- 1. A small rounded mass within the cell nucleus where ribonucleoprotein is produced; it is usually single, but there may be several accessory nucleoli besides the principal one. The n. is composed of a meshwork (nucleolonema) of microfilaments and granules and the pars amorpha, now shown to have microfilaments also. 2. A more or less central body in the vesicular nucleus of certain protozoa in which an endosome is lacking but one or more Feulgen-positive (DNA+) nucleoli are present; characteristic of certain sporozoans, flagellates, opalinids, dinoflagellates, and radiolarians among the Protozoa. The chromatin material is distributed throughout the nucleus rather than peripherally, as in the endosome type of nucleus of Entamoeba. [L. dim of nucleus, a nut, kernel]- false n. SYN: karyosome.
* * *nu·cle·o·lus n(y)ü-'klē-ə-ləs n, pl -li -.lī a spherical body of the nucleus of most eukaryotes that becomes enlarged during protein synthesis, is associated with a nucleolus organizer, and contains the DNA templates for ribosomal RNA called also plasmosome
* * *n. (pl. nucleoli)a dense spherical structure within the cell nucleus that disappears during cell division. The nucleolus contains RNA for the synthesis of ribosome and plays an important part in RNA and protein synthesis.
* * *nu·cle·o·lus (noo-kleґə-ləs″) gen. and pl. nucleґoli [L., dim. of nucleus] 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. nucleolar adj
Prominent nucleoli (arrows) in the eccentric nuclei of ganglion cells.
Medical dictionary. 2011.