The Menstrual Cycle

= a monthly cycle that occurs in women, involving morphological changes in the _______________ and the ____________________________________ of the uterus, caused by ___________________________________________________ of the hypothalamus, pituitary gland and ovaries

Click here for diagram of the hormonal feedback loop between the brain and ovaries.

Length of the cycle is extremely variable:

  • Usually ranges from 25 - 30 days, with an average of _______ days.
  • Cycles <21 days or >35 days are considered irregular.
  • Length is more variable in women younger than 20 and older than 40.
  • Tends to be longer in teenagers (ave. 35 days), becoming gradually shorter with age (30 days at 30 yrs and 28 days at 35 yrs).
  • Variation in length may be reduced when sexual intercourse is more frequent.

Menstrual cycle actually consists of two interrelated and interdependent cycles:

  1. Ovarian Cycle: functions to _____________________________________________

  2. Endometrial Cycle: functions to __________________________________________ to nourish and maintain that egg, should it become fertilized

Ovarian cycle can be divided into 3 phases:

  1. follicular phase*
  2. ovulation
  3. luteal phase**

Endometrial cycle can also be divided into 3 phases:

  1. menstrual phase*
  2. proliferative phase*
  3. secretory phase**

*The menstrual and proliferative phases occur during the follicular phase of the ovarian cycle.
**The secretory phase is synchronized with the luteal phase of the ovarian cycle.

Click here for a diagrammatic overview of the two cycles.

The Ovarian Cycle in Detail

1.      Follicular Phase: duration is highly variable, but typically lasts 11-16 days. Click for diagram.

    • Hypothalamus releases hormone GnRH, which signals the pituitary gland to secrete hormones ______________ and ____________.FSH stimulates __________________________________ and LH stimulates synthesis of _______________________________ by the developing follicles. Rising level of estrogen has a negative feedback on GnRH secretion, which inhibits further increases in FSH.
    • By day 13, one dominant (Graffian) follicle (2 - 2 cm in diameter) is established and the remaining tertiary follicles degenerate and undergo atresia.
    • Estrogen level reaches a maximum peak at day 12 or 13, triggering a surge of _____________ from the pituitary (accompanied by a smaller surge in FSH).

2.      Ovulation: the shortest phase; an instantaneous event; typically occurs on day 14 (of 28 day cycle). Click for diagram.

    • The surge in LH initiates a resumption of ____________________________ in the oocyte contained within the dominant follicle and subsequent ______________________________ of the mature egg (9-12 hours after the peak burst in LH).

  1. Luteal Phase: duration is remarkably consistent among women; usually about ______________ long. Click for diagram.
    • After ovulation, a ______________________________________ is formed inside the ovary from the remnants of the collapsed follicle that released the egg.
    • This structure secretes both _____________________________and ______________________________________, which together inhibit GnRH secretion from the hypothalamus.
    • This, in turn, results in a drop in ___________ (and FSH) levels, thereby preventing a second ovulation from occurring.
    • The corpus luteum reaches a peak of activity about 7 - 8 days after ovulation. If the egg was fertilized, the blastocyst implants in the endometrium at that time.
    • In the absence of fertilization, there are no (hCG) hormones from the developing embryo to maintain the corpus luteum, so it _________________________________________ about 10 days after ovulation. Blood levels of estrogen and progesterone therefore _______________, resulting in degeneration of the endometrial layer and the onset of menstruation about 4 days later.

The Endometrial Cycle in Detail

1.      Menstrual Phase: period of bleeding; duration usually 4 - 6 days. Click for diagram.

    • The rapid drop in blood levels of estrogen and progesterone (due to the degeneration of the corpus luteum in the ovary) causes the upper 2/3 of the endometrial layer of the uterus to degenerate. It is then shed from the body.
    • As the lining degenerates, the underlying blood vessels bleed, leading to a loss of 1 - 8 oz of blood (ave. 1 - 3 oz) over the menstrual period.

2.      Proliferative Phase: proliferation of new cells in uterine lining; duration usually 8 10 days

    • The rising level of estrogen in the bloodstream (produced by the next crop of growing follicles) promotes rapid _______________________ and ____________________________ of the uterine lining within a few days after the end of the menstrual phase.
    • The new layer proliferates rapidly during the days before ovulation, but nutrient secretion is minimal during this time.

  1. Secretory Phase: thickening of the uterine lining after ovulation; duration is usually 2 wks

    • Following ovulation, the glands of the endometrium begin secreting large amounts of _______________________________ that will be used by the developing embryo if fertilization and implantation occur.
    • In preparation for the possibility of pregnancy, the endometrium becomes twice as thick as it was during the previous phase, resulting in a thick, nutritive layer, suitable for implantation. _______________________________ supply to the endometrium also increases.

Click here for a diagram illustrating the fluctuating hormone levels and sequential events of the menstrual cycle. (You may need to also click on the image to enlarge the diagram.)

Click on the following two links for a two-part summary of all the events that occur during the menstrual cycle:

Summary, part 1 and Summary, part 2

Click here for a brief chronological listing of the events of the menstrual cycle.

Click here for an overview of the hormones involved and their functions. (But note that this table fails to acknowledge the role of LH in stimulating ovarian follicles to synthesize and secrete estrogen.)

Menstrual Problems

1.     Dysmenorrhea = _______________________ menstruation

2.     Amenorrhea = ________________________ of menstruation

3.     Endometriosis = abnormal growth of endometrial tissues ________________________________________________

o        Endometrial tissue may dislodge from uterus and grow in the ovaries, oviducts, bladder, urethra, intestines, rectum, cervix, vagina or lymph glands

o        Symptoms vary depending on location, but usually involve severe pain and excessive bleeding during each menstruation period

4.     Toxic Shock Syndrome

o        Symptoms: high fever, sun-burn like rash, vomiting, diarrhea, muscle aches, drop in blood pressure, kidney failure, liver abnormalities, death (10% of cases)

o        Caused by buildup of specific type of _______________________, which produce ______________________ that enter bloodstream

o        Associated with use of high absorbency tampons, particularly if left in for long periods of time (but can also occur in children and men)

5.     Premenstrual Syndrome (_____________) = a combination of ________________________ ailments and __________________________ difficulties exhibited (most often) during the week before menstruation

o        Cause is still _____________________________. Given such a wide variety of symptoms, there is not likely to be a single cause. Probably multifactorial. There are two prevailing theories, each with supporting evidence:

1.      changes in hormone levels drop in progesterone (P)?, simultaneous drop in P and E?

1.      psychological effects of cultural influences

o        Physical Symptoms:
Same as for dysmenorrhea, in addition to dizziness, breast tenderness, acne, migraine headaches, muscle aches and edema (swelling) in the hands and feet (due to water retention)

o        Emotional symptoms:
Fatigue, tension, anger, irritability, depression, anxiety, nervousness, insomnia, food cravings, difficulty concentrating, memory loss

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