Chromosome condensation activity (CCA) in bisected C57BL/6JxCBA mouse oocytes

Chromosome condensation activity (CCA) has been analysed in C57BL/6JxCBA mouse oocytes bisected (i) shortly after germinal vesicle breakdown (GVBD), (ii) in metaphase I (MI) and (iii) in metaphase II (MII) into two equal halves (nucleated, enucleated) which were thereafter fused to S- or G2-phase 4-cell-stage mouse blastomeres. In nucleated halves, premature chromosome condensation (PCC) in transplanted nuclei was always induced irrespective of the cell cycle stage of the blastomere, whereas in enucleated halves only G2 nuclei underwent PCC after transplantation. Premature chromosome condensation in S-phase nuclei was induced only in enucleated halves produced shortly after GVBD. Although S-phase nuclei transplanted to MI or MII enucleated halves remained intact, their capacity to synthesize DNA was invariably suppressed. When spindles were destroyed by preincubation of the oocytes in colcemid before bisection, both nucleated and enucleated halves produced at MI or MII induced PCC of both G2- or S-phase nuclei. These results demonstrate that chromosome condensation activity in mammalian oocytes is compartmentalized rather than uniformly distributed across the cell, and that the enucleation of mammalian oocytes before nuclear transplantation may, under some conditions, influence the levels of CCA and subsequent response of introduced nuclei to cytoplasmic factors.