Temporal relationships among LH, estradiol, and follicle vascularization preceding the first compared with later ovulations during the year in mares.

This research explored the relationships between Luteinizing Hormone (LH), estradiol, and follicle vascularization during preovulatory periods in mares. The study found that both LH levels and follicle vascularization were lower during the first preovulatory period of the year, though estradiol levels remained unchanged.

Study Design

The researchers undertook an examination of the preovulatory follicle — the structure within an ovary where an egg grows before ovulation — examining three primary factors:

• The diameter of the follicle
• The concentration of LH, a hormone that triggers ovulation, in the bloodstream
• How well-vascularized (supplied with blood vessels) the follicle was.

They studied these aspects in 40 pony mares over six days leading up to ovulation.

Comparisons and Findings

The purpose of the study was to compare these factors in the preovulatory periods before the first ovulation of the year with a later ovulation in the same year. They sought to understand the relationships between LH and estradiol (a form of estrogen) and between follicle vascularization and estradiol.

The findings were as follows:

• The preovulatory follicle was larger prior to the first ovulation of the year (P<0.02).
• The concentration of LH was lower before the first ovulation (P<0.02).
• The concentration of estradiol didn’t change between the first and later ovulation periods.
• The rate of increase in the follicle’s vascularized area was slowed during the first preovulatory period, resulting in a smaller area on the day before the first ovulation (1.4+/-0.1cm(2)) compared to before later ovulation (2.2+/-0.2cm(2)).

Implications of the Study

These results showed that follicle vascularization and the LH increase were reduced before the first ovulation of the year. The data did not suggest that estradiol played a role in the differences observed between the first and subsequent ovulations. The study enhances our understanding of equine reproduction — particularly the role hormones and vascularization play in the ovulation cycle. The results could potentially be applied towards improving fertility treatments or managing reproductive health in mares.