Miniature ponies: 2. Endocrinology of the oestrous cycle.

This study analyzes how FSH, LH, oestradiol and progesterone hormone levels vary during the oestrous cycle in Miniature ponies, highlighting differences in hormone levels and follicle size when compared with larger horse breeds.

Details of the Study

• The research was done on nine Miniature ponies. It examined the variations in the plasma concentrations of four key hormones—FSH, LH, oestradiol, and progesterone—during specific periods of the ponies’ oestrous cycle. These periods included 12 interovulatory intervals (the time between two ovulations) and 21 periovulatory periods (the time around ovulation).

Key Findings

• The study found that the FSH surge, which corresponds with the emergence of the ovulatory follicle, peaked when the follicle was about 9mm in size. This is smaller when compared to the reported 13mm diameter at which this occurs in larger horse breeds.
• The research also identified a three-phase increase in the ovulatory LH surge. There is a slow rise between days 13 and 18, a plateau (or minimal increase) from days 18 to 21, and then a rapid increase after day 21.
• The shift from the plateau phase to the rapid increase phase of the LH surge coincided with the day of the highest oestradiol concentration during the preovulatory surge. In other words, the rapid increase in LH kicked off following the peak in oestradiol levels.
• In what was termed as an “unexpected” finding, there was a mean increase and decrease in LH levels between days 5 to 9.
• The concentrations of oestradiol and progesterone seemed to align with results reported in studies involving larger horse breeds, suggesting that these hormone patterns are consistent across different horse sizes.

Implications of the Research

• These findings suggest that, even though Miniature ponies and larger horse breeds may have similar oestradiol and progesterone patterns, there are distinct differences related to FSH and LH levels and ovulatory follicle size.
• The three-phase nature of the LH surge and the correlation with oestradiol levels provide more insight into the fine-tuned orchestration of hormones during the oestrous cycle in Miniature ponies.
• The unexpected rise and fall in LH levels during the early luteal phase adds another layer of nuance to our understanding of Miniature ponies’ reproductive processes.