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Chapter 27
Human Reproduction
PowerPoint Lectures for
Campbell Biology: Concepts & Connections, Seventh Edition
Reece, Taylor, Simon, and Dickey
© 2012 Pearson Education, Inc.
Lecture by Edward J. Zalisko
27.3 Reproductive anatomy of the human female
 There are anatomical differences between the human male and female reproductive
systems, but have some important similarities.
1. A pair of gonads, the organs that produce gametes.
2. Ducts that store and deliver gametes
3. Structures that facilitate copulation.
 A woman’s gonads, ovaries, are each about an inch long, with a bumpy surface (Figure
27.3 A).
 The bumps are follicles, each consisting of one or more layers of cells that surround,
nourish, and protect a single developing egg cell.
 The ovaries produce hormones specifically, the follicle cells produce the female sex hormone
estrogen.
 A female is born with 1–2 million follicles, but only several hundred will release egg cell
during her reproductive years. Starting at puberty, one follicle (or, rarely, more than one)
matures and releases an immature egg after every 28 days until menopause, usually around
age 50.
 An immature egg cell is ejected from the follicle in a process called ovulation.
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 After ovulation, the follicular
tissue grows within the ovary
form a solid mass called the
corpus luteum; It will secrete
additional estrogen and
Progesterone (maintain the
uterine lining during
pregnancy)
 If the released egg is not
fertilized, the corpus luteum
degenerates, and a new
follicle matures during the
next cycle.
 Each ovary lies next to the
opening of an oviduct, also
called a fallopian tube.
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 After ovulation, the follicular
tissue grows within the ovary
form a solid mass called the
corpus luteum; It will secrete
additional estrogen and
Progesterone (maintain the
uterine lining during
pregnancy)
 If the released egg is not
fertilized, the corpus luteum
degenerates, and a new
follicle matures during the
next cycle.
 Each ovary lies next to the
opening of an oviduct, also
called a fallopian tube.
 The surface of the ovary is
separated from the opening of
the oviduct by a tiny space.
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When ovulation occurs, the egg cell passes across
the space and into the oviduct, where cilia sweep it
toward the uterus.
 If sperm are present, fertilization may occur in the
upper part of the oviduct.
The resulting zygote starts to divide, thus becoming
an embryo, as it moves along within the oviduct.
Uterus, the actual site of pregnancy
The uterus, also known as the womb, is
the actual site of pregnancy. The uterus is
about 3 inches long, but during pregnancy it
expands considerably as the baby develops.
The uterus has a thick muscular wall, and its
inner lining, the endometrium, is richly
supplied with blood vessels. An embryo
implants in the endometrium, and development
is completed there.
The term embryo is used for the stage in
development from the first division of the
zygote until body structures begin to appear,
about the 9th week in humans.
 From the 9th week until birth, a developing
human is called a fetus.
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In about 1% of pregnancies, the
embryo implants somewhere
else, resulting in an ectopic
pregnancy.
Most
ectopic
pregnancies occur in the oviduct
and
are
called
tubal
pregnancies. It is a serious
medical
emergency.
That
requires surgical intervention;
otherwise,
it
can
rupture
surrounding tissues, causing
severe bleeding and even death
of the mother.
Cervix and Vagina
The narrow neck at the bottom of the
uterus is the cervix, which opens into the
vagina.
Vagina is a thin-walled, but strong,
muscular chamber that serves as the birth
canal through which the baby is born.
The vagina is also the repository for
sperm during sexual intercourse.
Glands near the vaginal opening secrete
mucus during sexual arousal, lubricating the
vagina and facilitating intercourse.
The vagina opens to the outside just behind
the opening of the urethra, the tube through
which urine is excreted.
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It is recommended that women
have a yearly Pap test in which cells
are removed from around
the cervix and examined under a
microscope for signs of cervical
cancer.
Vulva is the collective term for the external female genitalia.
A pair of slender skin folds, the labia minora, border the
openings,
A pair of thick, fatty ridges, the labia majora, protect the vaginal
opening.
A thin piece of tissue called the hymen partly covers the vaginal
opening.
 Several female
reproductive structures are
important in sexual
arousal, and stimulation of
them can produce highly
pleasurable sensations.
 The vagina, labia minora,
and a small erectile organ
called the clitoris all
engorge with blood and
enlarge during sexual
activity.
 The clitoris consists of a
short shaft supporting a
rounded glans, or head,
covered by a small hood
of skin called the prepuce.
 The clitoris, especially the
glans, has an enormous
number of nerve endings
and sensitive to touch.
