Chapter 22: Development and
Aging
22-1
Early Developmental Stages
Fertilization occurs when the sperm and
egg interact to produce a zygote.
During fertilization, the acrosome of a
sperm releases enzymes that digest a
hole in the corona radiata, then in the
zona pellucida around the egg.
Several sperm penetrate the corona
radiata, several attempt to penetrate the
zona pellucida, but only one sperm
enters the egg.
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Depolarization of the egg’s plasma
membrane after the sperm touches
the egg and separation of the zona
pellucida prevent a second sperm
from fertilizing the egg.
The sperm enters the egg and the
sperm nucleus fuses with the egg
nucleus.
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Fertilization
22-4
Embryonic Development
Development is all the changes that
occur during the life cycle of an
organism.
The embryo is the first stage in human
development.
Following fertilization, the zygote
undergoes cleavage, a period of cell
division without growth.
Cleavage leads to a ball of cells called
the morula.
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The morula becomes a blastula when an
internal cavity, the blastocoel, appears.
At the gastrula stage, invagination of
cells into the blastocoel results in
formation of the germ layers: ectoderm,
mesoderm, and endoderm; mesoderm
arises from pouches in endoderm.
Two layers of mesoderm form, and the
space between them becomes the
coelom.
The three germ layers will have different
developmental fates.
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Lancelet early development
22-7
The Effect of Yolk
Yolk is a dense nutrient material found in
various amounts in the eggs of
animals.
The amount of yolk affects the process of
gastrulation, the formation of the three
germ layers.
The presence of yolk causes cells to
cleave more slowly.
The aquatic frog and lancelet need less
yolk as they develop rapidly.
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A chick egg has so much yolk that the
embryo lies flat and endoderm formation
does not occur by invagination.
Instead an upper layer of cells becomes
ectoderm, and a lower layer becomes
endoderm; mesoderm invaginates
between the two layers, and the furrow
that develops is called a primitive streak.
Because of a shared evolutionary history,
gastrulation in humans is like that of the
chick even though the human egg has
little yolk.
22-9
Comparative animal
development
22-10
Neurulation and the Nervous
System
The notochord forms from mesoderm.
During neurulation, the nervous system
develops from midline ectoderm, just
above the notochord; the notochord
induces formation of the nervous system.
A neural plate is seen first, then a neural
tube; the anterior neural tube becomes
the brain.
At the neurula stage, cross sections of all
chordate embryos appear similar.
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Development of neural tube and
coelom in a frog embryo
22-12
Chordate embryo, cross section
22-13
Developmental Processes
Development requires growth, cellular
differentiation, and morphogenesis.
Cells differentiate when they become
specialized in structure an function.
Morphogenesis produces shape and form.
There is no loss of genes during
development; each cell remains
totipotent, containing all instructions for
any other specialization.
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Cytoplasmic Segregation
The cytoplasm of an egg is not uniform
but contains maternal determinants
that are parceled out during mitosis.
Cytoplasmic segregation helps
determine how the various cells of the
morula will develop.
The gray crescent in a frog’s egg is
required for an embryo to develop.
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Cytoplasmic segregation
22-16
Cytoplasmic influence on
development
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Induction
Induction occurs when embryonic cells
influence one another to develop in a
particular way.
A molecular concentration gradient may
act as a chemical signal to induce germ
layer differentiation.
The presumptive (potential) notochord
tissue induces the formation of the
nervous system.
The vertebrate eye likewise forms by a
series of inductions.
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Control of nervous system
development
22-20
22-21
Model Organisms
The Roundworm Experiments
The roundworm Caenorhabditis elegans
develops into an adult of 959 cells;
researchers have traced every cell
division from the first fertilized egg and
developed a fate map.
Work with the roundworm shows that
induction involves signals that activate
new genes that provide new signals, and
that induction requires the regulation of
genes in a particular sequence.
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Programmed cell death (apoptosis) plays
a role during development.
A good example is the development of
fingers and toes in humans due to
death of cells between the digits.
The fate maps of C. elegans indicate that
apoptosis occurs in 131 cells as
development takes place.
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Development of C. elegans, a
small worm
22-24
The Fruit Fly Experiments
Research with fruit flies has shown how
morphogenesis comes about; that
morphogen genes determine the
pattern of an animals and its parts.
