Developmental competence of bovine oocytes selected based on follicle size and using the brilliant cresyl blue (BCB) test

Background: Many studies reported that follicle size has an essential role in developmental potential of oocytes. Also, the brilliant cresyl blue (BCB) test is one of the most important criteria in selection of more competent oocytes.
Objective: Selection of developmentally competent bovine oocytes.
Materials and Methods: A total of 1730 bovine cumulus oocyte complexes (COCs) were recovered from the ovaries by follicles isolation and classified into 3 categories according to the diameters of the follicles (small, <3 mm; medium 3-6 mm and large >6 mm). Oocytes were exposed to the BCB stain, diluted in Dulbecco's phosphate-buffered saline, modified with 0.4% bovine serum albumin (BSA) for 90 min. Oocytes with or without blue coloration of the cytoplasm were designated as BCB⁺ and BCB^-, respectively.
Results: The BCB⁺ and control oocytes originated from large and medium follicles exhibited a higher (p<0.0001) cleavage and blastocyst rate than BCB^- oocytes. Furthermore, the BCB⁺ oocytes from large and medium follicles had the highest (p<0.0001) proportion of blastocyst than other treatment groups. In contrast, the BCB^- oocytes from small follicles had the lowest (p<0.0001) proportion of blastocyst than other treatment groups. Interestingly, the percentage of the BCB⁺ oocytes from the large and medium ovarian follicles was significantly higher (p<0.0001), than the BCB⁺ oocytes from the small follicles.
Conclusion: Current results confirmed that each BCB⁺ oocyte could not lead to perfect embryo development and the BCB test is not sufficient enough for the identification of oocytes that are competent for in vitro embryo development.