The nucleolus has been proposed as the paradigm of nuclear functional compartmentalization (Strouboulis and WolVe 1996) . It is the site of ribosome biogenesis and in addition the nucleolar machineries are distributed in diVerent compartments. When observed by EM, three main nucleolar components (compartments) can be discerned in mammalian cells: the Wbrillar centers (FCs), the dense Wbrillar component (DFC) and the granular component (GC) (Fig. 1a) . The FCs are clear areas, partly or entirely surrounded by a highly contrasted region (Goessens et al. 1987) , the DFC. The FCs and the DFC are embedded in the GC, mainly composed of granules of 15-20 nm in diameter. The most contrasted structures in the EM sections stained with uranyl and lead correspond to high concentrations of nucleic acids. The condensed chromatin surrounding part of the nucleolus is visible using standard or preferential staining methods and also as a network within the nucleolus (Fig. 1b) . The global amount of intra-nucleolar chromatin is probably low since by light microscopy, DNA staining by DAPI excludes the nucleolus.