Maturational and developmental competence of immature oocytes retrieved from bovine ovaries at different phases of folliculogenesis

Immature oocytes recovered from bovine ovaries were studied to determine if their maturational and developmental competence is affected by phase of folliculogenesis. Ovaries (a total of 39 pairs) were collected from a local abattoir. Following examination, each pair of ovaries was assigned to one of three groups, according to follicle size and with or without a corpus luteum: (i) early phase ( n = 13 pairs): all follicles were ≤8 mm in diameter; (ii) late phase ( n = 13 pairs): the largest follicle was ≥15 mm in diameter; (iii) luteal phase ( n = 13 pairs): all follicles were ≤8 mm in diameter and there was a corpus luteum on one of the ovaries. All follicles were aspirated and cumulus–oocytes complexes (COC) were cultured in 1.0 ml maturation medium, TC-199 medium supplemented with 10% fetal bovine serum (FBS), 0.2 mmol pyruvate, and 75 mIU/ml FSH and LH (Humegon) at 38.5°C in a humidified atmosphere of 5% CO 2 and 95% air for 24 h. Following maturation in vitro, the oocytes were inseminated with frozen–thawed semen and cultured for further development in vitro. The mean number of oocytes retrieved from early phase ovaries was 32 ± 8.1 which was significantly higher ( P < 0.05) than other phases ( n = 20.1 ± 5.4 and 22.7 ± 6.9). The numbers of degenerated oocytes from early, late and luteal phase ovaries were not different ( n = 1.6 ± 0.7, 1.8 ± 0.5 and 1.5 ± 0.5, respectively). The rates of oocyte maturation (84.4%, 89.2% and 83.6%) and fertilization (53.5%, 53.3% and 51.3%) were not significantly different across the three groups. In addition, there were no significant differences in the rates of cleavage (77.9%, 79.9% and 73.9%) and blastocyst formation (27.4%, 33.0% and 30.4%) in the three groups. These results indicate that the maturational and developmental competence of immature oocytes is not affected by the phase of folliculogenesis.