Final answer:
The structures that serve as nucleation sites for the formation of microtubules in a centrosome are the centriole microtubules and the gamma tubulin rings.
Step-by-step explanation:
The structures that serve as nucleation sites for the formation of microtubules in a centrosome are the centriole microtubules and the gamma tubulin rings. The centrosome is composed of a pair of centrioles, which are cylindrical structures made up of nine triplets of microtubules. These centrioles act as origin points for the microtubules to extend from and form the mitotic spindle during cell division. The gamma tubulin rings are also found in the centrosome and provide a template for the assembly of microtubules.