CONTROL OF CLEAVAGE AND FURTHER DEVELOPMENT IN-VITRO IN RECONSTITUTED2-CELL MOUSE EMBRYOS

Nuclear-cytoplasmic interactions during the second cell cycle of mouse embryos were examined by assessing the timing of cleavage in reconsti tuted two-cell embryos and their ability to develop further to the bla stocyst stage in vitro. Nuclear transplantations were performed either within or across the cell cycle and at different stages of the cell c ycle to assess the effect of 'asynchrony' on development. In most case s, cleavage occurred at an intermediate time between nuclear and cytop lasmic controls indicating an interaction in their mechanisms for cont rolling the timing of cleavage. Early nuclei extended the cleavage tim ing of late cytoplasm for a short period, possibly to allow for comple tion of DNA synthesis, while early cytoplasm delayed the expected clea vage time of late nuclei, possibly to enable proper maturation of cyto plasmic 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 transcript ional 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, indepe ndent effects of cytoplast stage and, to a lesser extent, of karyoplas t cell cycle stage were predominant in transplantations using eight-ce ll karyoplasts.