The Nucleolus in the Mouse Oocyte is Required for the Early Step of Both Female and Male Pronucleus Organization

Abstract

Abstract. During oocyte growth in the ovary, the nucleolus is mainly responsible for ribosome biogenesis. However, in the fully-grown oocyte, all transcription ceases, including ribosomal RNA synthesis, and the nucleolus adopts a specific monotonous fibrillar morphology without chromatin. The function of this inactive nucleolus in oocytes and embryos is still unknown. We previously reported that the embryo lacking an inactive nucleolus failed to develop past the first few cleavages, indicating the requirement of a nucleolus for preimplantation development. Here, we reinjected the nucleolus into oocytes and zygotes without nucleoli at various time points to examine the timing of the nucleolus requirement during meiosis and early embryonic development. When we put the nucleolus back into oocytes lacking a nucleolus at the germinal vesicle (GV) stage and at second metaphase (MII), these oocytes were fertilized, formed pronuclei with nucleoli and developed to full term. When the nucleolus was reinjected at the pronucleus (PN) stage, most of the reconstructed zygotes cleaved and formed nuclei with nucleoli at the 2-cell stage, but the rate of blastocyst formation and the numbers of surviving pups were profoundly reduced. Moreover, the zygotes without nucleoli showed a disorder of higher chromatin organization not only in the female pronucleus but also, interestingly, in the male pronucleus. Thus, the critical time point when the nucleolus is required for progression of early embryonic development appears to be at the point of the early step of pronucleus organization. Key words: Mouse, Nucleolus, Oocyte, Pronucleus organization (J. Reprod. Dev. 56: [495][496][497][498][499][500][501] 2010) he nucleolus is the site of ribosomal RNA (rRNA) synthesis and processing and ribosomal subunit assembly [1]. During growth in the ovary, the oocyte possesses a fibrillo-granular nucleolus with active transcription and ribosome biogenesis and stockpiles the proteins and RNAs for early embryonic development. When the oocyte reaches full size, indicating that it has acquired developmental competence, all its transcription ceases, including the transcription of rRNA. Apparently, its nucleolus no longer contains DNA at this point and shows a highly compacted, fibrillar morphology [2,3]. As the nucleolus is eliminated during spermiogenesis [4][5][6], the spermatozoa do not have nucleoli. After fertilization, this maternally derived, transcriptionally inactive fibrillar nucleolus appears in both male and female pronuclei and in the embryonic nuclei of each interphase until the 4-or 8-cell stage; that is, it is present until the time at which zygotic RNA polymerase I activity is fully established [7]. The nucleolus in the oocyte or zygote is sometimes called the nucleolar precursor body (NPB) or nucleolar-like body (NLB), but we will use the term nucleolus in this study for the sake of simplicity.When we produced an embryo lacking this inactive nucleolus, the embryo failed to develop past the first few cleavages, indicating the require...

Transplantation of nucleoli into human zygotes: not as simple as expected?

“…However, as the protein composition of this unique structure is unknown it is also not known to what extent NPBs play an active role in this process. Ogushi and Saitou [19] have demonstrated that zygotic and GV-stage oocyte NPBs are not equivalent in composition and/or function. Exchanging zygotic NPBs for oocyte NPBs in mouse onecell stage embryos lead to a lag in cell cycle progression and severe developmental arrest.…”

“…An interesting observation was made by Ogushi and Saitou in 2010 [19]. These authors have transplanted the NPBs from GV stage oocytes into different oocyte maturation Fig.…”

“…Interestingly, Ogushi and Saitou [19] also show alteration of duration of the first mitotic cell cycle in embryos derived from enucleolated oocytes. Whether this observation reflects defects in centromere and pericentric heterochromatin establishment and function is currently unknown.…”

Cytoplasmic Inheritance Redux

“…After fertilization, the maternally derived, transcriptionally inactive nucleolus appears in both male and female pronuclei and in the embryonic nuclei until the four-cell or eightcell stage; that is, it is present until EGA (Zatsepina, Baly, Chebrout, & Debey, 2003). Although the nucleolus at this stage appears to be inactive and shows a highly compacted structure, studies with mice have revealed that it is essential for early embryonic development (Ogushi et al, 2008;Ogushi & Saitou, 2010). Embryos originating from enucleolated oocytes arrest between the two-cell and four-cell stages (Ogushi et al, 2008).…”

Production of Giant Mouse Oocyte Nucleoli and Assessment of Their Protein Content

“…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 development [12][13][14][15]. As mentioned above, contrary to somatic cells, the composition of NPBs is almost unknown.…”