That is, it is what is to become a set of triplets of microtubules destined to form spindles of fibres that will separate chromosomes during mitosis and in order to act as anchoring sites for some proteins.
Procentrioles are generally assembled canonically in association with pre-existing centrioles that are loosely connected to each other.
Free microtubules elongate the initial cylindrical structure of procentrioles to form centrioles.
Instead of forming in the vicinity of pre-exisintg centrioles, this “de novo” pathway is activated during ciliogenesis, in which many procentrioles are synthesized in a single cell from electron-dense areas of the cytoplasm.
However, studies indicate that procentriole assembly via the canonical pathway is more stable for proliferating cells.