PowerLecture: Chapter 17

1
PowerLectureChapter 17

• Development and Aging

2
Learning Objectives

• Describe early embryonic development and
  distinguish each of the following oogenesis,
  fertilization, cleavage, gastrulation, and organ
  formation.
• Correlate the three germ layersectoderm,
  mesoderm, and endodermwith the tissues that
  eventually form from each.
• Outline the principal events of prenatal
  development.

3
Learning Objectives (contd)

• Describe some of the risks to the early
  development of the fetus.
• Describe the events of aging.

4
Impacts/Issues

• Fertility Factors and Mind-Boggling Births

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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.

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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.

7
How Would You Vote?

• To conduct an instant in-class survey using a
  classroom response system, access JoinIn Clicker
  Content from the PowerLecture main menu.
• 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.

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Section 1

• The Six Stages of Early Development An Overview

9
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 childs
  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.

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Fig. 17.1, p. 314
zygote after first cleavage
beginning of the ball of cells called a morula
11
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 eggs 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.

12
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 bodys various tissues and organs.

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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.

15
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.

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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.
Organ Formation
Fertilization
Cleavage
Gastrulation
Growth, Tissue Specialization
e Subpopulations of cells are sculpted into
specialized organs and tissues in spatial
patterns at prescribed times.
d Cell divisions, migrations, and rearrangements
produce two or three primary tissues, the start
of specialized tissues and organs.
f Organs increase in size and gradually assume
their specialized functions.
c Cell divisions carve up different regions of
egg cytoplasm for daughter cells.
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
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Fig. 17.2, p. 315
top view
a Eggs form and mature in female reproductive
organs. Sperm form and mature in male
reproductive organs.
e Subpopulations of cells are sculpted into
specialized organs and tissues in spatial
patterns at prescribed times.
d Cell divisions, migrations, and rearrangements
produce two or three primary tissues, the start
of specialized tissues and organs.
f Organs increase in size and gradually assume
their specialized functions.
c Cell divisions carve up different regions of
egg cytoplasm for daughter cells.
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
Stepped Art
18
Animation Stages of Frog Development
CLICKTO PLAY
19
Section 2

• The Beginnings of YouFertilization to
  Implantation

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The Beginnings of You Fertilization to
Implantation

• Fertilization unites sperm and oocyte.
• 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.

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The Beginnings of You Fertilization to
Implantation

• Only one sperm will successfully enter the
  cytoplasm of the secondary oocyte because of
  changes to the eggs 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.

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Animation Fertilization
CLICKTO PLAY
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Fig. 17.3a-d, p. 316
FERTILIZATION
oviduct
ovary
uterus
OVULATION
follicle cell
opening of cervix
egg nucleus
vagina
zona pellucida
sperm enter vagina
a
b
fusion of sperm nucleus with egg nucleus
nuclei fuse
c
d
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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 cellsthe trophoblastand 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.

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The Beginnings of You Fertilization to
Implantation

• Implantation gives a foothold in the uterus.
• 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
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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 mothers urine is the
  basis for home pregnancy tests.

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Animation Cleavage and Implantation
CLICKTO PLAY
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Fig. 17.4, p. 317
trophoblast (surface layer of cells of
the blastocyst)
endometrium
fertilization
implantation
endometrium
blastocoel
inner cell mass
fluid
inner cell mass
uterine cavity
Days 1-2
Day 3
Day 4
Day 5
a
b
c
d
Days 6-7
e
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.
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.
Some of the blastocysts surface cells attach
themselves to the endometrium and start to burrow
into it. Implantation has started.
After the third cleavage, cells form a compact
ball
The first cleavage furrow extends between the
two polar bodies.
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Fig. 17.4, p. 317
trophoblast (surface layer of cells of
the blastocyst)
endometrium
fertilization
implantation
endometrium
blastocoel
inner cell mass
fluid
inner cell mass
uterine cavity
Days 1-2
Day 3
Day 4
Day 5
a
b
c
d
Days 6-7
e
Stepped Art
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Section 3

• How the Early Embryo Takes Shape

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How the Early Embryo Takes Shape

• First, the basic body plan is established.
• 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.

