A Releasing Factor of the Hypothalamus which Influences the Menstrual Cycle
GnRF (gonadotropin “releasing factor”)
GnRF is a special kind of hormone called a “releasing factor” located in the hypothalamus. A “releasing factor” causes another gland or organ to release a different hormone(s) into the blood stream. For example, GnRF causes the anterior pituitary gland to produce, store and release FSH (follicle stimulating hormone) and LH (luteinizing hormone).
Hormones of the Anterior Pituitary Gland that Influence the Menstrual Cycle
FSH (follicle stimulating hormone)
FSH stimulates the growth of the ovarian follicles (which contain ova). As the ovarian follicles develop, FSH also stimulates the follicle cells to secrete large amounts of estrogen.
LH (luteinizing hormone)
A surge, or sudden release, of LH causes ovulation, the release of a mature ovum from the dominant ovarian follicle. After ovulation, LH stimulates the empty follicle to develop into the corpus luteum. LH then causes the corpus luteum to secrete increasing amounts of progesterone and small amounts of estrogen.
Hormones of the Ovaries that Influence the Menstrual Cycle
The ovaries contain the ovarian follicles which produce estrogen while maturing. After ovulation, the dominant ovarian follicle becomes the corpus luteum which produces progesterone and small amounts of estrogen.
Every month, the endometrium is built up under the influence of estrogen produced by the ovarian follicles. Estrogen stimulates glands in both the endometrium and the cervical canal. Changes in the cervical glands cause changes in the cervical mucus, making it clear, stretchy and slippery so that sperm can pass easily. The endometrial blood supply is increased in preparation for a possible fertilized ovum, and a thickened layer of endometrial tissue develops. Estrogen, along with FSH, also promotes the growth of the ovum in the ovarian follicle. Estrogen causes “feedback” to the anterior pituitary gland for the regulation of FSH and LH.
When the estrogen level increases to a certain level, it gives feedback to the anterior pituitary gland, causing a surge of stored LH that triggers ovulation. When the amount of estrogen in the blood becomes low, it causes feedback to the anterior pituitary gland to produce more FSH and LH in order to start a new menstrual cycle. Estrogen also has other important functions in the body, such as:
- It initiates the growth and development of the uterus and other reproductive organs during puberty and pregnancy.
- Estrogen promotes the growth of mammary ducts and fat deposits in the breasts during puberty and pregnancy.
- It promotes bone growth and helps retain calcium in the bones throughout a woman's life.
- It gives protection from atherosclerosis and cardiovascular disease by causing blood vessels to dilate and by limiting the formation of atherosclerotic plaques from lipids.
After the dominant ovarian follicle releases a mature ovum, it changes into a corpus luteum and begins to secrete progesterone. Progesterone and estrogen further develop the endometrium by increasing the maturation of blood vessels in the endometrium. They cause the endometrial glands to enlarge and to begin secreting nutrients into the uterine cavity (in case the ovum is fertilized). Progesterone, however, also limits the volume of the endometrium; without progesterone, estrogen stimulation of the endometrium would be too great. Progesterone affects hormone release from the hypothalamus and anterior pituitary gland. Through this “feedback” system, high levels of progesterone inhibit GnRF secretion and decrease FSH and LH secretions.
Progesterone also has other important functions in the body, such as:
- It sustains early pregnancy until the placenta develops (in approximately 10 weeks).
- The decline of progesterone helps initiate uterine contractions in labor.
- It provides a protective effect from breast cancer and endometrial cancer.