last updated 13-07-2021

Ovarian follicle

The ovarian follicle

Gonadotropin-releasing hormone (GnRH), which is secreted from the hypothalamus, stimulates the gonadotropic cells in the anterior pituitary to release FSH and LH. In the beginning of the follicular phase a small amount of FSH stimulates the proliferation of the estradiol producing granulosa cells of the follicle (1). The local estradiol concentrations promote the number of FSH receptors and thus amplify their own production process (2). FSH now also increases the number of LH receptors in the growing follicle, which makes the granulosa and thecal cells more sensitive to LH. Meanwhile, FSH levels will drop due to the negative feedback by estradiol. The primary follicle is now maturing.
Small amounts of LH induce proliferation and androgen production in the dominant follicle. The androgens are converted into estrogens

in the granulosa cells. LH also stimulates progesterone production in the granulosa cells. These two events result in a very high estradiol production. Both estradiol and progesterone (and other factors) take care of the oocyte maturation. When estradiol levels pass a certain threshold, they exert a positive feedback on GnRH and LH secretion. LH levels peak (3) and the ovulation process occurs. One of the factors produced by granulosa cells is progesterone (4), which has a negative feedback on LH secretion (indirectly via GnRH). The remaining follicle becomes the corpus luteum: luteal phase of the cycle. When fertilization of the oocyte is absent or fails, the corpus luteum degenerates and progesterone levels drop towards the end of the cycle (5).
By understanding the ovulatory cycle and its hormonal regulation, one can modify the reproductive system by inhibiting it (contraception) and by stimulating it (IVF).