The nucleolus is an important nucleus sub-organelle found in almost all eukaryotic cells. On the one hand, it is known as a differentiated active site of ribosome biogenesis in somatic cells, but on the other hand, in fully grown oocytes, zygotes, and early embryos (up to the major embryonic genome activation), it is in the form of a particular homogenous and compact structure called a fibrillar sphere. Nowadays, thanks to recent studies, we know many important functions of this, no doubt, interesting membraneless nucleus sub-organelle involved in oocyte maturation, embryonic genome activation, rRNA synthesis, etc. However, many questions are still unexplained and remain a mystery. Our aim is to create a comprehensive overview of the recent knowledge on the fibrillar sphere and envision how this knowledge could be utilized in further research in the field of biotechnology and nucleolotransfer therapy.

Ecole Nationale Vétérinaire d'Alfort BREED 94700 Maisons Alfort France

Faculty of Natural Sciences Constantine the Philosopher University in Nitra Nabrezie mladeze 91 94974 Nitra Slovakia

Faculty of Veterinary Medicine St Cyril and Methodius University in Skopje 1000 Skopje North Macedonia

Institute of Animal Physiology and Genetics The Czech Academy of Sciences 27721 Libechov Czech Republic

Université Paris Saclay UVSQ INRAE BREED 78350 Jouy en Josas France

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