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gut cavity
epidermis
peritoneum
lined body cavity (coelom) lining also holds
internal organs in place
Fig. 17.5a, p. 318
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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.

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Animation Weeks 3-4 of Development
CLICKTO PLAY
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Fig. 17.5b, p. 318
yolk sac
pharyngeal arches
chorionic cavity
future brain
embryonic disk
primitive streak
amniotic cavity
neural tube
somites
a DAY 15. A primitive streak appears along the
axis of the embryonic disk. This thickened band
of cells marks the onset of gastrulation.
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.
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How the Early Embryo Takes Shape

• Next, organs develop and take on the proper shape
  and proportions.
• 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.

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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
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Animation Neural Tube Formation
CLICKTO PLAY
39
Animation Formation of Human Fingers
CLICKTO PLAY
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Fig. 17.6, p. 319
ectoderm at gastrula stage
climbing nerve cell
neural plate formation
b
a
neural tube
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Section 4

• Vital Membranes Outside the Embryo

42
Vital Membranes Outside the Embryo

• Four extraembryonic membranes form.
• 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.

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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.

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Animation First Two Weeks of Development
CLICKTO PLAY
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Fig. 17.8, p. 320
chorionic cavity
chorionic villi
start of amniotic cavity
start of embryonic disk
blood-filled spaces
chorion
amniotic cavity
yolk sac
start of chorionic cavity
start of yolk sac
connecting stalk
a DAYS 10-11. The yolk sac, embryonic disk, and
amniotic cavity have started to form from parts
of the blastocyst.
b DAY. 12 Blood filled spaces form in maternal
tissue. The chorionic cavity starts to form.
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.
46
Vital Membranes Outside the Embryo

• The placenta is a pipeline for oxygen, nutrients,
  and other substances.
• The placenta is a combination of endometrial
  tissue and embryonic chorion.
• The maternal tissue consists of tissues rich in
  arterioles and venules.
• The embryos chorion extends into the maternal
  tissue as tiny chorionic villi.

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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.

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Fig. 17.9 (1), p. 321
4 weeks
MATERNAL CIRCULATION
FETAL CIRCULATION
embryonic blood vessels
mothers blood vessels
8 weeks
blood passes to and from mothers vessels
umbilical cord
space between chorionic villi
12 weeks
chorionic villus
AMNIOTIC FLUID
tissues of uterus
appearance of the placenta at full term
fused amniotic and chorionic membranes
49
Animation Structure of the Placenta
CLICKTO PLAY
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Section 5

• The First Eight WeeksHuman Features Emerge

51
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
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Animation Fetal Development
CLICKTO PLAY
53
WEEK 4
yolk sac
embryo
connecting stalk
future lens
pharyngeal arches
forebrain
developing heart
upper limb bud
somites
neural tube forming
lower limb bud
a
tail
Fig. 17.10a, p. 322
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Fig. 17.10b, p. 322
WEEKS 56
head growth exceeds growth of other regions
future external ear
retinal pigment
umbilical cord forms between weeks 4 and
8 (amnion expands, forms tube that encloses
the connecting stalk and a duct for blood vessels)
upper limb differentiation (hand plates develop,
then digital rays of future fingerswrist,
elbow start forming)
foot plate
b
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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
56
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.

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Fig. 17.11, p. 323
7 weeks
Y chromosome present
Y chromosome absent
10 weeks
penis
vaginal opening
birth approaching
birth approaching
58
Fig. 17.11, p. 323
7 weeks
Y chromosome present
Y chromosome absent
10 weeks
Stepped Art
birth approaching
birth approaching
59
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.

60
Section 6

• Development
• of the Fetus

61
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.

62
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
63
Fig. 17.12(1), p. 324
WEEK 16 Length 16 cm (6.4 inches) Weight 200
gm (7 ounces)
placenta
WEEK 29
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)
64
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.

