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GRAVITY DEPENDENCE OF MICROTUBULE PREPARATIONS.  James Tabony.  Commissariat а l'Energie Atomique, Dйpartement Rйponse et Dynamique Cellulaires, Laboratoire d'Immunochimie, INSERM U548, D.S.V, C.E.A. Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France.

   This article presents a molecular mechanism by which individual cells might be affected by weightlessness. Weak external fields, such as gravity, are not normally considered as capable of intervening in chemical and biochemical reactions. One possible manner by which they might effect them is by participating in certain types of reaction-diffusion mechanisms that lead to a macroscopic self-organisation of the preparation. Under appropriate conditions, self-organisation can depend on the presence of a weak external field, such as gravity, at a critical moment early in the process.
   The in vitro formation of microtubules, an important element of the cellular skeleton, show this type of behaviour. Samples progressively self-organise by way of a combination of microtubule reaction dynamics and molecular diffusion. The morphology of the state that forms depends upon the presence of a small external factor, such as gravity, magnetic fields, or sheering, at a critical time early in the process. In this system, ground experiments with a rotating clinostat, and under conditions of magnetic levitation, demonstrate that the effect of weightlessness can be studied without the expense of space flight.
   At a molecular level, the gravity dependence of self-organisation results from an interaction of gravity with macroscopic concentration and density fluctuations created by microtubule contraction and elongation. Numerical simulations predict macroscopic self-organisation in qualitative agreement with experiment and account for how self-organisation is triggered by weak external factors.