Evaluation of equine oocyte developmental competence using polarized light microscopy.

The research probes into the use of polarized light microscopy as an objective method to examine the maturity of equine (horse) oocytes, in addition to the traditional morphological evaluation done with light microscopy. The study also determines the appropriate maturation time for the oocytes and gauges their quality and meiotic competence.

Objective of the Study

• The primary goal of this study was to develop an objective and computerized technique to observe the maturation of equine oocytes.
• The researchers wanted to compare the results from a polarized light microscope (PLM) to those obtained from a classic light microscope (LM).

Methodology

• The oocytes, enclosed in Cumulus Oocyte Complexes (COCs), were subjected to different maturation times – 24 hours, 36 hours, and 45 hours.
• Only the oocytes matured for 36 hours were examined using the computerized light microscope (CLM).
• To evaluate the quality and meiotic competency, the matured oocytes were parthenogenetically activated. This activation process is a way to trigger the development of an egg without fertilization.

Findings

• The study found that the average maturation percentages were highest (47.01% ± 9.90) for the group with 36-hour maturation, followed by the groups of 24-hour and 45-hour maturation.
• On the other hand, the percentage of immature oocytes was significantly lower (7.83 ± 5.51%) in the 36-hour maturation group compared to the other groups, asserting the effectiveness of the 36-hour maturation period.
• The birefringent properties of the zona pellucida (ZP), an outer layer of the oocyte, were measured and correlated with the activation outcome. ZP’s thickness and retardance were noticeably higher in immature oocytes than in mature ones.
• The comparison between parthenogenetically activated (without fertilization) and non-activated oocytes revealed a significant increase in the area and thickness of the ZP in activated oocytes.

Conclusion

• Based on the findings, the 36-hour In Vitro Maturation (IVM) protocol was identified as the efficient window for equine oocytes to achieve maturity.
• Furthermore, the PLM technique offers an effective method to differentiate between mature and immature oocytes, as well as between activated and non-activated oocytes, based on the birefringent properties of the ZP.