SPATIAL ATTRACTORS IN AGGREGATION PATTERNS OF DICTYOSTELIUM-DISCOIDEUM

Chemotactic cell motion in aggregation patterns of the slime mould Dic tyostelium discoideum is analyzed by a computerized cross-correlation method. In this excitable medium the movement of amoebae is selforgani zed by signals of cAMP that propagate as rotating spirals or expanding concentric circles (target patterns). A vortex-like rotation of cells is found close to the core of spiral waves, with a maximum velocity o f 15 mu m/min. Cell motion and spiral tip orbiting follow an opposite sense of rotation. The calculation of streamlines of the most probable trajectories reveals the exsitence of a closed curve (radius approxim ate to 130 mu m) corresponding to the boundary of the spiral core. Thi s spatial-limit cycle attracts the amoebae on rotational paths and lea ds to the formation of a cell-free disk in the centre of the pattern. In contrast, pacemakers of target patterns organize eel movement in a radial, star-shaped motion, leading directly to the formation of centr al mounds.