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.he 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.