Review ArticleFollicle Selection in Mares: 90 Years from Observation to Theory

- Deviation in growth rates between the two largest follicles occurs on about day 16.
- The deviation process represents the theory of follicle selection in mares.
- The intrafollicular insulin-like growth factor system is the initiator of follicle selection.

The day of selection of the ovulatory follicle was first observed in any species by an equine practitioner during transrectal palpation in 1926. It was not until 1997 that ultrasonic imaging demonstrated the nature of selection. A wave-stimulating follicle stimulating hormone (FSH) surge reaches a peak on about day 13 postovulation when the growing follicles are about 13 mm. The FSH declines initially from secretion of inhibin by the multiple growing follicles. After about 2 days, estradiol is secreted by the future dominant follicle (DF) and synergistically contributes to a continuing FSH decline. On about day 16, the two largest follicles have grown to 23 and 20 mm, respectively, and begin to deviate in growth. The deviation process begins a day before diameter deviation as indicated in the future DF by (1) an increase in intrafollicular estradiol, inhibin-A, insulin-like growth factor 1 (IGF1), and vascular endothelial growth factor (VEGF) and (2) changes in echotexture and increased blood flow in the wall of the future DF. The IGF1 increases the sensitivity of the cells of the future DF to the reduced FSH, and VEGF presumably increases the vascular perfusion of the DF wall. The future DF is thus prepared to be the only follicle able to respond to the reduced levels of FSH. Ablation of various follicles at expected deviation indicates that several follicles have dominance capacity. And so, 90 years after the original observation of selection it can be proposed that the deviation process represents the theory of follicle selection in mares.