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Chapter 18
Lecture Outline
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Human Development
and Aging
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Points to ponder
• What is fertilization?
• Describe the steps in fertilization.
• What is cleavage? Growth? Morphogenesis?
Differentiation?
• What are the four extraembryonic membranes?
• Be familiar with what happens during preembryonic development, fetal development,
and development after birth.
• Follow the path of fetal circulation.
• What determines the sex of an individual? Be
sure to understand the three hormones
involved and the SRY gene.
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Points to ponder
• What are two conditions in which sex
determination is ambiguous, and two
conditions in which the sex organs do not
develop normally?
• What are the three stages of birth?
• What can you do to help prevent birth defects?
• What are the hypotheses of aging?
• What are the effects of aging on the body?
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18.1 Fertilization
Fertilization
• Fertilization is the union of the sperm and egg
to form a zygote.
• Egg is surrounded by an outer matrix called the
zona pellucida.
• Outside this matrix it has a few layers of
follicular cells collectively called the corona
radiata.
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18.1 Fertilization
Fertilization
• Steps of fertilization
1. Several sperm penetrate the corona radiata.
2. Acrosomal enzymes digest a portion of the
zona pellucida.
3. Sperm binds to and fuses with the egg’s
plasma membrane.
4. Sperm nucleus enters the egg.
5. Sperm and egg nuclei fuse.
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18.1 Fertilization
Fertilization
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microvilli of oocyte plasma membrane
tail
1. Sperm makes its
way through the
corona radiata.
middle
piece
2. Acrosomal enzymes
digest a portion of
zona pellucida.
3. Sperm binds to and
fuses with egg
plasma membrane.
corona
radiata
sperm
4.Sperm nucleus enters
cytoplasm of oocyte.
plasma
membrane
nucleus
5. Cortical granules
release enzymes;
zona pellucida
becomes fertilization
membrane.
head
acrosome
fertilization membrane
sperm pronucleus
cortical granule
6. Sperm and egg
pronuclei are enclosed
in a nuclear envelope.
egg plasma membrane
egg pronucleus
zona Pellucida
(top right): © David M. Phillips/Visuals Unlimited
Figure 18.1 The steps in the fertilization of an egg.
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18.1 Fertilization
What prevents more than one
sperm from entering?
• The egg’s plasma membrane changes to
prevent other sperm from binding.
• Vesicles within the egg release enzymes that
cause the zona pellucida to become
impenetrable and sperm cannot bind.
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18.2 Pre-Embryonic and Embryonic Development
What are the main processes of
development?
• Cleavage – cells undergo division without the
embryo increasing in size
• Growth – cells undergo division as well as
increase in size
• Morphogenesis – the embryo begins to take
shape as cells migrate
• Differentiation – when cells take on specific
structure and function (the nervous system is
the first visible system)
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18.2 Pre-Embryonic and Embryonic Development
What are the functions of the
extraembryonic membranes in humans?
• Chorion – fetal half of the placenta, the organ that
provides the embryo with nourishment and gets rid
of wastes
• Allantois – gives rise to the bladder and the blood
vessels of the umbilical cord that carry blood to and
from the fetus
• Yolk sac – contains many blood vessels and where
blood cells first form (there is little yolk in humans)
• Amnion – contains amniotic fluid that cushions and
protects the embryo
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18.2 Pre-Embryonic and Embryonic Development
Extraembryonic membranes
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extraembryonic
cavity
amnion
amnionic
cavity
chorion
allantois
umbilical
cord
yolk sac
Figure 18.3 The
extraembryonic
membranes.
maternal
blood vessels
developing
placenta
endometrium
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18.2 Pre-Embryonic and Embryonic Development
What are the stages of
development?
1.
Pre-embryonic development - 1st week of
development after fertilization
2.
Embryonic development – 2nd week after
fertilization until the end of the 2nd month
3.
Fetal development – the 3rd through the 9th months
of development
4.
Development after birth – stages of life including
infancy, childhood, adolescence, and adulthood
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18.2 Pre-Embryonic and Embryonic Development
1. Pre-embryonic development
•
Cleavage – cell division that increases the
number of cells
•
Morula – compact ball of embryonic cells
•
Early blastocyst – inner cell mass that
becomes the embryo, covered by a layer of
cells that becomes the chorion
•
Implantation – embryo embeds into the uterus
around day six
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18.2 Pre-Embryonic and Embryonic Development
1. Pre-embryonic development: Week 1
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egg nucleus
2. Fertilization
sperm nucleus
egg
zona pellucida
corona radiata
5. Early blastocyst
single cell=
zygote
1. Ovulation
fimbriae
inner cell
mass
ovary
Uterine tube
(oviduct)
2-cell
stage
3. Cleavage
6. Implantation
4-cell
stage
8-cell
stage
early chorion
4. Morula
Figure 18.2 The stages of pre-embryonic development.
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18.2 Pre-Embryonic and Embryonic Development
2. Embryonic development: Week 2
•
•
•
•
Pregnancy begins after implantation.
Human chorionic gonadotropin (HCG) is
secreted, maintaining the corpus luteum and
the endometrium.
HCG is the basis for a pregnancy test.
