Factors regulating the growth of the preovulatory follicle in the sheep and human.

Around the time of luteal regression in monovular species a single dominant follicle, which will eventually ovulate, is selected from the population of antral follicles. The dominant follicle is characterized by its progressive increase in diameter due to increase in antral fluid volume as well as an increased number of granulosa cells. The crucial factor in the continued development of the dominant follicle is its ability to synthesize oestradiol under the influence of LH and FSH. In the sheep FSH secretion continues throughout the luteal phase while LH is suppressed. Thus development of large antral follicles continues so that when luteal regression occurs and LH secretion increases the final stages of development of the pre-antral follicle occur within 3 days. In the human, however, both FSH and LH are suppressed during the luteal phase and only rise when the levels of progesterone and oestradiol fall a few days before menstruation. This rise in FSH and LH which occurs at this time stimulates the further development of a small antral follicle (1-2 mm diameter). Within 7 days the favoured follicle establishes dominance over the other asynchronously developing follicles probably by inhibiting the secretion of FSH. As in the sheep, once aromatase enzyme(s) has been fully activated the dominant follicle is able to utilize the increased androgen precursor produced by the theca under LH stimulation.