Transcript Fertilization

PowerLecture:
Chapter 17
Development and Aging
Learning Objectives



Describe early embryonic development and
distinguish each of the following: oogenesis,
fertilization, cleavage, gastrulation, and
organ formation.
Correlate the three germ layers—ectoderm,
mesoderm, and endoderm—with the tissues
that eventually form from each.
Outline the principal events of prenatal
development.
Learning Objectives (cont’d)


Describe some of the risks to the early
development of the fetus.
Describe the events of aging.
Impacts/Issues
Fertility Factors and
Mind-Boggling Births
Fertility Factors and Mind-Boggling Births

Multiple births are becoming more common;
twins, triplets, quads, and so on are usually
the result of the administration of fertility
drugs to the prospective mother.
Fertility Factors and Mind-Boggling Births
The rise in higher order
multiple births worries some
doctors.




The risk of miscarriage,
premature delivery, and delivery
complications is increased.
Multiples’ birth weights are lower
and mortality rates higher.
Parents face more physical,
emotional, and financial
burdens.
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 Should
we restrict the use of fertility drugs to
conditions that could limit the number of
embryos that form?


a. Yes, multiple pregnancies are too risky and
can lead to serious disabilities or death for
infants.
b. No, reproductive decisions belong to
individuals. There are other ways to reduce
multiple births.
Section 1
The Six Stages of Early
Development: An Overview
The Six Stages of Early Development
In the first three stages, gametes form, an
egg is fertilized, and cleavage occurs.




Development begins when gametes (sperm
and eggs) form and mature in the prospective
child’s parents.
Fertilization occurs when a sperm penetrates
an egg; after a series of steps, fertilization
produces a zygote.
Cleavage converts the zygote into a ball of cells
called a morula.
zygote after
first cleavage
beginning of
the ball of
cells called
a morula
Fig. 17.1, p. 314
The Six Stages of Early Development
•
•
The number of cells increases but not individual cell
size.
Each new cell (blastomere) contains a particular
portion of the egg’s cytoplasm, which will determine
its developmental fate.
In stage four, three primary tissues form.


Gastrulation lays out the organizational
framework for the body as the cells are
arranged into three primary germ layers.
The Six Stages of Early Development
•
•
•

Ectoderm is the outer layer; it gives rise to the
nervous system and the outer layers of the
integument.
Mesoderm is the middle layer; muscles as well as
organs of circulation, reproduction, excretion, and
the skeleton are derived from it.
Endoderm is the inner layer; it gives rise to the lining
of the digestive tube and organs derived from it.
Each layer will split into subgroups to give rise
to the body’s various tissues and organs.
The Six Stages of Early Development
In stages five and six, organs begin to form,
then grow and become specialized.



Organogenesis begins as germ layers
subdivide into populations of cells destined to
become organs and tissues that are unique in
structure and function.
Growth and tissue specialization allow
organs to grow and acquire functional
capabilities.
The Six Stages of Early Development

