Nucleoli

The nucleoli are non-membrane bound organelles inside the nucleus, which are responsible for the synthesis, processing and assembly of ribosomes. They are also involved in several other cellular processes, such as mitosis, stress response and cell cycle regulation. Structurally, the nucleolus consists of three sub-regions; the fibrillar center, dense fibrillar components and granular components (FC, DFC and GC).

Nucleoli Fibrillar center

The fibrillar center is a sub-compartment 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.

Immunofluorescent staining

In the Human Protein Atlas the chemical 4',6-diamidino-2-phenylindole (DAPI) is used to counterstain the nucleoplasm, as it will bind strongly to DNA, and hence not stain the nucleoli. Therefore it is easy to identify the nucleoli by taking caution to the DAPI staining. Nucleoli are usually stained as small, somewhat elongated circles in the nucleus. The number and shape of the nucleoli depends 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 of the fibrillar center and/or dense fibrillar components varies between cell lines. Usually it appears as a spotty cluster in the nucleoli but it can also appear as a single, bigger spot in some cell lines.

Read more about the proteome of the nucleoli.