Female Reproduction 1 and 2: The Menstrual Cycle
R.J. Witorsch, Ph.D.
OBJECTIVES:
At the end of this block of lectures, the student should be able to:
1. Describe the functional anatomy of ovary with particular reference to events prior
to, during and after ovulation.
2. Describe the functions of female sex accessory organs.
3. Describe and explain the events and regulatory mechanisms involved in the
human menstrual cycle.
4. Describe the events involved in the secretion, circulation, and biological half-life
of estrogen and progestin in the adult cycling female.
5. Describe the physiological actions and mechanisms of action of estrogen and
progestin.
6. Describe and explain the various srategies involved in female contraception.
7. Explain the etiologies, symptomologies, and diagnostic strategies of the following
gonadal disorders in the female:
a. hypogonadism
b. precocious puberty
c. menstrual disorders.
Suggested Reading: Costanzo 2nd Edition, pp. 417-424
I.
OVARY
A.
Female reproduction is characterized by monthly cycles, culminating in
the extrusion of a single egg from the ovary by the process known as
ovulation.
B.
The major structures of the ovary and their functions are (Fig. 1):
Figure 1. Mammalian Ovary
1. Hilus, the area where lymphatics, nerves and blood vessels enter and
exit the ovary.
2. Germinal epithelium: a misnomer, in reality a specialization of
peritoneal mesothelium.
3. Primordial follicle: a single egg cell (oocyte) surrounded by a single
layer of cells (granulosa cells). This is the basic reproductive unit of
ovary. The primordial follicle is separated from adjacent stroma by a
basement membrane surrounding the granulosa cells. Primordial
follicles are found primarily in outer cortex just beneath fibrous
capsule of ovary. The following are noteworthy features of the
primordial follicles:
a. The oocyte is small (< 25μm) and in prophase of meiosis I.
b. Primordial follicles mature and subsequently release a single
oocyte from the ovary at ovulation. Primordial follicles provide
a resting pool from which follicles develop.
c. About 1-2 x 106 primordial follicles are present in each ovary at
birth.
i.
ii.
Groups of follicles undergo partial development and
atresia (degeneration) during late fetal life and through
prepubertal years.
At puberty there are approximately 4 x 105 primordial
follicles in each ovary. The rest have undergone
development and atresia in earlier years.
d. From the beginning of female fertility (menarche) to the end of
female fertility (menopause) there are approximately 400
ovulatory cycles. About 1,000 primordial follicles/ovary are
mobilized (i.e., undergo development) during each cycle.
i.
ii.
Only 1 follicle fully matures while the remainder
undergo atresia.
Recent observations (2004) suggest that mammalian
ovaries contain stem cells that can generate new eggs
through adulthood. This challenges the assumption that
the full complement of eggs are present at birth.
4. Follicles in various stages of development
a. The primary follicle is an early stage of development beyond
the primordial stage. It contains an enlarged oocyte (80100μm) which is surrounded by a zona pellucida. In the
primary follicle the granulosa cells become cuboidal (having
been previously flat).
b. With further development granulosa cells proliferate and a
vesicle (antrum) forms within granulosa cell region from a
plasma exudate. Connective tissue layers formed around the
follicle are called the theca interna and theca externa.
5. The mature (Graafian) follicle is the structure of the follicle just prior
to ovulation. Fluid infiltration and antrum formation are maximal and
the follicle has a diameter of 5 mm.
6. At ovulation the follicle ruptures. The oocyte (plus some granulosa
cells) is extruded from the ovary, leaving behind most of the follicle.
a. Just prior to or at ovulation, meiosis I is completed and the
primary oocyte becomes the secondary oocyte + first polar
body.
7. The corpus hemorrhagicum is a blood-filled remnant of the ruptured
follicle retained by the ovary immediately after ovulation.
8. The corpus luteum is the structure transformed from the corpus
hemorrhagicum. Granulosa cells are increased in size and lipid-laden
(luteinized or yellow in color). Granulosa cells are the major source of
estradiol and progesterone after ovulation.
9. Regressing corpora lutea: Regression of the corpus luteum begins
toward the end of an ovarian cycle and continues into the subsequent
cycle.
10. Corpus albicans is scar tissue resulting from corpus luteum regression.
11. Atretic (or degenerating) follicles result from developing follicles that
never fully mature and undergo regression prior to reaching the
Graafian stage.
12. Ovarian stroma is fibroelastic connective tissue and interstitial cells.
Stromal cells contain lipid derived from atretic follicles. They secrete
sex steroids in response to LH and human chorionic gonadotrophin
(hCG).
II.
