Activation and early parthenogenesis of bovine oocytes treated with ethanol and strontium

Abstract

Efficient artificial activation is indispensable for the success of cloning programs. Strontium has been shown to effectively activate mouse oocytes for nuclear transfer procedures, however, there is limited information on its use for bovine oocytes. The present study had as objectives: (1) to assess the ability of strontium to induce activation and parthenogenetic development in bovine oocytes of different maturational ages in comparison with ethanol; and (2) to verify whether the combination of both treatments improves activation and parthenogenetic development rates. Bovine oocytes were in vitro matured for 24, 26, 28, and 30 h, and treated with ethanol (E, 7% for 5 min) or strontium chloride (S, 10 mM SrCl 2 for 5 h) alone or in combination: ethanol+strontium (ES) and strontium+ethanol (SE). Activated oocytes were cultured in vitro in synthetic oviductal fluid (SOF) medium and assessed for pronuclear formation (15–16 h), cleavage (46–48 h) and development to the blastocyst stage (D7). Treatment with ethanol and strontium promoted similar results regarding pronuclear formation (E, 20–66.7%; S, 26.7–53.3%; P>0.05) and cleavage (E, 12.8–40.6%; S, 16.1–41.9%; P>0.05), regardless of oocyte age. The actions of both strontium and ethanol were influenced by oocyte age: ethanol induced greater activation rates after 28 and 30 h of maturation (48.4 and 66.7% versus 20.0 and 23.3% for 24 and 26 h, respectively; P<0.05) and strontium after 30 h (53.3%) was superior to 24 and 26 h (26.7% for both). Blastocyst development rates were minimal in all treatments (0.0–6.3%; P>0.05), however, when the mean (±S.D.) cell number in blastocysts at the same maturational period was compared, strontium treatment was superior to ethanol for activation rates (82±5.7 and 89.5±7.8 versus 54 and 61, at 28 and 30 h, respectively). Improved results were obtained by combined treatments. The combination of ethanol and strontium resulted in similar pronuclear formation (ES, 36.7–83.9%; SE, 53.1–90.3%) and cleavage rates (ES, 31.3–81.3%; SE, 65.6–80.7%). Regarding embryo development, there was no difference ( P>0.05) between treatments, and blastocysts were only obtained in treatment SE at 24 and 26 h (6.5% for both). It is concluded that, SrCl 2 induces activation and parthenogenetic development in bovine oocytes.