microtubule-organizing centres
The number, length, distribution and polarity of microtubules are largely controlled by microtubule-organizing centres, which nucleate and anchor microtubule minus ends in a process that requires gamma-tubulin.
References
Luders J, Stearns T. Microtubule-organizing centres: a re-evaluation. Nat Rev Mol Cell Biol. 2007 Feb;8(2):161-7. PMID: 17245416