During the first several weeks of development
three key processes are at work:
•
•
•
During cell determination, the eventual
developmental path is established.
In cell differentiation, cells come to have specific
structures, products, and functions associated with a
specific purpose in the body.
Morphogenesis is the organization of differentiated
cells into tissues and organs by means of localized
cell division, movements of tissues, folding, and the
like.
Gamete
Formation
top view
a Eggs form and mature
in female reproductive
organs. Sperm form
and mature in male
reproductive organs.
Fertilization
b A sperm and an
egg fuse at their
plasma membrane.
Then the nucleus of
one fuses with the
nucleus of the other
to form the zygote
Cleavage
c Cell divisions
carve up different
regions of egg
cytoplasm for
daughter cells.
Gastrulation
d Cell divisions,
migrations, and
rearrangements
produce two or
three primary
tissues, the start of
specialized tissues
and organs.
Organ
Formation
e Subpopulations
of cells are
sculpted into
specialized organs
and tissues in
spatial patterns at
prescribed times.
Growth, Tissue
Specialization
f Organs
increase in
size and
gradually
assume their
specialized
functions.
Fig. 17.2, p. 315
Gamete
Formation
top view
a Eggs form and mature in
female reproductive
organs. Sperm form
and mature in male
reproductive organs.
Fertilization
b A sperm and an
egg fuse at their
plasma membrane.
Then the nucleus of
one fuses with the
nucleus of the other
to form the zygote
Cleavage
c Cell divisions
carve up different
regions of egg
cytoplasm for
daughter cells.
Gastrulation
d Cell divisions,
migrations, and
rearrangements
produce two or three
primary tissues, the
start of specialized
tissues and organs.
Organ
Formation
e Subpopulations of
cells are sculpted
into specialized
organs and tissues
in spatial patterns at
prescribed times.
Growth, Tissue
Specialization
f Organs
increase in size
and gradually
assume their
specialized
functions.
Stepped Art
Fig. 17.2, p. 315
Animation: Stages of Frog Development
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TO PLAY
Section 2
The Beginnings of You—
Fertilization to Implantation
The Beginnings of You –
Fertilization to Implantation
Fertilization unites sperm and oocyte.
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
Of the millions of sperm deposited in the vagina
during coitus, only a few hundred ever reach
the upper region of the oviduct where
fertilization occurs.
•
•
The acrosome of the sperm becomes structurally
unstable in a process called capacitation.
Many sperm will bind to the zona pellucida of the
egg.
The Beginnings of You –
Fertilization to Implantation

Only one sperm will successfully enter the
cytoplasm of the secondary oocyte because of
changes to the egg’s membrane that prevent
entry by additional sperm.
•
•
The arrival of that sperm inside stimulates the
completion of meiosis II in the secondary oocyte,
which yields a mature ovum and a second polar
body.
The sperm nucleus fuses with the egg nucleus to
restore the human diploid chromosome number of
46.
Animation: Fertilization
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TO PLAY
FERTILIZATION
oviduct
ovary
follicle cell
uterus
OVULATION
opening of cervix
egg nucleus
zona
pellucida
a
vagina
sperm
enter
vagina
b
nuclei
fuse
fusion of
sperm
nucleus with
egg nucleus
c
d
Fig. 17.3a-d, p. 316
The Beginnings of You –
Fertilization to Implantation

Cleavage produces a multicellular embryo.



Repeated divisions of the zygote produce the morula;
the cells are not necessarily larger but differ in size,
shape, and activity.
When the morula reaches the uterus, it transforms into
a blastocyst, consisting of a surface layer of cells—the
trophoblast—and an inner cell mass, from which the
embryo develops.
Identical twins are the result of a separation of the two
cells produced by the first cleavage; fraternal twins are
not identical because they are the result of two separate
fertilizations.
The Beginnings of You –
Fertilization to Implantation
Implantation gives a foothold in the uterus.
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
Implantation into the wall of the uterus takes
place about a week after fertilization.
•
•
The blastocyst contacts and invades the
endometrium; eventually the endometrium will
close over it.
Sometimes an ectopic (tubal)
pregnancy occurs; this is where the
fertilized egg implants outside of the
uterus, often in the oviduct, and must
be surgically removed.
Figure 17.25
The Beginnings of You –
Fertilization to Implantation

