Removal of remodeling/reprogramming factors from oocytes and the impact on the full-term development of cloned embryos.

The reason for the poor development of cloned embryos is not yet clear. Several reports have suggested that some nuclear remodeling/reprogramming factors (RRFs) are removed from oocytes at the time of enucleation, which might cause low success rate of animal cloning. However, there is currently no method to manipulate the volume of RRFs in oocytes. Here, we developed techniques to gradually reduce RRFs in oocytes by injecting somatic cell nuclei into oocytes. These injected nuclei were remodeled and reprogramed using RRFs, and then RRFs were removed by subsequent removal of somatic nuclei from oocytes. The size of metaphase II spindle reduced immediately, but did recover when transferred into fresh oocytes. Though affected, the full-term developmental potential of these RRF-reduced oocytes with MII-spindle shrinkage was not lost after fertilization. When somatic cell nuclear transfer was performed, the successful generation of cloned mice was somewhat improved and abnormalities were reduced when slightly RRFs-reduced oocytes were used. These results suggest that the amount of RRFs in oocyte is important but not the main reason for the incomplete reprogramming of somatic cell nuclei.