Centrosome.

The major microtubule organising centre (MTOC) in animal cells is the centrosome. The centrosome is composed of two centrioles surrounded by pericentriolar material (centrosome matrix) to which g-tubulin ring complexes (g- TURCs) are associated. The g-TURCs are responsible for nucleation of microtubules. Recruitment of g-TURCs to the centrosome explains the nucleation pattern of microtubules seen previously. However, although microtubule nucleation always requires g-TURC, not all microtubules are nucleated from the centrosome, as seen in the epithelial cell on the right.