Activation of ribosomal RNA genes in porcine embryos produced in vitro or by somatic cell nuclear transfer

The onset of ribosomal RNA (rRNA) synthesis occurs during the second half of the third cell cycle, that is, at the four-cell stage, in porcine embryos developed in vivo. In the present study the onset of rRNA synthesis was investigated in porcine embryos produced in vitro (IVP) or by somatic cell nuclear transfer (SCNT) using fluorescence in situ hybridization (FISH) with an rDNA probe and subsequent visualization of the nucleolar proteins by silver staining. In the 205 IVP embryos investigated, all two-cell embryos (n=34) were categorized as transcriptionally inactive. At the late four-cell stage (n=45), 38% of the embryos contained 1-3 nuclei with signs of rRNA transcription, indicating an asynchronous transcription initiation. This pattern continued in the following stages, as 78% (n=47), 47% (n=42) and 83% (n=37) of the embryos revealed a mixture of transcriptionally inactive and active cells at the eight-cell, 16-cell and blastocyst stage, respectively. In the 143 SCNT embryos investigated, all two-cell embryos (n=34) and early four-cell embryos (n=12) were also transcriptionally inactive. At the late four-cell stage (n = 33) and at the eight-cell stage (n = 24) there were equal proportions of transcriptionally active and inactive embryos and essentially all embryos that developed to the 16-cell stage (n=21) and further to the blastocyst stage (n= 19) contained only transcriptionally active cells. In conclusion, porcine embryos produced in vitro had an asynchronous pattern of rRNA transcription initiation when compared to SCNT and in vivo developed porcine embryos.