Biology in Focus - Chapter 36

CAMPBELL BIOLOGY IN FOCUS
© 2014 Pearson Education, Inc.
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
36
Reproduction
and Development

Overview: Pairing Up for Sexual Reproduction
 Each sea slug produces sperm and eggs and thus
acts as both mother and father to the next generation
 Animal reproduction takes many forms
 A population outlives its members only by
reproduction, the generation of new individuals
from existing ones

Figure 36.1

Concept 36.1: Both asexual and sexual
reproduction occur in the animal kingdom
 Sexual reproduction is the creation of an offspring
by fusion of a male gamete (sperm) and female
gamete (egg) to form a zygote
 Asexual reproduction is the creation of offspring
without the fusion of egg and sperm

Mechanisms of Asexual Reproduction
 Many invertebrates reproduce asexually by budding,
in which new individuals arise from outgrowths of
existing ones
 Also common among invertebrates is fission, the
separation of a parent organism into two individuals
of approximately equal size
Video: Hydra Budding

Figure 36.2

 Fragmentation is breaking of the body into pieces,
some or all of which develop into adults
 Fragmentation must be accompanied by
regeneration, regrowth of lost body parts
 Parthenogenesis is the development of a new
individual from an unfertilized egg

Sexual Reproduction: An Evolutionary Enigma
 Sexual females have half as many daughters as
asexual females; this is the “twofold cost” of sexual
reproduction
 Despite this, almost all eukaryotic species reproduce
sexually

Figure 36.3-1
Asexual reproduction
Female
Female
Male
Sexual reproduction
Generation 1
Generation 2

Figure 36.3-2
Female
Female
Male
Sexual reproduction
Generation 1
Generation 2
Generation 3

Figure 36.3-3
Female
Female
Male
Sexual reproduction
Generation 1
Generation 2
Generation 3
Generation 4

 Sexual reproduction results in genetic recombination
 The resulting increased variation among offspring
may enhance the reproductive success of parents in
changing environments

Reproductive Cycles
 Most animals exhibit reproductive cycles related to
changing seasons
 Reproductive cycles are controlled by hormones and
environmental cues
 Because seasonal temperature is often an important
cue in reproduction, climate change can decrease
reproductive success

Figure 36.4

 In some species of whiptail lizards, the members of
breeding pairs alternate roles
 Reproduction is exclusively asexual, and there are
no males
 However, courtship and mating behavior still take
place, with one female of each pair mimicking male
mating behavior
 The two alternate roles two or three times during
the breeding season

Figure 36.5
(a) A. uniparens females
Male-
like
Ovulation
Female Male-
like
Female
Time
(b) The sexual behavior of A. uniparens is
correlated with the cycle of ovulation.Behavior
Hormone
level
Ovary
size
Ovulation
Progesterone
Estradiol

Figure 36.5a
(a) A. uniparens females

Figure 36.5b
Male-
like
Ovulation
Female Male-
like
Female
Time
(b) The sexual behavior of A. uniparens is
correlated with the cycle of ovulation.
Behavior
Hormone
level
Ovary
size Ovulation
Progesterone
Estradiol

 For many animals, finding a partner for sexual
reproduction may be challenging
 One solution is hermaphroditism, in which each
individual has male and female reproductive systems
 Any two hermaphrodites can mate, and some
hermaphrodites can self-fertilize
Variation in Patterns of Sexual Reproduction

 Individuals of some species undergo sex reversals
 For example, in a coral reef fish, the bluehead
wrasse, a lone male defends a group of females
 If the male dies, the largest female in the group
transforms into a male
 Within a few weeks, this individual can begin to
produce sperm instead of eggs

External and Internal Fertilization
 Sexual reproduction requires fertilization, the
union of sperm and eggs
 In external fertilization, eggs shed by the female
are fertilized by sperm in the external environment
 In internal fertilization, sperm are deposited in or
near the female reproductive tract, and fertilization
occurs within the tract

 A moist habitat is almost always required for external
fertilization to prevent gametes from drying out and to
allow sperm to swim to eggs
 Many aquatic invertebrates simply shed gametes into
the surrounding water
 In this case, timing of release of gametes is crucial to
ensure that sperm and egg encounter one another
 If external fertilization is not synchronous across a
population, courtship behaviors might lead to
fertilization

