Revealing the unseen: the organizer region of the nucleolus

We carried out a high-resolution ultrastructural analysis of the nucleolus in mouse P815 cells by combining specific DNA and RNA staining, anti-fibril larin immunolabeling, contrast enhancement by energy filtering TEM and phos phorus mapping by ESI to visualize nucleic acids. We demonstrated that spec ifically contrasted DNA, fibrillarin and phosphorus overlap within the nucl eolar dense fibrillar component. Moreover, we describe a 'DNA cloud' consis ting of an inner core of DNA fibers (fibrillar center) and a periphery made of extremely thin fibrils overlapping the anti-fibrillarin immunolabeling (dense fibrillar component). This highly sensitive approach has allowed us to demonstrate, for the first time, the exact distribution of DNA within th e decondensed interphase counterpart of the NOR, which includes both the fi brillar center and the dense fibrillar component.