Centrosomes are organelles responsible for the organization and nucleation of microtubules in animal cells and also regulate the cell cycle during cellular division. When the nuclear membrane breaks down during mitosis, the chromosomes interact with the centrosome nucleated microtubules to build the mitotic spindle. The centrosome plays a key role in efficient mitosis, but it is not considered essential.
The centrosome is a microtubule organizing center, or MTOC, comprised of two centrioles surrounded by a mass of protein called pericentriolar material, or PCM. During the prophase stage of mitosis, the centrosomes migrate to opposite ends of the cell and the mitotic spindle forms between them. Plants and fungi cells do not contain centrosomes and rely upon other MTOC structures when organizing their microtubules.