Bioscientifica Proceedings

In mammals, growing oocytes have characteristically high levels of RNA synthesis. After the initiation of meiosis, that is germinal vesicle breakdown, this RNA synthesis ceases. Although there is limited evidence for RNA synthesis by the zygote, significant amounts of RNA synthesis do not occur until a species-specific cell stage. In pigs, significant amounts of mRNA synthesis cannot be detected before the four-cell stage. There appear to be three qualitatively different periods of transcription during the four-cell stage. The first occurs during a short ( < 2 h) Gl phase. The second, occurs after completion of DNA synthesis (S phase) about 16 h after cleavage to the four-cell stage, and the third occurs about 24 h after cleavage to the four-cell stage. Correlated with these changes in RNA synthesis are changes in nucleolar morphology, amino acid transport characteristics, protein production, mitochondrial morphology, and metabolism of the embryo. The mechanisms that regulate initiation of RNA synthesis in early mammalian embryos appear to repress transcription. A state of transcription permissiveness then follows that sets into motion the differentiation programme.