The implanted embryo releases HCG (human
chorionic gonadotropin), which stimulates the
corpus luteum to continue secreting estrogen
and progesterone to maintain the uterine lining;
the presence of HCG in the mother’s urine is
the basis for home pregnancy tests.
Animation: Cleavage and Implantation
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TO PLAY
trophoblast (surface
layer of cells of the
blastocyst)
endometrium
fertilization
implantation
endometrium
blastocoel
inner cell mass
fluid
uterine
cavity
a Days 1-2
The first cleavage
furrow extends
between the two
polar bodies.
b Day 3
After the third
cleavage,
cells form a
compact ball
inner cell
mass
c Day 4
d Day 5
e Days 6-7
By 96 hours
there is a ball of 16
to 32 cells. This is the
morula. Cells of the
surface layer will
function in implantation
and will give rise to a
membrane, the chorion.
A fluid-filled
cavity forms in the morula.
By the 32-cell stage,
differentiation is occurring
in an inner cell mass that
will give rise to the embryo.
This embryonic stage is
the blastocyst.
Some of
the blastocyst’s
surface cells attach
themselves to the
endometrium and
start to burrow into
it. Implantation has
started.
Fig. 17.4, p. 317
trophoblast (surface
layer of cells of the
blastocyst)
endometrium
fertilization
implantation
endometrium
blastocoel
inner cell mass
fluid
uterine
cavity
a Days 1-2
b Day 3
c Day 4
d Day 5
inner cell
mass
e Days 6-7
Stepped Art
Fig. 17.4, p. 317
Section 3
How the Early Embryo
Takes Shape
How the Early Embryo Takes Shape
First, the basic body plan is established.
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By the time of implantation, the inner cell mass
has transformed into a pancake-shaped
embryonic disk.
Gastrulation rearranges the cells into the three
germ layers and the primitive streak;
ectoderm thickens around the streak to
establish the neural tube and notochord,
which eventually forms the brain, spinal cord,
and vertebral column.
gut cavity
epidermis
peritoneum
lined body cavity (coelom);
lining also holds internal
organs in place
Fig. 17.5a, p. 318
How the Early Embryo Takes Shape

By week three, blocks of mesoderm called
somites form and will give rise to connective
tissues, bones, and muscles; pharyngeal
arches (face, neck, and associated parts) and
the coelom (body cavity) also begin to form.
Animation: Weeks 3-4 of Development
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yolk sac
chorionic cavity
embryonic
disk amniotic primitive
cavity
streak
neural
tube
a DAY 15. A primitive
streak appears along
the axis of the
embryonic disk. This
thickened band of
cells marks the onset
of gastrulation.
future brain
pharyngeal
arches
somites
b DAYS 19-23. Cell migrations, tissue
folding, and other morphogenic events
lead to the formation of a hollow
neural tube and to somites (bumps of
mesoderm). The neural tube gives rise
to the brain and spinal cord. Somites
give rise to most of the axial skeleton,
skeletal muscles, and much of the
dermis.
c DAYS 24-25. By now,
some cells have given
rise to pharyngeal
arches, which
contribute to the face,
neck, mouth, nasal
cavities, larynx, and
pharynx.
Fig. 17.5b, p. 318
How the Early Embryo Takes Shape
Next, organs develop and take on the
proper shape and proportions.
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
Neurulation is the first stage in the
development of the nervous system.
•
•
Ectodermal cells at the midline of the embryo
elongate to form a neural plate.
Cells of the neural plate fold over and meet at the
midline to form a neural tube that will eventually form
the spinal cord and brain.
How the Early Embryo Takes Shape

The folding of sheets of cells is an
important part of morphogenesis.
•
•
Cells migrate from one place to another
by sending out pseudopods that guide
them along prescribed routes using
adhesive and chemical cues.
Body parts are sculpted by apoptosis,
a mechanism of genetically
programmed cell death.
Figures 17.6b and 17.7
Animation: Neural Tube Formation
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TO PLAY
Animation: Formation of Human Fingers
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ectoderm at gastrula stage
“climbing”
nerve cell
neural plate formation
b
a
neural tube
Fig. 17.6, p. 319
Section 4
Vital Membranes
Outside the Embryo
Vital Membranes Outside the Embryo
Four extraembryonic membranes form.
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
The inner cell mass becomes the embryonic
disk; some cells will give rise to the embryo,
others to the extraembryonic membranes.
•
•
The yolk sac gives rise to the digestive tube and is a
source of early blood cells.
The amnion is a fluid-filled sac that keeps the
embryo from drying out and acts as a shock
absorber; the fluid is amniotic fluid.
Vital Membranes Outside the Embryo
•
•
The allantois gives rise to the blood vessels that will
become enclosed in the umbilical cord, linking the
embryo to the placenta.
The chorion, a protective membrane around the
embryo, secretes HCG to maintain the uterine lining
after implantation.
Animation: First Two Weeks
of Development
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TO PLAY
start of
amniotic
cavity
start of
embryonic
disk
blood-filled spaces
chorionic
cavity
chorionic
villi
chorion
amniotic
cavity
yolk sac
start of
yolk sac
a DAYS 10-11. The yolk
sac, embryonic disk, and
amniotic cavity have
started to form from parts
of the blastocyst.
start of
chorionic cavity
b DAY. 12 Blood
filled spaces form
in maternal tissue.
The chorionic
cavity starts to
form.
connecting
stalk
c Day 14 A connecting stalk has
formed between the embryonic
disk and chorion. Chorionic villi
which will be features of a
placenta start to form.
Fig. 17.8, p. 320
Vital Membranes Outside the Embryo
The placenta is a pipeline for oxygen,
nutrients, and other substances.
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
The placenta is a combination of endometrial
tissue and embryonic chorion.
•
•
The maternal tissue consists of tissues rich in
arterioles and venules.
The embryo’s chorion extends into the maternal
tissue as tiny chorionic villi.
Vital Membranes Outside the Embryo

