Scale bar represents 10µm
The nucleolus is a non-membrane bound organelle inside the nucleus and it is responsible for the synthesis, processing, and assembly of ribosomes. It is also involved in several other cellular processes, such as mitosis, stress response, and cell cycle regulation. Structurally, the nucleoli consists of three subregions: the fibrillar centers, dense fibrillar components, and granular components (FC, DFC and GC).
Nucleoli Fibrillar center
The fibrillar center is a subcompartment of the nucleolus. It is the site of the first step of ribosome synthesis, where pre-ribosomal RNA (pre-rRNA) is transcribed from rDNA. The pre-rRNA is later modified in the dense fibrillar components before the ribosomes are assembled in the granular components.
Nucleoli are usually stained as small, slightly elongated circles in the nucleus. The number and shape of the nucleoli depend a lot on the cell type. Some proteins localize to the rim of the nucleolus, which is visible as a thin circle around the nucleolus.
The staining for fibrillar center and/or dense fibrillar components appears as a spotty cluster in most cell lines but can also appear as a single, bigger spot in other cell lines.
Read more about the proteome of the nucleoli.