Follicle and endocrine dynamics during experimental follicle deviation in mares.

The research article investigates the changes in follicle sizes and endocrine concentrations in mares during a specific reproductive cycle stage known as the follicular wave. It reveals that certain hormones and growth factors contribute to one follicle becoming dominant over others.

Objective of the Research

• The study aimed to determine and understand the intrafollicular changes that occur when the largest follicle (F1) in mares reaches a mean diameter of 22.5 mm – a stage which marks the start of the follicular wave.
• The follicular wave is a part of the mares’ reproductive cycle where one follicle becomes dominantly larger (F1), while the size of the second largest follicle (F2) regresses.
• The researchers used F1 ablation – a process of removing or destroying tissue – at the expected beginning of the follicular wave (Hour 0) to serve as a reference point for observing the changes occurring between F2 and the third follicle (F3).

Methodology and Findings

• To observe the changes, the diameters and concentrations of follicular fluid factors in F2 and F3 were measured in F1-ablated mares at different stages: Hours 0, 12, 24, 48, and 72. The sample included eight mares per group.
• It was found that the circulating FSH (Follicle-stimulating hormone) concentrations were higher in the mares belonging to the hour 72 ablation group 12 hours after ablation.
• The diameters of F2 and F3 increased from Hours 0 to 24. After that point, the size of F2 continued to grow while F3 did not grow further. This indicated that the experimental deviation, the process where one follicle becomes dominant over the other, began at Hour 24.
• Furthermore, at Hour 24, the diameter of F2 and the FSH concentration were similar to that of F1 and the FSH concentration at Hour 0.

Involvement of Hormones and Growth Factors

• No detectable differences occurred between F2 and F3 in terms of follicular fluid concentrations for the hormones estradiol, inhibin-A, and activin-A at the start of experimental deviation.
• However, the concentration of estradiol was higher in F2 at the start and 12 hours in, and inhibin-A remained higher in F2 throughout the experiment. These two hormones could potentially influence deviation.
• Additionally, free insulin-like growth factor-1 (IGF-1) in F2 started to increase from hour 12 and remained higher than in F3 by the start of experimental deviation. This result suggested that intrafollicular IGF-1 played a part in determining which follicle became dominant.