Each morphogen gene codes for a
protein that is present in a gradient.
Homeotic genes control the organization
of differentiated cells into specific
three-dimensional structures.
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Pattern formation in the fruit fly
22-26
Homeotic genes form protein gradients
that determine if a segment will bear
antennae or legs or wings.
The same sequence of genes is found in
many organisms; the same sequence
of nucleotides is a homeobox.
A homeobox codes for a sequence of 60
amino acids called a homeodomain.
Homeodomain proteins bind to DNA and
determine which genes are turned on.
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22-28
Homeotic mutations
22-29
Human Embryonic and Fetal
Development
Nine months of human development is
divided into the embryonic period (first
2 months) and the fetal period (months
3–9).
A human embryo is surrounded by four
extraembryonic membranes:
1) The amnion envelops the embryo/fetus
in a protective amniotic fluid.
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2) The yolk sac is the first site of red
blood cell formation.
3) The blood vessels of the allantois
become the umbilical blood vessels.
4) The chorion contributes to the
placenta.
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Extraembryonic membranes
22-32
Embryonic Development
The First Week
Fertilization occurs in the upper third of
the oviduct; a zygote is produced.
The embryo is ball of cells called a
morula when it reaches the uterus on
the third day.
By the fifth day, the morula is
transformed into a blastocyst which
consists of an outer trophoblast and an
inner cell mass.
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Human development before
implantation
22-34
The Second Week
The embryo begins to implant in the
uterine lining at end of first week.
The trophoblast secretes human chorionic
gonadotropin (HCG), a hormone that
maintains the corpus luteum.
The yolk sac and amnion form.
Gastrulation occurs and the inner cell
mass becomes the embryonic disk while
the trophoblast becomes the chorion.
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Human embryonic development
22-36
The Third Week
Neurulation occurs and the nervous
system is the first visible organ system.
The heart begins to form and pump blood
when right and left heart tubes fuse.
The Fourth and Fifth Weeks
The allantois forms and is contained
within the umbilical cord.
Limb buds appear and sense organs
develop.
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Human embryo at beginning of
fifth week
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The Sixth Through Eighth Weeks
By end of eight weeks, the embryo is
only 38 mm (1.5 inches) long but is
easily recognized as human.
All organ systems are established, even
though the embryo weighs no more
than an aspirin tablet at this point.
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Fetal Development and Birth
The Third and Fourth Months
During the third and fourth months, the
body increases in size, and epidermal
refinements (eyelashes, nipples)
become apparent.
Bone is replacing cartilage.
It is now possible to distinguish males
from females, and the heartbeat is
audible with a stethoscope.
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The three-to four-month-old
fetus looks human
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The Fifth Through Seventh Months
The thin skin is covered with lanugo and
coated with a vernix caseosa.
The eyelids open fully.
At the end of seven months, the fetus can
possibly survive if born prematurely.
The fetus is now 300 mm (12 inches) in
length and weighs 1,380 grams (3 lb).
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Fetal Circulation
Blood passes from the right to the left
atrium through an oval opening, the
foramen ovale, and an arterial duct, the
ductus arteriosus, shunts blood between
the pulmonary trunk and aorta.
These features enable blood to bypass the
non-funtioning lungs.
Two umbilical arteries that branch off the
iliac arteries lead to the placenta.
22-44
One umbilical vein takes nutrients to the
systemic system when the umbilical
vein joins the vena cava by a venous
duct.
If the oval opening fails to close, it
causes a “blue baby” that receives a
mixture of oxygenated and
unoxygenated blood.
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Fetal circulation and the placenta
22-46
The Structure and Function of
the Placenta
Chorionic villi project into maternal
tissue as the placenta develops.
By the tenth week, the placenta is fully
formed and secretes estrogen and
progesterone that maintains the lining
and prevents further menstrual cycling
and ovulation.
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Fetal and maternal blood cells do not mix
within the placenta.
Carbon dioxide and wastes diffuse from
the fetal to the maternal side, and
oxygen and nutrients diffuse from the
maternal to the fetal side.
Harmful chemicals can cross the
placenta and some alter normal fetal
development.
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Anatomy of the placenta in a
fetus at six to seven months
22-49
Birth
Stage 1
Prior to parturition (giving birth),
contractions of labor move the baby’s
head downward, causing effacement
and dilation of the cervix.
