Systemic and intrafollicular components of follicle selection in mares

- Diameter of 2 largest follicles begins to deviate when they were 20 to 23 mm.
- The deviation process begins 1 d before diameter deviation.
- Intrafollicular IGF1 but not estradiol is directly involved in deviation.

Mares are superb models for study of follicle selection owing to similarities between mares and women in relative follicle diameters at specific events during the follicular wave and follicle accessibility for experimental sampling and manipulation. Usually, only 1 major follicular wave with a dominant follicle (DF) greater than 30 mm develops during the 22 to 24 d of the equine estrous cycle and is termed the primary or ovulatory wave. A major secondary wave occasionally (25%) develops early in the cycle. Follicles of the primary wave emerge at 6 mm on day 10 or 11 (day 0 = ovulation). The 2 largest follicles begin to deviate in diameter on day 16 when the future DF and largest subordinate follicle (SF) are 23 mm and 20 mm, respectively. The deviation process begins the day before diameter deviation as indicated in the future DF but not in the future SF by (1) increase in prominence of an anechoic layer and vascular perfusion of the wall and (2) increase in follicular-fluid concentrations of IGF1, vascular endothelial growth factor, estradiol, and inhibin-A. A systemic component of the deviation process is represented by suppression of circulating FSH from secretion of inhibin and estradiol from the developing DF. Production of inhibin is stimulated by IGF1 and LH, and estradiol is stimulated by LH and not by IGF1 in mares. A local intrafollicular component involves the production of IGF1, which apparently increases the responsiveness of the future DF to FSH. The roles of the IGF system have been well studied in mares, but the effect of IGF1 on increasing the sensitivity of the follicle cells to FSH is based primarily on studies in other species. The greater response of the future DF than the SF to the low concentrations of FSH is the essence of selection. During the common growth phase that precedes deviation, diameter of the 2 largest follicles increases in parallel on average when normalized to emergence or retrospectively to deviation. Study of individual waves indicates that (1) the 2 follicles change ranks (relative diameters) during the common growth phase in about 30% of primary waves and (2) after ablation of 1, 2, or 3 of the largest follicles at the expected beginning of deviation, the next largest retained follicle becomes the DF indicating that several follicles have the capacity for dominance; therefore, it is proposed that the deviation process represents the entire mechanism of follicle selection in mares.