Follicular growth and estradiol influence on luteal function in mares.

This research study examined the interrelationship between luteolysis, follicular growth, and estradiol concentrations in mares by monitoring their reproductive cycles. The findings suggest that during the diestrus phase, follicular growth and the resulting estradiol production may lead to luteolysis via endometrial prostaglandin F2 alpha (PGF2 alpha) production.

Objective and Methodology

• The aim of the study was to investigate the relationships between follicular growth, levels of circulating estradiol, and endometrial prostaglandin F2 alpha (PGF2 alpha) production, particularly regarding luteolysis in mares.
• The researchers monitored the follicular growth of 16 mares through rectal palpation every other day during diestrus. They also measured plasma estradiol on a daily basis during all estrous cycles.
• Endometrial tissue was collected at various times (d 10, 12, 14, or 16) during several normal cycles and after d 30 during spontaneously prolonged cycles for PGF2 alpha analysis via radioimmunoassay.

Findings

• The study found a strong correlation between circulating estradiol concentrations and follicular diameter.
• In normal cycles, the researchers found that follicular growth was initiated before day 14, leading to a significant rise in estradiol during the diestrus phase preceding luteolysis. In contrast, during spontaneously prolonged cycles, no correlation between follicular size and circulating estradiol was found, follicular growth started on average 19 days after ovulation, and there was no increase in estradiol levels.
• The estradiol concentrations during diestrus in normal cycles were significantly raised immediately (72 to 24 h) prior to the time when peak endometrial PGF2 alpha production occurred (on days 12 and 14).
• However, during spontaneously prolonged cycles, estradiol levels did not rise at the time of expected luteolysis, and endometrial PGF2 alpha production was minimal in day-30 biopsies.

Implications

• The results suggest that the growth of follicles during diestrus and the associated production of estradiol might play a critical role in initiating luteolysis, potentially through endometrial PGF2 alpha production.
• A delay or failure in this process could lead to prolonged functioning of the corpus luteum, highlighting potential therapeutic targets or diagnostic indicators for reproductive health issues in mares.