The Female Menstrual Cycle

LESSON X:

THE FEMALE MENSTRUAL CYCLE

To Address NYS Standards:

2 (Organisms inherit genetic information in a variety of ways that result in continuity of structure and function between parents and offspring) and 4 (The continuity of life is sustained through reproduction and development).

Behavioural Objective:

The students will be able to understand the hormonal control that is the driving force behind the menstrual cycle, and will be able to read and correctly interpret a graph of the hormones progesterone, estrogen, FSH, LH, and GnRH. The students will also completely understand what effects these hormones have on the reproductive system and body in general.

Explanation of Lesson Plan:

Students will learn about the hormonal basis behind the menstrual cycle, and compare and contrast its evolutionary significance and benefit with that of the estrus cycle. The hormones that drive the cycle will be the principal idea, with the effects of these hormones on the reproductive system and other body systems discussed as well.

Hook: (3 minutes)

“When can a human female have a baby? When can a dog or cat have a puppy or kitten? What is the difference between these animals? One has a menstrual cycle (derived from moon, month, etc.), while cats and dogs have estrus cycles. That is, there are only certain times during the year that they are able to reproduce. How do you think male of species with an estrous cycle can tell if a female is receptive or “in heat?” What do you think is the benefit of one system vs. the other?”

Test of Prior Learning: (5 minutes)

1. How do you think the menstrual cycle is beneficial on an individual level, a population level, a species level?

2. What kind of organisms would benefit from an estrus cycle?

New Learning: (30 minutes)

1. Let’s remember to keep the big picture in mind: The goal here is to prepare the female for pregnancy, and then de-prepare her if a pregnancy doesn’t happen.

2. A female becomes reproductively mature at menarch, the first onset of the menstrual cycle. After that, a female will ovulate roughly 12 times per year, until menopause, when the cycle stops.

3. The entire thing is controlled by five hormones. LH, FSH, estrogen, progesterone, and GnRH. Remember them by FELP.

4. GnRH is the master hormone here. It is secreted by, you guessed it, the hypothalamus. Gonadotropin Releasing Hormone stimulates the anterior pituitary, which releases lutenizing hormone and follicle stimulating hormone. These, in turn, stimulate the indirect production of estrogen and progesterone.

5. Interestingly, LH and FSH are present in males as well. There LH stimulates the production of testosterone, while FSH promotes spermatogenesis.

- draw the following diagram of hormone release

6. In the female, FSH stimulates a follicle to develop. It is secreted by the anterior pituitary, which is stimulated by GnRH.

7. LH in the female stimulates ovulation, and the production of the corpus luteum. LH is also made by the anterior pituitary. The corpus luteum makes estrogen, which feedsback to inhibit GnRH.

- draw a diagram of hormone feedback on the board

8. Let’s begin at the beginning of the cycle. Look in your workbooks on page 91 and follow along:

- At the beginning, estrogen levels are very low, so GnRH is not inhibited, and small amounts of FSH and LH are released. At this point, the follicle doesn’t have receptors for LH, just for FSH, so it starts to grow and secrete estrogen slowly.

- This slow secretion of estrogen inhibits FSH and LH. The follicle, however, is continuing to grow. It gets to a certain point, and starts to secrete a lot of estrogen quickly. This larger secretion stimulates FSH and LH production, instead of inhibiting it! Weird, right?

- By now, the follicle has developed receptors for LH, so it can respond to it. Here we see another example of positive feedback. The increase in estrogen output from the follicle stimulates more LH and FSH release, which causes the follicle to release more estrogen.

- The loop is broken with ovulation. The high levels of LH cause the empty follicle to become a corpus luteum. The job of the corpus luteum is to produce estrogen and progesterone to keep the lining of the uterus intact. Again, the slow increase in the production of estrogen inhibits the production of LH and FSH, due to repressed GnRH.

- The lower levels of FSH and LH mean that no other follicles will start to mature now. This is critical, because the body is preparing for the fertilization and implantation of the first ovum. The corpus luteum reaches maturity in about 6-8 days.

- If the ovum is fertilized and implants, the placenta begins to secrete its own hormones, to trigger the body to exit the menstrual cycle, and start the pregnancy cycle. If not, the corpus luteum dies, and stops estrogen and progesterone production. This sudden drop triggers menstruation, when the lining of the uterus slides off to be replaced.

- The sudden estrogen drop also stops the inhibition of GnRH, and FSH and LH can be produced again, starting the cycle over.

9. In animals with an estrus cycle, the lining of the uterus is reabsorbed, not shed. What does this tell you about evolution? That we can afford the energy loss, while lower animals cannot.

Test of New Knowledge: (5 minutes)

1. What would happen if the placenta failed to produce enough hormones after implantation?

2. Do males have FSH and LH? Yes. FSH stimulates spermatogenesis in Semi. Tub., while LH stimulates testosterone in Lydig cells.

Assignment:

Syracuse’s Nifflenork worksheet