Selection of oocytes for in vitro maturation by brilliant cresyl blue staining: a study using the mouse model

Selecting oocytes that are most likely to develop is crucial for in vitro fertilization and animal cloning. Brilliant cresyl blue (BCB) staining has been used for oocyte selection in large animals, but its wider utility needs further evaluation. Mouse oocytes were divided into those stained (BCB+) and those unstained (BCB-) according to their ooplasm BCB coloration. Chromatin configurations, cumulus cell apoptosis, cytoplasmic maturity and developmental competence were compared between the BCB+ and BCB- oocytes. The effects of oocyte diameter, sexual maturity and gonadotropin stimulation on the competence of BCB+ oocytes were also analyzed. In the large- and medium-size groups, BCB+ oocytes were larger and showed more surrounded nucleoli (SN) chromatin configurations and higher frequencies of early atresia, and they also gained better cytoplasmic maturity (determined as the intracellular GSH level and pattern of mitochondrial distribution) and higher developmental potential after in vitro maturation (IVM) than the BCB- oocytes. Adult mice produced more BCB+ oocytes with higher competence than the prepubertal mice when not primed with PMSG. PMSG priming increased both proportion and developmental potency of BCB+ oocytes. The BCB+ oocytes in the large-size group showed more SN chromatin configurations, better cytoplasmic maturity and higher developmental potential than their counterparts in the medium-size group. It is concluded that BCB staining can be used as an efficient method for oocyte selection, but that the competence of the BCB+ oocytes may vary with oocyte diameter, animal sexual maturity and gonadotropin stimulation. Taken together, the series of criteria described here would allow for better choices in selecting oocytes for better development.