A functional nucleolus (the organelle that makes ribosome subunits) is essential for embryonic development. Nucleologenesis, which involves transformation of an inactive nucleolar precursor body (NBP) into a mature nucleolus, coincides with embryonic genome activation, which occurs at the late 2-cell stage in mouse embryos. Here (), Edgar Vogt, Rüdiger Behr and colleagues investigate the involvement of the pluripotency factor LIN28 in nucleologenesis in mouse embryos. LIN28, which is abundantly expressed in early embryonic tissue, is an RNA-binding protein that can contribute to the reprogramming of somatic cells to pluripotent stem cells. The researchers report that LIN28 accumulates at the periphery of NPBs and mature nucleoli in preimplantation embryos and that initiation of LIN28 expression coincides with embryonic genome activation. Following LIN28 knockdown, they report, most embryos arrest between the 2- and 4-cell stages. Notably, presumptive NPB sites are not enriched with the nucleolar marker nucleophosmin in these arrested embryos. Thus, the researchers conclude, LIN28 is essential for NPB assembly and nucleologenesis during early embryonic development.