Figure 36.6

 However fertilization occurs, the mating animals
may use pheromones, chemicals released by one
organism that can influence physiology and
behavior of another individual of the same species

Ensuring the Survival of Offspring
 Internal fertilization is typically associated with
production of fewer gametes but the survival of a
higher fraction of zygotes
 Internal fertilization is also often associated with
mechanisms to provide protection of embryos and
parental care of young

 The embryos of some terrestrial animals develop in
eggs with calcium- and protein-containing shells and
several internal membranes
 Some other animals retain the embryo, which
develops inside the female
 In many animals, parental care helps ensure survival
of offspring

Figure 36.7

Concept 36.2: Reproductive organs produce and
transport gametes
 Sexual reproduction in animals relies on sets of
cells that are precursors for eggs and sperm

Variation in Reproductive Systems
 Many (but not all) animals have gonads, organs that
produce gametes
 Some simple systems do not have gonads, but
gametes form from undifferentiated tissue
 More elaborate systems include sets of accessory
tubes and glands that carry, nourish, and protect
gametes and sometimes developing embryos

 Most insects have separate sexes with complex
reproductive systems
 In many insects, the female has a spermatheca in
which sperm is stored during copulation

 A cloaca is a common opening between the external
environment and the digestive, excretory, and
reproductive systems
 A cloaca is common in nonmammalian vertebrates;
mammals usually have a separate opening to the
digestive tract

Human Male Reproductive Anatomy
 The male’s external reproductive organs are the
scrotum and penis
 The internal organs are gonads that produce sperm
and reproductive hormones, accessory glands that
secrete products essential to sperm movement, and
ducts that carry sperm and glandular secretions
Animation: Male Reproductive Anatomy

Figure 36.8
Seminal
vesicle
(Rectum)
Ejaculatory
duct
Bulbourethral
gland
Erectile
tissue
Vas deferens
Prostate gland
Epididymis
Scrotum
Vas deferens
Testis Prepuce
Penis
Glans
Urethra
(Pubic bone)
(Urinary duct)
(Urinary bladder)

Testes
 The male gonads, or testes, produce sperm in
highly coiled tubes called seminiferous tubules
 Production of normal sperm cannot occur at the
body temperatures of most mammals
 The scrotum, a fold of the body wall, maintains testis
temperature at about 2o
C below the core body
temperature

Ducts
 From the seminiferous tubules of a testis, sperm
pass into the coiled duct of the epididymis
 During ejaculation, sperm are propelled through
the muscular vas deferens and the ejaculatory
duct and then exit the penis through the urethra

Accessory Glands
 Semen is composed of sperm plus secretions from
three sets of accessory glands
 The two seminal vesicles contribute about 60% of
the total volume of semen
 The prostate gland secretes its products directly
into the urethra through several small ducts
 The bulbourethral glands secrete a clear mucus
before ejaculation that neutralizes acidic urine
remaining in the urethra

Penis
 The human penis is composed of three cylinders of
spongy erectile tissue
 During sexual arousal, erectile tissue fills with blood
from the arteries, causing an erection
 The head of the penis, or glans, has a thinner skin
covering than the shaft
 The prepuce, or foreskin, is a fold of skin covering
the glans; it is removed when a male is circumcised

Human Female Reproductive Anatomy
 The female external reproductive structures include
the clitoris and two sets of labia
 The internal organs are a pair of gonads and a
system of ducts and chambers that carry gametes
and house the embryo and fetus
Animation: Female Reproductive Anatomy

Figure 36.9
Oviduct
(Rectum)
Cervix
Major vestibular
gland
Vagina
Vaginal opening
Ovary
Uterus
(Urinary
bladder)
(Pubic bone)
(Urethra)
Body
ClitorisGlans
Prepuce
Labia minora
Labia majora

Ovaries
 The female gonads, a pair of ovaries, lie in the
abdominal cavity
 Each ovary contains many follicles, which consist
of a partially developed egg, called an oocyte,
surrounded by support cells

Oviducts and Uterus
 Upon ovulation, a mature egg cell is released
 It travels from the ovary to the uterus via an oviduct
 Cilia in the oviduct convey the egg to the uterus,
also called the womb
 The uterus lining, the endometrium, has many
blood vessels
 The neck of the uterus is the cervix, which opens
into the vagina

