(1) Fibrous part - Central part of nucleolus composed of fibers called nucleonema. Nucleonema mainly composed of r-DNA and proteins.
(2) Granular part - Peripheral part of nucleolus composed of granules (r-RNA + proteins) which represents maturing ribosomes.
(3) Amorphous matrix or Pars amorpha - A proteinaceous matrix which contains both fibers and granules. r-RNA 5.8s, 18s and 28s are synthesized in nucleolus. Ribosomal protein is synthesized in cytoplasm. Organisation of r-RNA and protein to form ribosomes occurs in nucleolus. Completely formed ribosomes then move to cytoplasm through nuclear pores. Nucleolus also play important role in initiation of cell division.