Nuclear transplantation of ectodermal cells in pig oocytes: ultrastructure and radiography

This study investigated the changes in nucleolar fine structure and the synthesis of both heterogeneous nuclear RNA (hnRNA) and ribosomal RNA (rRNA) in pig reconstructed embryos obtained by electrofusion of a single pig ectodermal cell to an enucleated metaphase-II oocyte. The nucleolar morphological changes and the pattern of transcription were examined in the ectodermal cells before fusion and in the nuclear transfer reconstructed embryos 16-18 hr after fusion. Before fusion the ectodermal cells exhibited a reticulated nucleolus with active RNA synthesis. In the reconstructed embryos, modifications of the nucleolar structure were observed, as assessed by the presence of either round-shaped, compact, dense nucleolar precursor bodies, or reticulated nucleoli. However, in both cases there was no RNA synthesis. Blebbing activity of the nuclear envelope was not observed. These results indicate that the nucleus of pig ectodermal cells exhibited either a complete or an incomplete remodelling when transferred to an enucleated metaphase-II oocyte, with no detectable RNA synthesis. Cell-cycle synchronization of ectodermal cell donor nuclei may play an important role in nuclear reprogramming after fusion.