Chapter 22: Development and
Aging
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22-1
Early Developmental Stages
Fertilization occurs when the sperm and
egg interact to produce a zygote.
Several sperm may penetrate the corona
radiata, several may attempt to penetrate
the zona pellucida, but only one sperm
enters the egg.
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Fertilization – fusion of sperm and egg nuclei
Fig. 22.1
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.
Depolarization of the egg’s plasma membrane after the
sperm touches the egg prevents a second sperm from
fertilizing the egg.
22-3
Embryonic Development
Development - all the changes that occur
during the life cycle of an organism.
The embryo is the first stage in human
development.
22-4
Lancelet early development
Fig. 22.2
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.
22-5
Fig. 22.2
At the gastrula stage,
invagination of cells into
the blastocoel results in
formation of the germ
layers: ectoderm,
mesoderm, and
endoderm.
The three germ layers
will have different
developmental fates.
22-6
Neurulation and the Nervous
System
The notochord forms from mesoderm.
During neurulation, the nervous system
develops from midline ectoderm, just
above the notochord.
22-7
Development of neural tube and
coelom in a frog embryo
Fig. 22.4
A neural plate is seen first, then a neural tube; the
anterior neural tube becomes the brain.
22-8
Fig. 22.5
Chordate embryo, cross section
At the neurula
stage, cross
sections of all
chordate
embryos
appear similar.
22-9
Developmental Processes
Development requires:
1) Growth
2) Cell differentiation (become specialized in
structure and function).
3) Morphogenesis (produces shape and form).
**Each cell remains totipotent.
22-10
Human Embryonic and Fetal
Development
Human development is divided into the:
embryonic period (first 2 months) early
formation of the major organs
fetal period (months 3–9) refinement of
these structures.
22-11
Four Extraembryonic membranes
Fig. 22.11
1) The amnion envelops
the embryo/fetus in a
protective amniotic
fluid.
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.
22-12
Fertilization occurs
in the upper third
of the oviduct; a
zygote is produced.
The embryo is a
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.
Embryonic Development
The First Week
Fig. 22.12
22-13
The Second Week
Embryo begins to
implant at end of
first week.
Fig. 22.13
Trophoblast
secretes human
chorionic
gonadotropin
(HCG).
Gastrulation occurs and the inner cell mass becomes
the embryonic disk while the trophoblast becomes the
chorion.
The yolk sac and amnion form.
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The Third Week
Neurulation occurs and the nervous
system becomes the first visible organ
system.
The heart begins to form and pump
blood.
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The Fourth and Fifth
Weeks
The allantois forms
and is contained
within the umbilical
cord.
Limb buds appear
and sense organs
develop.
Fig. 22.13
22-16
Human embryo at beginning of
fifth week
Fig. 22.14
22-17
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
The body increases in
size, and epidermal
refinements (eyelashes,
nipples) become apparent.
Bone is replacing
cartilage.
Males can be
distinguished from
females, and the heartbeat
is audible.
Fig. 22.15
22-19
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).
22-20
Fetal circulation and the placenta
One umbilical vein
takes nutrients to
the systemic
system when the
umbilical vein joins
the vena cava by a
venous duct.
Two umbilical
arteries that branch
off the iliac arteries
lead to the
placenta.
Blood passes from the right to the left atrium
through the foramen ovale
Fig. 22.16
22-21
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.
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Anatomy of the placenta in a fetus at
six to seven months
Fig. 22.17
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.
22-23
Three stages of parturition (birth)
Fig. 22.18
1
2
3
22-24
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.
22-25
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.
22-26