In vitro developmental potential of nuclear transfer embryos cloned with enucleation methods using pre-denuded bovine oocytes

Enucleation of a recipient oocyte is an important essential process in the procedure of somatic cell nuclear transfer (SCNT). The present study investigated a method for the improvement of enucleation efficiency. Oocytes were denuded of cumulus cells before the completion of nuclear maturation (pre-denuded) after 12 h of culture at MI stage and subsequently cultured for additional 6 h until the completion of nuclear maturation and extrusion of the first polar body (PB1). The extrusion rate of PB1 was not significantly different in the pre-denuded oocyte group, compared with control oocyte group matured for 18 h. However, the number of oocytes showing the metaphase II (MII) located just underneath the PB1 was significantly higher (p<0.05) in the pre-denuded oocyte group than those in control oocyte group. To test the effect of pre-denuding on the enucleation rate and developmental potential of embryos to blastocyst stage, subsequent somatic cell nuclear transfer comparisons were made with three different methods of enucleation at MII stage using vital dyes (demicoline and Hoescht) or the PB1 (blind enucleation) to localize the chromosome plate. Enucleation rate of the oocytes with demicoline, Hoechst and pre-denuding enucleation groups were significantly higher (p<0.05) than those of blind enucleation groups. However, cleavage rate to two-cell stage and, developmental rate to blastocyst and hatched blastocyst stage, the mean numbers of total and ICM cells in the SCNT embryos with Hoechst enucleation groups were significantly decreased (p<0.05), compared to those of blind, demicoline and pre-denuding enucleation groups. Moreover, the level of telomerase activity was also significantly (p<0.05) decreased in SCNT blastocysts of Hoechst enucleation group, compared to those of blind, demicoline and pre-denuding enucleation groups. Taken together, pre-denuding enucleation group using pre-denuded oocytes was a useful and simple enucleation method for bovine SCNT embryos.