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High-resolution autoradiographic studies of comparative nucleologenesis and genome reactivation during early embryogenesis in pig, man and cattle.

Using high-resolution autoradiography, simultaneous studies of ultrastructure and nucleic acid dynamics were performed during nucleologenesis in early porcine, human and bovine embryos. In contrast to the early genome activation known to occur during the second cell cycle in the mouse, the onset of rRNA synthesis detected by (5-3H) uridine incorporation in the nucleolar compartment is delayed by cleavage of one cell cycle (to the third cell cycle) in the early pig embryo and by two cell cycles (to the fourth cell cycle) in human and cattle embryos. Extranucleolar RNA synthesis, as detected by nucleoplasm labelling, generally started shortly before rRNA synthesis. The timing of nucleolar labelling was well correlated with the penetration of embryonic DNA into the nucleolus precursor body and with nucleolus structure differentiation. Ultrastructural and/or autoradiographic techniques are suggested for the study of the onset of embryonic transcription, e.g. in embryos "reconstructed" by micromanipulation.