Fig. 3. (a) Ultrathin (25 nm) section through a nucleolus immunolabeled as in Fig. 2 and stained for DNA. Observation at 0 eV loss. Under these conditions, the nucleolus (Nu) appears as an ‘empty’ area in which DNA fibers are present as clumps scattered in the nucleolar body. A cloud of thin DNA fibers, the periphery of which overlaps the anti-fibrillarin labeling, can be seen in the center (arrowhead). Abbreviation: N, nucleoplasm. Bar, 0.5 µm. (b) At a higher magnification, it is clearly evident that the gold grains localizing fibrillarin are present at the periphery of the cloud (asterisk) and superimposed on the thinner fibers within the external part of the DNA cloud (arrows). (c) The same area as in b, observation at 250 eV energy loss. At this energy loss, it is clear that the inner core of the DNA cloud (asterisk) is formed by fibers thinner than chromatin clumps but thicker than those overlapping the anti-fibrillarin labeling (arrows). (d) The same area as in b and c, after phosphorus mapping. Phosphorus is present in all the locations that were positive in the DNA staining. Even the thin filaments at the periphery of the cloud are visible (arrows). Bar, 0.2 µm.
