Nucleologenesis and ribonucleic acid synthesis in preimplantation equine embryos.

This research explores the creation of the nucleolus and the initial stage of RNA synthesis in preimplantation equine embryos. It was discovered that a specific stage of development precipitates a significant amount of transcription activities and thus RNA synthesis.

Methodology

• The study used zygotes and embryos up to the blastocyst stages, obtained surgically from normally cycling horses.
• The horses were induced to ovulate by administering 3000 IU hCG, after which they were inseminated 20 and 34 hours later.
• At different points following ovulation, the horses were anesthetized, and their ovaries were surgically removed.
• The presumptive fertilized ova were then incubated with [3H]uridine for 20 minutes and then processed for light microscopy, transmission electron microscopy (TEM), and TEM autoradiography.

Results

• The research has identified the presence of electron-dense nucleolus precursor bodies in zygotes and 2- and 4-cell embryos.
• In the 6- and 8-cell embryos, the researchers observed reticulated fibrillo-granular nucleoli possessing both fibrillar and granular components.
• It is at this developmental stage that the first autoradiographic labeling was noticed over the dense fibrillar component of the nucleoli and also over the nucleoplasm in the 8-cell embryos.
• In the 16-cell embryos and beyond, fully active compact fibrillo-granular nucleoli displaying granular and fibrillar components as well as fibrillar centers were observed, with autoradiographic labeling detected over the dense fibrillar component of the nucleolus and over the nucleoplasm.

Conclusion

• The study concluded that the activation of the nucleolus, including the transcription of presumptive rRNA and heterogeneous nuclear RNA, is initiated during the fourth cell cycle.
• This research provides valuable insights into the biological processes occurring during the early stages of equine embryo development.