Materials are exchanged between the blood
capillaries of mother and fetus where these
vessels associate in the blood-filled spaces of
the endometrium; exchange is by diffusion.
•
•
Maternal and fetal bloods do not mix.
Harmful substances, such as alcohol, caffeine,
drugs, and even infectious agents such as HIV
can also cross the placenta.
4 weeks
MATERNAL
CIRCULATION
8 weeks
12 weeks
mother’s
blood
vessels
blood
passes to
and from
mother’s
vessels
FETAL
CIRCULATION
embryonic blood
vessels
umbilical cord
space between
chorionic villi
chorionic villus
appearance of the
placenta at full
term
tissues
of uterus
AMNIOTIC FLUID
fused amniotic and
chorionic membranes
Fig. 17.9 (1), p. 321
Animation: Structure of the Placenta
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Section 5
The First Eight Weeks—
Human Features
Emerge
The First Eight Weeks –
Human Features Emerge

The embryonic stage ends as the eighth
week draws to a close; by this time
morphogenesis has begun to form the
features that show us to be human.
Figure 17.10
Animation: Fetal Development
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TO PLAY
WEEK 4
yolk sac
future lens
forebrain
developing
heart
a
lower limb bud
tail
embryo
connecting stalk
pharyngeal
arches
upper limb bud
somites
neural tube
forming
Fig. 17.10a, p. 322
WEEKS 5–6
head growth exceeds
growth of other regions
retinal pigment
future external ear
upper limb differentiation
(hand plates develop,
then digital rays of
future fingers;wrist,
elbow start forming)
umbilical cord forms
between weeks 4 and 8
(amnion expands, forms
tube that encloses the
connecting stalk and a
duct for blood vessels)
b
foot plate
Fig. 17.10b, p. 322
WEEK 8
final week of embryonic
period; embryo looks
distinctly human
compared to other
vertebrate embryos
upper and lower limbs
well formed; fingers
and then toes have
separated
early tissues of all
internal, external
structures now
developed
tail has become
stubby
Fig. 17.10c, p. 322
The First Eight Weeks –
Human Features Emerge
Gonad development begins by the second
half of the first trimester.



An embryo with a Y chromosome will have a
sex-determining region on the chromosome
that triggers the development of testes; testes
will produce male hormones that will influence
further sex differentiation.
An XX embryo will become a female because
of the absence of testosterone; no other
hormones are necessary at this point.
Y chromosome 7 weeks
present
Y chromosome
absent
10 weeks
penis
vaginal
opening
birth approaching
birth approaching
Fig. 17.11, p. 323
Y chromosome
present
7 weeks
Y chromosome
absent
10 weeks
penis
vaginal
opening
birth approaching
birth approaching
Stepped Art
Fig. 17.11, p. 323
The First Eight Weeks –
Human Features Emerge
At the end of eight weeks of development,
the embryo is designated a fetus; a heart
monitor at this point can detect the fetal
heartbeat.
Miscarriage is the spontaneous expulsion
of an embryo or fetus.




This occurs in about 20% of all conceptions,
usually during the first trimester.
More than half of all spontaneous abortions
occur because of genetic disorders in the
embryo/fetus.
Section 6
Development
of the Fetus
Development of the Fetus
In the second trimester movements begin.




