Physiology of the Menstrual Cycle

I.          Overview

A.        Major glands and organs

1.         Hypothalamus (see Biological Overview Lecture)

2.         Anterior pituitary (see Biological Overview Lecture)

3.         Ovary (see Oogenesis Lecture)

4.         Uterus

1.         Characterisitics

            a.         Shaped like inverted pear

            b.         Muscular organ with thick walls

                        i.          Glandular lining called endometrium

            c.         Narrow end is called cervix

                        i.          Projects into vagina

ii.          Canal of the cervix is flattened from front to back and is somewhat larger in its middle part

                        iii.         Opening into the vagina is called the external os of the uterus

iv.         Lined by a mucous membrane containing numerous glands that secrete a clear, alkaline mucus

            d.         Two uterine tubes

                        i.          Fallopian tubes

                        ii.          Connect ovaries to the fundus of the uterus

            e.         The body of the uterus lies below the fundus

                        i.          Inverted triangular shape

                        ii.          Cavity is small except during pregnancy

            f.          Uterus is anteverted

                        i.          Lies at a right angle to vagina

            g.         Composed by three layers

                        i.          Serous coat of peritoneum

                        ii.          Myometrium—middle layer of muscular tissue

iii.         Endometrium—innermost layer of tissue in the uterus is the mucous membrane

            h.         Endometrium

i.          Contains numerous uterine glands that open into the uterine cavity

ii.          Exhibits dramatic changes during each menstrual cycle

iii.         Composed of three levels

iv.         Two inner most layers are shed at menstruation

B.         Hormones

1.         Definition—substance produced in one part of the body that affects another part of the body

            a.         Endocrine—travels through blood

2.         GnRH—gonadotropin releasing hormone

            a.         Produced by hypothalamus

                        i.          Hypothalamus is part of the brain

b.         Travels via portal vein to anterior pituitary to control release of LH and FSH

3.         FSH—follicle stimulating hormone

            a.         Produced by anterior pituitary

            b.         Controls gonadic function

4.         LH—luteining hormone

            a.         Produced by anterior pituitary

            b.         Controls gonadic function

5.         Estrogen

            a.         Produced by gonads

                        i.          Follicle in females

                        ii.          Sertoli cells in males

6.         Progesterone

            a.         Produced by corpus luteum

            b.         Prepares uterus for implantation and pregnancy

7.         Testosterone

            a.         Primarily a male hormone

                        i.          Also produced by adrenal gland

8.         HCG—human chorionic gonadotropin

            a.         Secreted by extramembryonic membranes during pregnancy

 

C.        Menstrual cycle

1.         Prototypic: 28 days

a.         Range is 20-36 days

i.          Typically 26-32

            b.         Only 10% of women have a 28 day cycle

2.         Day 1: first day of menstruation

            a.         Typical flow 2-7 days

i.          Duration, amount and pattern varies

3.         Cycles varies in length at different times during female lifespan

4.         Pheromones

a.         Olfactory behavioral signals

b.         More important to lower species

c.         Affect human behavior

i.          Evidence: synchronicity of cycles among women living together

 

II.         Overview

 

III.       Phases of the menstrual cycle: based on ovarian events

A.        Follicular phase

 

1.         Hypothalamus secretes GnRH

a.         Controls activity of anterior pituitary

2.         Anterior pituitary secretes FSH & LH

a.         Controls activity of ovary

3.         FSH stimulates ovarian follicles to begin to develop
4.         As follicle develops it begins to secrete estrogen

a.         Estrogen causes further follicular development

b.         Estrogen causes the endometrium to thicken

            c.         Estrogen acts to signal hypothalamus to stop releasing GnRH

                        i.          Negative feedback—reduced production

                        ii.          Causes reduced FSH release

            d.         Elevated estrogen levels causes LH release from anterior pituitary

                        i.          Positive feedback—increases production

5.         Although there are many follicles in the ovary, one becomes dominant

            a.         Matures while others do not

            b.         Fraternal twins, two become dominant

B.         Ovulation

1.         Events:

a.         Egg is released from follicle

b.         Follicle differentiates into corpus luteum

2.         Estrogen levels peak 1-1.5 days prior to ovulation

3.         Elevated estrogen causes a surge in LH release from anterior pituitary

4.         Ovulation is a response to LH surge

 

 

5.         Follicle differentiates into corpus luteum

6.         Corpus luteum produces progesterone

 

a.         Progesterone is thermogenic

i.          Causes an increase in basal body temperature

ii.          Following ovulation, body temperature will increase 0.5 degree and remain elevated until the beginning on the next cycle

            b.         Progesterone causes reduced LH levels

                        i.          Negative feedback—reduced production

            c.         Progesterone causes reduced GnRH levels

                        i.          Negative feedback—reduced production

7.         As follicle differentiates it produces testosterone

            a.         As in males, testosterone increases libido

8.         Rupture of the follicle

a.         Releases egg into fimbria

b.         Mittelschmerz—pain of ovulation in some women

i.          Stretching of the ovarian capsule and subsequent capsular rupture during expulsion of the egg

