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Menstrual Cycle

Background: When a human female is born, her ovaries already contain all the immature eggs that will later mature and produce functional eggs during her lifetime. Eggs usually begin to mature between the ages of 12 and 14, when a release of hormones triggers puberty and a young woman reaches sexual maturity. Most commonly, eggs mature every 28 days or so. They usually mature one at a time, in alternating ovaries. This rhythmic maturation of eggs and the other chemical and physical events that accompany the process are called the menstrual cycle.

As a reaction to increasing levels of hormone FSH (follicle stimulating hormone), eggs start to mature in a woman’s ovary. Each egg matures inside an egg sac, or follicle, near the surface of one of the ovaries. When the egg is fully mature, another hormone – LH (luteinizing hormone) – reaches peak level. As a reaction to these high hormone levels, the follicle bursts open and releases the egg. This process is called ovulation. Tiny microscopic hairs, called cilia, on the cells at the opening to the Fallopian tube or oviduct, sweep the egg into the tube which leads to the uterus.

As a reaction to increasing levels of the hormone estrogen, the lining of the uterus has been prepared to receive a fertilized egg by building up its lining with nurturing tissues and blood vessels.

After the egg is released from the follicle in the ovary, the remaining follicle tissue becomes a hormone secreting gland, the corpus luteum. The gland releases the hormone progesterone. High levels of progesterone help maintain the uterine in its build up, nurturing phase.

If the released egg remains unfertilized, it does not important in the uterus lining. This triggers further hormonal changes. Both estrogen levels and progesterone levels drop. This causes the lining of the uterus to deteriorate. As a result both unfertilized egg and uterus lining are shed and pass out of the body. This periodic loss of tissues and fluids form the uterus is a normal function known as menstruation. Menstruation is considered the beginning of the monthly menstrual cycle.

All of these changes are governed by coordinated hormones carried in the bloodstream from their releasing gland to their responding target cells. These hormones act through feedback mechanisms. The pituitary gland, at the base of the brain, secretes the two hormones that trigger the growth and development of the egg in the ovary – FSH and LH. In response, the ovary then secretes the two sex hormones that control development of the egg and uterus lining – estrogen and progesterone. When ovarian hormones reach low levels, this feedback mechanism stimulates the pituitary gland to once again secrete its hormones to stimulate the development of another egg for another cycle.

Follicular Phase Study

Within the ovaries are located many egg cells. Each egg is enclosed within a structure called a follicle. The follicle is aid to be immature. Under the influence of a hormone called FSH, the follicle matures.

1. Prepare a line graph of the data in Table 1.

Table 1: FSH Levels measured over a Menstrual Cycle

Day

1

3

5

7

9

11

13

15

17

19

21

23

25

27

Units of FSH

10

12

14

13

13

14

20

10

9

9

8

8

8

10

2. FSH is a hormone in the female body that causes a woman’s follicles to mature within the ovary. Using your graph, on which day of the cycle has the follicle reached maturity?

Day ____

3. What happens to the egg follicle in the ovary as FSH rises (during Days 1 – 12)?

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4. If the picture below is illustrating a cell secreting FSH, then draw what the receptors would look like on the target cell in the ovary.

5. Describe the overall trend of FSH over a menstrual cycle.

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Luteal Phase

Once a follicle is mature, it bursts open and the egg is released. This process is called ovulation. The egg passes into the oviduct where it may or may not become fertilized. Meanwhile, the mature follicle, once it loses its egg, forms a body within the ovary called the corpus luteum.

A hormone called luteinizing hormone is responsible for ovulation. Table 2 shows data obtained from blood samples taken from a female and analyzed for the amount of LH present.

1. What is the purpose of the corpus luteum?

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2. Describe how an egg goes from immature to a mature-released egg.

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3. Prepare a line graph of the data from Table 2.

Table 2: LH Levels measured over a Menstrual Cycle

Day

1

3

5

7

9

11

13

15

17

19

21

23

25

27

Units of FSH

12

14

14

14

14

16

70

12

12

12

12

12

8

8

4. Luteinizing hormone stimulates the follicle in the ovary to release an egg. Using your graph, on which day of the cycle will an egg be released? Day ____

5. What gland secrets LH? _____________________________

6. What happens to the egg in the ovary on Day 14 after LH levels reach their peak?

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7. If the picture below is illustrating a cell secreting LH, then draw what the receptors would look like on the target cell in the uterus.

8. Describe the overall trend of LH over a menstrual cycle.

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Changes in the Uterus

While the follicular and luteal stages are taking place in the ovaries, a series of changes is also occurring in the uterus. The uterus lining changes from being very thin to being very thick. This change in thickness occurs because the number of cells increases through rapid cell division. At one point, the uterus ceases to thicken. The buildup of cells begins to break apart. This breakdown of the uterine lining, both as tissue loss and bleeding, is called menstruation.

