Behaviour of blastomere nuclei fused to mouse oocytes is affected by oocyte enucleation and age

The influence of oocyte age and presence of oocyte meiotic apparatus on the behaviour of introduced blastomere nuclei was evaluated. Blastomeres from 4-cell mouse embryos were fused to intact (metaphase II) oocytes, demi-oocytes (nucleate) or cytoplast (anucleate). Fusion and simultaneous activation of the recipient oocytes were accomplished by a single electrical pulse at 20 or 24 h post human chorionic gonadotrophin (hCG) administration. The hybrids were fixed for evaluation 2 h after fusion. There was no difference in the behaviour of blastomere nuclei in whole oocytes and demi-oocytes. Most nuclei fused to the nucleate recipients at 20 h underwent breakdown of nuclear membrane (NMBD), chromosome condensation and consequently proceeded to telophase, in parallel with the resident meiotic chromosomes. Following fusion to cytoplasts, only a small portion of the blastomere nuclei underwent chromosome condensation and the vast majority (83%) of the nuclei remained in interphase. The influence of oocyte age on nuclear behaviour was assessed in oocyte-blastomere hybrids prepared by simultaneous fusion and activation at 20 and 24 h post-hCG administration. The introduced nuclei proceeded to telophase in 63% of the hybrids constructed at 20 h, but in only 28% of those constructed at 24 h. We conclude that nuclei introduced into aged or enucleated oocytes at the time of activation are predominantly remodelled in their interphase configuration.