FEMALE SEX ACCESSORY ORGANS (Figure 2)
Figure 2. Female Reproductive Organs
A. The female sex accessory organs provide a proper environment for the postovulatory egg, its fertilization and the development of the fetus. The major
female sex accessory organs and their roles are:
1. The fallopian tube (or oviduct) which receives the egg after being
extruded from the ovary at ovulation and is the organ where
fertilization occurs.
2. The uterus is composed mucosa (endometrium) and muscle wall
(myometrium). The uterus is the site where the fertilized egg
(blastocyst) implants, the placenta forms and embryo develops and is
the organ involved in parturition.
3. The cervix is the constricted structure of the lower uterus and serves as
the boundary between uterus and vagina. The cervix plays a passive
role in conception and is the major site of spermatozoan storage, postcoitus.
4. The vagina is the organ of sexual receptivity. The "sweating" response
of the vagina is the earliest response to sexual stimulation.
III.
HUMAN MENSTRUAL CYCLE (Figure 3)
A.
The human menstrual cycle is a recurring pattern of physiological changes
in the female reproductive system. The primary goal of the cycle is to
prepare the uterus for implantation of the fertilized ovum. On average the
menstrual cycle is of 28 days duration, although on an individual basis
cycles usually range from 25-35 days.
B.
The menstrual cycle exhibits the following phases: menses (menstrual
flow) which occurs from day 1 to 4 or 5 of the cycle; the proliferative
phase (referring to uterine events) or follicular phase (referring to ovarian
events) which occurs from days 5 to 14 of the cycle; ovulation or
extrusion of the egg from the ovary which occurs on day 14 of the cycle;
and the secretory phase (referring to uterine event) or luteal phase
(referring to an ovarian event which occurs during the last half (days 15 to
28) of the cycle.
C.
Cyclic changes are observed in pituitary hormone levels.
1.
2.
3.
FSH exhibits a slow decline in days 1-14. At midcycle (day 14) a
small surge in serum FSH occurs. From days 15-28, FSH is
maintained at levels lower than at days 1-14. An abrupt small
increase in FSH occurs on days 28-1 (at the end of the old cycle
and the start of the new cycle.
LH levels are stable but low during the first half of the cycle (days
1-14). On day 14, a large ovulatory surge in LH occurs which
lasts about 24 h. A slow decline in LH levels occurs from days 15
to 28 followed by an abrupt small increase between days 28 and 1.
PRL (prolactin) levels are higher during days 15-28 than days 1-14
(not shown), a pattern which is of questionable consistency.
Figure 3. Human Menstrual Cycle
D. Cyclic changes in ovarian histology
1. Days 1-14 (follicular phase) is characterized by development of
primordial follicles . About 1,000 primordial follicles are mobilized
however usually only one is destined to become the Graafian follicle.
2. Days 14-15 is the time when ovulation occurs. This is 36-38 hr after
the start of the LH surge.
3. Days 15-21 is the time when a portion of the follicle retained in the
ovary becomes the corpus hemorrhagicum for a 1-2 day period. This
structure then becomes the corpus luteum for the duration of the
period.
4. Days 21-25 is the time when peak corpus luteum development and
function occurs.
5. Days 26-28 is the time when regression of corpus luteum initiated and
the cycle ends.
6. After 28 days (subsequent cycles) luteal regression continues and the
corpus luteum ultimately becomes a corpus albicans.
E. Cyclic changes in plasma steroid levels
1. Estradiol
a. From days 1-14 of the cycle a gradual increase in plasma
estradiol level occurs. The estradiol is produced by the
developing follicle. The theca interna makes testosterone which
is then aromatized to estradiol by the granulosa cells.
i.
Thirty to sixty hrs prior to the LH surge (during the
second week of cycle) plasma estradiol rises at a more
rapid rate.
b. Days 14-15 exhibits an elevation in estradiol corresponding to
the LH surge and post-ovulation, estradiol levels drop.
c. Days 15-25 exhibits a second rise in estradiol. This estradiol is
now being produced by the corpus luteum.
d. Days 26-28 is characterized by a decline in estradiol levels
which is concomitant with the regression of the corpus luteum.
2. Progesterone
a. On a molar basis, the output of progesterone exceeds that of
estradiol. However, there are no dramatic changes observed
during the follicular phase of the menstrual cycle. From Days
1-14 serum progesterone levels are low.
b. A periovulatory increase is seen in plasma 17αhydroxyprogesterone (not shown in Fig. 3).
c. On days 15-25 (post-ovulation) gradual increase in
progesterone production from corpus luteum occurs.
i.
