Growing up is a stressful process, particularly during embryonic development. The embryo starts out as a single symmetric cell. This cell can be considered as an active fluid bound by the cell membrane. Flow within this fluid leads to the build-up of stresses, the formation of patterns and asymmetries within the cell. Stephan Grill, at the Max-Planck Insitute for the Physics of complex systems and the Max-Planck Institute for Molecular Cell Biology and Genetics, Dresden, is interested in understanding how these flows lead to the polarisation observed in the Caenorhabditis elegans zygote.
Grill has shown that the changes in cell polarity are driven by myosin flow on the surface of the cell. The cortex can be considered as a dynamic self-contracting polymer gel surface, which lies underneath the cell membrane. This polymer gel layer behaves as a thin film of an active fluid. Passive advective transport of molecules – in this case myosin – embedded in the fluid can occur depending on the diffusivity and the flow velocities of the molecules. In C. elegans the flows are fast enough that advection does play a role, influencing the distribution of the molecules as they diffuse on the cortex. Modelling shows that the passive advective transport by flow of such a mechanically active materials acts as a trigger for the segregation of the proteins, resulting in the polarisation of the zygote.
The movement of myosin across the surface of the cell also results in anisotropies in the cortical tension. These so called active stresses cause isolated sections of the cortex to self-contract. Grill has developed a novel method for locally determining the stresses, by cutting the cortex with a laser and measuring the recoil. The cortical tension is found to be greatest in the direction orthogonal to the flow.
Grill suggests that advective transport in active fluids is a general mechanism for the formation of patterns in developmental biology.
For more information see:
Goehring, N.W. et al., Polarization of PAR Proteins by Advective Triggering of a Pattern-Forming System, Science, 2011.
Bois, S, et al., Pattern Formation in Active Fluids, Phys. Rev. Lett., 2011.
Mayer. M, et al., Anisotropies in cortical tension reveal the physical basis of polarizing cortical flows, Nature, 2010.
The image above is taken from: Cyclodextrin/dextran based drug carriers for a controlled release of hydrophobic drugs in zebrafish embryos, Soft Matter, 2011, and shows C. elegans embryo 30hrs after fertilisation.