Physical aspects of the growth and regulation of microtubule structures.

The growth of microtubules through the so-called dynamical instability is analyzed within a simple theoretical model in which the polymers are nucleated by a flat surface. For an isolated microtubule the model predicts the existence of a transition between bounded and unbounded growth. It is also shown that this transition alters the assembly of dense structures, e.g., by drastically limiting the number of long microtubules grown from the surface. Coupled to the microscopic biochemical control of the growth, such physical eff'ects seem to play an important role in the regulation of the formation of cellular structures (such as the mitotic spindle).