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 t hrough 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-organi sation occurs when the microtubules are assembled under low gravity conditi ons. At a molecular level this behaviour results from an interaction of gra vity with macroscopic concentration and density fluctuations arising from p rocesses of microtubule contraction and elongation. Numerical simulations o f reaction and diffusion in a population of microtubules, leads to macrosco pic self-organisation when a small effect somehow creates an asymmetry in t he direction of microtubule growth. This can arise either by a force that s lightly favours some microtubule orientations or by an asymmetry in molecul ar transport and diffusion such as may arise in the presence of gravity,