15.3 - Menstrual Cycle

Role of Hormones in Menstrual Cycle

The menstrual cycle is the cyclic changes that occur in the endothelium of the uterus to prepare it for a fertilized ovum and its subsequent development. The endometrium thickens with rich blood vessels for possible implantation of a fertilized ovum. If fertilization does not occur, the secondary oocyte will die and the endometrium lining will break down. The discharge of blood, epithelial tissues and mucus through the vagina is called menstruation.

Hormones, Glands and its' Functions

• Follicle-stimulating hormone (FSH)

  • Pituitary gland
    • Stimulates the development of follicles in the ovary.
    • Stimulates the production of estrogen.

• Luteinizing hormone (LH)

  • Pituitary gland
    • Stimulates ovulation.
    • Stimulates the development of corpus luteum.
    • Promotes the secretion of progesterone.

• Estrogen

  • Ovary
    • Stimulates the further growth of the follicles until maturation.
    • Promotes the repair and thickening of the endometrium.
    • Stimulates secretion of FSH and LH before ovulation.

• Progesterone

  • Ovary
    • Stimulates the thickening the endometrium to prepare for implantation.
    • Inhibits the secretion of FSH and LH to prevent the development of the Graafian follicle and ovulation.

Figure 7 - Sequence of activities in the menstrual and ovarian cycles.

• DAY 0 -5

  • The absence of the hormone estrogen and progesterone causes the endometrium lining to shed and menstruation begins. One day before the menstruation begins, the gonadotrophin-releasing hormone (GnRH) secreted by the hypothalamus stimulates the release of FSH and LH into the bloodstream. FSH promotes follicle growth in the ovaries. The primary oocyte grows to become secondary oocyte in the Graafian follicle. Growing follicles produce estrogen which stimulates the repair and growth of the uterine lining. Estrogen also promotes the continued development of the follicle. The low estrogen level exerts a negative feedback on FSH and LH, keeping their levels low to prevent the growth of new follicles.

• DAY 6 - 14

  • The level of estrogen rises steeply and peak on day 12. This stimulates the secretion of GnRH from the hypothalamus via a positive feedback mechanism. High level of GnRH also promotes the secretion of FSH and LH. The level of LH continues to increase and peaks on day 13, which stimulates ovulation from Graafian follicle on day 14. LH also stimulates the remaining follicle tissue to develop into a corpus luteum.

• DAY 15 - 21

  • The corpus luteum secretes estrogen and progesterone, stimulated by LH. Both estrogen and progesterone inhibit the release of FSH and LH from the hypothalamus via a negative feedback mechanism. This prevents the development of more follicles and ovulation. Progesterone stimulates the uterine wall to become thicker, more folded and highly vascular in preparation for the implantation of a fertilized ovum.

• DAY 22 - 28

  • If fertilization does not occur, low levels of LH cause the corpus luteum to disintegrate. This causes the levels of estrogen and progesterone to drop. Without the stimulation of estrogen and progesterone, the endometrium breaks down and menstruation will occur. The reduced levels of estrogen and progesterone no longer inhibit the hypothalamus and pituitary gland. Hence, GnRH is once again released by the hypothalamus, stimulating the secretion of FSH and LH. A new cycle begins again with new follicle growth. If fertilization occurs, the corpus luteum will not degenerate. The levels of estrogen and progesterone continue to rise. The uterine walls continue to grow and nourish the embryo.

Role of Hormone in Pregnancy and Miscarriage

1. Estrogen and progesterone production from the corpus luteum continue after pregnancy for 3 or 4 months. When the corpus luteum degenerates, the production is taken over by the placenta until the baby is born.
2. Progesterone stops the secretion of FSH and LH so that ovulation does not take place during the pregnancy.
3. Imbalance between the hormones estrogen and progesterone can lead to miscarriage. Reduced in progesterone level causes the uterus to shrink.

Premenstrual Syndrome and Menopausal Syndrome

Premenstrual syndrome (PMS) is a cyclic disorder of a severe physical, mental and emotional distress. The symptoms normally appear 5 to 11 days before the start of menstruation. The causes of PMS is due to imbalance in the estrogen and progesterone levels.

Menopause is time when the menstrual cycle stops. It occurs in women between age of 46 to 50 years when the ovaries are less stimulated by FSH and LH. At this stage, ovulation stops and women can no longer have babies.