RNA synthesis in 2-32 cell embryos, as assessed by alpha-amanitin-sensitive 3H-uridine incorporation, was first detectable in 4-cell stage rabbit and 8-cell stage sheep embryos. In the rabbit, uridine incorporation was detectable at the 2-cell stage but was unaltered by alpha-amanitin, indicating synthesis of non-polymerase II-dependent RNA species. Initiation of mRNA synthesis as determined by in situ hybridization with 3H-poly (U) probe was first detectable in late 2-cell stage rabbit and 4-cell stage sheep embryos. In the rabbit embryos, nuclear labelling increased from the late 2-cell stage to the 16-cell stage, following a pattern similar to that of 3H-uridine incorporation. In contrast, the intensity of cytoplasmic labelling decreased from the 1- to the 8-cell stage and then increased up to the 32-cell stage. In sheep embryos, nuclear labelling by the poly (U) probe increased from the 4- to the 16-cell stage. It is concluded that initiation of transcription of the embryonic genome (mRNA) can be detected via the current methods used at the 4-cell stage in the rabbit and the 8-cell stage in the sheep.

Institute of Animal Physiology and Genetics Czech Academy of Sciences Libĕchov

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