Post thawing in vitro maturation rate was evaluated for partially matured vitrified oocytes from Chilean Red Friesian cattle. Cumulus-Oocytes Complexes were obtained by follicular aspiration, classified by morphology and randomly in vitro matured in TCM199 (10 % Bovine Fetal Serum (BFS), 50 mg/mL gentamicine, 0.2 mM sodium piruvate, 0.08 μg/ml FSH, 1 μg/mL LH and 1 μg/mL estradiol) in the following groups: a) control (n= 137), matured for 24 h at 38.5 oC, 5 % CO2 y 99 % humidity, and b) treatment (n= 156), matured for 6 h, partially denuded, and incubated until completion of 20 h. Then, oocytes were vitrified by the Open Pulled Straws (OPS) method. Oocytes were exposed to vitrification solution one (VS1) (Phosphate Buffered Saline (PBS), 10 % Ethylen glycol (EG), 10 % DMSO) for 30 s, then they were exposed to VS2 (PBS, 20% EG, 20% DMSO) for 25 s. Afterwards, oocytes were loaded into open pulled straws and submerged into liquid nitrogen. Oocytes were thawed by exposure to sequential solutions with decreasing concentrations of sucrose (0.3; 0.15 y 0 M respectively). Finally, oocytes continued the in vitro maturation for 4 additional hours. After completion of maturation period oocytes from both groups were fixated, stained and evaluated. The proportion of lost and damaged, MI, MII, and degenerate oocytes were compared between groups by Chi square test. Vitrification procedure increased (p ≤0.05) the percentage of oocytes lost and damaged when compared to control group. Additionally, vitrification increased (p ≤0.05) the proportion of MI and degenerated oocytes, and decreased the proportion of MII oocytes. Therefore, vitrification by the OPS method of partially matured bovine oocytes is a reliable alternative for the conservation of germinal cells from Chilean Red Friesian females.

KEY WORDS: Oocytes; Vitrification; in vitro maturation; Chilean Red Friesian cattle.