Order Out of Chaos by Ilya Prigogine & Isabelle Stengers

Author:Ilya Prigogine & Isabelle Stengers
Language: eng
Format: epub
Publisher: Verso Books

Figure A

When coming out of spores the amoebae grow and multiply as unicellular organisms. This situation extends until food, principally furnished by bacteria, becomes scarce. Then the amoebae cease to reproduce and enter into an interphase that lasts some eight hours. At the end of this period the amoebae begin to aggregate around cells that behave as aggregation centers. The aggregation occurs in response to chemotactic signals emitted by the centers. The aggregate thus formed migrates until the conditions for the formation of a fruiting body are satisfied. Then the mass of cells differentiates to form a stalk surmounted by a mass of spores.

In Dictyostelium discoideum, the aggregation proceeds in a periodic manner. Movies of aggregation process show the existence of concentric waves of amoebae moving toward the center with a periodicity of several minutes. The nature of the chemotactic factor is known: it is cyclic AMP (cAMP), a substance involved in numerous biochemical processes such as hormonal regulations. The aggregation centers release the signals of cAMP in a periodic fashion. The other cells respond by moving toward the centers and by relaying the signals to the periphery of the aggregation territory. The existence of a mechanism of relay of the chemotactic signals allows each center to control the aggregation of some 105 amoebae.

The analysis of a model of the process of aggregation reveals the existence of two types of bifurcations. First the aggregation itself represents a breaking of spatial symmetry. The second bifurcation breaks the temporal symmetry.

Initially the amoebae are homogeneously distributed. When some of them begin to secrete the chemotactic signals, there appear local fluctuations in the concentration of cAMP For a critical value of some parameter of the system (diffusion coefficient of cAMP, motility of the amoebae, etc.), fluctuations are amplified: the homogeneous distribution becomes unstable and the amoebae evolve toward an inhomogeneous distribution in space. This new distribution corresponds to the accumulation of amoebae around aggregation centers.

To understand the origin of the periodicity in the aggregation of D. discoideum, it is necessary to study the mechanism of synthesis of the chemotactic signal. On the basis of experimental observations one can describe this mechanism by the scheme of Figure B.

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