| Actions of FSH and LH on the ovary
|
In the female, FSH promotes estradiol synthesis leading to follicular maturation, while LH leads to follicle rupture and oocyte release
|
| In the mature female the GnRH pulse generator is responsible for the control of all the hormonal changes seen in the normal menstrual cycle (Figs. 37.9 and 37.12). The initiation of follicular growth begins in the last few days of the preceding menstrual cycle and terminates at the time of ovulation. FSH is the dominant hormone acting through its receptors on the ovarian granulosa cells. Rising FSH concentrations stimulate estradiol synthesis through induction of the aromatase and other enzymes (see Fig. 37.7). As estradiol is secreted so FSH falls, and this combination leads to the selection of a dominant follicle for further development. Follicular maturation continues under the influence of rising estradiol concentrations. At midcycle there is a further estradiol surge, which causes positive feedback at the pituitary to initiate the LH surge. This LH binds to its receptors on the dominant follicle and, in tandem with steroid hormones and other factors such as prostaglandins, results some 36 hours later in rupture of the follicle and release of the oocyte, i.e. ovulation. At this time there is a sharp fall in plasma estradiol, followed by a fall in LH. The ruptured follicle transforms into the corpus luteum, which secretes progesterone and lesser amounts of estradiol both to sustain the oocyte and to prepare the estrogen primed uterine endometrium for implantation of a fertilized ovum and the establishment of early pregnancy. In the absence of fertilization, corpus luteum function declines, and progesterone and estradiol secretion fall. This brings about vascular changes in the endometrium leading to tissue death and menstruation. The decline in steroid secretion stimulates FSH secretion, setting the stage for initiation of the next cycle.
|
|
