Strontium-Induced Activation Regimen for Rat Oocytes in Somatic Cell Nuclear Transplantation.

The objective of this study was to determine the parthenogenetic activation regimen of ovulated rat oocytes using strontium (Sr2+). Spontaneous activation of Wistar rat oocytes was triggered immediately after dissociation from the animals, but such activation seemed to be incomplete because no cleavage occurred even after 34 h culture in modified KRB medium. The survival rate of oocytes treated with 5 mM Sr2+ for 6 h (28%) was lower than that of those treated with 0, 0.31 and 1.25 mM Sr2+ (91-99%). Overall activation efficiency, as determined by the proportion of cleaved oocytes at 28 h of subsequent culture, was the highest in the oocytes treated with 1.25 mM Sr2+ (57%), followed by those treated with 0.31 mM Sr2+ (45%), 5 mM Sr2+ (20%) and 0 mM Sr2+ (8%). When oocytes from Sprague-Dawley (SD) or Wistar rats were incubated with 1.25 mM Sr2+ for 6 h, the oocytes from the SD strain tended to be more resistant to the Sr2+ treatment than the oocytes from the Wistar strain (survival rate 99 vs 87%), and the overall activation efficiency for SD oocytes was higher than that for Wistar oocytes (74 vs 45%). Therefore, nuclei of cumulus cells were injected into enucleated SD oocytes using piezo-driven micropipettes and the reconstructed oocytes were then activated with 1.25 mM Sr 2+ for 6 h. The cleavage rate of reconstructed oocytes 18 h post-activation was 22%. One implantation site but no live offspring was observed in the 9 surrogate mothers receiving a total of 224 presumptive cloned zygotes. This study contains fundamental data concerning oocyte activation and somatic cell nuclear transplantation in rats.