Selection of developmentally competent oocytes through brilliant cresyl blue stain enhances blastocyst development rate after bovine nuclear transfer.

The aim of the present investigation was to study the effect of oocyte selection on the efficiency of bovine nuclear transfer in terms of increased blastocyst production. For this purpose, prior to in vitro maturation (IVM), oocytes were selected for their developmental competence on the basis of glucose-6-phosphate dehydrogenase (G6PDH) activity indicated by brilliant cresyl blue (BCB) staining. It has been hypothesized that growing oocytes have a higher level of active G6PDH in comparison to the mature oocytes. Compact cumulus oocyte complexes (COCs) were recovered from slaughterhouse-collected bovine ovaries and classified either as control group, which were placed immediately into culture without exposure to BCB stain, or treatment group, which were stained with BCB for 90min before culture. Treated oocytes were then divided into BCB- (colourless cytoplasm, increased G6PDH) and BCB+ (coloured cytoplasm, low G6PDH) based on their ability to metabolize the stain. After IVM, oocytes were subjected to nuclear transfer procedure for the production of cloned embryos which were then cultured for a period of 8 days to determine the blastocyst rate. The BCB+ oocytes yielded a significantly higher blastocyst rate (39%) than the control (21%) or BCB- oocytes (4%). These results show that the staining of bovine cumulus-oocyte complexes with BCB before in vitro maturation could be used to select developmentally competent oocytes for nuclear transfer. In addition, G6PDH activity could prove to be a useful marker for determining the oocyte quality in future.