9.         Egg moved by cilia and motility of fallopian tube

            a.         Egg is viable for approximately 36 hours

            b.         Sperm is viable for approximately 3 to 5 days

            c.         Window for pregnancy can be as large as 7 days

                        i.          5 days prior to ovulation

                        ii.          1-1.5 days after ovulation

C.        Luteal phase

1.         Last 14 days of the menstrual cycle

            a.         Corresponds to the life of the corpus luteum

2.         Corpus luteum—“Yellow body”

            a.         Derived from the follicle

3.         Corpus luteum produces

a.         Progesterone

b.         Estrogen (estradiol)

c.         Testosterone

4.         Progesterone

            a.         Suppresses new follicle growth

                        i.          Prevents ovulation of other follicles

            b.         Maturation of glandular and blood supply to endometrium of uterus

5.         If no pregnancy:

            a.         Corpus luteum degenerates into corpus albicans (“White body”)

            b.         Estrogen and progesterone levels fall

                        i.          Causes endometrial lining to degenerate—menstruation

6.         If pregnant:

            a.         Developing embryo produces HCG

                        i.          Maintains corpus luteum

                        ii.          Progesterone maintains uterus

 

IV.       Phases of the menstrual cycle: based on uterine/cervical events

 

 

A.        Proliferative phase

1.         Corresponds to follicular phase in ovary

2.         Uterine events

a.         Endometrium thickens

b.         Glands and blood vessels grow in response to estrogen

3.         Cervical events

a.         Estrogen causes changes to cervical mucus

i.          Thinner

ii.          Clearer

iii.         More elastic (spinnbarkeit)

iv.         Changes facilitate passage of sperm into uterus

v.         Cervix may feel softer and more open

B.         Ovulation—egg released from follicle

C.        Secretory phase

1.         Corresponds to luteal phase in ovary

2.         Uterine events

a.         Endometrial glands and blood supply mature

i.          Prepared to secrete glycogen—nutrition for egg

b.         Progesterone maintains endometrium

i.          Increasing levels of progesterone cause a reduced quantity of thick and tacky cervical mucus

D.        Menstruation

1.         Occurs if fertilization does not occur

2.         Progesterone and estrogen levels fall

3.         Uterine events

a.         Endometrial blood vessels constrict

i.          Loss of blood supply causes endometrium to be shed

b.         Menstrual flow is a combination of endometrial tissue and blood

c.         Cramps are associated with lack of progesterone and presence of prostaglandins secreted in uterus

 

V.        Toxic shock syndrome

A.        Caused by a strain of Staphylococcus aureus

1.         Characteristic of S. aureus

a.         Resistant to penicillin and related antibiotics

b.         Grows faster than other staphylococci

i.          100 – 2500 times faster

B.         Pathology

1.         Bacteria colonize and begin to grow on a mucosal area

a.         Vagina is a common site for this infection

b.         Affects men and non-menstruating women as well as menstruating women

2.         Bacteria secrete toxins

a.         Toxins enter bloodstream

3.         Initial exposure to toxin cause mild flu-like symptoms

            a.         Initial exposure sensitizes patient to toxin

4.         Infected individual recovers from symptoms but bacteria are still present

            a.         Bacteria can infect again

5.         With subsequent infection, response to toxin is sensitized 

6.         Compromises immune system function

C.        Bacterial growth cycle parallels menstrual cycle

1.         Sensitizing exposure occurs during one menstrual cycle

a.         Patient recovers from bacterial infection

b.         Bacteria still remain in the vagina

2.         During a subsequent menstrual cycle, bacterial population dramatically increases

            a.         Large amounts of toxin is produced

            b.         Immune system is compromised

D.        Symptoms of disease

1.         High fever

            a.         Greater than 102

2.         Scarlet fever-like rash on chest and neck

3.         Skin on palms and soles of feet peels

4.         Low blood pressure

5.         Present with at three of the following:

            a.         Diarrhea and vomiting

            b.         Muscle aches

            c.         Vaginal or throat infection

            d.         Kidney malfunction

            e.         Liver failure

            f.          Disorientation or confusion

E.         Prevention

1.         Create less permissive environments for bacterial growth

a.         Frequently change tampons

i.          Every 3 – 4 hours

ii.          Never more than 8 hours (i.e., overnight)

            a.         Use the lowest absorbent tampon that is convenient

i.          Alternate care products

 


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