Symmetry breaking, aggregation and pattern formation

The fascinating collective behavior of Dictyostelium discoideum is known since the 1940s. [1]. Physicists became interested in Dictyostelium discoideum in the early '90s to investigate the patterns formed by the cells under starvation conditions that the cells formed when starved. [2] After about 4 hours of starvation, individual Dictyostelium begin to emit cAMP and become sensitive to external cAMP gradients. A cell that senses a cAMP gradient also emits cAMP in turn and crawls up the gradient that it has sensed. As a result of the cell shape changes induced hereby, spiral waves and target patterns of cAMP can be observed. (Fig. 1) The cells aggregate and form streams and then mounds. (Fig. 2)

Figure 1:

The spirals, as seen under a dark field microscope, are formed after a few hours of starving the Dictyostelium discoideum cells.

Figure 2:

The streaming and aggregation patterns of Dictyostelium discoideum cells.

Our work on aggregation consists of varying the experimental conditions of starvation and explaining the patterns observed. We vary the cell density, the substrate stiffness and the average concentration of cAMP already in the medium. We also expose the cells to a gentle flow of buffer to see how the aggregation pattern evolves. Another part of the work consists of observing cell aggregation in 3D.

We observe that, as agar concentration in the substrate increases, the spiral waves give way to target patterns (youtube video links 1 and 2) and that the time taken for synchronisation of the amoebae increases. The increase in concentration of agar in the substrate is associated with changes in the diffusion coefficient, increase in Young's modulus of the substrate, cell-substrate adhesion, oscillation time period, entropy, wave vector etc. This can be compared to existing theoretical models. [3] To estimate the effect of these parameters, we perform further experiments like FRAP, single cell force spectroscopy with AFM and simulations based on the reaction-diffusion system of Martiel – Goldbeter [4].

Contact: Kaumudi Prabhakara, Azam Gholami, Eberhard Bodenschatz

[1]. F. Alcantara and M. Monk, Journal of General Microbiology 85 321-334 (1974).
[2] J.L. Martiel and A. Goldbeter, Biophysical Journal 52 807-828 (1987).
[3]. J.J. Tyson et al, Physica D 34 193-207 (1989).
[4]. J. Lauzeral et al, Proc. Natl. Acad. Sci., 94 9153-9158 (1997).