Nucleolus precursor body makeover

Unlike somatic cells, the nucleus of the oocyte and very early embryo contains a morphologically distinct nucleolus called the nucleolus precursor body (NPB). Although this enigmatic structure has been shown to be essential for normal mammalian development, its precise function remains unclear. In this issue, Helena Fulka and Alena Langerova now demonstrate (p. 1694) a crucial role for the NPB in regulating major and minor satellite DNA sequences and chromosome dynamics in the mouse. Absence of the NPB during the first embryonic cell cycle causes a significant reduction in satellite DNA sequences, and the authors also observe extensive chromosome bridging of these sequences during the first embryonic mitosis. The authors further demonstrate that the NPB is unlikely to be involved in ribosomal gene activation and processing as previously believed, since this process can still occur in NPB-depleted early embryos. This study uncovers an interesting and novel role for the NPB in early embryogenesis.