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Nucleolar re‐activation is delayed in mouse embryos cloned from two different cell lines

Aim of this study was to evaluate and compare embryonic genome activation (EGA) in mouse embryos of different origin using nucleolus as a marker. Early and late 2‐cell and late 4‐cell stage embryos, prepared by in vitro fertilization (IVF), parthenogenetic activation (PG), and nuclear transfer of mouse embryonic fibroblast (MEF) and mouse HM1 embryonic stem cells (HM1), were processed for autoradiography following 3H‐uridine incubation (transcriptional activity), transmission electron microscopy (ultrastructure) and immunofluorescence (nucleolar proteins; upstream binding factor, UBF and nucleophosmin, B23). All early 2‐cell embryos showed transcriptional activity only in nucleoplasm, not over nucleolar precursor bodies (NPBs). UBF was diffusely localized to cytoplasm and B23 to cytoplasm and nucleoplasm. Late 2‐cell IVF and PG embryos displayed transcription over nucleoplasm and NPBs. Ultrastructurally, the latter were developing into functional nucleoli. NT‐MEF and NT‐HM1 embryos displayed transcription over nucleoplasm, but not over NPBs. Development of NPBs into nucleoli was lacking. UBF was in both groups localized to nucleoplasm or distinctly to presumptive NPBs. B23 was distinctly localized to NPBs. All 4‐cell embryos presented nucleoplasmic transcription and developing fibrillo‐granular nucleoli. UBF and B23 were distinctly localized to nucleoli. However, whereas fully transformed reticulated fibrillo‐granular nucleoli were found in IVF and PG embryos, NT‐MEF and ‐HM1 embryos displayed early NPBs transformation. In conclusion, despite normal onset of EGA in cloned embryos, activation of functional nucleoli was one cell cycle delayed in NT embryos. NT‐MEF embryos displayed normal targeting but delayed activation of nucleolar proteins. Contrary, in NT‐HM1 embryos, both of these processes were delayed. Mol. Reprod. Dev. 76: 132–141, 2009. © 2008 Wiley‐Liss, Inc.