“…Ultrastructural studies have shown that in mammalian cell nucleoli, a part from intranucleolar chromatin, three main components are constantly present: the fibrillar centers (FCs), which are light electron-opaque, roundish structures composed by thin, loosely distributed fibrils, the dense fibrillar component (DFC) constituted by tightly packed fibrils, which surround and are intimately associated to the fibrillar centers, and the granular component (GC), constituted by granules with a diameter of 15-20 nm, which embeds both fibrillar components (Hadjiolov, 1985). There is evidence that ribosomal genes in an extended, ready-to-be-transcribed configuration are mainly located in the FCs and, in part, within the DFC (Derenzini et al, 1993a;Mosgoeller et al, 1993) and all the substances necessary for rRNA transcription are present both in the FCs and in the DFC (Derenzini et al, 2006). In the DFC nascent pre-rRNA molecules undergo to early processing and maturation reactions which continue in the GC where pre-ribosomes in an advanced stage of maturation are located (Scheer and Hock, 1999).…”