Effect of Activation Regimens for Rat Oocytes on Full-term Development after Round Spermatid Injection

The present study was designed to compare the effect of two activation regimens for rat oocytes on full-term development after round spermatid injection (ROSI). Oocytes were treated either with 1.25 mM SrCl₂ for 20 min or with two direct-current (DC) pulses at 100 V/mm for 99 μsec followed by exposure to 2 mM 6-dimethylaminopurine (6-DMAP) for 20 min. Single spermatids then were injected into the oocytes that were ready for polar body extrusion. The proportions of oocytes surviving injection and exhibiting pronuclear formation were comparable between the SrCl₂ group (76.8% and 45.6%, respectively) and the DC+6-DMAP group (78.3% and 34.9%, respectively). More SrCl₂-activated ROSI oocytes (44.2%) cleaved into the two-cell stage than did those activated with DC+6-DMAP (34.7%). However, the proportions of the two-cell embryos developing to morulae/blastocysts and live offspring were significantly (P = 0.0005 and 0.042) lower in the SrCl₂ group (12.3 and 6.1%, respectively) than the DC+6-DMAP group (36.1 and 14.6%, respectively). Our results indicate DC+6-DMAP is better than SrCl₂ for oocyte activation prior to successful ROSI of rats.