Differences in Cytoplasmic Maturation Between the BCB+ and Control Porcine Oocytes Do Not Justify Application of the BCB Test for a Standard IVM Protocol

The Brilliant Cresyl Blue (BCB) test relies on G6PDH activity and a simple protocol for the selection of higher quality oocytes. Although the BCB+ oocytes of all the species that have been investigated are characterized by superior quality when compared to BCB- counterparts, application of the test for embryo production still remains an open issue. The aim of our study was to compare BCB+ and the control oocytes (not subjected to the BCB test) in terms of selected aspects of cytoplasmic maturation (mtDNA copy number, mitochondria distribution, relative transcript abundance of six marker genes). The results of our study revealed more relevant differences within the BCB+ and the control oocytes (before and after IVM) than between the two categories of oocytes. There was no difference in the transcript abundance of the BCB+ and the control oocytes in 5 out of 6 analyzed genes (BMP15, GDF9, ATP5A1, EEF1A, ZAR1) and in mtDNA content (pre-IVM 179609 vs. 176595 and post-IVM 187243 vs. 246984, respectively). With regard to mitochondria distribution in pre- and post-IVM oocytes, there was nonsignificant tendency for a more frequent occurrence of the expected patterns in the BCB+ group. The results of the present study do not support the application of BCB staining in a routine IVM protocol due to relatively high similarity in selected parameters characterizing cytoplasmic maturation of BCB+ and control oocytes. This high similarity may results from the limited amount of less competent BCB- oocytes (10%) still present among nonselected oocytes of proper morphology.