Effect of follicle size of Boran cows on glucose 6 phosphate dehydrogenase activity and developmental competence

Abstract

In vitro embryo production (IVEP) has been used to improve of cattle genotypes in many parts of the world. However IVEP is not without some challenges. Oocyte developmental competence is a key area affecting IVEP and it is associated with the glucose 6 phosphate dehydrogenase (G6PDH) activity and follicle size. Ovarian follicular dynamics differ between B. indicus and B. taurus , which may influence oocyte competence. This study sought to evaluate the effect of follicle size on G6PDH activity and oocyte developmental competence in the Boran (African Zebu breed) cattle using brilliant cresyl blue (BCB) staining. Oocytes were collected from Boran cows at slaughter, their follicles were measured and classified into 3 groups; 1-3mm, >3-6mm and >6mm in diameter. Cumulus oocytes complexes (COC) were exposed to BCB stain for 1 hour and observed for cytoplasmic coloration. Those that retained the blue coloration (BCB positive) were deemed competent (low activity of G6PDH) while those that did not have any cytoplasmic coloration (BCB negative) were deemed inco mpetent (high activity of G6PDH). Higher proportion (P<0.01) of BCB positive oocytes was found in >6mm follicles (81.1%) than in 1-3mm (73.1%) and >3-6 mm (76.5%) follicles. BCB positive oocytes from 1-3 mm follicle s had higher (P<0.05) blastocyst rate (18.94%) than BCB negative oocytes (9.7%), however no significant difference was found in their cleavage rates (P > 0.05). No difference in cleavage and blastocyst rates was found between BCB positive and negative oocytes from follicles with >3-6 mm in diameter. The cleavage and blastocysts rates with BCB positive oocytes increased as follicle diameter increased. In conclusion, in Boran cows as follicle size increases the activity of G6PDH decreases in the whereas COC developmental competence increases and follicles as small as 1-3 mm in diameter are able of producing developmentaly competent oocytes.