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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

10

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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