Chapter 22: Aging
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Fertilization
• 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.
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.
• 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.
Lancelet early development
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.
• 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.
Comparative animal development
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.
Development of neural tube and
coelom in a frog embryo
Chordate embryo, cross section
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.
Cytoplasmic segregation
Cytoplasmic influence on
development
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.
Control of nervous system
development
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.
• 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.
Development of C. elegans, a small
worm
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 threedimensional structures.
Pattern formation in the fruit fly
• 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.
Homeotic mutations
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.
Human development before
implantation
• 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.
Human embryonic development
• 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.
Human embryo at beginning of fifth
week
• 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.
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.
The three-to four-month-old fetus
looks human
• 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).
• 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.
• 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.
Fetal circulation and the placenta
• 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.
• 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.
Anatomy of the placenta in a fetus
at six to seven months
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.
• 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.
Three stages of parturition (birth)
•
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.
• 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.
Female breast anatomy
Aging
Theories of Aging: WHY oh WHY?
• 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.
• 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.
• 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.
Extrinsic Factors: chasing youth!
• 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.
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.
• 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.
Remaining active