Transcription and translation in bovine nuclear transfer embryos

The development of bovine embryos reconstructed by nuclear transfer (NT) is poor compared to that of embryos produced by in vitro fertilization. One reason for this could be incomplete reprogramming of the transferred nucleus. Therefore, with a view to optimizing the conditions for NT, the reprogramming of blastomere nuclei from 16- to 32-cell-stage in vitro-fertilized (IVF) embryos was investigated following NT by fusion of individual blastomeres with cytoplasts prepared from oocytes at two different stages of maturation. Heterogeneous RNA (hnRNA) production, nucleolar ultrastructure, and protein profiles of the NT embryos up to the 8-cell stage were analyzed. In all NT embryos analyzed for their hnRNA production (n = 133), [3H]uridine incorporation was higher at the 1-, 2-, and 4-cell stages than in control IVF embryos (n = 50). Ultrastructural examination of 11 NT embryos revealed evidence of transcriptional activity; fibrillar and granular components were seen in the nucleolus at the 1-cell stage. At the 2-, 4-, and 8-cell stages, fibrillar components were still evident but granular components had become scarce. The hnRNA synthesis, however, was not reflected in the one-dimensional electrophoretic patterns of protein production in the NT embryos (n = 56); these were largely similar to those of IVF embryos (n = 34) of corresponding stages. Thus, NT embryos made in this way do not behave like equivalent IVF embryos, suggesting that reprogramming of the transferred nucleus is absent or incomplete.