Nature Journal Club

Dirk Brockmann

Max-Planck Institute for Dynamics and Self-Organization, Göttingen, Germany

A physicist enthuses about criticality in biological development.

Physicists often overestimate the impact of their work on biological research. A biologist recently joked to me that physicists are rather like consultants: they appear without being asked and don’t tell you anything new. As a physicist studying the spread of infectious diseases, I reckon there is some truth in this.

But biologists can underestimate our insights, too. The joke turned my mind to a paper by three physicists who applied the theory around spontaneous symmetry breaking to the development of body axes (J. Soriano et al. Phys. Rev. Lett. 97, 258102; 2006).

Spontaneous symmetry breaking occurs in, for example, a cooling magnetic material. At high temperatures, magnetic spins are randomly arranged, but as the material cools patches form in which the spins are aligned. At a critical temperature, the spins align throughout the material. A small, external magnetic field can then determine the system’s fate, setting all the spins in a particular direction.

Soriano and his team studied symmetry breaking in developing hydra — multicellular organisms with clearly defined head and foot ends. Hydra can establish their body axis from a jumbled ball of cells, reminiscent of the way a magnetic material orders its spins as it cools. Patches of cells develop similar gene-expression profiles. This creates a system that is critically sensitive to tiny temperature gradients, which determine the direction of the body axis.

Impressively, Soriano and his team worked out the exponent in the size distribution of cell patches expressing a particular gene as a function of the age of the developing hydra. Through this, they related axis development to other self-organized critical systems physicists study, such as forest fires.

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