The meiotic stage of preovulatory oocytes in mares.

The research aimed to clarify some confusion around the meiotic stage at which oocytes—eggs—of the domestic horse are released during ovulation. The study concluded that, like most mammals, these horse oocytes are ovulated after completing the first meiotic division—a stage called Metaphase II (MII).

Objective of the Study

• The purpose of the research was to understand the meiotic stage at which domestic horse oocytes are ovulated, addressing a lack of clear information as to whether it occurs at the first meiotic metaphase (MI) or the second (MII).

Methodology

• Eight oocytes were obtained from the preovulatory follicles of sixteen mares.
• The mares had been treated with human chorionic gonadotropin (hCG) 36 hours prior to the collection of the oocytes.
• Each oocyte’s meiotic stage was determined by examining the presence or absence of the first polar body, a cell structure that forms during meiosis.

Findings

• Out of the eight collected oocytes, six were classified to be at the MII stage due to the presence of the first polar body. This assessment was later confirmed by semithin sectioning—a method of microscopic analysis—on one of these six oocytes.
• Out of the remaining two oocytes without a polar body, one was determined to be at the MII stage after conducting chromosomal analysis via the fixation method.
• The meiotic stage of the last oocyte could not be ascertained, leaving it as an undetermined case within the study’s findings.

Conclusion

• All seven oocytes that could be conclusively evaluated were at the MII stage. Based on this, the researchers concluded that, much like how it happens in most mammals, domestic horse oocytes are ovulated after the completion of the first meiotic division (MI) and the formation of the first polar body, putting them at the MII stage during ovulation.