The amnion (bag of waters) breaks,
releasing amniotic fluid.
The cervix is dilated completely at the
end of this stage.
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Stage 2
Uterine contractions occur each 1–2
minutes and the mother experiences a
desire to push.
An episiotomy may be performed to
prevent tearing.
The baby is pushed out during this stage,
and the umbilical cord is cut and tied.
Stage 3
The afterbirth (placenta) is delivered.
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Three stages of parturition (birth)
22-52
Female Breasts and Lactation
The breast contains 15–25 lobules with
milk ducts.
No milk is produced during pregnancy,
but milk ducts and alveoli proliferate
during that time, and breasts enlarge.
Once the baby is delivered, the pituitary
secretes prolactin, and milk is
produced.
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Suckling of the baby at the breast
stimulates the hypothalamus to direct
the pituitary to secrete oxytocin that, in
turn, causes milk to flow.
Breast milk contains antibodies that
supplements the baby’s immature
immune system.
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Female breast anatomy
22-55
Human Development After Birth
Development continues throughout all
the stages of life: infancy, childhood,
adolescence, and adulthood.
Aging encompasses these progressive
changes.
The study of aging is gerontology; its
goal is to increase the health span.
The human life span extends a maximum
of 120-125 years.
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Aging
22-57
Theories of Aging
Genetic in Origin
Evidence suggests aging has a genetic
basis:
1) Cells of a species divide only a set
number of times.
2) As we grow older, it may be that more
cells age, become non-functional, or die
due to mutations.
3) In addition, offspring of long-lived people
also tend to be long-lived.
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Whole-Body Process
A second theory of aging suggests that a
hormonal decline can affect many
different organ systems.
Type II diabetes is due to cells lacking
receptors to take up insulin;
menopause is a similar failure by
ovaries to take up the folliclestimulating hormone.
The thymus gradually gets smaller with
age.
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The immune system no longer performs
as well, which is perhaps why cancer
and autoimmune diseases are more
prevalent in the elderly.
Aging may also be due to a tissue
change that affects all organs
throughout the body.
Collagen fibers become cross-linked
which leads to loss of elasticity
throughout many body organs.
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Extrinsic Factors
A third theory on aging suggests that
years of poor health habits contribute
most to aging.
Insufficient calcium intake and smoking
increase osteoporosis, for example.
Exercise and adequate servings of fruits
and vegetables help eliminate
cardiovascular disease.
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Effect of Age on Body Systems
Skin
Skin loses elasticity and becomes
thinner with age, resulting in sagging
and wrinkling.
Fewer sweat glands are present, so
temperature regulation is less efficient.
The number of oil glands is reduced, so
skin tends to crack.
Pigmented blotches appear on the skin.
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Processing and Transporting
Cardiovascular disorders are the leading
cause of death among the elderly; the
heart shrinks with age, and fatty
deposits clog arteries.
Lungs lose elasticity, so ventilation is
reduced.
A reduced blood supply to the kidneys
results in the kidneys becoming
smaller and less efficient.
The digestive tract may lose muscle tone
but still absorbs nutrients efficiently.
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Integration and Coordination
Normal aging results in the loss of few
nerve cells; short-term memory may
decline but overall cognitive skills
remain.
After age 50, there is a decline in the ability
to hear higher frequencies, and the lens
of the eye does not accommodate as
well.
Loss of skeletal muscle mass and bone
density is common but can be controlled
through exercise and adequate calcium
intake.
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The Reproductive System
Females undergo menopause and are no
longer reproductive.
In males, sperm production declines after
age 50 but continues until death.
Women as a group outlive men.
Conclusion
Good health habits, started when young,
slow the aging process and contribute
to a long, healthy life span.
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Remaining active
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Chapter Summary
The first stages of human embryonic
development lead to establishment of
embryonic germ layers.
Differentiation and morphogenesis are
two processes that occur when
specialized organs develop.
The roundworm and fruit fly have served
as model organisms to determine the
function of certain genes during
development.
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Extraembryonic membranes make life on
land possible.
Human embryonic and fetal development
is a series of steps that end at
childbirth.
Breasts develop mammary glands to
supply nutrients to the infant.
Research on the processes of aging may
identify underlying causes of
degeneration and prolong the health
span of individuals.
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