Vagina and Vulva
 The vagina is a muscular but elastic chamber that
is the repository for sperm during copulation and
serves as the birth canal
 The vagina opens to the outside at the vulva, which
consists of the labia majora, labia minora, hymen,
and clitoris

Mammary Glands
 The mammary glands are not part of the
reproductive system but are important to mammalian
reproduction
 Within the glands, small sacs of epithelial tissue
secrete milk

Gametogenesis
 Gametogenesis is the production of gametes
 There is a close relationship between the structure
and function of the gonads
Video: Flagella in Sperm
Video: Sperm Flagellum

Figure 36.10a
Epididymis
Seminiferous tubule
Testis
Cross section of
seminiferous tubule
Sertoli cell
nucleus
Plasma
membrane
Lumen of
seminiferous
tubule
Mitochondria
Nucleus
Acrosome
Midpiece
Tail
Head
Neck
Primordial germ cell in embryo
Spermatogonial
stem cell
Mitotic divisions
Spermatogonium
Mitotic divisions
Mitotic divisions
Primary spermatocyte
Secondary spermatocyte
Spermatids
(two stages)
Early
spermatid
Differentiation
(Sertoli cells
provide nutrients)
Meiosis I
Meiosis II
n n
2n
2n
2n
n n n n
n n n nSperm cell

Figure 36.10aa
Epididymis
Seminiferous tubule
Testis
Cross section of
seminiferous tubule
Sertoli cell
nucleus
Spermatogonium
Primary
spermatocyte
Secondary
spermatocyte
Spermatids
(two stages)
Lumen of
seminiferous tubule
Sperm cell

Figure 36.10ab
Primordial germ cell in embryo
Spermatogonial
stem cell
Mitotic divisions
Spermatogonium
Mitotic divisions
Mitotic divisions
Primary spermatocyte
Secondary spermatocyte
Early
spermatid
Differentiation
(Sertoli cells
provide nutrients)
Meiosis I
Meiosis II
n n
2n
2n
2n
n n n n
n n n nSperm cell

Figure 36.10ac
Plasma
membrane
Mitochondria
Nucleus
Acrosome
Midpiece
Tail
Head
Neck

Figure 36.10b
Primordial germ cell
Completion of meiosis I
and onset of meiosis II
Mitotic divisions
In embryo
Mitotic divisions
Mature follicle
Primary oocyte
(present at birth), arrested
in prophase of meiosis I
Ovulation, sperm entry
First
polar
body
Degenerating
corpus luteum
Ruptured
follicle
Ovulated
secondary oocyte
Fertilized egg
Ovary
Secondary oocyte,
arrested at metaphase of
meiosis II
Oogonium2n
n
Second
polar
body
Completion of meiosis II
2n
n
n
n
Primary oocyte
within follicle
Growing
follicle
Corpus luteum

Figure 36.10ba
Mature
follicle
Degenerating
corpus
luteum
Ruptured
follicle
Ovulated
secondary
oocyte
Ovary
Primary
oocyte
within
follicle
Growing
follicle
Corpus
luteum

Figure 36.10bb
Primordial germ cell
Mitotic divisions
In embryo
Mitotic divisions
Primary oocyte
(present at birth), arrested
in prophase of meiosis I
Oogonium2n
2n

Figure 36.10bc
Completion of meiosis I
and onset of meiosis II
Ovulation, sperm entry
First
polar
body
Fertilized egg
Secondary oocyte,
arrested at metaphase of
meiosis II
n
Second
polar
body
Completion of meiosis II
n
n
n

 Spermatogenesis, the development of sperm, is
continuous and prolific (millions of sperm are
produced per day); each sperm takes about 7 weeks
to develop
 Oogenesis, the development of a mature egg, is a
prolonged process
 Immature eggs form in the female embryo but do not
complete their development until years or decades
later

 Spermatogenesis differs from oogenesis in three
ways
 All four products of meiosis develop into sperm, while
only one of the four becomes an egg
 Spermatogenesis occurs throughout adolescence and
adulthood
 Sperm are produced continuously without the
prolonged interruptions in oogenesis

