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Medical Term:


Pronunciation: nu-kle'o-lus, -li


  1. A small, rounded mass within the cell nucleus where ribosomal ribonucleoprotein is produced; it is usually single, but there may be several not bound to membrane accessory nucleoli besides the principal one. The nucleolus 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]

© Copyright 2016 Wolters Kluwer. All Rights Reserved. Review Date: Sep 19, 2016.