Nucleoli are rather large non-membrane bound subcompartments in the nucleoplasm. Nucleoli are responsible for the synthesis, processing, and assembly of ribosomes, but they are also involved in several other cellular processes, including stress responses and cell cycle regulation, through sequestration of factors by nucleolar detention. Nucleoli are assembled around nucleolar organizing regions (NORs), found at rDNA loci along chromosomes. Structurally, the nucleoli consist of three subregions: the fibrillar center (FC) where rDNA transcription occurs, the dense fibrillar component (DFC) where rRNA is modified, and the granular component (GC) where ribosomes are assembled.
Nucleoli can be seen as slightly elongated circular areas in the nucleoplasm, the number and size of which varies between cell types. Nucleoli are only weakly stained by DAPI because of a 3-fold lower concentration of DNA as compared to the surrounding nucleoplasm.
Read more about the proteome of nucleoli.