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Chapter 27
The
Reproductive
System
Figure 27.19 Events of oogenesis.
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Meiotic events Follicle development
in ovary
Before birth
Oogonium (stem cell)
Mitosis Follicle cells
Primary oocyte
Oocyte
Infancy and
childhood
(ovary functionally
inactive)
Primary oocyte
(arrested in prophase I;
present at birth)
Primordial follicle
Each month from
puberty to menopause
Primary oocyte (still
arrested in prophase I)
Primary follicle
Spindle
Primordial follicle
Meiosis I (completed by
one primary oocyte each
month in response to
LH surge) Secondary oocyte
(arrested in metaphase II)
Vesicular (antral)
follicle
Secondary follicle
First polar body
Ovulation
Ovulated secondary
oocyte
Meiosis II of polar body
(may or may not occur) Sperm
Polar bodies
(all polar bodies
degenerate) Second
polar body Ovum
Meiosis II
completed
(only if sperm
penetrates
oocyte)
In absence of fertilization,
ruptured follicle becomes
a corpus luteum and
ultimately degenerates.
Degenerating
corpus luteum
2n
2n
2n
2n
n
n n n n
2
Figure 27.20 Schematic and microscopic views of the ovarian cycle: development and fate of ovarian
follicles.
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Primordial
follicles
Primary
follicle Secondary
follicle
Late secondary
follicle Forming antrum
Theca folliculi
Theca folliculi
Primary oocyte
Zona pellucida Antrum
Secondary oocyte
Secondar
y
oocyte Corona radiata
Zona pellucida
Antrum
Corpus luteum
(forms from ruptured
follicle)
Follicle ruptures;
secondary oocyte
ovulated
Mature vesicular follicle
carries out meiosis I; ready
to be ovulated
Slide 1
1 2
3a
3b
4
5
3b 3a
2
7
6
6
6 5 4
3
Ovulation
• Ovary wall ruptures, expels secondary oocyte with its corona radiata to peritoneal cavity
• Mittelschmerz - twinge of pain sometimes felt at ovulation
• 1–2% of ovulations release more than one secondary oocyte, which, if fertilized, results in fraternal twins
• Identical twins result from fertilization of one oocyte, then separation of daughter cells
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Figure 27.20 Schematic and microscopic views of the ovarian cycle: development and fate of ovarian
follicles.
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Primordial
follicles
Primary
follicle Secondary
follicle
Late secondary
follicle Forming antrum
Theca folliculi
Theca folliculi
Primary oocyte
Zona pellucida Antrum
Secondary oocyte
Secondar
y
oocyte Corona radiata
Zona pellucida
Antrum
Corpus luteum
(forms from ruptured
follicle)
Follicle ruptures;
secondary oocyte
ovulated
Mature vesicular follicle
carries out meiosis I; ready
to be ovulated
Slide 1
1 2
3a
3b
4
5
3b 3a
2
7
6
6
6 5 4
5
Establishing the Ovarian Cycle
• During childhood, ovaries grow and secrete
small amounts of estrogens that inhibit the
hypothalamic release of GnRH
• At puberty
– Leptin from adipose tissue decreases the
estrogen inhibition
– GnRH, FSH, and LH are released, act on
ovaries
– In about four years, an adult cyclic pattern is
achieved and menarche occurs
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Hormonal Interactions During a 28-Day
Ovarian Cycle
• Day 1: GnRH release of FSH and LH
– FSH and LH growth of several follicles, and
estrogen release
– estrogen levels
• Inhibit the release of FSH and LH
• Stimulate synthesis and storage of FSH and LH
• Enhance further estrogen output
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Hormonal Interactions During a 28-Day
Ovarian Cycle
• Effects of LH surge
– Completion of meiosis I (secondary oocyte
continues on to metaphase II)
– Day 14 – LH Triggers ovulation
– Transforms ruptured follicle into corpus
luteum
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Hormonal Interactions During a 28-Day
Ovarian Cycle
• Functions of corpus luteum
– Produces inhibin, progesterone, and estrogen
– These hormones inhibit FSH and LH release
• Declining LH and FSH ends luteal activity and inhibits
follicle development
• Days 26–28: corpus luteum degenerates and ovarian
hormone levels drop sharply
– Ends the blockade of FSH and LH
– The cycle starts anew
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Figure 27.21 Regulation of the ovarian cycle.
