Density Wave Propagation Research Articles

Analysis of Optical Density Wave Propagation and Cell Movement during Mound Formation inDictyostelium discoideum

Aggregation fields ofDictyosteliumamoebae are organized by propagating concentric or spiral waves of cAMP. These waves coordinate cell movement directed toward the aggregation center. We now systematically investigated dark-field wave propagation and chemotactic cell movement during late aggregation and mound formation. The period and the signal propagation velocity decreased continuously during aggregation leading to a 15-fold decrease of the chemical wavelength. By analyzing the behavior of single GFP-labeled cells in aggregates and mounds we measured cell movement velocity, changes in cell shape, periodicity of cell movement, and cell trajectories. In early mounds of strain AX-3 dark-field waves propagated frequently as multiarmed (high-frequency) spirals. During the high-frequency waves observed in the early mound stage, cell movement speed is low and cell movement rather undirected. During tip formation the wave period decreased again and the cells started to rotate in the mound at unusually high average speeds of 40 μm/min. The rotation was almost monotonic with no clear periodicity. Since at this time the majority of the cells had already differentiated into prespore cells, this implies that prespore cells moved faster than aggregation stage cells. At 12 hr of development cell movement velocity dropped again and became highly periodic. These measurements show that the relay system is characterized by a specific temporal evolution, which is closely correlated with cellular differentiation. The remarkable changes in cell movement speed and period indicate a qualitative change in signal and movement parameters which might well be caused by the observed switch from high- to low-affinity cAMP receptors during mound formation. This switch might be required to copy with the increase in cell density and most likely plays a crucial role in the process of cell sorting.

Analysis of optical density wave propagation and cell movement in the cellular slime mould Dictyostelium discoideum

We have studied optical density wave propagation during aggregation of the cellular slime mould Dictyostelium discoideum in a quantitative manner by digital image analysis. The waves are mostly single ended spiral waves starting from an aggregation center. We can measure a variety of parameters such as oscillation frequency, wave propagation velocity and wave shape. This allows the construction of dispersion curves under a variety of experimental conditions. During later development where the optical density waves are no longer visible we have started to measure movement of fluorescently labelled cells. Our main conclusions from these measurements are that the cells continue to move chemotactically to periodic signlas both in aggregates and in slugs. There is a dramatic difference in the movement pattern of prestalk and prespore cells: Prestalk cells move perpendicular to the long axis of the slug, they are most likely organized by a scroll wave. Prespore cells seem to move almost perpendicular to the prestalk cells, in the direction of the tip. This behaviour is explained on the basis of different relay properties of prespore and prestalk cells.

Analysis of optical density wave propagation and cell movement in the celllular slime mould [formula omitted

Analysis of optical density wave propagation and cell movement in the celllular slime mould [formula omitted

Digital image processing of optical density wave propagation in Dictyostelium discoideum and analysis of the effects of caffeine and ammonia

Waves of chemotactic movement during the early phase of aggregation in Dictyostelium discoideum were analyzed by digital image processing in a manner that immediately shows the following parameters: wave propagation velocity, period length, wave amplitude und wave shape. We have characterized the aggregation of AX-2 and the streamer F mutant NP 377 in terms of these parameters and investigated the influence of caffeine and ammonia. It was found that during normal aggregation oscillation frequency increases while at the same time wave propagation velocity decreases. Caffeine, a known inhibitor of cyclic AMP relay, reduces oscillation frequency and wave propagation velocity in a dose-dependent manner but most notably leads to the appearance of bimodal (harmonic) oscillations. These bimodal waves are also found in streamer F mutants without caffeine during early aggregation. The effect of caffeine is interpreted as an increase in the average chemotactic deadaptation time due to elevated cyclic GMP levels after a cyclic AMP stimulus. This increased deadaptation time results in some cells responding to every chemotactic signal, while others respond only to every second signal, leading to mixed population behavior and hence biphasic optical density waves. Ammonia has no significant influence on oscillation frequency and wave propagation velocity but shows a clear increase in the amplitude of the optical density waves. This may indicate a more vigorous chemotactic response by individual cells or a better synchronization of the responding cell populations due to shortened chemotactic deadaptation times.

