Investigation: The Human Menstrual Cycle Research Question: How do hormones control the menstrual cycle?

Introduction: The menstrual cycle (changes within the uterus) is an approximately 28-day cycle that coincides

with the ovarian cycle (changes within the ovary). Hormones released by the anterior pituitary gland, which

is in turn regulated by the hypothalamus, regulate the ovarian and menstrual cycles.

The three phases of the ovarian cycle are the follicular phase, the ovulatory phase and the luteal

phase. The follicular phase involves the growth of the follicle, which surrounds the egg. In the ovulatory

phase, the egg is ovulated (ruptures from the follicle). During the luteal phase of the cycle, the uterine lining,

also called the endometrium, is prepared for possible pregnancy. Hormonal fluctuations of the ovarian cycle

drive the events of the menstrual cycle. The menstrual cycle begins with the menstrual phase. The menstrual

phase is followed by the proliferative phase and then the secretory phase.

Follicular/Ovulatory and Menstrual/Proliferative Phases (Days 1-14):

At the beginning of the cycle, menstruation occurs and the endometrial lining is shed. At the same time, two

pituitary hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), stimulate the growth

of a few primary follicles in the ovary. These growing follicles begin to secrete estrogen, one of the two main

female hormones. The bigger the follicle, the more estrogen is secreted.

The endometrial lining of the uterus begins to thicken due to estrogen.

Rising estrogen levels have two effects: slowly rising estrogen levels suppress secretion of FSH and LH

from the pituitary gland (a negative-feedback loop), and they stimulate secretion of more estrogen by the ovary

(a positive-feedback loop). Rapidly rising levels of estrogen trigger an abrupt, midcycle spike in LH and FSH

output. LH triggers the ripening of one follicle, which is followed by ovulation, or the expulsion of an egg from

the ovary.

Luteal/Secretory Phase (Days 15-28):

The egg now travels down the fallopian tube towards the uterus.

The estrogen level falls as the secretion of estrogen by the follicle is interrupted by ovulation.

The endometrial lining of the uterus continues to thicken.

Immediately after ovulation, LH continues to act on the ovary, causing the follicular cells to enlarge,

producing the corpus luteum, a mass of enlarged follicular cells.

The corpus luteum resumes large-scale secretion of estrogen and begins to secrete progesterone, the

second main female hormone (these are positive-feedback loops).

Rising levels of progesterone and, to a lesser degree, estrogen, inhibit the release of FSH and LH (these

are negative-feedback loops).

If implantation of a fertilized egg does not occur, the corpus luteum disintegrates and estrogen and

progesterone levels decline as a result. The decrease in estrogen and progesterone cause the blood vessels

supplying the thickened uterine lining to constrict. This results in decrease of oxygen and nutrients to those

tissues. The endometrial lining will then be shed during the process of menstruation at the beginning of the

next cycle. A mixture of blood and discarded endometrial cells leaves the uterus through the vagina.

Procedure: 1. Table 1 was used to graph the concentrations of estrogen and progesterone in the blood on days 1-28 in the cycle (Graph 1) as well as FSH and LH (Graph 2). Analyze the graphs and data table below and use the information provided, to answer the 4 analysis questions. Table 1: Concentrations of Four Different Hormones in Female Blood

Day FSH (ng/mL) Estrogen (ng/mL) LH (ng/mL) Progesterone (ng/mL)

1 260 .1 80 .6

2 280 .1 110 .6

3 270 .1 115 .6

4 260 .11 110 .6

5 240 .13 105 .6

6 220 .15 100 .6

7 210 .175 95 .6

8 200 .2 90 .6

9 190 .25 85 .6

10 180 .3 80 .6

11 170 .4 100 .6

12 200 .7 160 .6

13 420 .68 400 1.0

14 400 .4 380 1.3

15 290 .12 200 2.0

16 200 .18 120 3.0

17 190 .25 110 4.0

18 180 .30 100 5.5

19 160 .37 80 6.4

20 140 .395 60 7.0

21 130 .4 50 7.5

22 120 .395 40 7.4

23 115 .35 30 6.5

24 110 .28 30 5.0

25 120 .21 30 3.0

26 140 .19 40 2.0

27 180 .1 60 1.0

28 200 .08 80 0.5

Analysis Questions: 1.

On what day does each of the following hormones reach its

maximum concentration?

On what day(s) does each of the following hormones

reach its lowest concentration?

From which gland/structure is this hormone secreted?

a) estrogen:

b) progesterone:

c) FSH:

d) LH:

2. Why is FSH so low during the luteal phase of the ovarian cycle? 3. Specifically, what causes the end of a cycle and the onset of a new cycle? 1) the egg is not fertilized, therefore…

2)

3)

4)

4. Why are the menstrual and ovarian cycles considered to be made of feedback mechanisms?