Concept 36.3: The interplay of tropic and sex
hormones regulates reproduction in mammals
 Human reproduction is coordinated by hormones
from the hypothalamus, anterior pituitary, and
gonads
 Gonadotropin-releasing hormone (GnRH) is secreted
by the hypothalamus and directs the release of FSH
(follicle-stimulating hormone) and LH (luteinizing
hormone) from the anterior pituitary

 FSH and LH regulate processes in the gonads and
the production of sex hormones
 The major androgen is testosterone and major
estrogens are estradiol and progesterone
 Both males and females produce androgens and
estrogens but differ in their blood concentrations of
particular hormones
 For example, males have testosterone levels about
10 times higher than females

 Sex hormones regulate gamete production both
directly and indirectly
 They serve many additional functions including
sexual behavior and the development of primary and
secondary sex characteristics
 Androgens are responsible for male vocalizations
and courtship displays

Figure 36.11

 FSH and LH act on different cells in the testes to
direct spermatogenesis
 Sertoli cells, found in the seminiferous tubules,
respond to FSH by nourishing developing sperm
 Leydig cells, connective tissue between the tubules,
respond to LH by secreting testosterone and other
androgens, which promote spermatogenesis
Hormonal Control of the Male Reproductive
System
Animation: Male Hormones

Figure 36.12
Hypothalamus
Anterior pituitary
GnRH
Sertoli cells Leydig cells
Inhibin
Testis
Spermatogenesis Testosterone
FSH LH
Negativefeedback
Negativefeedback

 Two negative feedback mechanisms control sex
hormone production in males
 Testosterone regulates the production of GnRH,
FSH, and LH through negative feedback
mechanisms
 Sertoli cells secrete the hormone inhibin, which
reduces FSH secretion from the anterior pituitary

Hormonal Control of Female Reproductive Cycles
 Females produce eggs in cycles
 Prior to ovulation, the endometrium thickens with
blood vessels in preparation for embryo implantation
 If an embryo does not implant in the endometrium,
the endometrium is shed in a process called
menstruation

 Hormones closely link the two cycles of female
reproduction
 Changes in the uterus define the menstrual cycle
(also called the uterine cycle)
 Changes in the ovaries define the ovarian cycle
Animation: Ovulation
Animation: Post-Ovulation

Figure 36.13
Anterior pituitary
Hypothalamus
GnRH
FSH LH
Control by hypothalamus(a)
FSH and LH stimulate
follicle to grow
Inhibited by low levels of
estradiol
FSH
LH
Pituitary gonadotropins
in blood
(b)
Growing follicle
Follicular phase Ovulation
Estradiol secreted
by growing follicle in
increasing amounts
Ovarian cycle(c)
Stimulated by high levels
of estradiol
Inhibited by combination of
estradiol and progesterone
LH surge triggers
ovulation
Maturing
follicle
Corpus
luteum
Degenerating
corpus luteum
Luteal phase
Progesterone and
estradiol secreted
by corpus luteum
Ovarian hormones
in blood
(d)
Uterine (menstrual) cycle(e)
Estradiol
Estradiol level
very low
Peak causes
LH surge
(see )
Progesterone
Progesterone and estra-
diol promote thickening
of endometrium
Endometrium
Menstrual flow phase Proliferative phase Secretory phase
0
Days
1
2
3
4
8
6
7
5
6
9
10
105 14 15 20 25 28

Figure 36.13a
Anterior pituitary
Hypothalamus
GnRH
FSH LH
Control by hypothalamus(a)
follicle to grow
Inhibited by low levels of
estradiol
FSH
LH
in blood
(b)
Stimulated by high levels
of estradiol
Inhibited by combination of
estradiol and progesterone
LH surge triggers
ovulation
1
2
3
6
Days
0 105 14 15 20 25 28

4
Figure 36.13b
follicle to grow
FSH
LH
in blood
(b)
Growing follicle
Estradiol secreted
increasing amounts
Ovarian cycle(c)
LH surge triggers
ovulation
Maturing
follicle
Corpus
luteum
Degenerating
corpus luteum
Luteal phase
Progesterone and
estradiol secreted
by corpus luteum
3
8
6
7
Days
0 105 14 15 20 25 28

