Quick Facts
The nucleolus is a rounded, dense body, present in the nucleus of eukaryotic cells during interphase, that is the site of ribosomal RNA (rRNA) synthesis and ribosomal subunit assembly. It consists of (a) a fibrillar center, containing the rRNA genes, surrounded by (b) the pars fibrosa, a dense fibrillar component where transcription occurs, and (c) the pars granulosa, a granular component where the ribosomal subunits are assembled. Multiple nucleoli occur in some cells (Dorland, 2011).
Structure and/or Key Features
The most predominant, non-membranous structure in the nucleus is the spherical nucleolus. It surrounds rRNA genes that are transcriptionally active. The nucleolus is the site of synthesis of ribosomal ribonucleic acid (rRNA), ribosomal production and assembly. The diameter of the nucleolus is 1–2 mm, but this varies within different populations of cells. In cells that are most active in synthesizing proteins, the nucleolus is well developed and prominent. It is not uncommon for some cells to contain more than one nucleolus (Mescher, 2013).
Like heterochromatin, the nucleolus stains with basic dyes and hematoxylin.
Anatomical Relations
The nucleolus is located at the center of the nucleus.
Function
The nucleolus synthesizes and assembles ribosomal ribonucleic acid (rRNA).
Clinical Correlates
Nucleoli house proteins for ribosome synthesis and oncogenes and tumor suppressor genes that control cell proliferation. Structural and functional faults in nucleoli have been the basis of a number of disease processes including some types of cancers (McKinley, O'Loughlin and Pennefather-O'Brien, 2016).
References
Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.
McKinley, M. P., O'Loughlin, V. D. and Pennefather-O'Brien, E. E. (2016) Human Anatomy. 5th edn.: McGraw-Hill Education.
Mescher, A. (2013) Junqueira's Basic Histology: Text and Atlas. 13th edn.: McGraw-Hill Education.