Digital image processing of optical density wave propagation in Dictyostelium discoideum and analysis of the effects of caffeine and ammonia

ABSTRACT Waves of chemotactic movement during the early phase of aggregation in Dictyostelium discoideum were analyzed by digital image processing in a manner that immediately shows the following parameters: wave propagation velocity, period length, wave amplitude und wave shape. We have characterized the aggregation of AX-2 and the streamer F mutant NP 377 in terms of these parameters and investigated the influence of caffeine and ammonia. It was found that during normal aggregation oscillation frequency increases while at the same time wave propagation velocity decreases. Caffeine, a known inhibitor of cyclic AMP relay, reduces oscillation frequency and wave propagation velocity in a dose-dependent manner but most notably leads to the appearance of bimodal (harmonic) oscillations. These bimodal waves are also found in streamer F mutants without caffeine during early aggregation. The effect of caffeine is interpreted as an increase in the average chemotactic deadaptation time due to elevated cyclic GMP levels after a cyclic AMP stimulus. This increased deadaptation time results in some cells responding to every chemotactic signal, while others respond only to every second signal, leading to mixed population behavior and hence biphasic optical density waves. Ammonia has no significant influence on oscillation frequency and wave propagation velocity but shows a clear increase in the amplitude of the optical density waves. This may indicate a more vigorous chemotactic response by individual cells or a better synchronization of the responding cell populations due to shortened chemotactic deadaptation times.

Three‐dimensional acoustic modeling by the Fourier method

A three‐dimensional forward modeling algorithm, allowing arbitrary density and arbitrary wave propagation velocity in lateral and vertical directions, directly solves the acoustic wave equation through spatial and temporal discretization. Spatial partial differentiation is performed in the Fourier domain. Time stepping is performed with a second‐order differencing operator. Modeling includes an optional free surface above the spatial grid. An absorbing boundary is applied on the lateral and bottom edges of the spatial grid. Three‐dimensional forward modeling represents a challenge for computer technology. Computation of meaningfully sized models requires extensive calculations and large three‐dimensional data sets which must be retrieved and restored during the computation of each time step. The computational feasibility of the Fourier method is demonstrated by implementation on the multiprocessor CRAY X‐MP computer system using the large secondary memory of the solid‐state storage device (SSD). Calculations use vectorization and parallel processing architecture. The similarity of numerical and analytical results indicates sufficient accuracy for many applications.

Negative Momentum Relaxation Rate and Transport in Polar Semiconductors

AbstractThe possibility of a negative momentum relaxation rate by absorption of polar optical phonons in a non‐degenerate semiconductor is examined. Charge density wave propagation is predicted as a result of this. The wave sets up an average dc field in which the electrons respond in such a way that a dc current independent of the external field flows. This implies that a persistent current could flow in the solid.

On the tidal interaction between protoplanets and the primordial solar nebula. II - Self-consistent nonlinear interaction

A method to analyze the full nonlinear response and physical processes associated with the tidal interaction between a binary system and a thin disk in the steady state is presented. Using this approach, density wave propagation, induced by tidal interaction, may be studied for a wide range of sound speeds and viscosities. The effect of self-gravity may also be incorporated. The results of several calculations relevant to the tidal interaction between a protoplanet and the primordial solar nebula are also presented.

The role of dissipation in shepherding of ring particles

In the absence of any damping mechanism, a shepherd satellite would force oscillations in the motion of a ring particle (relative to circular motion) that are symmetrical with respect to the encounter geometry. No net torque would be exerted by the satellite on the rings. Only in the presence of some damping mechanism (such as density wave propagation or a dissipative medium) can a particle's response lag so as to provide the asymmetry that permits a torque. Remarkably, the standard formula for the confining torque exerted by a sheperd satellite seems to be independent of damping. Moreover, many heuristic derivations of the formula tend to obscure the role of damping. In fact, the torque on any given particle does depend on the degree of damping, but that dependence disappears when the torque is averaged over a range of orbits that span resonances if the degree of damping is within a certain range. If damping is too weak or too strong, the torque can be much less than is given by the standard formula.