Activation regimens for full-term development of rabbit oocytes injected with round spermatids

The present study was designed to investigate the effect of activation regimens on full-term development of rabbit oocytes after round spermatid injection (ROSI). In the first series, rabbit oocytes were treated with 5 microM ionomycin before ROSI, after ROSI, or before and after ROSI. In addition, non-treated oocytes were subjected to intracytoplasmic sperm injection (ICSI) using ejaculated spermatozoa. Cleavage rate of ROSI oocytes activated before and after ROSI (55%) was comparable with that of ICSI oocytes (60%), and significantly higher than those of ROSI oocytes activated either before or after ROSI (29-39%; P < 0.05). No offspring were produced by transfer of the cleaving ROSI oocytes, while 8% of the cleaving ICSI oocytes transferred gave birth to offspring. In the second series, oocytes were exposed to 5, 10, or 20 microM ionomycin, followed by ROSI, 5 microM ionomycin treatment, and incubation with 5 microg/ml cycloheximide (CHX) + 2 mM 6-dimethylaminopurine (DMAP). Significantly higher cleavage rates were derived from oocytes activated with 10 and 20 microM ionomycin before ROSI (91% and 82%, respectively; P < 0.05) compared to those activated with 5 microM ionomycin before ROSI (53%). Live offspring were obtained when the cleaving ROSI oocytes with the initial ionomycin treatment at 5 and 10 microM were transferred (offspring rate 2% and 4%, respectively). These activation regimens, however, were not valid for the ROSI using cryopreserved round spermatids. In conclusion, rabbit ROSI oocytes were capable of developing into full-term when the oocytes were activated with a combined treatment of ionomycin and CHX/DMAP.