Injecting enucleolated oocytes (left) with isolated nucleolar materials causes nucleoli to reform (right).
OGUSHI/AAAS
Unlike somatic cell nucleoli, those in fully developed but unfertilized oocytes are transcriptionally inactive and have an odd, compact form, which led Ogushi to ask what contribution they make to the embryo. Using a fine needle, she removed the nucleoli from mouse oocytes, taking the internal contents while leaving the surrounding heterochromatin. The seemingly unharmed enucleolated oocytes matured properly. But when they were fertilized, no new nucleoli formed in the embryos, which stopped developing after a few rounds of cleavage. If Ogushi put the nucleoli back in before maturation, functional nucleoli developed. She could even wait until after maturation to reinject and still obtain viable embryos and healthy mouse pups.
Whatever the essential material was, it was only present in oocyte nucleoli. When Ogushi injected somatic cell nuclei and their nucleoli into enucleated oocytes, the reconstructed oocytes failed to develop normal nucleoli, and they too stopped dividing shortly after fertilization.
Sperm have no nucleoli, so it makes sense that the zygote relies on the oocyte for its nucleolar beginnings. But what do they provide? “Little is known about nonmammalian embryonic nucleoli,” says Ogushi. “But if this is a mammal-specific phenomenon, we might suspect that the maternal nucleolus organizes the three-dimensional chromatin structure in the nucleus of zygotes and very early embryos, influencing mammal-specific gene expression, such as for imprinted genes.” The contents of the oocyte nucleolus are unknown, and identifying them will be the next step in the group's research. 
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