5
9
10
Figure 36.13c
Growing follicle
Estradiol secreted
increasing amounts
Ovarian cycle(c)
Maturing
follicle
Corpus
luteum
Degenerating
corpus luteum
Luteal phase
Progesterone and
estradiol secreted
by corpus luteum
Ovarian hormones
in blood
(d)
Estradiol
Estradiol level
very low
Peak causes
LH surge
(see )
Progesterone
of endometrium
4
87
6
Days
0 105 14 15 20 25 28

Figure 36.13d
Uterine (menstrual) cycle(e)
Endometrium
Menstrual flow phase Proliferative phase Secretory phase
Days
5
9
10
Ovarian hormones
in blood
(d)
Estradiol
Estradiol level
very low
Peak causes
LH surge
(see )
Progesterone
of endometrium
6
0 105 14 15 20 25 28

The Ovarian Cycle
 The sequential release of GnRH then FSH and LH
stimulates follicle growth
 Follicle and oocyte growth and an increase in the
hormone estradiol characterize the follicular phase
of the ovarian cycle
 Estradiol causes GnRH, FSH, and LH levels to rise
through positive feedback
 The final result is the maturation of the follicle

 At the end of the follicular phase, the follicle
releases the secondary oocyte
 The follicular tissue left behind transforms into the
corpus luteum in response to LH; this is the luteal
phase
 The corpus luteum disintegrates, and ovarian steroid
hormones decrease by negative feedback
mechanisms

The Uterine (Menstrual) Cycle
 Hormones coordinate the uterine cycle with the
ovarian cycle
 Thickening of the endometrium during the
proliferative phase coordinates with the follicular
phase
 Secretion of nutrients during the secretory phase
coordinates with the luteal phase
 Shedding of the endometrium during the menstrual
flow phase coordinates with the growth of new
ovarian follicles

Menopause
 After about 500 cycles, human females undergo
menopause, the cessation of ovulation and
menstruation
 Menopause is very unusual among animals
 Menopause might have evolved to allow a mother to
provide better care for her children and grandchildren

Menstrual Versus Estrous Cycles
 Menstrual cycles are characteristic only of humans
and some other primates
 The endometrium is shed from the uterus in a
bleeding called menstruation
 Sexual receptivity is not limited to a time frame

 Estrous cycles are characteristic of most mammals
 The endometrium is reabsorbed by the uterus
 Sexual receptivity is limited to a “heat” period
 The length and frequency of estrus cycles vary from
species to species

 Two reactions predominate in both sexes
 Vasocongestion, the filling of tissue with blood
 Myotonia, increased muscle tension
 The sexual response cycle has four phases:
excitement, plateau, orgasm, and resolution
 Excitement prepares the penis and vagina for coitus
(sexual intercourse)
Human Sexual Response

 Direct stimulation of genitalia maintains the plateau
phase and prepares the vagina for receipt of sperm
 Orgasm is characterized by rhythmic contractions of
reproductive structures
 In males, semen is first released into the urethra and
then ejaculated from the urethra
 In females, the uterus and outer vagina contract

 During the resolution phase, organs return to their
normal state and muscles relax

Concept 36.4: Fertilization, cleavage, and
gastrulation initiate embryonic development
 Across animal species, embryonic development
involves common stages occurring in a set order
 First is fertilization, which forms a zygote
 During the cleavage stage, a series of mitoses
divide the zygote into a many-celled embryo
 The resulting blastula then undergoes
rearrangements into a three-layered embryo called
a gastrula

Figure 36.14
EMBRYONIC DEVELOPMENT
FERTILIZATION
GASTRULATION
CLEAVAGE
ORGANO-
GENESIS
Metamorphosis
Zygote
Blastula
Gastrula
Tail-bud
embryo
Larval
stages
Adult
frog
Sperm
Egg

Fertilization
 Molecules and events at the egg surface play a
crucial role in each step of fertilization
 Sperm penetrate the protective layer around the egg
 Receptors on the egg surface bind to molecules on
the sperm surface
 Changes at the egg surface prevent polyspermy, the
entry of multiple sperm nuclei into the egg

Video: Cortical Granule
Video: Calcium Release of Egg
Video: Calcium Wave of Egg

Figure 35.15-1
Basal body
(centriole)
Sperm
head
Sperm-
binding
receptors
Acrosome
Jelly coat
Egg plasma membrane
Vitelline layer

