The oocyte (maternal) nucleolus is essential for early embryonic development and embryos originating from enucleolated oocytes arrest at the 2-cell stage. The reason for this is unclear. Surprisingly, RNA polymerase I activity in nucleolus-less mouse embryos, as manifested by pre-rRNA synthesis, and pre-rRNA processing are not affected, indicating an unusual role of the nucleolus. We report here that the maternal nucleolus is indispensable for the regulation of major and minor satellite repeats soon after fertilisation. During the first embryonic cell cycle, absence of the nucleolus causes a significant reduction in major and minor satellite DNA by 12% and 18%, respectively. The expression of satellite transcripts is also affected, being reduced by more than half. Moreover, extensive chromosome bridging of the major and minor satellite sequences was observed during the first mitosis. Finally, we show that the absence of the maternal nucleolus alters S-phase dynamics and causes abnormal deposition of the H3.3 histone chaperone DAXX in pronuclei of nucleolus-less zygotes.

No abstract

Abstract. Compared with advanced developmental stage embryos and somatic cells, fully grown mammalian oocytes contain specific nucleolus-like structures (NPB -nucleolus precursor bodies). It is commonly accepted that they serve as a store of material(s) from which typical nucleoli are gradually formed. Whilst nucleoli from somatic cells can be collected relatively easily for further biochemical analyses, a sufficient number of oocyte nucleoli is very difficult to obtain. We have found that isolated oocytes nucleoli fuse very efficiently when contact is established between them. Thus, well visible giant nucleoli can be obtained, relatively easily handled and then used for further biochemical analyses. With the use of colloidal gold staining, we estimated that a single fully grown mouse oocyte nucleolus contains approximately 1.6 ng of protein. We do believe that this approach will accelerate further research aiming at analyzing the composition of oocyte nucleoli in more detail. Key words: Fusion, Nucleolus, Oocyte, Protein content (J. Reprod. Dev. 58: [371][372][373][374][375][376] 2012) T he nucleolus is the most prominent structure in a cell nucleus [1]. In developmentally advanced embryos (i.e., blastocysts) and in somatic cells, it contains three basic components: fibrillar centers (FC), a dense fibrillar component (DFC) and a granular component (GC) [2]. Originally, it has been accepted that the nucleolus is exclusively the site of ribosomal RNAs (rRNAs) synthesis, processing and their assembly with ribosomal proteins, but now it is clear that nucleoli play an essential role(s) in many cellular processes [3]. Within the last few years, the composition of somatic cell nucleoli has been characterized, and it is now evident that somatic cell nucleoli contain thousands of proteins [4].Contrary to differentiated cells, fully grown mammalian oocytes and very early developing embryos prior to major embryonic genome activation do not contain typical tripartite nucleoli. Instead, socalled "nucleolus precursor bodies" (NPBs) are visible in germinal vesicles (GVs) and in pronuclei (PNs) of some mammalian species, i.e., rodents (mouse, rat), humans and pigs [5]. NPBs contain only a dense fibrillar mass. Contrary to somatic cell nucleoli, very little is known about the composition of NPBs. The only protein convincingly demonstrated to be present in them is nucleoplasmin 2 [6]. The function of NPBs in oocytes and embryos has been characterized, especially after the invention of so-called "enucleolation" [7]. This method represents the microsurgical removal of nucleoli from fully grown and growing oocytes as well as from one-cell stage embryos [8][9][10]. Thus, it has been convincingly demonstrated that NPBs are dispensable for maturation of fully grown oocytes. The enucleolation method also showed that nucleoli (NPBs) in zygotes are formed from the original oocyte nucleolar material [11]. Further detailed analysis demonstrated that the presence of nucleoli in pronuclei is absolutely essential for normal embryonic devel...

No abstract

No abstract