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The nucleolus is believed to be the active site of rRNA synthesis in all eukaryotic cells. In preimplantation embryos, the embryonic genome is apparently more or less silent up to a species-specific developmental stage at which a major burst of transcription occurs. Here we report on nucleologenesis and some ultrastructural aspects of the onset of RNA synthesis in equine embryos during in vivo development. The zygotes and embryos up to blastocyst stages were surgically recovered from normally cycling mares. Mares were induced to ovulate by treatment with 3000 IU hCG and inseminated 20 and 34 h later. At different time intervals postovulation, mares were anesthetized and ova were collected from oviducts removed through a flank incision. The presumptive fertilized ova were incubated for 20 min with [3H]uridine and processed for light microscopy, transmission electron microscopy (TEM), and TEM autoradiography. Ultrastructurally, electron-dense nucleolus precursor bodies were observed in zygotes and 2- and 4-cell embryos. In 6- and 8-cell embryos, reticulated fibrillo-granular nucleoli displaying both fibrillar and granular components were observed. At this stage of development, the first autoradiographic labeling was observed over the dense fibrillar component of the nucleoli as well as over the nucleoplasm in the 8-cell embryos. In the 16-cell embryos and beyond, fully transcriptionally active compact fibrillo-granular nucleoli displaying granular and fibrillar components as well as fibrillar centers were observed, and autoradiographic labeling was detected over the dense fibrillar component of the nucleolus as well as over the nucleoplasm. In conclusion, nucleolar activation, including transcription of presumptive rRNA and heterogeneous nuclear RNA, is initiated during the fourth cell cycle.

Nucleologenesis and Ribonucleic Acid Synthesis in Preimplantation Equine Embryos1