH in Leydig cell bioassay, estradial, hLH and progesterone levels were determined on different days throughout a menstrual cycle. The hormonal profiles were normal