Figure 1.

Chromatin States of rRNA Genes in ESCs and Differentiated Cells

The hyperactive state of the nucleolus in ESCs is characterized by the lack of silent rRNA genes and elevated ribosome biogenesis. In contrast, upon exit from pluripotency, a fraction of rRNA genes acquire epigenetic silencing, ribosome biogenesis is reduced, and clusters of heterochromatin blocks surround the nucleolus and the nuclear periphery. The red arrows indicate the link between the nucleolar chromatin state and the rest of the genome. Representative electron microscopy images show the distinct chromatin organization between an mESC and a neural progenitor cell (NPC) 8 days after differentiation. Generally, ESCs display a single, large nucleolus (Nu), whereas differentiated cells have more nucleoli of smaller size compared with ESCs. The contrast procedure reveals in dark large and condensed heterochromatic structures (Het) particularly evident close to the nucleolus of the NPC.