The nucleolus is contained within the cell nucleus. The nucleolus (/nuːˈkliːɵləs/ ; plural nucleoli /nuːˈkliːɵlaɪ/) is the largest structure in the nucleus of eukaryotic cells, where it primarily serves as the site of ribosome synthesis and assembly.ucleoli also have other important functions like assembly of signal recognition particles and playing a role in the cell's response to stress. Nucleoli are made of proteins and RNA and form around specific chromosomal regions.