Peak levels of progesterone occur between days 21-25
concomitant with maximal corpus luteum activity.
d. On days 26-28 progesterone levels decline concomitant with
corpus luteum regression. The post-ovulatory pattern of 17αhdroxyprogesterone is similar to that of progesterone.
F. Cyclic changes in the uterus
1. During menses (Days 1-4) there is a sloughing off of necrotic
endometrium with blood and uterine fluid discharged as menstrual
flow. This occurs because of spasmotic closure of arteries due to local
release of prostaglandin-F2α. Menses is due to withdrawal of
hormonal support (estradiol and progesterone) as a consequence of
regression of corpus luteum on days 26-28 of the preceding cycle.
2. The proliferative phase occurs from days 5-14 of the cycle and is
characterized by proliferation of uterine epithelium, glandular tissue,
stroma and vascularity. Uterine thickness increases several fold.
Uterine proliferation due to estradiol secreted by developing follicle.
3. The secretory phase occurs from days 15-25 of the cycle. During this
period the uterus exhibits increased vascularity (coiled arteries),
stromal hypertrophy, fluid accumulation (edematous tissue), further
glandular development, increased glandular secretory activity
(proteins, amino acids, carbohydrates), and glycogen accumulation.
Secretory stage changes that occur in the uterus are a consequence of
the combined effects of estradiol and progesterone after ovulation.
Secretory changes in the uterus prepare the organ for implantation of
a fertilized egg, if fertilization occurs. If fertilization does not occur,
the corpus luteum regresses and the withdrawal of hormonal support
leads to menses on day 1 of the next cycle.
G. Cyclic changes in the cervix
1. At midcycle: changes in cervical mucus occur due to the actions of
estradiol. At this time, cervical mucus is highest in quantity, most
penetrable to sperm (less hostile and less acidic), most transparent, and
least viscous (has a high degree of elasticity), a property called
"spinnbarkeit." When smeared on a slide and allowed to dry, cervical
mucus exhibits a ferning pattern.
H. Cyclic changes in the vagina
1. At midcycle, the vagina exhibits cytological (exfoliative) changes due
to the effects of estradiol. Among these changes are increased mitotic
activity and exfoliation of squamous epithelial cells.
I. Cyclic changes in basal body temperature (BBT)
1. BBT during luteal phase (post-ovulation) is approximately 1°F greater
than that during the follicular phase. The increase in BBT is attributed
to an action of progesterone on the thermoregulatory center of the
hypothalamus.
IV.
HORMONAL CONTROL OF THE MENSTRUAL CYCLE (Figure 4.)
Figure 4. Feedback Control of Ovulation
A. Throughout most of the menstrual cycle, estrogen and progestin exert a
negative feedback effect producing suppressed and/or declining levels of
FSH and LH during the follicular phase and suppression of FSH and LH
during the luteal phase. This negative feedback is responsible, in part, for
luteal regression which occurs at the end of the cycle.
1. The small but abrupt rise in FSH and LH at the beginning of a new
cycle is due to corpus luteum regression and withdrawal of estradiol
and progesterone at the end of preceding cycle.
B. A rise in plasma estradiol 30-60 hours prior to the LH surge appears to
stimulate LH and FSH surge by a positive feedback mechanism. The positive
feedback effect of estradiol on LH and FSH release appears to occur at the
pituitary level. Anterior pituitary responsiveness to GnRH increases 10-50
fold. Therefore, estradiol increases pituitary sensitivity to pulsatile GnRH
(accordingly, clomiphene, a nonsteroidal estrogen, is an inducer of ovulation).
C. The ovulatory surge in LH and FSH is controlled by a single follicle which
emerges, called the dominant follicle. The dominant follicle is selected from
a cohort of 8-12 cells that are most sensitive to FSH.
1. The dominant follicle (from left or right ovary) achieves dominance
over other follicles in both ovaries.
2. Dominance of a single follicle is evident by day 7 of the cycle.
3. The dominant follicle is the source of most of the estradiol secreted
during the follicular phase.
a. It is responsible for the pre-ovulatory rise in estradiol which
then produces the positive feedback stimulus for the ovulatory
surge in LH and FSH
D. Intra-ovarian mechanisms responsible for cyclic changes that occur in the
ovary
1. During the follicular phase, the theca interna develops LH receptors
and produces androgens from cholesterol in response to LH.
