Transcription of ribosomal RNA genes is initiated in the third cell cycle of bovine embryos.

Transcription from the embryos own ribosomal genes is initiated in most species at the same time as the maternal-embryonic transition. Recently data have indicated that a minor activation may take place during the third embryonic cell cycle in the bovine, one cell cycle before the major activation of the embryonic genome. In the present study, ribosomal RNA (rRNA) transcription was investigated by visualization of the rRNA by fluorescent in situ hybridization, and subsequent visualization of the argyrophilic nucleolar proteins by silver staining. A total of 145 in vivo developed and 200 in vitro produced bovine embryos were investigated to allow comparison of transcription initiation. Signs of active transcription of rRNA were observed in the third cell cycle in 29% of the in vitro produced embryos (n = 35) and in 58% of the in vivo developed embryos (n = 11). Signs of active transcription of rRNA were not apparent in the early phase of the fourth cell cycle but restarted later on. All embryos in the fifth or later cell cycles were all transcribing rRNA. The signs of rRNA synthesis during the third and fourth embryonic cell cycle could be blocked by actinomycin D, which is a strong inhibitor of RNA polymerase I. In conclusion, rRNA transcription is initiated during the third cell cycle at a low level in both in vivo developed and in vitro produced bovine embryos. Transcription seems to be interrupted during the G1 phase of the fourth cell cycle, but reinitiates in the late half of the cycle and persists during subsequent cell cycles.