(redirected from nucleoli)
Also found in: Dictionary, Thesaurus, Medical.


see cellcell,
in biology, the unit of structure and function of which all plants and animals are composed. The cell is the smallest unit in the living organism that is capable of integrating the essential life processes. There are many unicellular organisms, e.g.
..... Click the link for more information.



the dense light-refracting body within the cell nucleus of eukaryotes. The nucleolus consists mainly of ribo-nucleoproteins, that is, proteins that contain ribonucleic acid (RNA). Usually there are one to three nucleoli in a nucleus. In a few organisms (amoebas, testaceans) and in the growing egg cells of certain vertebrates there are many nucleoli in the nucleus. Sometimes the nucleolus consists of two differently staining parts (amphinucleoli).

The morphology of the nucleolus depends on the functional state of the cell. During mitosis, the nucleolus usually decomposes; an exception is the nucleolus of many protozoans. In the telophase of mitosis, the nucleolus forms anew in special areas of the nucleolar chromosomes, in which the genes that encode riboso-mal RNA are located. The numerous marginal nucleoli of egg cells are not directly connected to the chromosomes; they are formed on nonchromosomal copies of the ribosomal gene, which are first synthesized on the nucleolar chromosome.

The following inclusions can be seen in the nucleolus with an electron microscope: strands of intranucleolar chromatine containing DNA and corresponding either to the nucleolar chromosome itself or to its nonchromosomal copies, a zone of ribonu-cleoprotein fibers 5–10 nanometers (nra) thick, and a zone of ribonucleoprotein granules 15–20 nm thick (usually on the periphery of the nucleolus).

Molecules of preribosomal RNA, with a sedimentation constant of 45S and located in the fibrillar zone of the nucleolus, are synthesized on the genes of the intranucleolar chromatin. The preribosomal-RNA molecules are cleaved to produce molecules of ribosomal RNA with sedimentation constants of 18S and 28S; upon uniting with proteins, the molecules coalesce into granules, which are the precursors of the small and large subunits of the ribosomes. The granules separate from the nucleolus and enter the cytoplasm through the pores of the nuclear membrane. The ribosomes are assembled in the cytoplasm.


Busch, H., and K. Smetana. The Nucleolus. New York-London, 1970.
See references under .



(cell and molecular biology)
A small, spherical body composed principally of protein and located in the metabolic nucleus. Also known as plasmosome.
References in periodicals archive ?
Cytologic examination of fine-needle aspirates in birds can show pleomorphic cells with anisonucleosis and irregular nuclear membranes, dense chromatin, and visible nucleoli as well as mitotic figures.
While a study conducted by Sahaet al in 201325 concluded that all the cytological parameters except cell size and nucleoli had high significance in predicting the tumor grade.
Some of the tumour cells had irregular nuclear membrane and inconspicuous nucleoli.
The small, dark pyknotic nuclei had disrupted nuclear membranes and were devoid of nucleoli (Fig.
Breast incisional biopsy showed monomorphic population large cell with multiple hyperchromatic nucleoli along with breast tissue (Figure 2).
This stage could be divided into early, middle and late phases according to the size and the shape of the oocytes, the appearance of ovarian lamellae, and the size and number of the nucleoli.
On fine needle aspiration cytology adenomatoid tumors reveal abundant cellularity and comprise sheets and multilayered clusters of monotonous cells with round or ovoid, eccentric nuclei containing small central nucleoli.
The team found that deletion of the Sirt1 gene in mice resulted in PIN formation; features included cellular hyperplasia, increased Ki67 staining, hyperchromatic nuclei and prominent nucleoli, as well as a reduced size in prostate.
The technique of Ag-NOR (silver--nucleolar organiser region), widely used in cytogenetics, was originally used by Goodpasture and Boom (1975) in order to detect nucleolar organiser regions (NORs) in chromosomes and nucleoli in interphase nuclei, thus providing a visualisation of their functional state.