Figure 35.15-2
Basal body
(centriole)
Sperm
head
Sperm-
binding
receptors
Acrosome
Jelly coat
Egg plasma membrane
Hydrolytic enzymes
Vitelline layer

Figure 35.15-3
Sperm
nucleus
Basal body
(centriole)
Sperm
head
Acrosomal
process
Actin
filament
Sperm-
binding
receptors
Acrosome
Jelly coat
Egg plasma membrane
Hydrolytic enzymes
Vitelline layer

Figure 35.15-4
Sperm plasma
membrane
Sperm
nucleus
Basal body
(centriole)
Sperm
head
Acrosomal
process
Actin
filament
Sperm-
binding
receptors
Acrosome
Jelly coat
Fused
plasma
membranes
Egg plasma membrane
Hydrolytic enzymes
Vitelline layer

Figure 35.15-5
Sperm plasma
membrane
Sperm
nucleus
Basal body
(centriole)
Sperm
head
Acrosomal
process
Actin
filament
Sperm-
binding
receptors
Acrosome
Jelly coat Fertilization
envelope
Perivitelline
space
Cortical
granuleFused
plasma
membranes
Egg plasma membrane
Hydrolytic enzymes
Vitelline layer

 The events of fertilization also initiate metabolic
reactions that trigger the onset of embryonic
development, thus “activating” the egg
 Activation leads to events such as increased protein
synthesis that precede the formation of a diploid
nucleus
 Sperm entry triggers a release of Ca2+
, which
activates the egg and triggers the cortical reaction,
the slow block to polyspermy

Figure 36.16
First cell division
Onset of DNA synthesis
Fusion of egg and sperm nuclei complete
Increased protein synthesis
Formation of fertilization envelope complete
Binding of sperm to egg
Cortical reaction (slow block to polyspermy)
Increased intracellular calcium level
Acrosomal reaction: plasma membrane
depolarization (fast block to polyspermy)
MinutesSeconds
1
2
3
4
6
8
10
20
30
40
50
1
2
3
4
5
10
20
30
40
60
90

Cleavage and Gastrulation
 Fertilization is followed by cleavage, a period of
rapid cell division without growth
 Cleavage partitions the cytoplasm of one large cell
into many smaller cells
 The blastula is a ball of cells with a fluid-filled cavity
called a blastocoel
 The blastula is produced after about five to seven
cleavage divisions
Video: Cleavage of Egg

Figure 36.17
(a) Fertilized egg (b) Four-cell stage (c) Early blastula (d) Later blastula
50 µm

Figure 36.17a
(a) Fertilized egg
50 µm

Figure 36.17b
50 µm
(b) Four-cell stage

Figure 36.17c
50 µm
(c) Early blastula

Figure 36.17d
50 µm
(d) Later blastula

 After cleavage, the rate of cell division slows
 The remaining stages of embryonic development
are responsible for morphogenesis, the cellular and
tissue-based processes by which the animal body
takes shape

 During gastrulation, a set of cells at or near the
surface of the blastula moves to an interior location,
cell layers are established, and a primitive digestive
tube forms
 The hollow blastula is reorganized into a two- or
three-layered embryo called a gastrula

 The cell layers produced by gastrulation are called
germ layers
 The ectoderm forms the outer layer and the
endoderm the inner layer
 In vertebrates and other animals with bilateral
symmetry, a third germ layer, the mesoderm,
forms between the endoderm and ectoderm

Figure 36.18
Mesenchyme cells
Vegetal plate
Blastocoel
Blastocoel
Blastocoel
Blastopore
Blastopore
Mesenchyme cells
Vegetal
pole
Animal
pole
Filopodia
Archenteron
ArchenteronEctoderm
Mouth
50 µm
Digestive tube (endoderm)
Anus (from blastopore)
Future ectoderm
Future endoderm
Future mesoderm
Key
Mesenchyme
(mesoderm forms
future skeleton)

Figure 36.18a-1
Mesenchyme
cells
Vegetal plate
Blastocoel
Vegetal
pole
Animal
pole
Future ectoderm
Future endoderm
Future mesoderm

