“…Since it became evident that cell nuclei are compartmentalized organelles (Spector, 1993;Strouboulis & Wolffe, 1996;Singer & Green, 1997;Lamond & Earnshaw, 1998), one of the major goals in cell biology has been to understand their functional organization+ In particular, the nucleolus provides the fascinating possibility of linking morphologically distinct structures such as those seen in the electron microscope with biochemical features of the synthesis of rRNAs and stepwise maturation of ribosomes+ In eukaryotic cells, the synthesis and maturation of cytoplasmic rRNAs mainly take place in the nucleolus (Hadjiolov, 1985;Sollner-Webb et al+, 1996)+ The 18S, 5+8S, and 25/28S rRNAs are synthesized as simple precursor RNAs (pre-rRNAs), which contain long external and internal transcribed spacers+ The primary transcripts are processed through a complex pathway of endonucleolytic cleavages, exonucleolytic digestions, and covalent modifications+ These pre-rRNAs are assembled with ribosomal and nonribosomal proteins in addition to the separately transcribed 5S rRNAs+ In this way, the large and small preribosomal particles are formed and subsequently released from the nucleolus into the cytoplasm, where they assembled into fully functional ribosomes+ Using cytological methods, considerable efforts have been devoted to the identification of the nucleolar sites at which the various biochemical events in rRNA synthesis take place+ In the mammalian cell nucleolus, three major substructures have been visualized by electron microscopy: the fibrillar center (FC), the dense fibrillar component (DFC), and the granular component (GC) + Most studies dealing with the nucleolus have been focused on the identification of rDNA transcription sites (Shaw & Jordan, 1995;Scheer & Hock, 1999)+ There is a general consensus that transcription occurs in the fibrillar regions of the nucleolus, but whether it is within the DFC, or at the border of the DFC and the FC, or even within the FC, remains a matter of much debate+ Kinetic information concerning the synthesis and maturation of pre-rRNA molecules within the nucleolus of intact, active cells is equally sparse+ Autoradiographic studies (Fakan, 1978) have consistently shown that when rRNAs are labeled with tritiated uridine for short periods of time, the signal is uniquely localized on the fibrillar part of the nucleolus+ The GC only becomes labeled after longer periods of radioactive incubation and/or after a cold chase+ In complementary biochemical studies (Daskal et al+, 1974;Royal & Simard, 1975), 45S pre-rRNAs appear restricted to the fibrillar component while 32S (i+e+, pre-28S) rRNAs are also located in the GC+ Together, these data suggest a migration of pre-rRNA molecules from the fibrillar component into the GC+ However, autoradiography displays a projection onto a plane of the three-dimensional data, thus discarding the possibility of dealing with the question of the spatial organization of the labeling+…”