The second trimester encompasses the fourth
through sixth months.
Fuzzy hair (lanugo) and a cheesy coating
(vernix caseosa) cover the body.
The sucking reflex is evident, as is movement
of the arms and legs; the fetus is about 4-5
inches long at this point.
Development of the Fetus
Organ systems mature during the third
trimester.



The third trimester extends from month seven
until birth; the earliest delivery in which survival
on its own is possible is the middle of this
trimester.
Babies born before seven months’ gestation
often suffer from respiratory distress
syndrome.
Figure 17.12
WEEK 16
Length: 16 cm
(6.4 inches)
Weight: 200 gm
(7 ounces)
WEEK 29
placenta
Length: 27.5 centimeters
(11 inches)
Weight: 1,300 grams
(46 ounces)
WEEK 38 (full term)
placenta
Length: 50 centimeters
(20 inches)
Weight: 3,400 grams
(7.5 pounds)
Fig. 17.12(1), p. 324
Development of the Fetus
The blood and circulatory system of a fetus
have special features.




Deoxygenated blood is carried from the fetus to
the placenta in two umbilical arteries;
oxygenated blood is returned to the fetus via
the umbilical vein.
The lungs are bypassed due to the foramen
ovale and the ductus arteriosus.
The ductus venosus allows blood to proceed
directly from the placenta to the heart,
bypassing the liver.
arterial duct (ductus arteriosus)
aorta
pulmonary
vessels
superior
vena cava
foramen
ovale
heart
liver
umbilical
vein
umbilical
cord
venous duct
(ductus venous)
inferior vena cava
allantois
umbilical arteries
placenta
urinary bladder
Fig. 17.13a, p. 325
ligament
pulmonary artery
closed
foramen
ovale
(fossa
ovalis)
hepatic vein
pulmonary veins
ligament
umbilicus (navel)
umbilical ligaments
hepatic portal vein
serving the liver
degenerated allantois
(urinary bladder)
Fig. 17.13b, p. 325
Section 7
Birth and Beyond
Birth and Beyond
Hormones trigger birth.




Birth (parturition) usually takes place about 39
weeks after fertilization.
The process of “labor” begins when the smooth
muscles of the uterus begin to contract,
stimulated by the hormones oxytocin and
prostaglandin.
Labor has three stages.
Birth and Beyond



In the first stage, contractions of the uterine
muscles push the fetus against the cervix; the
cervical canal dilates to about 10 centimeters,
and the amniotic sac ruptures.
In the second stage, birth occurs and the fetus
is forcefully expelled from the uterus because of
contractions and the mother’s urge to push; the
baby usually comes out head first (bottom first
is called breech position).
The third stage occurs after birth; continued
contractions force fluid, blood, and the placenta
(afterbirth) from the mother’s body and the
umbilical cord is severed.
placenta
uterus
detaching
placenta
umbilical
cord
umbilical
cord
dilating
cervix
a
b
c
Fig. 17.14, p. 326
Animation: Birth
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Birth and Beyond
Hormones also control milk production in a
mother’s mammary glands.



The mammary glands produce a special fluid
(colostrum) for the newborn for the first few
days; then, under the influence of prolactin, milk
production (lactation) occurs.
Suckling by the baby stimulates the pituitary to
release oxytocin, which in turn forces milk into
the ducts of the breast tissue in a positive
feedback circuit.
milk-producing
mammary gland
nipple
adipose
tissue
(a) Breast anatomy.
milk duct
(b) Breast of lactating female.
Fig. 17.15, p. 327
Animation: Anatomy of the Breast
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Section 8
Potential Disorders of
Early Development
Potential Disorders of Early Development
Good maternal nutrition is vital.




Maternal diet, especially vitamins and minerals,
is important throughout pregnancy for the
proper development of the fetal tissues.
Folic acid (folate) is vital for preventing spina
bifida, a condition where the neural tube does
not form properly and the baby is born with an
exposed spine.
Severe restriction of the maternal diet can
result in underweight babies; a pregnant
woman should expect to gain between 20 and
35 pounds, on average, during pregnancy.
Potential Disorders of Early Development
Infections present risks.



