One of the central questions in biology concerns the formation of complex highly organized microscopic structures from initially disordered states. Complex ordered structures are established and maintained through a dynamic interplay between self-assembly and regulatory processes.

Here we introduce a model for spatio-temporal self-organization of an ensemble of microtubules interacting via molecular motors. Starting from a generic stochastic model of inelastic polar rods with an anisotropic interaction kernel we derive a set of equations for the local rods concentration and orientation. At large enough mean density of rods and concentration of motors, the model describes orientational instability. We demonstrate that the orientational instability leads to the formation of vortices and (for large density and/or kernel anisotropy) asters seen in recent experiments. The corresponding phase diagram of vortex-asters transitions is in qualitative agreement with experiment.