2. Granulosa cells develop FSH receptors. In response to FSH, androgen
is converted to estradiol by the aromatase system. Estradiol stimulates
the proliferation, induces FSH receptors (and increases FSH
responsiveness) of granulosa cells. Toward the end of the follicular
phase (late antral stage) FSH and estradiol stimulate development of
LH receptors granulosa cells. Antral fluid contains androgen, estradiol
and progesterone as a result of the hormonal interactions described
above. Through most of the cycle, granulosa cells are not vascularized,
just prior to ovulation granulosa cells become vascular, hyperemic,
and edematous.
3. Ovulation involves plasmin-assisted degradation of the wall of the
follicle.
4. Post-ovulation granulosa cells become luteinized, cease to proliferate,
hypertrophy and secrete estradiol and progesterone in response to LH.
5. Luteal failure (luteolysis) at day 26 occurs by a mechanism that has
not been fully characterized. However, LH withdrawal via negative
feedback is implicated. Intraovarian estrogens and prostaglandin F2α
may also participate.
6. The GnRH surge is not necessary for ovulation. The ovulatory surge in
LH and FSH requires pulsatile GnRH and positive feedback effect due
to the pre-ovulatory rise of estradiol. Pulsatile GnRH (every 90 min)
delivered via a portable infusion pump is used in the treatment of
anovulatory conditions. Nevertheless, a GnRH surge occurs at
midcycle. This surge by itself appears too small to be responsible for
the surge in LH and FSH. Pulsatile GnRH may facilitate the ovulatory
surge by providing a self-priming effect.
7. Pulsatile GnRH rate can be modified. Estradiol increases and
progesterone tends to decrease pulse frequency. As ovulation
approaches, pulse frequency increases and decreases during luteal
phase.
8. The periovulatory release of 17α-hydroxyprogesterone may enhance
anterior pituitary sensitivity to GnRH.
V.
ESTROGENS
A.
The principal circulating estrogen is estradiol-17ß which is in
equilibrium with its 17-ketosteroid derivative, estrone.
B.
Plasma binding: Ninety-nine % of circulating estradiol-17β is bound to
plasma proteins, 48% bound to albumin and 51% to sex hormone binding
globulin (SHBG which same as testosterone binding globulin TeBG).
C.
Metabolism, conjugation and excretion: The major metabolite of estradiol17β is a 16α -hydroxylated derivative, estriol (Fig. 5). Hydroxylation of
estradiol-17β can also occur at the C-2 position forming catechol estrogen
(Fig. 5). In humans the 16α-hydroxylation and 2-hydroxylation
pathways are inversely related to one another. Tobacco smoking is
associated with increased 2-hydroxylation, whereas alcohol consumption
associated with increased 16α-hydroxylation. Sixteen α-hydroxylated
estrogens are biologically active while 2-hydroxylated estrogen are
relatively inactive. In women as well as in certain strains of mice,
increased hepatic 16α-hydroxylation of estradiol is linked with increased
incidence of breast cancer.
Estrogens (secreted and metabolites) are excreted primarily as
glucuronide and sulfate conjugates.
Figure 5. Two Major Metabolites of Estradiol-17β
D. Among the physiological actions of estrogens are the following: regulation of
gonadotrophin secretion (positive and negative feedback), influences sexual
behavior, cyclic changes in uterus, cervix and vagina, granulosa cell
proliferation and development, maturation and maintenance of sex accessory
organs and secondary sex characteristics, mild anabolic effects, maintenance
of pregnancy, lowered threshold of uterus to contractile stimuli during
parturition, development of the tubular mammary gland, stimulation of
prolactin release, decrease in serum LDL cholesterol levels, an antiosteoporotic effect, and induction of progestin receptors.
1. Estrogen induction of progestin receptors indicates that estrogens are
required for progestin action. This is the basis for estrogen priming
effect for progestin action. Steroid receptor analysis of breast biopsy
specimens reveals important information about the status of a breast
cancer. A tumor that is positive for both estrogen receptor (ER) + and
progesterone receptor (PR) + suggests that the tumor is hormone
dependent. ER+, PR- suggests that tumor is hormone independent (i.e.,
nonfunctional estrogen receptors).
VI.
PROGESTINS
A.
The principal circulating progestin is progesterone. The corpus luteum
also produces 17α-hydroxy progesterone.
B.
Plasma binding: Ninety-eight % of circulating progesterone is bound to
plasma proteins, 48% bound to albumin and 50% to corticosteroid binding
globulin (CBG).
C.
Metabolism and excretion (Fig 6): The principal metabolic conversions of
progesterone are A ring and 20-ketone reduction in liver which forms
pregnanediol from progesterone, and pregnanetriol from 17α-hydroxy
progesterone. Pregnanetriol may be converted further to etiocholanolone, a
17 keto derivative. Etiocholanolone is also a product of cortisol and
testosterone metabolism. Progestin derivatives are excreted either in
unconjugated form or as glucuronide or sulfate conjugates.
