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Increased FSH levels stimulate follicular secretion of estradiol by inducing androgen synthesis by the theca externa and upregulating expression of the aromatase enzyme receptor on granulosa cells. Aromatase enzyme converts androgens into estrogens. FSH also induces expression of FSH receptors on follicles. The effect of circulating FSH is thus enhanced and follicles are induced to produce more estradiol. These actions cause a peak in estradiol secretion during days 10-13 of the menstrual cycle, shortly preceding ovulation. |