Nucleolus
From Ganfyd
A subnuclear organelle where ribosomal RNA is transcribed in the interphase cell from rDNA and processed as part of subunit ribosome assembly. The nucleolus is disassembled during mitosis at which time preribosomal RNA transcription and processing are suppressed, and it is reassembled at the end of mitosis.
Other functions include:
- Regulation of mitosis
- Cell-cycle progression and control of aging
- Cell proliferation (there tend to be more prominent nucleoli in actively proliferating cells like cancer cells, especially with classic silver staining)
- Many forms of stress response
- Viral infections
- Biogenesis of multiple ribonucleoprotein particles.
- Nuclear export
- Sequestration of regulatory molecules
- Modification of small nucleolar RNAs
The position of the nucleoli in the nucleus is determined by the position of a discrete chromosomal locus known as a nucleolus organizer region (NOR) . The NOR is a loci at which repetitive, nearly identical, rRNA genes, several hundred in number, are clustered in long tandem arrays, typically spanning millions of basepairs. Each rDNA sequence encodes a precursor transcript that is processed to generate one molecule each of 18S, 5.8S and 28S rRNA. Another key mechanism involving rRNA regulation is nucleolar dominance in which one parents NOR is suppressed.
Structure
In hematoxylin and eosin (H&E)-stained histological sections, the nucleoli are intensely stained by eosin because of their very high protein content. As such their existence inside the nucleus, if not function has been known for over 200 years. On electron microscopy the nucleolus has three substructures:
- Fibrillar centers, which appear as roundish structures of varying size, with a very low electron opacity
- These are areas of rRNA transcription from the NOR.
- A dense fibrillar component, which frequently constitutes a rim intimately associated with the fibrillar centers, composed of densely packed fibrils
- These are areas of rRNA transcription from the NOR.
- The granular component, composed of granules that surround the fibrillar components
- These are where the rRNA is further processed into the subcomponents of ribosomes that are exported