27.4 Reproductive anatomy of the human male
The male gonads, or testes (singular,
testis), are each housed outside the
abdominal cavity in a sac called the scrotum.
A testis and scrotum together are called
a testicle.
Sperm cannot develop at human body
temperature; the scrotum keeps the spermforming cells about 2°C cooler, which allows
them to function normally.
In cold conditions, muscles around the
scrotum contract, pulling the testes toward
the body, thereby maintaining the proper
temperature.
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1. From each testis, sperm pass into a coiled tube called the epididymis, which stores the
sperm while they continue to develop.
2. Sperm leave the
epididymis during
Ejaculation (the expulsion
of sperm-containing fluid
from the penis).
3. At that time, muscular
contractions propel the
sperm from the epididymis
through another duct called
the vas deferens.
4. The vas deferens joins a
short duct from a gland,The
seminal vesicle.The two
ducts unite to form a short
Ejaculatory duct, joins
its counterpart conveying
sperm from the other testis.
5. Each ejaculatory duct
empties into the urethra,
which conveys both
urine and sperm out through
the penis,
6. The human penis consists mainly of erectile tissue essential for
insertion of the penis into the vagina. Like the clitoris, the penis consists of
a shaft that supports the glans, or head. The glans is richly supplied with
nerve endings and is highly sensitive to stimulation. A fold of skin called the
prepuce, or foreskin, covers the glans.
The reproductive system of human males contains three sets of glands:
1. The seminal vesicles (two seminal vesicles secrete a thick fluid that contains
fructose, provides most of the energy to sperm as they propel themselves through the
female reproductive tract.),
2. The prostate gland (secretes a thin fluid that further nourishes the sperm),
3. The bulbourethral glands (two bulbourethral glands secrete a clear, alkaline
mucus.
Together, the sperm and the glandular secretions make up semen, ejaculated
from the penis during orgasm and is about 2–5 mL semen/during a typical
ejaculation.
About 95% of the fluid consists of glandular secretions and 5% is made up of
sperm (typically 200–500 million of them), only one of which may eventually
fertilize an egg.
The alkalinity of the semen balances the acidity of urine in the urethra and
neutralizes the acidic environment of the vagina, protecting the sperm and
increasing their motility.
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Process of Ejaculation occurs in two stages.
1. At the peak of sexual arousal, muscle contractions in multiple glands force
secretions into the urethra and propel sperm from the epididymis. At the same time, a
sphincter muscle at the base of the bladder contracts, preventing urine from leaking
into the urethra from the bladder. Another sphincter also contracts, closing entrance of
the urethra into penis. The section of the urethra between the two sphincters fills with
semen and expands.
2. The expulsion stage, the sphincter at base of penis relaxes, admitting semen into
the penis. At the same time, a series of strong muscle contractions around the base of
the penis and along the urethra expels the semen from the body.
How hormones control sperm production by the testes
The hypothalamus secretes a releasing
hormone that regulates release of folliclestimulating hormone (FSH) and luteinizing
hormone (LH) by the anterior pituitary
FSH increases sperm production by the
testes, while LH promotes the secretion of
androgens, mainly testosterone.
Androgens stimulate sperm production.
In addition, androgens carried in the blood help
maintain homeostasis by a negative-feedback
mechanism (red arrows), inhibiting secretion of
both the releasing hormone and LH
Under the control of this chemical regulating
system, the testes produce hundreds of millions of
sperm every day, from puberty well into old age.
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27.5 The formation of sperm and egg cells requires meiosis
Both sperm and egg are haploid (n) cells that develop by
meiosis from diploid (2n) cells in the gonads.
The diploid chromosome number in humans is 46; that
is, 2n = 46.
The formation of gametes is called gametogenesis
There are significant differences in gametogenesis
between human males (spermatogenesis) and females
(oogenesis)
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Sperm develop in the testes in coiled tubes called
the seminiferous tubules
 Diploid cells that begin the process are located near the outer wall of the tubules
 These cells multiply continuously by mitosis, and each day about 3 million of them
differentiate into primary spermatocytes, the cells that undergo meiosis
 Meiosis I of a primary spermatocyte produces two secondary spermatocytes, each
with the haploid number of chromosomes (n = 23)
 Meiosis II then forms four cells, each with the haploid number of chromosomes
 A sperm cell develops by differentiation of each of these haploid cells and is gradually
pushed toward the center of the seminiferous tubule
 From there it passes into the epididymis, where it matures, becomes motile, and is stored
until ejaculation
 In human males, spermatogenesis takes about 10 weeks
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Oogenesis, the development of a mature egg
 Oogenesis actually begins prior to birth, when a diploid cell in each developing follicle begins
meiosis.