The inner cell mass detaches itself and
becomes the embryonic disk that will go
through gastrulation to become three primary
germ layers (ectoderm, mesoderm, and
endoderm).
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18.2 Pre-Embryonic and Embryonic Development
2. Embryonic development: Week 2
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amniotic cavity
embryonic disk
yolk sac
chorionic villi
chorion
a. 18 days
Figure 18.4a The stages of embryonic development.
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18.2 Pre-Embryonic and Embryonic Development
3 primary germ layers
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endoderm
mesoderm
amnion
amniotic cavity
notochord
ectoderm
yolk sac
Endoderm
Primary
Germ Layer
Ectoderm
(outer layer)
Mesoderm
(middle layer)
Ectoderm
Endoderm
(inner layer)
Figure 18.5 The embryonic germ layers.
Mesoderm
Human Adult Structures
Epidermis of skin; epithelial lining of oral
cavity and rectum; nervous system
Skeleton; muscular system; dermis of
skin; cardio vascular system; urinary
system; reproductive system; outer layers
of respiratory and digestive systems
Epithelial lining of digestive tract and
respiratory tract; associated glands of
these systems; epithelial lining of urinary bladder
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18.2 Pre-Embryonic and Embryonic Development
2. Embryonic development: Week 3
•
Nervous system
begins to develop.
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body stalk
•
The posterior neural
tube will become
the spinal cord and
brain.
amniotic cavity
allantois
embryo
yolk sac
chorionic villi
21 days
•
Development of the
heart begins.
Figure 18.4b The stages of embryonic development.
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18.2 Pre-Embryonic and Embryonic Development
2. Embryonic development: Weeks 4 and 5
•
4th week
– The embryo is slightly larger than the height of
the print in your book.
– Chorionic villi form.
– The umbilical cord forms.
– Limb buds form (later develop into legs and
arms).
– Hands and feet are apparent.
•
5th week
– The head enlarges.
– Eyes, ears, and nose become prominent.
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18.2 Pre-Embryonic and Embryonic Development
2. Embryonic development: Weeks 4 and 5
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chorion
amnion
amniotic cavity
allantois
yolk sac
chorionic villi
amniotic cavity
c. 25 days
chorion
digestive tract
chorionic villi
amnion
umbilical cord
d. 35+ days
Figure 18.4c-d The stages of embryonic development.
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18.2 Pre-Embryonic and Embryonic Development
The embryo at week 5
Figure 18.6 The human embryo after five weeks of development.
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18.2 Pre-Embryonic and Embryonic Development
2. Embryonic development: Weeks 6-8
•
The embryo begins to look like a human being.
•
Reflex actions occur.
•
All organ systems have been established.
•
The embryo is 38 mm by the end of this period
and weighs about the same as an aspirin
tablet.
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18.3 Fetal Development
3. Fetal development: Months 3 and 4
•
•
•
•
•
•
Hair develops.
The head slows in growth so that the body size
can catch up.
Cartilage begins to be replaced by bone.
It is possible to distinguish female from male
(month 3).
The heartbeat can be heard with a stethoscope
(month 4).
By the end of this period, the fetus is ~6 inches
and ~6 ounces.
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18.3 Fetal Development
3. Fetal development: Months 5-7
•
•
•
•
Fetal movement
can be felt by the
mother.
Fetus is in fetal
position.
Eyelids are fully
open.
Fetus size has
increased to ~12
inches and ~3
pounds.
Figure 18.8 A five- to seven-month-old fetus.
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18.3 Fetal Development
3. Fetal development: Months 8 and 9
•
Weight gain is about a pound per week.
•
Fetus rotates so the head is pointed toward
the cervix.
•
At the end of fetal development, the fetus
weighs ~7.5 pounds and ~20.5 inches.
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18.3 Fetal Development
Preventing birth defects
•
Get physical exams by a trained doctor.
•
Have good health habits: proper nutrition and
adequate sleep and exercise.
•
Avoid smoking, alcohol, and drug abuse.
•
Avoid having X-rays.
•
Avoid certain medications and supplements.
•
Avoid sexually transmitted diseases or know if
you have one.
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18.3 Fetal Development
Fetal circulation
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ductus arteriosus
(becomes ligamentum
arteriosum)
pulmonary artery
aortic arch
superior vena cava
pulmonary veins
pulmonary trunk
foramen ovale
(becomes fossa ovalis)
left atrium
right atrium
left ventricle
inferior vena cava
right ventricle
abdominal aorta
ductus venosus
(becomes ligamentum
venosum)
common iliac artery
umbilical arteries
(become medial
umbilical ligaments)
umbilical vein
(becomes ligamentum
teres)
Internal iliac artery
Umbilical vein
takes O2 –high blood
to fetus.
placenta
umbilical vein
chorionic villi
Umbilical arteries
take O2- low blood
to placenta.
umbilical arteries
Figure 18.7 Fetal
circulation and the
placenta.
Decreasing
blood
oxygen
level
maternal blood vessels
Note: Fetal and maternal blood do not mix because exchange of materials
between the two occurs at the chorionic villi.
