Nucleolus

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Overview of the cell and its contents.

The nucleolus (plural: nucleoli) is a dark-staining, typically spherical body within the nucleus of a cell. It is a dense non-membrane-bound structure which forms a specialized subdomain of the nucleus that is responsible for the synthesis of ribosomal subunits.

Each nucleolus is composed of an aggregate of ribosomal genes, newly synthesized ribosomal RNA (rRNA), ribosomal proteins and ribonucleoproteins. The densely concentrated rRNA that is complexed into ribosomal subunits is responsible for the intense basophilia (dark-staining) of nucleolus.

The main function of the nucleolus is to synthesize both small and large subunits of ribosomes. These ribosomal subunits are ultimately transported out of the nucleus into the cytoplasm, where they are assembled into ribosomes.

It is important to note that not all cells have a nucleolus and that the presence and number of nucleoli is an indication of the protein synthetic activity of a cell. Thus, cells such as nerve cells contain nucleoli whereas sperm cells have no nucleoli as they do not produce proteins. Additionally, the nucleolus may change in appearance depending on the functional state of the cell. For example, an inactive fibroblast will usually have a small nucleolus while a fibroblast activated during wound healing, for instance, has a prominent nucleolus.