Risk of infection in the fetus is
minimized by maternal antibodies
that cross over into the fetal blood.
However, viral diseases in the
mother (such as rubella, or
German measles) can cause fetal
malformations; such agents act as
teratogens.
Potential Disorders of Early Development
Prescription drugs can harm.



Thalidomide can cause limb deformities;
retinoic acid, such as is found in anti-acne
creams, increases the risk of facial and cranial
deformities.
Antibiotics can be a problem also: tetracycline
causes yellowed teeth, and streptomycin
causes hearing problems.
Potential Disorders of Early Development
Alcohol and other drugs can also harm.


Alcohol can cross the
placenta and cause
many effects collectively
known as fetal alcohol
syndrome (FAS), which
is one of the most common
causes of mental retardation
in the U.S.; children with
FAS never catch up,
physically or mentally.
Figure 17.18
Potential Disorders of Early Development


Cocaine, especially crack cocaine, prevents a
child’s nervous system from developing
normally; affected children are chronically
irritable and small for their chronological age.
Cigarette smoking can cause miscarriage,
stillbirth, and premature delivery; long term
studies show that toxic substances build up in
the fetuses of nonsmokers who are exposed to
second-hand smoke.
Animation: Sensitivity to Teratogens
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Sensitivity to Teratogens
Figure 17.17
Video: Mermaid Baby
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TO PLAY

From ABC News, Human Biology in the Headlines, 2006 DVD.
Section 9
Prenatal Diagnosis:
Detecting Birth Defects
Prenatal Diagnosis:
Detecting Birth Defects
Medical technology now allows us to detect
more than 100 genetic disorders before
birth.


Amniocentesis samples the fluid within the
amnion surrounding the fetus to retrieve
sloughed off cells, which can be analyzed for
genetic abnormalities.
Fig. 17.19a, p. 330
Removal of about
20ml of amniotic
fluid containing
suspended cells
that were sloughed
off from the fetus
A few biochemical
analyses with some
of the amniotic fluid
Centrifugation
Quick determination of
fetal sex and analysis
of purified DNA
Fetal cells
Biochemical analyses
for the presence of
genes that cause many
different metabolic
disorders
Growth for
weeks in
culture
medium
Additional analysis
Fig. 17.19b, p. 330
Animation: Amniocentesis
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Prenatal Diagnosis:
Detecting Birth Defects


Chorionic villus sampling (CVS) carefully
harvests tissue from the placenta for cell
analysis.
In preimplantation diagnosis, an embryo
conceived by IVF is analyzed for genetic
defects before it is implanted into the uterus to
begin gestation.
Prenatal Diagnosis:
Detecting Birth Defects


Fetoscopy allows
direct visualization
of the developing
fetus using a
fiber-optic device.
All of these
procedures carry
risks to the unborn
fetus.
Figure 17.20
Video: Pre-implantation Genetics
 This
video clip is available in CNN Today
Videos for Genetics, 2005, Volume VII.
Instructors, contact your local sales
representative to order this volume, while
supplies last.
Section 10
From Birth to Adulthood
From Birth to Adulthood
There are many transitions from birth to
adulthood.



Prenatal development occurs before birth; a
newborn is referred to as a neonate.
The stages of postnatal development are:
neonate (first two weeks) >>> infant (two weeks
to 15 months) >>> child (to 12 years) >>>
pubescent (individual at puberty) >>>
adolescent (from puberty to 3–4 years later)
>>> adult >>> old age.
From Birth to Adulthood

