Fig. 2. (a) Immunolabeling for fibrillarin and specific DNA staining with osmium ammine. The labeling for fibrillarin, a marker of the DFC, is present at the periphery of a stained area formed by thin fibers of DNA. Clumps of intranucleolar chromatin are also stained (arrow). Notice that the different nucleolar components shown in Fig. 1 are no more recognizable. Bar, 0.2 µm. (b) Control sample submitted to anti-fibrillarin labeling, hydrolysis with 5 N HCl for 20 minutes and staining with uranyl acetate. The DFC surrounding the FC (asterisk) is specifically labeled (arrows). Bar, 0.2 µm. (c) After immunolabeling with an anti-RNA-polymerase-I antibody, the gold particles are located over the DFC (arrows). The FCs (asterisks) and the GC (G) are devoid of labeling. Bar, 0.5 µm.