Ultrastructural changes in bovine oocytes cryopreserved by vitrification

Area of Science:

• Reproductive Biology
• Cryobiology
• Oocyte Cryopreservation

Background:

• Oocyte cryopreservation is crucial for assisted reproduction and conservation.
• Vitrification offers a promising approach but requires optimized cryoprotectant solutions and protocols.
• Understanding ultrastructural changes post-vitrification is key to improving survival rates.

Purpose of the Study:

• To evaluate the effects of vitrification using DAP213 cryoprotectant on oocyte ultrastructure and developmental competence.
• To compare the impact of vitrification on oocytes at the germinal vesicle (GV) stage versus in vitro matured (IVM) oocytes.
• To assess the efficacy of one-step versus three-step cryoprotectant dilution procedures.

Main Methods:

• Oocytes from abattoir ovaries were vitrified using DAP213 at GV or IVM stages.
• Vitrified oocytes were thawed and survival assessed by in vitro fertilization and blastocyst development.
• Transmission electron microscopy was used to examine ultrastructural integrity post-thawing.

Main Results:

• Cleavage rates for vitrified IVM oocytes were low (4.5-6.7%), with no blastocyst formation.
• GV oocytes showed no cleavage, while controls developed to blastocysts (19.2%).
• Vitrification induced significant ultrastructural damage in GV oocytes; IVM oocytes were better preserved, especially with three-step dilution.

Conclusions:

• Vitrification with DAP213 significantly alters oocyte ultrastructure, compromising developmental potential.
• In vitro matured oocytes exhibit greater resilience to vitrification compared to GV stage oocytes.
• Three-step cryoprotectant dilution better preserves organelle integrity than one-step dilution, and zona pellucida damage may relate to premature cortical granule release.