Almost every living cell contains a nucleus and, within that, a nucleolus. It is commonly accepted that the nucleolus of somatic cells arises from the nucleolar precursor body (NPB), a large, compact nucleolus present in the oocyte, but whether this is true remains to be definitively shown. Now, on p. , Hirohisa Kyogoku and colleagues reveal that the NPB in the zygote, which is inherited from the oocyte, is not required for embryonic and full-term development in the mouse. Instead, the nucleolus in the developing embryo is formed de novo. Using micromanipulation techniques, the authors show that removal of the NPB from the late-stage zygote did not cause any disruption to normal development: the resulting zygote was able to develop to the blastocyst stage and upon implantation gave rise to phenotypically normal pups. Several cell divisions after NPB removal, the authors observed de novo formation of nucleoli from newly synthesized materials. Taken together, these results indicate that the NPB of late-stage zygotes is dispensable for subsequent embryonic development, and that the nucleoli of developing and somatic cells form from de novo synthesized material.