Figure 36.18a-2
Mesenchyme
cells
Vegetal plate
Blastocoel
Vegetal
pole
Animal
pole
Filopodia
Archenteron
Future ectoderm
Future endoderm
Future mesoderm

Figure 36.18a-3
Mesenchyme
cells
Vegetal plate
Blastocoel
Blastocoel
Blastopore
Vegetal
pole
Animal
pole
Filopodia
Archenteron
Archenteron
Future ectoderm
Future endoderm
Future mesoderm

Figure 36.18a-4
Mesenchyme
cells
Vegetal plate
Blastocoel
Blastocoel
Blastopore
Vegetal
pole
Animal
pole
Filopodia
Archenteron
ArchenteronEctoderm
Mouth
Digestive tube (endoderm)
Anus (from blastopore)
Future ectoderm
Future endoderm
Future mesoderm
Mesenchyme
(mesoderm forms
future skeleton)

Figure 36.18b
Blastocoel
Blastopore
Mesenchyme
cells
Filopodia
Archenteron
50 µm

 Cell movements and interactions that form the germ
layers vary among species
 One distinction is whether the mouth develops at the
first opening that forms in the embryo (protostomes)
or the second (deuterostomes)
 Sea urchins and other echinoderms are
deuterostomes, as are chordates

 Each germ layer contributes to a distinct set of
structures in the adult animal
 Some organs and many organ systems derive from
more than one germ layer

Figure 36.19
ECTODERM (outer layer of embryo)
ENDODERM (inner layer of embryo)
MESODERM (middle layer of embryo)
• Epithelial lining of digestive tract and associated organs
(liver, pancreas)
• Epithelial lining of respiratory, excretory, and reproductive tracts
and ducts
• Thymus, thyroid, and parathyroid glands
• Skeletal and muscular systems
• Circulatory and lymphatic systems
• Excretory and reproductive systems (except germ cells)
• Dermis of skin
• Adrenal cortex
• Epidermis of skin and its derivatives (including sweat glands,
hair follicles)
• Nervous and sensory systems
• Pituitary gland, adrenal medulla
• Jaws and teeth
• Germ cells

Human Conception, Embryonic Development,
and Birth
 Conception, fertilization of an egg by a sperm,
occurs in the oviduct
 The resulting zygote begins cleavage about 24 hours
after fertilization and produces a blastocyst after
4 more days
 A few days later, the embryo implants into the
endometrium of the uterus

 The condition of carrying one or more embryos in
the uterus is called gestation, or pregnancy
 Human pregnancy averages 38 weeks from
fertilization
 Gestation varies among mammals, from 21 days in
many rodents to more than 600 days in elephants
 Human gestation is divided into three trimesters of
about three months each

Figure 36.20
Cleavage
Fertilization
Ovulation
Ovary
Uterus
Endometrium
Implantation
Cleavage
continues.
TrophoblastBlastocyst
Endometrium Inner
cell
mass
(a) From ovulation to implantation
(b) Implantation of blastocyst
1
2
3
4
5
Cavity

 During the first trimester, the embryo secretes
hormones that signal its presence and regulate the
mother’s reproductive system
 Human chorionic gonadotropin (hCG) acts to
maintain secretion of progesterone and estrogens by
the corpus luteum through the first few months of
pregnancy

 Occasionally an embryo splits during the first month
of development, resulting in identical or monozygotic
twins
 Fraternal, or dizygotic, twins arise when two eggs
are fertilized and implant as two genetically distinct
embryos
 Some embryos cannot complete development due to
chromosomal or other abnormalities
 These spontaneously abort, often before a woman is
aware of her pregnancy

 During its first 2 to 4 weeks, the embryo obtains
nutrients directly from the endometrium
 Meanwhile, the outer layer of the blastocyst, called
the trophoblast, mingles with the endometrium and
eventually forms the placenta
 Materials diffuse between maternal and embryonic
circulation to supply the embryo with nutrients,
immune protection, and metabolic waste disposal

 The first trimester is the main period of
organogenesis, development of the body organs
 All the major structures are present by 8 weeks, and
the embryo is called a fetus
Video: Ultrasound of Fetus

Figure 36.21
(a) 5 weeks (c) 20 weeks(b) 14 weeks

Figure 36.21a
(a) 5 weeks

Figure 36.21b
(b) 14 weeks

Figure 36.21c
(c) 20 weeks

 During the second trimester the fetus grows and is
very active
 The mother may feel fetal movements as early as
one month into the second trimester

