The mammalian oocyte is a round cell arrested at prophase I of meiosis. It is characterized by the presence of a large nucleus, called the germinal vesicle, in the middle of which is the nucleolus. Before it can be fertilized, the oocyte must resume meiosis, enter metaphase II and be ovulated. The nucleolus is dissolved during this process. However, the nucleoli of the male and female pronuclei in the zygote are both of maternal origin. A recent paper1 demonstrates that the (...) maternal nucleolus, together with other nucleoplasmic elements, is essential for early embryonic development. These nucleolar and nucleoplasmic factors remain undetermined. BioEssays 30:613–616, 2008. © 2008 Wiley Periodicals, Inc. (shrink)