Control of cleavage and further development in vitro in reconstituted two-cell mouse embryos

Nuclear-cytoplasmic interactions during the second cell cycle of mouse embryos were examined by assessing the timing of cleavage in reconstituted two-cell embryos and their ability to develop further to the blastocyst stage in vitro. Nuclear transplantations were performed either within or across the cell cycle and at different stages of the cell cycle to assess the effect of 'asynchrony' on development. In most cases, cleavage occurred at an intermediate time between nuclear and cytoplasmic controls indicating an interaction in their mechanisms for controlling the timing of cleavage. Early nuclei extended the cleavage timing of late cytoplasm for a short period, possibly to allow for completion of DNA synthesis, while early cytoplasm delayed the expected cleavage time of late nuclei, possibly to enable proper maturation of cytoplasmic components. However, a block of cleavage was observed in most across cell cycle transplantations and also when fusing early two-cell karyoplasts to enucleated late two-cell blastomeres. It is suggested that this incompatibility is caused by major changes in the transcriptional status of donor and recipient cells. Although the development of reconstituted embryos to the blastocyst stage was clearly affected by cell cycle 'asynchrony' in within cell cycle transplantations, independent effects of cytoplast stage and, to a lesser extent, of karyoplast cell cycle stage were predominant in transplantations using eight-cell karyoplasts.