Two hormones are responsible for the thickening of the uterus, estrogen and progesterone. The amount of these hormones in the bloodstream influences the changes just described.

1. Prepare a line graph of the data from Table 3 on the next page.

Table 3: Amount of Estrogen and Progesterone Presence

Day of Cycle

Units of Estrogen Present in Blood

Day of Cycle

Units of Progesterone Present in Blood

1

50

1

5

3

50

3

5

5

50

5

5

7

75

7

5

9

125

9

5

11

225

11

5

13

200

13

10

15

75

15

40

17

100

17

60

19

100

19

110

21

100

21

150

23

100

23

150

25

50

25

100

27

50

27

30

2. Based on the graph, when do estrogen and progesterone reach their peaks?

a. Estrogen ___

b. Progesterone ___

3. What gland secretes estrogen? __________________________

4. What happens to the uterus lining during days 1 – 12, as estrogen is rising?

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5. If the picture below is illustrating a cell secreting estrogen, then draw what the receptors would look like on the target cell in the uterus.

6. Describe the overall trend of estrogen over a menstrual cycle.

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7. On day 14 the egg is released (ovulation). After that the corpus luteum forms in the remaining follicle and it starts releasing progesterone. On what day does the progesterone reach its peak concentration? ____

8. While progesterone stays at a high level what happens to the lining of the uterus?

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9. If the woman does not get pregnant, then the corpus luteum breaks down and the level of progesterone starts declining. Once progesterone decreases what happens to the lining of the uterus?

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Figure 1

Answer the following questions using Tables 1, 2, 3, Graphs 1, 2, 3, and Figure 1.

1. After a follicle has released an egg it turns into a corpus luteum. This in turn releases estrogen and progesterone, which cause the uterine lining to thicken and prepare to receive a fertilized egg. On which day would the uterine lining be the thickest? Day____

2. Look at the LH graph. How much time passes from the time the egg is released until when the uterine lining is the thickest? ____ Days

3. What might account for this? (Hint: Where does egg start and where is the uterus?)

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4. If an egg is not fertilized, estrogen and progesterone levels ___________. When this occurs, the thick lining of the uterus falls off and is released. This causes menstrual flow, or the menstrual period. During which days in the cycle does menstruation occur? Days ____

5. If an egg is fertilized, it will attach to the thick __________________ wall and estrogen and progesterone will continue to be released. What two things do these hormones prevent from happening?

a. _____________________________________

b. _____________________________________

6. There are four stages/phases in a given menstrual cycle. Match these stages/phases with the days below. Draw a line connecting the stages/phases and their corresponding days.

a. Follicular Phase (follicle is developing)~Day 14

b. Ovulation (egg is released)~Day 1-5

c. Luteal Phase (corpus luteum releases E and P)~Day 15-28

d. Menstrual Period (excess uterine lining and egg released)~Day 6-13

7. How might one trick the female body into thinking it was pregnant?

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8. After an egg implants on the thick uterine lining, the egg releases a hormone called hCG, or human chorionic gonadotrophin (home pregnancy kits test for this hormone). What might be the purpose of this hormone?

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9. Under which situations/conditions would the menstrual cycle stop?

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10. Why would the menstrual cycle stop during pregnancy?

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11. Is it possible to become pregnant during menstruation? Why or why not?

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12. If you miss your period, does that mean that you are pregnant? Explain.

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13. In Figure 1 Events in Lining of Uterus chart, what process is occurring between Days 3 – 6?

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14. Describe the changes that take place during the menstrual cycle from Day 5 – 13 to the following:

a. Unfertilized egg in ovary ________________________________________________

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b. Uterus lining __________________________________________________________

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15. Describe what happens to the egg during the menstrual cycle on Day 14.

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16. Describe the changes that take place to the an egg:

a. From Day 15 – 28, if no fertilization occurs __________________________________

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b. From Day 15 – 21, if fertilization occurs ____________________________________

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c. From Day 21 – 26, if fertilization does occur _________________________________

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17. Explain why the female needs a thick uterus lining if fertilization does occur. ____________

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18. At what time during a woman’s menstrual cycle is it easiest to become pregnant?

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No Fertilization of Egg

Cut out individual boxes and place on the appropriate empty date on the accompanying calendar labeled “Figure 2B Day by Day Changes in the Menstrual Cycle: No Fertilization of Egg”. Note: Do not paste over the days with writing in them. Paste into the empty day next to the correct description.

Fertilization of Egg

Cut out individual boxes and place on the appropriate empty date on the accompanying calendar labeled, “Figure 3B Day by Day Changes in the Menstrual Cycle: Fertilization of Egg”. Note: Do not paste over the days with writing in them. Paste into the empty day next to the correct description.