 The first step in spindle formation is the nucleation of microtubules by microtubule-organizing centers, which then grow in all directions.Microtubules that attach to a chromosome are stabilized and are therefore retained to form part of the spindle.Because of dynamic instability, some individual microtubules that are not stabilized are at risk of collapse (or “catastrophe” as Kirschner named it), allowing re-use of the tubulin monomers.