Figure 6. Major Products of Progestin Metabolism
D. Among the physiological actions of progestins are regulation of gonadotophin
secretion, cyclic changes in uterus, and maintenance of pregnancy. During
pregnancy progestins increase the threshold of the uterus to contractile stimuli
and promote development of the alveolar mammary gland.
VII.
FEMALE ORAL CONTRACEPTIVE
A.
Female oral contraception are available in several forms.
1.
2.
Synthetic progestins (derivatives of 19-nortestosterone, such as
norethindrone) + estrogens (such as ethinyl estradiol). The
original rationale for this approach was based inhibition of the
ovulatory surge of LH by negative feedback. However, in the
presence of estrogens, lower doses of progestins are contraceptive
even when they do not block ovulation. This is because, cervical
mucus becomes hostile to sperm, as well as adverse effects of
progestin on the endometrium and oviduct. Thus contraception
may be the result of local effects on cervical mucus and other
components of the female reproductive tract.
RU-486 is a steroid analogue (11β-(4-dimethyl-amino phenyl)17β-hydroxy-17α)prop-1-ynyl)-estra-4,9-dien-3-one) that has a
high affinity for the progesterone receptor but appears unable to
initiate transcriptional events. As a result, RU 486 would prevent
trophoblastic implantation.
a.
3.
VIII.
RU-486 has similar antagonistic actions on the
glucocorticoid receptor system and has utility in the
treatment of disorders associated with glucocorticoid
excess such as Cushing's syndrome.
Plan B, an emergency contraceptive containing the progestin,
levonorgestrel given as two tablets 12 hrs apart. The procedure is
about 90% effective in preventing pregnancy if taken within 72 hrs
of intercourse. Among the possible contraceptive mechanisms of
this method are: 1) inhibition or delay of ovulation, 2) altered tubal
transport of sperm or ova, or 3) inhibition of implantation of the
blastocyst by alteration of the endometrium.
FEMALE GONADAL DISORDERS
A.
Hypogonadism and precocious puberty can occur in females by the
same mechanisms described previously for males. Pulsatile GnRH has
utility for treatment of hypothalamic (secondary) hypogonadism. Longacting GnRH analogues have utility for treatment of complete precocious
puberty of hypothalamic origin. Adrenogenital disorders produce virilizing
syndromes in females by mechanisms described in adrenal cortex lectures.
IX.
B.
Menstrual abnormalities, are the most common disorders in women and
are due to defects in the menstrual cycle, such as failure to ovulate,
shortened follicular phase, defects in timing of luteal regression, too
abrupt fall in estrogen levels at time of ovulation, hypothalamic-pituitary
dysfunction, and ovarian or uterine defects.
C.
Several diagnostic tests are useful in evaluating menstrual irregularities,
among these are: basal body temperature (should increase 1°F during
luteal phase); evaluation of cervical mucus (for viscosity, ferning),
evaluation of vaginal smear (for exfoliated squamous cells), serum
hormone levels (e.g., LH, FSH, gonadal steroids) and provocative tests
(LH, FSH; GnRH) to evaluate target organ responsiveness.
STUDY QUESTIONS
1. The dominant follicle is producing most of its estradiol during which days
of the menstrual cycle:
A.
B.
C.
D.
E.
Days 1-4
Days 5-14
Days 15-20
Days 21-25
Days 26-28
2. The fertilized egg goes through the morula (or pre-blastocyst) stage during
which days of the menstrual cycle?
.
A.
B.
C.
D.
Days 1-4
Days 5-14
Days 15-20
Days 21-25
Days 26-28
3. The corpus luteum undergoes regression during which days of the
menstrual cycle?
.
A.
B.
C.
D.
Days 1-4
Days 5-14
Days 15-20
Days 21-25
Days 26-28
4. The uterus exhibits maximum secretory activity during which days of the
menstrual cycle?
.
A.
B.
C.
D.
Days 1-4
Days 5-14
Days 15-20
Days 21-25
Days 26-28
5. The corpus hemorrhagicum is evident during which days of the menstrual
cycle?
.
A.
B.
C.
D.
Days 1-4
Days 5-14
Days 15-20
Days 21-25
Days 26-28
6. Granulosa cells are proliferating and the antrum is being formed (i.e.
follicular development), during which days of the menstrual cycle?
.
A.
B.
C.
D.
Days 1-4
Days 5-14
Days 15-20
Days 21-25
Days 26-28
Answer Key: 1. B; 2. C; 3. E; 4. D; 5. C; 6. B