Somatic and embryonic cell nucleus transfer into intact and enucleated immature mouse oocytes

The aim of our study was to evaluate the possibility of embryonic or somatic cell haploidization after fusion with intact or enucleated immature oocytes which were subsequently cultured in vitro. Embryonic or somatic cell nuclei do not undergo premature chromosome condensation when fused to intact or enucleated immature oocytes whose maturation is prevented by dibutyryl cyclic AMP (dbcAMP). The presence of dbcAMP permits, however, the completion of DNA replication in somatic cell nuclei.

The chromosomes condensed when the reconstructed cells were released from the dbcAMP block. When somatic or embryonic nuclei were introduced into intact immature meiotically competent oocytes and subsequently cultured their chromosomes assembled on a common spindle with meiotic chromosomes and proceeded through the meiotic-like division, judged according to the presence of the first polar body extruded. When embryonic cell nuclei were introduced into cytoplasts obtained from immature meiotically competent oocytes, polar bodies were extruded in about 75% of reconstructed cells but the metaphase plates were abnormal in almost all cases. When somatic cell nuclei were inserted into the above cytoplasts, polar bodies were extruded only very exceptionally and in these cells chromosomes were arranged in abortive metaphase plates.

Our results suggest that somatic cell nuclei are unable to proceed through the reduction division (haploidization) when introduced into an immature oocyte meiotic cytoplasm.