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Fig. 17.13a, p. 325
arterial duct (ductus arteriosus)
pulmonary vessels
aorta
superior vena cava
foramen ovale
heart
liver
venous duct (ductus venous)
umbilical vein
umbilical cord
inferior vena cava
allantois
umbilical arteries
urinary bladder
placenta
66
ligament
pulmonary artery
closed foramen ovale (fossa ovalis)
hepatic vein
pulmonary veins
hepatic portal vein serving the liver
ligament
umbilicus (navel)
umbilical ligaments
degenerated allantois
(urinary bladder)
Fig. 17.13b, p. 325
67
Section 7

• Birth and Beyond

68
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.

69
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 mothers 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 mothers body and the
  umbilical cord is severed.

70
Fig. 17.14, p. 326
placenta
uterus
detaching placenta
umbilical cord
umbilical cord
dilating cervix
a
b
c
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Animation Birth
CLICKTO PLAY
72
Birth and Beyond

• Hormones also control milk production in a
  mothers 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.

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Fig. 17.15, p. 327
milk-producing mammary gland
nipple
adipose tissue
milk duct
(a) Breast anatomy.
(b) Breast of lactating female.
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Animation Anatomy of the Breast
CLICKTO PLAY
75
Section 8

• Potential Disorders of Early Development

76
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.

77
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.

78
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.

79
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
80
Potential Disorders of Early Development

• Cocaine, especially crack cocaine, prevents a
  childs 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.

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Animation Sensitivity to Teratogens
CLICKTO PLAY
82
Sensitivity to Teratogens
Figure 17.17
83
Video Mermaid Baby
CLICKTO PLAY

• From ABC News, Human Biology in the Headlines,
  2006 DVD.

84
Section 9

• Prenatal Diagnosis Detecting Birth Defects

85
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.

86
Fig. 17.19a, p. 330
87
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
88
Animation Amniocentesis
CLICKTO PLAY
89
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.

90
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
91
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.

92
Section 10

• From Birth to Adulthood

93
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) gtgtgt infant (two weeks to 15
  months) gtgtgt child (to 12 years) gtgtgt pubescent
  (individual at puberty) gtgtgt adolescent (from
  puberty to 34 years later) gtgtgt adult gtgtgt old
  age.

94
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
95
Animation Proportional Changes During
Development
CLICKTO PLAY
96
From Birth to Adulthood

• Adulthood is also a time of bodily change.
• Aging (senescence) is the progressive cellular
  and bodily deterioration built into the life
  cycle of all organisms.
• 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.

97
Stages of Human Development
98
Section 11

• Times Toll
• Everybody Ages

99
Times Toll Everybody Ages

• Aging is the gradual loss of vitality as body
  functions become less and less efficient.
• 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
100
Times Toll Everybody Ages

• Genes may determine the maximum human life span.
• 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.

101
Times Toll Everybody Ages

• Cumulative damage to DNA may also play a role in
  aging.
• A cumulative assaults hypothesis suggests that
  aging results from mounting damage to DNA
  combined with a lack of DNA repair.
• 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.

102
Section 12

• Aging Skin, Muscle, Bones, and Reproductive
  Systems

103
Aging Skin, Muscle, Bones, and Reproductive
Systems

• Changes in connective tissue affect skin,
  muscles, and bones.
• 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.

104
Aging Skin, Muscle, Bones, and Reproductive
Systems

• 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
105
Aging Skin, Muscle, Bones, and Reproductive
Systems

• 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.

106
Section 13

• Age-Related Changes in Some Other Body Systems

107
Age-Related Changes in Some Other Body Systems

• The nervous system and senses decline.
• 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 Alzheimers
• disease (AD).

Figure 17.24a-b
108
Age-Related Changes in Some Other Body Systems

• 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 Alzheimers 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
109
Age-Related Changes in Some Other Body Systems

• The cardiovascular and respiratory systems
  deteriorate.
• 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.

110
Age-Related Changes in Some Other Body Systems

• The immune, digestive, and urinary systems become
  less efficient.
• The numbers of T cells drops, B cells become less
  active, and autoimmune diseases can occur,
  possibly due to mutations in self-markers.
• 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.

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