 During the third trimester, the fetus grows and fills
the space within the embryonic membranes

 Childbirth begins with labor, a series of strong,
rhythmic contractions that push the fetus and
placenta out of the body
 Once labor begins, local regulators, prostaglandins
and hormones, induce and regulate further
contractions of the uterus

 Lactation, the production of milk, is an aspect of
maternal care unique to mammals
 Suckling stimulates the release of prolactin, which in
turn stimulates mammary glands to produce milk,
and the secretion of oxytocin, which triggers milk
release from the glands

Contraception
 Contraception, the deliberate prevention of
pregnancy, can be achieved in a number of ways
 Contraceptive methods fall into three categories
 Preventing development or release of eggs and sperm
 Keeping sperm and egg apart
 Preventing implantation of an embryo

 A health-care provider should be consulted for
complete information on the choice and risks of
contraception methods

Figure 36.22
Male Female
Method Event MethodEvent
Vasectomy
Abstinence
Condom
Coitus
interruptus
(very high
failure rate)
Tubal ligation
Abstinence
Female condom
Combination
birth control
pill (or injection,
patch, or
vaginal ring)
Spermicides;
diaphragm;
progestin alone
(as minipill
or injection)
Morning-after
pill; intrauterine
device (IUD)
Production of
sperm
Production of
primary oocytes
Sperm transport
down male
duct system
Oocyte
development
and ovulation
Sperm
deposited
in vagina
Capture of the
oocyte by the
oviduct
Sperm
movement
through female
reproductive
tract
Transport
of oocyte in
oviduct
Meeting of sperm and oocyte
in oviduct
Implantation of blastocyst
in endometrium
Union of sperm and egg

Figure 36.22a
Male Female
Vasectomy
Abstinence
Condom
Coitus
interruptus
(very high
failure rate)
Tubal ligation
Abstinence
Female condom
Combination
birth control
pill (or injection,
patch, or
vaginal ring)
Spermicides;
diaphragm;
progestin alone
(as minipill
or injection)
Production of
sperm
Production of
primary oocytes
Sperm transport
down male
duct system
Oocyte
development
and ovulation
Sperm
deposited
in vagina
Capture of the
oocyte by the
oviduct

Figure 36.22b
Morning-after
pill; intrauterine
device (IUD)
Sperm
movement
through female
reproductive
tract
Transport
of oocyte in
oviduct
Meeting of sperm and oocyte
in oviduct
Implantation of blastocyst
in endometrium
Union of sperm and egg
Male Female

 The rhythm method, or natural family planning, is
temporary abstinence when conception is most
likely; it has a pregnancy rate of 10–20%
 Coitus interruptus, the withdrawal of the penis before
ejaculation, is unreliable
 Barrier methods block fertilization with a pregnancy
rate of less than 10%
 A condom fits over the penis
 A diaphragm is inserted into the vagina before
intercourse

 Intrauterine devices (IUDs) are inserted into the
uterus and interfere with fertilization and implantation;
the pregnancy rate is less than 1%
 Female birth control pills are hormonal
contraceptives with a pregnancy rate of less than 1%

Infertility and In Vitro Fertilization
 Infertility, the inability to conceive offspring, affects
about one in ten couples in the United States and
worldwide
 The causes are varied and equally likely to affect
men and women
 Sexually transmitted diseases (STDs) are the most
significant preventable causes of infertility

 Some forms of infertility are treatable
 Hormone therapy can sometimes increase sperm or
egg production
 In vitro fertilization (IVF) mixes oocytes with sperm
in culture dishes and returns the embryo to the uterus
at the eight-cell stage
 Sperm may be injected directly into an oocyte as well

Figure 36.UN01

Figure 36.UN02
Fertilized
egg
Polar body
Secondary
oocyte
Secondary
spermatocytes
Primary
oocyte
Primary
spermatocyte
Polar
body
Sperm
Spermatids
Spermatogenesis Oogenesis
Human gametogenesis
2n 2n
n
n
n
n
n n
n n n n
n n n n

Biology in Focus - Chapter 36

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Biology in Focus - Chapter 36

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