[Ultrastructural changes in nucleoli during embryogenesis of Marsilea vestita]

The nucleoli of oospheres and young embryos of Marsilea vestita exhibited a typical ultrastructure with segregation into several compact nucleolus-like bodies and small granules of similar appearance. When embryos reached the 16 cell stage, the granules disappeared and the nucleolar-like bodies changed into classical nucleolar components. When the embryos were submitted to hypothermic shock, the nucleoli returned to the characteristic ultrastructure of the very young stages. In ageing oospheres the nucleus was full of very compact clumps. No detectable RNA synthesis occurred in the oospheres and young embryos, and the same phenomenon applied in ageing oospheres and after cold shock treatment. Transcriptional activity started with the 8-16 cell stage in the reference embryos. These results suggest that nucleolar ultrastructure is correlated with rRNA synthesis.