Effect of meiotic stages during In vitro maturation on the survival of vitrified-warmed buffalo oocytes

The present study was conducted to investigate the effect of meiotic stages during in vitro maturation (IVM) on the survival of vitrified-warmed buffalo oocytes, vitrified at different stages of IVM. Cumulus oocyte complexes obtained from slaughterhouse ovaries were randomly divided into 6 groups: control (non-vitrified, matured for 24 h at 38 +/- 1 degrees C, 5% CO2 in humidified air), and those matured for 0 h (vitrified before IVM) or 6, 12, 18 and 24 h before vitrification. Cumulus oocyte complexes were vitrified in solution consisting of 40% w/v propylene glycol and 0.25 mol/L trehalose in phosphate-buffered saline supplemented with 4% w/v bovine serum albumin. Vitrified cumulus oocyte complexes were stored at -196 degrees C (liquid nitrogen) for at least 7 days and then thawed at 37 degrees C; cryoprotectant was removed with 1 mol/L sucrose solution. Cumulus oocyte complexes in the 0, 6, 12, 18 and 24 h groups were then matured for an additional 24, 18, 12, 6 and 0 h, respectively, to complete 24 h of IVM. Among the five vitrification groups, 89-92% of cumulus oocyte complexes were recovered, after warming, of which 84-91% were morphologically normal. Overall survivability of vitrified cumulus oocyte complexes was lower (p < 0.05) than that of non-vitrified cumulus oocyte complexes (94.5%). Survival rates of cumulus oocyte complexes matured 24 h prior to vitrification (61.3%) were higher (p < 0.05) than those matured for 12 h (46.7%), 6 h (40.6%) and 0 h (37.6%). Nuclear status following 24 h IVM was assessed. A higher proportion of non-vitrified (control) oocytes (72.7%) reached metaphase II (M-II) stage in control than oocytes vitrified for 24 h (60.0%), 18 h (54.4), 12 h (42.3%), 6 h (33.3%) and 0 h (31.6%) (p < 0.05). The results suggest that length of time in maturation medium prior to vitrification influences post-thaw survivability of buffalo oocytes; longer intervals resulted in higher survival rates.