Changes in embryonic 8-cell nuclei transferred by means of cell fusion to mouse eggs.

Metaphase II and activated mouse oocytes were fused with 8-cell blastomeres, and morphological changes in the transferred nuclei were followed using light and electron microscopy. In metaphase II oocytes, blastomere nuclei underwent premature chromosome condensation (PCC) typical for S-phase nuclei: chromatin pulverization. Then an abortive spindle was formed without evident microtubule organizing centers. Blastomere chromosomes condensed to a lesser degree than meiotic chromosomes and lacked mature functional, trilaminar kinetochores. After parthenogenetic activation of these oocytes, blastomere chromosomes followed, in synchrony with oocyte chromatin, a similar route of changes (anaphase, telophase) and then reformed interphase nuclei of the pronuclear type. Remodeling of 8-cell nucleus thus occurred, but the integrity of the chromatin set was frequently disturbed by formation of micronuclei. If blastomere fusion with oocytes was done close to activation (either before or after parthenogenetic stimulation), the chances of remodeling of the nuclei decreased, because PCC was not regularly induced in all oocytes. In hybrids produced 60 min or later after oocyte activation, blastomere nuclei were maintained in interphase without any structural modifications. Multiple experiments in the mouse have shown that the nuclei from 8-cell stage transferred to enucleated oocytes and egg cells are not capable of substituting for pronuclear functions. Possible reasons for impaired functional reprogramming of 8-cell nucleus in the mouse are discussed in light of our present findings on the morphology of nuclei transferred before and after oocyte activation.