The effect of gravity on microtubule self-organisation

The molecular processes by which gravity biological systems are effected by gravity are not understood. Theoreticians have proposed this might arise through the bifurcation proper-ties of certain types of non-linear chemical reactions that self-organise by reaction and diffusion. We have found that in-vitro preparations of microtubules, an important element of the cellular skeleton, show this type of behaviour. The solutions self-organise and the morphology that arises depend upon the sample orientation, with respect to gravity, at a critical moment at an early stage in the development of the self-organised state. As predicted by theories of this type, no self-organisation occurs when the microtubules are assembled under low gravity conditions. At a molecular level this behaviour results from an interaction of gravity with macroscopic concentration and density fluctuations arising from processes of microtubule contraction and elongation. Numerical simulations of reaction and diffusion in a population of microtubules, leads to macroscopic self-organisation when a small effect somehow creates an asymmetry in the direction of microtubule growth. This can arise either by a force that slightly favours some microtubule orientations or by an asymmetry in molecular transport and diffusion such as may arise in the presence of gravity,