Post thaw quality and viability of vitrified immature sheep oocytes using different cryoprotectant concentrations.

BACKGROUND: The cryopreservation of oocytes through vitrification is quite successful but oocyte vitrification is still being standardized because of their structural and molecular sensitivity to the cooling and freezing processes. OBJECTIVE: To evaluate the effect of different cryoprotectant concentrations on post-thaw morphology and viability of immature oocytes in sheep. MATERIALS AND METHODS: Vitrification was achieved in three vitrification solutions comprised of different concentrations of the cryoprotectants ethylene glycol + DMSO, viz., (G1) 20%, (G2) 30%, (G3) 40% ethylene glycol + DMSO in equal ratio. Cryopreservation was in open pulled straws. Post vitrification evaluation was done after 1 week's storage in liquid nitrogen based on morphological evaluation and viability using trypan blue dye. RESULTS: The present study revealed non-significantly higher morphologically normal oocytes in G3 (74.7%) followed by G2 (70.3%), and the lowest in G1 (66.6%). Morphological defects were observed in 33.3 %, 29.6% and 25.2% of oocytes after cryopreservation in 20% (G1), 30% (G2) and 40% (G3) vitrification solutions, respectively. The results were non-significantly different between vitrification solution groups. However, the viability of post thaw immature oocytes was 95.6%%, 84.4% and 81.1% after vitrification in G1 (20%), G2 (30%) and G3 (40%), with viability being significantly highest (P<0.05) in G1 (20%) and lowest in G3 (40%). CONCLUSION: Cryoprotectant concentrations enable the maintenance of normal morphology and minimize cryoinjury during vitrification of immature oocytes.