The localization of the nucleolar machineries is related to their function in the production of the small and large ribosome subunits. These Wndings have led to assigning speciWc functions to speciWc compartments of the nucleolus. Nascent transcripts appear at the junction between the FCs and DFC and accumulate in the DFC (Cmarko et al. 2000; Guillot et al. 2005; Hozàk et al. 1994; Puvion-Dutilleul et al. 1997; Shaw and Jordan 1995) . This was recently conWrmed in the GFC since no transcripts can be detected in these large structures (Casafont et al. 2007 ). Processing of the 47S pre-rRNA starts at the site of transcription in the DFC (Cmarko et al. 2000) and continues during the intranucleolar migration of the RNA towards the GC. The nucleolar proteins that participate in the early stages of rRNA processing, localize in the DFC, such as Wbrillarin and nucleolin along with the U3 snoRNAs (Biggiogera et al. 1989; Ginisty et al. 1998; Ochs et al. 1985b; Puvion-Dutilleul et al. 1991) , whereas proteins B23/NPM (nucleo- Fig. 1 The nucleolus of mammalian cells as seen by electron microscopy. a In the human HeLa cell, the three main nucleolar components are visible in a section of material Wxed in glutaraldehyde and osmium tetroxyde, embedded in Epon and the section contrasted with uranyl acetate and lead citrate. FCs of diVerent sizes are visible and the largest is indicated by an asterisk. The FCs are surrounded by the DFC and are embedded in the GC. b Preferential contrast of DNA using NAMA-Ur staining in a PtK1 cell (courtesy J. Gébrane-Younès). The nucleolus is the gray structure surrounded by highly contrasted chromatin (arrow). Some chromatin Wlaments are also visible inside of the nucleolus (Nu). c, d Nucleolus of rat neurones (courtesy M. J. Pébusque) in the day (c), and during the night (d) which is the active period for the nucleolus of the rat. In the nonactive period (c), the nucleolus is reticulated with small FCs (asterisk). In the active period, one giant FC is visible (d, asterisk) . Bar in a: 0.5 m and bars in b, c and d: 1 m phosmin) and PM-Scl 100 (rrp6 in yeast) that are involved in intermediate or later stages of processing have been localized to the GC (Biggiogera et al. 1989; Gautier et al. 1994) . Recent advances in the isolation of large RNP complexes by tandem aYnity puriWcation and the characterization of their constituents demonstrated that two largely independent processing machineries exist in yeast nucleoli, the SSU processome (Dragon et al. 2002; Grandi et al. 2002) and the LSU processing/assembly factors (Raué 2004) . The SSU/90S processome is localized in the DFC and most of the 60S processing occurs in the GC. There is no particular domain characterized in the GC corresponding to the 43S subunit. This is most probably due to the limited events of 40S processing in the GC since the last step of processing occurs in the cytoplasm. In conclusion it seems that in the nucleoli, the vectorial distribution of the machineries successively involved in ribosome biogenesis correlates with the diVerent processing steps of the biogenesis of the ribosome subunits.