Vitrifying immature equine oocytes impairs their ability to correctly align the chromosomes on the MII spindle.

The research suggests that the process of vitrification (freezing) of immature horse oocytes (unfertilized eggs) negatively affects their ability to properly align chromosomes during the development phase, which can lead to developmental complications. The study also found that contrary to expectations, the vitrification process impacted oocytes from younger mares more severely than those from older mares.

Investigating the Effect of Vitrification

• The study was motivated by the observation that while vitrified-warmed immature equine oocytes can complete the first meiotic division, the developmental competence of these oocytes is compromised. This observation prompted the researchers to specifically investigate the impact of vitrification on the chromosome and spindle configuration of these oocytes after in-vitro maturation (IVM).

The Research Method

• The researchers collected Cumulus-oocytes complexes (COCs) and categorized them into two groups based on the age of the mare: young (≤14 years) and old (≥16 years).
• The COCs were either matured directly in-vitro or made to undergo vitrification and warming before IVM.
• The researchers used immunofluorescent staining, confocal microscopy and three-dimensional image analysis to assess the spindle morphology and chromosome alignment within the MII stage oocytes.

Findings

• The process of vitrification reduced the ability of the oocytes to reach the MII stage and resulted in ultrastructural changes to the meiotic spindle such as shortening of its long axis.
• The research also observed an increased occurrence of chromosomes failing to align properly at the metaphase plate in vitrified oocytes, which likely contributes to their reduced developmental competence.
• Interestingly, the study found that oocytes from young mares were more negatively affected by vitrification than those from older mares, contrary to initial expectations.

Conclusion

• The researchers hypothesized that this unexpected result could be due to the possibility of pre-existing compromises in spindle assembly checkpoint control mechanisms in oocytes from older mares. This could potentially make these oocytes less susceptible to the impact of vitrification.