Figure 8. An initially homogeneous and isotropic solution of microtubules might grow into an oriented state if growth of a given filament requires not finding other filaments on its path as it grows.
We study  a microtubule growth model where the increase in size of the plus-end is perturbed by collisions with other microtubules and show that such a simple mechanism of constrained growth can induce ordered structures and patterns from an initially isotropic and homogeneous suspension.
 Baulin, V.A.; Marques, C.M.; Thalmann, F. Collision induced spatial organization of microtubules, Biophys. Chemist., 128, 231 (2007).