At birth, each follicle contains a dormant primary oocyte, a diploid cell that is resting in prophase of
meiosis I
A primary oocyte can be hormonally triggered to develop further. Between puberty and menopause,
about every 28 days.
FSH (follicle-stimulating hormone) from the pituitary stimulates one of the dormant follicles to
develop. The follicle enlarges, and the primary oocyte completes meiosis I and begins meiosis II.
Meiosis then halts again at metaphase II
In the female, the division of the cytoplasm in meiosis I is unequal, with a single secondary oocyte
receiving almost all of it. The smaller of the two daughter cells, called the first polar body, receives
almost no cytoplasm.
The secondary oocyte is released by the ovary during ovulation. it enters the oviduct, and if a sperm
cell penetrates it, the secondary oocyte completes meiosis II.
Meiosis II is also unequal, yielding a second polar body and the mature egg (ovum). The haploid
nucleus of the mature egg can then fuse with the haploid nucleus of the sperm cell, producing a
zygote.
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About the time the secondary oocyte forms, the pituitary hormone LH
(luteinizing hormone) triggers ovulation, the rupture of the follicle and
expulsion of the secondary oocyte.
The ruptured follicle then develops into a corpus luteum (“yellow body”).
Unless fertilization occurs, the corpus luteum degenerates before another
follicle starts to develop.
27.6 Hormones synchronize cyclic changes in the ovary and
uterus
Actually one integrated cycle
involving two organs
Hormonal messages links these
two cycles—synchronizing
follicle development in the
ovaries and establishment of
uterine lining for supporting
growing embryo
For simplicity, ovarian cycle is
divided into two phases,
separated by ovulation
Reproductiv
e cycle
Menstrual
cycle in the
uterus –3-5
days
Ovarian
cycle in
ovaries of
28 days
Preovulatory
phase
Postovulatory
phase
The first
day of
period is
day 1 of
menstrual
cycle
The
endometrium
ruptures and
leave the body
through vagina
After menstruation,
regrows and max thicken
on 20-25 days
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Hormonal Events Before Ovulation
1. Releasing hormone from the hypothalamus
stimulates the anterior pituitary to increase
its output of FSH and LH.
2. FSH stimulates the growth of an ovarian
follicle, in effect starting the ovarian cycle. In
turn, the follicle secretes estrogen.
3. As the follicle grows, secretes more and
more estrogen, and the rising but still
relatively low estrogen exerts negative
feedback on the pituitary keeps blood levels
of FSH and LH low.
4. As the time of ovulation approaches,
hormone levels change drastically, with
estrogen reaching a critical peak (Part D)
just before ovulation. This high level of
estrogen exerts positive feedback on the
hypothalamus makes the pituitary—secrete
surges of FSH and LH
Hormonal Events After Ovulation
1. LH stimulates the completion of meiosis I,
transforming the primary oocyte in the follicle into a
secondary oocyte.
2. It also signals enzymes to rupture the follicle,
allowing ovulation to occur, and triggers the
development of the corpusluteum from the ruptured
follicle (hence its name, luteinizing hormone).
3. LH also promotes the secretion of progesterone
and estrogen by the corpus luteum.
4. High levels of estrogen and progesterone in the
blood following ovulation have a strong influence on
both the ovary and uterus. The combination of the
two hormones exerts negative feedback on the
hypothalamus and pituitary, producing falling FSH
and LH levels.
5. The drop in FSH and LH prevents follicles from
developing and ovulation from occurring during the
post-ovulatory phase. Also, the LH drop is followed
by the gradual degeneration of the corpusluteum.
6. Near the end of the post-ovulatory phase, the
corpus luteum stops secreting estrogen and
progesterone---the hypothalamus --stimulate the
pituitary to secrete more FSH and LH, and a new
cycle begins.
Control of the Menstrual Cycle
1. Hormonal regulation of the menstrual cycle is
simpler than that of the ovarian cycle.
2. The menstrual cycle (Part E) is directly
controlled by estrogen and progesterone alone.
3. Starting around day 5 of the cycle, the
endometrium thickens in response to the rising
estrogen and, later, progesterone. When the
levels of these hormones drop, the endometrium
begins to slough off.
4. Menstrual bleeding begins soon after, on day 1
of a new cycle.
What happens in the human
ovary and uterus in the absence of
fertilization and pregnancy?
Early in pregnancy, the developing embryo, implanted in
the endometrium, releases a hormone (human chorionic
gonadotropin, or hCG). This hormone acts like LH in that
it maintains the corpusuteum, which continues to secrete
progesterone and estrogen, keeping the endometrium
intact.