Immunofluorescent images of formaldehyde-fixed cell lines are shown. Three different organelle markers are displayed as different channels in the multicolor images - nucleus stained in blue, microtubules in red and ER in yellow. The various cell structures that are demonstrated are always shown in the green channel using an antibody found in the Human Protein Atlas. The antibody id is linked to the corresponding Cell Atlas protein page. By using the "toggle channels"-buttons, the different channels can be turned on and off. Most cell structures can be highlighted in the cell illustration by hovering over them with the exception of the aggresome. Cytoplasmic bodies are highlighted as cytosol; cytokinetic bridge, midbody, midbody ring and mitotic spindle are highlighted as microtubules, cell junctions are highlighted as plasma membrane and nucleus is highlighted as nucleoplasm.
Staining of fibrillar center in human cell line HepG2 (HPA049086)
Scale bar represents 10µm
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.
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.