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Hypothalamus
GnRH
Travels via portal blood
Anterior pituitary
FSH LH
Thecal cells
Androgens
Granulosa cells
Convert androgens to estrogens
Mature vesicular follicle Ovulated
secondary oocyte
Corpus luteum
Ruptured
follicle
LH surge
Stimulates Inhibits
Early and
midfollicular phases
Late follicular and
luteal phases
Positive feedback exerted by large in estrogen output by maturing follicle.
Slightly elevated estrogen and rising inhibin levels inhibit FSH secretion.
GnRH
Slide 1
1
1
2 2
2
2
2
Estrogens
4 4
6
4
5 6
5
Progesterone Estrogens Inhibin
Inhibin
Hypothalamus
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Uterine (Menstrual) Cycle
• Cyclic changes in endometrium in
response to ovarian hormones
• Three phases
1. Days 1–5: menstrual phase
2. Days 6–14: proliferative (preovulatory) phase
3. Days 15–28: secretory (postovulatory) phase
(constant 14-day length)
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Figure 27.23 Correlation of anterior pituitary and ovarian hormones with structural changes of the ovary and uterus.
Pla
sm
a h
orm
on
e le
ve
l
Pla
sm
a h
orm
on
e le
ve
l
LH
FSH
Dominant
follicle
Ovulation Corpus
luteum
Degenerating corpus luteum
Follicular
phase Ovulation
(Day 14)
Luteal
phase
Estrogens
Progesterone
Endometrial
glands
Menstrual
flow
Blood vessels
Functional layer
Basal layer
Days
Menstrual
phase
Proliferative
phase
Secretory
phase
1 5 10 15 20 25 28
Fluctuation of gonadotropin levels:
Fluctuating levels of pituitary gonadotropins
(follicle-stimulating hormone and luteinizing
hormone) in the blood regulate the events of
the ovarian cycle.
Ovarian cycle: Structural changes in
vesicular ovarian follicles and the corpus luteum are correlated with changes in the endometrium of the uterus during the uterine cycle (d). Recall that only vesicular follicles (in their antral phase) are hormone dependent —primary and secondary follicles are not.
Fluctuation of ovarian hormone levels:
Fluctuating levels of ovarian hormones (estrogens and progesterone) cause the endometrial changes of the uterine cycle. The high estrogen levels are also responsible for the LH/FSH surge in (a).
The three phases of the uterine cycle: • Menstrual: The functional layer of the endometrium
is shed.
• Proliferative: The functional layer of the
endometrium is rebuilt.
• Secretory: Begins immediately after ovulation.
Enrichment of the blood supply and glandular
secretion of nutrients prepare the endometrium
to receive an embryo.
Both the menstrual and proliferative phases occur before ovulation, and together they correspond to the follicular phase of the ovarian cycle. The secretory phase corresponds in time to the luteal phase of the ovarian cycle. 12
Uterine Cycle
• If fertilization does not occur
– Corpus luteum degenerates
– Progesterone levels fall
– Spiral arteries kink and spasm
– Endometrial cells begin to die
– Spiral arteries constrict again, then relax and open wide
– Rush of blood fragments weakened capillary beds and the functional layer sloughs
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Premenstrual Syndrome - PMS
• Symtoms – food cravings, headaches,
depression, aggression, mood swings,
sore breasts, etc.
• Cause – excessive response of body to
fluctuations of estrogen and progesterone
and prostaglandins
• Estrogen mimics aldosterone
• Relief of PMS????
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