Nucleus but not nucleoli
The nucleus is a membrane-bound organelle, which spatially take up a large proportion of the cell's total volume. It contains almost all the DNA of eukaryotic cells (the rest being found in mitochondria). It is enclosed by a nuclear envelope (here denoted nuclear membrane), which is directly linked to the endoplasmic reticulum. Apart from the DNA the nucleus also contains proteins that are associated with the DNA and make up chromatin, which in turn will form chromosomes.
There are several sub-nuclear structures within the nucleus that, just as the nucleoli, also lack membranes: for instance Cajal bodies, Gemini of Cajal bodies (GEMS), interchromatin granule (speckles) and promyelotic leukaemia (PML) nuclear bodies. These structures are involved in RNA processing and are highly dynamic, often changing appearance during the progression of the cell cycle.
The chemical 4',6-diamidino-2-phenylindole (DAPI) is used to counterstain the nucleus, as it will bind strongly to DNA, and hence not stain the nucleoli.
The immunofluorescent staining of the nucleus varies between different antibodies, in some cases staining the whole nucleus and in other the nucleus but not nucleoli. The staining characteristic can vary from smooth, to granular, speckled or spotty. The speckled and spotty patterns may represent sub-nuclear structures.