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18.3 Fetal Development
Development of the sex organs
• Sex of an individual is determined at conception
(XX is female and XY is male).
• If the SRY (the sex-determining region on the Y
chromosome) gene is present at week 6 then
the embryo develops into a male.
• Anti-Mϋllerian hormone secreted by the testes
prevents the development of female sex
organs.
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18.3 Fetal Development
Development of the sex organs
• At 14 weeks, primitive testes and ovaries are
already developing.
• The development of the external organs is
dependent on the presence or absence of
dihydrotestosterone (DHT) produced by the
testes.
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18.3 Fetal Development
Development of the sex organs
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gonads
Figure 18.9
Development
of the internal
sex organs.
1
limb bud
Müllerian duct
Wolffian duct
probladder
kidney
ureter
cloaca
4 weeks
Figure 18.10
Development
of the external
sex organs.
1
cloaca
6 weeks
urogenital groove
SRY absent
SRY present
6 weeks
kidney testes
ovaries
SRY present
SRY absent
glans
urinary
bladder
urethra
2
urogenital
groove
uterus
vaginal
opening
14 weeks
anus
3
9 weeks
14 weeks
9 weeks
2
uterus
glans of penis
prepuce
glans of clitoris
ovary
urethra
testis
a.
penis
vagina
scrotum
labium minorus
labium majoris
4
vagina
5
14 weeks
b.
3
14 weeks
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18.3 Fetal Development
Abnormal development of the
sex organs
•
XY female syndrome - an individual develops
into a male because a piece of the Y
chromosome containing the SRY gene is
missing
•
XX male syndrome – an individual develops
into a male because the same small piece of
the Y containing the SRY gene is present on
an X chromosome
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18.3 Fetal Development
Ambiguous sex determination
•
Results from the absence of testosterone, antiMϋllerian hormone and/or DHT
– Androgen insensitivity syndrome: all hormones
are made, but testosterone receptors on cells
are ineffective; thus the individual has testes
that do not descend and outwardly appears to
be female
– Male pseudo-hermaphroditism: an individual
appears female until puberty when anti-Mϋllerian
hormone is produced, but the testes never
produce testosterone or DHT
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18.3 Fetal Development
Androgen insensitivity syndrome
Figure 18.11 Androgen insensitivity
affects sexual development.
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18.4 Pregnancy and Birth
What changes occur in the mother’s
body?
•
•
•
•
•
•
•
•
•
Nausea and vomiting are common symptoms early
on (morning sickness).
Some mothers report an overall increase in energy
levels and sense of well-being.
Acid reflux and constipation are common problems.
There is an increase in vital capacity.
Edema and varicose veins can result.
Incontinence is not uncommon.
The placenta produces peptide hormones that make
cells resistant to insulin, so diabetes can result.
Stretch marks are common.
Melanocyte activity increases in some areas.
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18.4 Pregnancy and Birth
Birth
•
True labor is characterized by uterine
contractions that occur every 15-20 minutes
and last for at least 40 seconds each.
•
Three stages
– Stage 1
• Effacement occurs, in which the cervical
canal slowly disappears and the baby’s
head acts as a wedge to cause cervical
dilation.
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18.4 Pregnancy and Birth
Birth
– Stage 2
• Uterine contractions occur every 1-2
minutes, lasting for 1 minute each.
• An incision (episiotomy) is made to the
opening to help the baby as its head
reaches the exterior.
• Once the baby is born, the umbilical cord
is cut and tied.
– Stage 3
• The afterbirth is delivered, usually about
15 minutes after the birth of the baby.
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18.4 Pregnancy and Birth
Birth
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placenta
ruptured
amniotic
sac
a. First stage of birth: Cervix dilates.
b. Second stage of birth: Baby emerges.
placenta
uterus
umbilical
cord
d. Third stage of birth: Afterbirth is expelled.
Figure 18.13 The stages of birth.
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18.5 Aging
Aging
•
Stages of life: infancy, childhood, adolescence,
and adulthood
•
Hypotheses of aging
– Cellular aging: there may be hormonal and
genetic influences on aging; mitochondrial
activity and caloric intake may be involved
– Damage accumulation: aging may result from
the accumulation of damage—some avoidable,
some unavoidable—over time
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18.5 Aging
What are the effects of age on
body systems?
• Skin becomes thinner, less elastic, and dry.
• Less adipose tissue in the skin, so one feels cold
more easily.
• Decrease in melanocytes leading to gray hair, while
some of the remaining cells are larger leaving “age
spots” (dark spots on the skin).
• Heart shrinks and arteries become more rigid.
• Reaction time slows and senses are muted.
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18.5 Aging
What are the effects of age on
body systems?
• Lenses in the eyes lose ability to accommodate.
• Blood pressure usually increases.
• Bone density declines.
• Muscle mass decreases.
• Weight gain results from a decrease in metabolism
and an increase in inactivity.
• Females undergo menopause and males andropause.
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18.5 Aging
Think about how you might prevent aging
Figure 17.15 What steps can an
individual take to increase health
span?
Note: Although many changes occur in the body as we age, some of them
can be tempered or even reversed by understanding what extrinsic factors
can be controlled to decrease these changes.
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