Visualization of nucleolar organization in DBA fibroblasts by electron microscopy. The 3 nucleolar components classically described are visible in control cells: fibrillar centers (stars), the meshwork of the highly contrasted dense fibrillar component (arrowheads) and, in between, the granular component (arrows). The dense fibrillar component appears abnormally compact and reticulated in MUTS-A cells, whereas it is scarce and disorganized in NOTS-A cells. Contrast enhancement and colors were applied to stress the differences in nucleolar organization (bottom): nucleolus in blue with fibrillar centers in yellow. The highly contrasted dense fibrillar component is highlighted in red in control cells. The nucleolus appears less fibrillar and more granular in NOTS-A and MUTS-B cells.