Summary

Freshly ovulated rabbit oocytes were activated parthenogenetically by periodically repeated calcium stimuli generated by electric field pulses applied onto the plasma membrane. Electric field pulses of 1.8 kV cm-1 were delivered every 4 min for 1 h 30 min (22 double pulses) in a specially designed chamber. Before each pulse, the culture medium was replaced by an isotonic glucose solution containing 10 microM Ca2+. The effects of modulating the ionic stimuli (by changing the duration of EF pulse) on a postactivation reaction, and/or on the pre- and postimplantation development, were studied. The rate of activation increased progressively as the pulse duration lengthened. For 22 pulses of 200 microseconds, 13% of oocytes were activated versus 100% for 1200 microseconds. The uniformity of the parthenogenetic response was obtained when oocytes were exposed to a series of pulses within which the reduction of pulse duration followed a negative exponential law. The influence of such activating treatment on the preimplantation development was tested using two treatments of 22 pulses with a total pulse duration equal to 14,868 and 11,228 microseconds, respectively. For the weaker treatment, a lower proportion of embryos underwent compaction and those that compacted were irregular. In contrast, the majority of embryos resulting from the stronger treatment compacted and developed into blastocysts. The most significant result that emerges from this study is that the level of stimulation affects in vitro developmental potency after the third cleavage division. The postimplantation viability of parthenogenetic eggs was tested and the results showed that parthenogenetic rabbit embryos died at a similar stage of development to the parathenogenetic mouse embryos. But, in the present series, high implantation rates and embryonic development (66%) till day 10–11 of pregnancy were obtained after the appropriate pulsatile EF treatment of oocytes. The parthenogenetic fetuses were of smaller size than the controls, but the development of the trophoblast tissue was proportional to the development of the fetuses. Anomalies of fetuses were also observed. This study reveals that activation is not a time-limited event and that the type of activating treatment has a marked effect on the ability of the resulting parthenogenetic embryos to develop to the early postimplantation stages. The sustained alteration of the cytoplasmic activity provides a useful tool to study the function of embryonic or somatic nuclei introduced during the earliest stages of activation.