Follicle-stimulating hormone and advanced follicle development in the human

The great majority of human oocytes is destined to undergo atresia. Only follicles able to respond to stimulation by follicle-stimulating hormone (FSH) will enter the final stage of development and ovulate. While the role of FSH in early follicle development is unclear, late follicular development is FSH-dependent. FSH levels increase during the luteo-follicular transition and give rise to continued growth of a cohort of follicles. In the normo-ovulatory cycle, one follicle achieves a diameter of >8 mm and produces high concentrations of estradiol. In response to negative feedback from rising estradiol and inhibin levels, FSH levels fall in the late follicle phase. The dominant follicle has increased sensitivity to the falling FSH levels and continues growing. Follicles that initiate the latter stages of development after FSH levels begin to fall undergo atresia. The duration of this FSH window during which FSH levels are above the threshold required to stimulate ongoing development determines the number of follicles that can develop to the pre-ovulatory stage. Recognition of this concept has resulted in new approaches in ovulation induction treatment and ovarian hyperstimulation therapy for in vitro fertilization (IVF).