Sheep embryo cryopreservation by vitrification and conventional freezing

Area of Science:

• Reproductive Biology
• Embryology
• Cryobiology

Background:

• Ovine embryo cryopreservation is crucial for livestock genetic resource preservation.
• Conventional freezing and vitrification are common methods, each with potential drawbacks.
• Understanding the impact of cryopreservation techniques on embryo viability is essential for improving success rates.

Purpose of the Study:

• To compare the in vitro survival and development rates of ovine embryos (morulae and blastocysts) cryopreserved by conventional freezing versus vitrification.
• To assess the toxicity of the vitrification solution.
• To investigate the effect of programmable freezer parameters, including automatic seeding, on embryo viability.

Main Methods:

• Ovine morulae and blastocysts were cryopreserved using either a vitrification solution (25% propylene glycol, 25% glycerol) or conventional slow freezing with 10% glycerol and 0.25 M sucrose.
• A simulated control group was used to evaluate vitrification solution toxicity.
• Programmable freezing involved automatic seeding at -7°C and cooling at -0.3°C/min to -25°C, with straws placed in different positions.
• In vitro development rates were assessed post-thaw.

Main Results:

• Vitrified embryos showed low in vitro development rates: 12% for morulae and 19% for blastocysts.
• Simulated control embryos (vitrification solution toxicity test) had higher survival (67% morulae, 63% blastocysts) than vitrified embryos.
• Conventional slow freezing resulted in higher blastocyst viability (67%) compared to morulae (31%).
• Embryo viability in conventional freezing varied significantly (51-75%) based on straw position in the programmable freezer, with automatic seeding on the meniscus yielding higher survival (65%) than central seeding (21%).
• Zona pellucida integrity damage did not affect in vitro survival rates.

Conclusions:

• Vitrification of ovine embryos in this study resulted in lower in vitro survival compared to conventional freezing.
• Conventional slow freezing demonstrated better viability for blastocysts than morulae.
• Programmable freezer parameters, particularly straw placement and automatic seeding, significantly influenced embryo survival rates during conventional freezing.
• Further optimization of vitrification protocols and controlled cooling rates are needed for improved ovine embryo cryopreservation.