Certain of these stages are characterized by
more noticeable changes such as the growth
spurt and the reproductive changes of puberty.
Figure 17.21
Animation: Proportional Changes
During Development
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From Birth to Adulthood
Adulthood is also a time of bodily change.
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Aging (senescence) is the progressive cellular
and bodily deterioration built into the life cycle
of all organisms.
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Beginning around age 40 there is a gradual decline
in bone and muscle mass, increased skin wrinkling,
and more fat deposition.
Metabolic rates decline, reflexes become slower, and
reduced collagen contents make tissues all over the
body less elastic.
The definitive causes of aging are not known.
Stages of Human
Development
Section 11
Time’s Toll:
Everybody Ages
Time’s Toll: Everybody Ages
Aging is the gradual loss of vitality as body
functions become less and less efficient.
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Skin begins to noticeably wrinkle and sag; body
fat accumulates; injuries
are more frequent and
harder to heal.
In the connective tissues,
more crosslinks form in
the collagen, making it less pliable.
Figure 17.22
Time’s Toll: Everybody Ages
Genes may determine the maximum human
life span.
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Cells may have some internal, biological clock
with a predetermined life span.
Support for this idea comes from our
knowledge of telomeres, which cap the ends of
chromosomes; at each cell division a small bit
of telomere is lost until none is left and cell
division is no longer possible.
Time’s Toll: Everybody Ages
Cumulative damage to DNA may also play a
role in aging.
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A “cumulative assaults” hypothesis suggests
that aging results from mounting damage to
DNA combined with a lack of DNA repair.
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Free radicals of oxygen could cause damage to
proteins and mitochondrial DNA.
There may be a decline in the ability of cells to repair
DNA.
Aging may ultimately be due to a wide range of
controlling factors.
Section 12
Aging Skin, Muscle,
Bones, and
Reproductive Systems
Aging Skin, Muscle, Bones, and
Reproductive Systems
Changes in connective tissue affect skin,
muscles, and bones.
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Changes in the skin include: slower
replacement of epidermis; elastin fibers are
replaced with more rigid collagen; fewer oil and
sweat glands are present, resulting in drier skin;
and loss of hair pigment.
Changes in muscle include: loss of mass and
strength; muscle replacement by fat.
Aging Skin, Muscle, Bones, and
Reproductive Systems
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Changes in the skeleton are
also seen: bones become
weaker, more porous, and
brittle due to loss of calcium;
intervertebral disks deteriorate,
leading to loss of height; joints
deteriorate from wear and tear.
Reproductive systems and sexuality
change.
Figure 17.23
Aging Skin, Muscle, Bones, and
Reproductive Systems
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Falling secretions of estrogen and progesterone
trigger menopause in women, whereas
declining testosterone in men causes reduced
fertility.
Because the effects of declining hormones may
be more troublesome in women, hormone
replacement therapy (HRT) may be
recommended.
Despite declines in hormones and other
potential problems, men and women both retain
their capacity for sexual response well into old
age.
Section 13
Age-Related Changes in
Some Other Body
Systems
Age-Related Changes in Some Other
Body Systems
The nervous system and senses decline.
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Neurons are generally not replaced when they
die, regardless of age.
Neurofibrillary tangles may
form inside the neurons, and
beta amyloid plagues may
form between neurons;
both of these are present in
people with Alzheimer’s
disease (AD).
Figure 17.24a-b
Age-Related Changes in Some Other
Body Systems
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AD manifests with progressive memory
loss and disruptive personality changes.
Low levels of acetylcholine and chronic
inflammation of brain tissue may also be
part of the cause of AD.
No effective treatment for AD currently
exists.
Persons who inherit one version of a gene that
codes for apolipoprotein E are at significantly
higher risk for Alzheimer’s disease.
All of us will experience some short-term
memory loss as we age, as well as less efficient
responses to many stimuli.
Figure 17.24c
Age-Related Changes in Some Other
Body Systems
The cardiovascular and respiratory systems
deteriorate.
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Changes to the respiratory system are mainly
due to the breakdown of the alveoli, resulting in
less respiratory surface.
Changes in the cardiovascular system include
reduction in heart pumping capacity, stiffening
of blood vessels, and deposition of plaque in
the vessels.
The combined effect of deterioration of these
systems is less efficient blood transport.
Age-Related Changes in Some Other
Body Systems
The immune, digestive, and urinary systems
become less efficient.
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The numbers of T cells drops, B cells become
less active, and autoimmune diseases can
occur, possibly due to mutations in selfmarkers.
Fewer digestive enzymes are produced in the
intestines and basal metabolic rate falls,
resulting in weight gain if not compensated for
by changes to diet and exercise.
Urinary incontinence may also occur,
particularly in women who have borne children.