Nucleolus

The nucleolus is a dark, dense, roughly spherical area of fibers and granules in the nucleus. Only plant and animal nuclei contain one or more nucleoli, although some do not. No membrane separates the nucleolus from the nucleoplasm. The nucleolus mediates the ribosomal RNA biogenesis. It is organized from the "nucleolar organizing regions" on a number of different chromosomes which cluster and transcribe ribosomal RNA. These regions are seen as circular areas surrounded by a rim of electron dense filaments, collectively called the pars fibrosa (PF), which are formed from newly transcribed ribosomal RNA. PF then link to proteins and leads to accumulation of ribonucleoprotein particles in the pars granulosa (PG). These particles form the large and small ribosomal subunits which are then transported out of the nuclear pores separately. The pores do not accommodate the assembled ribosomes, therefore, they cannot reenter.

Source:http://purl.uniprot.org/locations/188