The Expression and Localization of Pluripotency Transcription Factors OCT4 and SOX2 in 2-Cell Mouse Embryos Developed In Vitro

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The key scientific problem: the pluripotency transcription factors OCT4 and SOX2 are related to 2-cell block, which usually occurs in mouse(Mus musculus) embryos cultured in vitro. The main research methods: the expression of Oct4 and Sox2 was detected by real-time quantitative PCR in oocytes and embryos which were cultured in M16 medium. Meanwhile, the expression and localization of OCT4 and SOX2 were compared among 2-cell stage embryos, 2-cell arrested embryos and 4-cell stage embryos by real-time quantitative PCR and immunofluorescence analysis. Statistical analyses of real-time quantitative PCR results were conducted using an analysis of variance(ANOVA), and a difference of P 0.05 was considered significant. The research results: 24.8% of 2-cell embryos reached the 4-cell stage, and 75.2% of 2-cell embryos were blocked(Fig. 1). Oct4 and Sox2 were expressed throughout MII oocytes, pronuclear zygotes, 2-cell embryos, 4-cell embryos, morulae and blastocysts(Fig. 2a, b). Oct4 was expressed higher in 4-cell embryos than in 2-cell embryos and 2-cell arrested embryos(Fig. 2c)(P 0.05). Sox2 was expressed higher in 2-cell embryos than in 4-cell embryos and 2-cell arrested embryos(P 0.05), but there was no significant difference between 4-cell embryos and 2-cell arrested embryos(Fig. 2d)(P 0.05). OCT4 was co-localized with chromatin in 2-cell and 4-cell nuclei, while diffused in cytoplasm instead of in nuclei in 2-cell arrested embryos(Fig. 3a). SOX2 was co-localized with chromatin in all groups(Fig. 3b). The research conclusion: the expression and localization of OCT4 and SOX2 is related to 2-cell block in mouse embryos. The stable expression of maternal SOX2 plays an important role during zygotic genome activation(ZGA). The abnormal localization of maternal OCT4 may affect the activation of ZGA-related genes; moreover, the expression of zygotic Oct4 may impact embryo development after ZGA.

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