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8/13/2009
Outline - Development
I. Fertilization
II. Embryonic development
III. Fetal circulation
IV. Extraembryonic membranes
V. Fetal development
VI. Birth
VII. Aging
Human Development
Biol 105
Lecture 22
Chapter 18
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Terminology
Sperm
 Sperm have a cap called the acrosome which
contains enzymes. The environment of the
female body activates the acrosome
 When a sperm and an egg fuse = fertilization
 The product of fertilization is a zygote. The
zygote is a diploid cell.
 Several sperm may go through the corona
radiata, which is the outer layer surrounding
the secondary oocyte
 This zygote begins to divide into multiple
cells.
 After the zygote divides it is an embryo.
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Fertilization
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Fertilization
 The enzymes from the acrosome digest the
zona pellucida which surrounds the egg, the
sperm needs to penetrate this layer in order to
enter the egg.
 Sperm need to penetrate two layers
surrounding the secondary oocyte
1. The corona radiata, which is the outer layer
 One sperm enters the egg—immediately the
zona pellucida and the egg plasma membrane
changes.
2. The zona pellucida which surrounds the egg
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Fertilization
Fertilization
 The secondary oocyte now completes its
second meiotic division and is considered
an ovum
Figure 18.4a
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Zygote
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Morula
 The zygote divides and by day 4 it is now
a solid ball of 12 or more cells called a
morula
Figure 18.2a
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The Prenatal Period
Morula: Solid ball
of cells formed as
the zygote
undergoes cleavage
Early blastocyst:
Hollow ball of cells
with a fluidfluid-filled
cavity
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The Prenatal Period
Late blastocyst:
Pre-embryo, with the
Preembryonic disk, two
layers of cells that
become the embryo
proper
Gastrula: Embryo
with three primary
germ layers (ectoderm,
mesoderm, and
endoderm)
Step 3: Cleavage
The zygote soon undergoes rapid
mitotic cell divisions as it moves
along the oviduct toward the
uterus, becoming a prepre-embryo
that consists of two cells, then four
cells, then eight cells, and so on.
Step 2: Fertilization
A single sperm penetrates the
secondary oocyte. Eventually,
the sperm and egg nuclei will
fuse, resulting in a fertilized
egg, or zygote.
Uterus
Day 16
2-cell stage
4-cell stage
Day 10
Secondary
oocyte nucleus
Day 6
Oviduct
Zona pellucida
Day 4
Trophoblast
Inner
cell mass
8-cell stage
Morula
Step 4: Morula
By day 4, successive
divisions produce a
morula, a solid ball of cells
that enters the uterus.
Fimbriae
Ovary
Secondary oocyte
Inner cell mass
Step 1: Ovulation
A secondary oocyte is
released from the ovary
and swept into the
oviduct.
Yolk sac
Blastocyst cavity
Amniotic cavity
Ectoderm
Trophoblast
Embryonic disk
Mesoderm
Step 5: Blastocyst
By day 6, the prepre-embryo
becomes a blastocyst, a hollow
ball of cells with a fluidfluid-filled
cavity. The blastocyst has freed
itself from the zona pellucida
and can increase in size.
Step 6: Implantation
The blastocyst attaches to the
uterine lining (endometrium) and
begins to digest its way inward.
The cells of the inner cell mass
begin to form primary germ layers.
Endoderm
Blastocyst
Blastocyst
Endometrium
Inner cell mass
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Figure 18.9
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Figure 18.3
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Embryonic development
 Embryonic development is divided into three
phases:
1. Cleavage – division that results in a
hollow ball of cells = blastocyst.
2. Gastrulation – the cells rearrange into
three layers.
3. Organogenesis –organs begin to form.
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Stem Cells
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Stem Cells
 Early in embryonic development the cells are
very adaptable – they can become many
different types of cells = stem cells.
 But as the embryo develops and the cells
become committed to a certain type, they
loose the ability to be adaptable – they
become differentiated.
 Some cells in adults still retain this adaptability
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 For the first few days after fertilization the
cells in the embryo can form a whole new
embryo – this is how identical twins happen.
These cells are called totipotent cells
 It is these cells that are embryonic stem cells.
 These embryonic stem cells can be taken
from the embryo and cultured in a lab.
 They have shown more promise than adult
stem cells.
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Stem Cell Debate
 Where do researchers get embryonic stem
cells?
 One source is fertility clinics. Remember
the movie and the ethics concerning
creating embryos to be implanted in a
woman.
 Usually more embryos are created than
are implanted in the woman, the rest are
frozen in case they are needed later.
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It is the blastocyst that implants in the uterus, usually
on day 6.
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The blastocyst
is implanted in
the uterus.
Pregnancy
begins after
implantation.
The blastocyst is implanted in the uterus.
The blastocyst has
outer cells called
trophoblasts that
becomes the
embryonic part of
the placenta
placenta..
There are three layers: ectoderm, mesoderm, and
endoderm.
Three Layers of the Embryo
 Part of the ectoderm becomes the nervous
system and skin
 The mesoderm becomes support tissues
including bone, muscle, cardiovascular
system, urinary system
 The endoderm becomes many of the
digestive and respiratory organs
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The notocord is for support and the neural tube
becomes the nervous system.
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The Placenta
 The placenta contains both blood vessels
from both the mother and the embryo.
 This allow carbon dioxide, oxygen,
nutrients and wastes to pass between the
mother and embryo.
 The umbilical cord connects the embryo to
the placenta.
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Fetal Circulation

The placenta produces human chorionic
gonadotropin (HCG) to tell the body to
stop ovulating and stop menstruation

Blood from the mother and the fetus never
meet, the exchange of materials between
the maternal and fetal blood takes place
across the walls of the chorionic villi.

Harmful chemicals can pass from mother
to fetus
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Does the blood from the mother and
fetus mix?
1. Yes
2. No
50%
1
50%
2
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Extraembryonic Membranes
1. Chorion - develops into the fetal half of the
placenta.
2. Yolk sac - contains very little yolk and is the
first site of RBC formation.
3. Allantosis - the allantosis blood vessels
become the umbilical blood vessels.
4. Amnion - contains amniotic fluid to cushion
and protect the embryo.
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Embryonic Development
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Table 16.1a
Embryonic development—begins with the 2nd
week and lasts until the end of the 2nd month
of development.
 Nervous system—first to become
visually evident.
 Heart begins development the third
week and begins beating the 4th week.
 All organ system have been
established by the end of embryonic
development.
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Fifth Week
Fetal development
Fetal development 9th – 38th weeks of
development.
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 Fetus has visible human characteristics.
 Cartilage begins to be replaced by bone.
 3rd month—possible to distinguish males
from females.
 4th month—heartbeat is loud enough to
be heard with a stethoscope.
 9th month - Fetus is about 20 1/2 inches
long and weighs about 7 1/2 pounds at
the end of fetal development.
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Table 16.1b
Development of Male and Female Sex
Organs




Sex of an individual—determined at
conception. Males have XY chromosomes,
females have XX chromosomes
Gonads start developing at 7 weeks.
SRY gene—sex determining region of the Y
chromosome.
Hormones control the development of sex
organs.
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Note that the neural tube begins to form very early in
embryonic development. This is why proper nutrition is
important from the beginning of a pregnancy. Folic
acid in particular is needed early in the pregnancy.
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Comparison of different animals
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Limbs and organs
are formed by a
process of certain
cells dying.
Programmed cell
death is called
apoptosis
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Pregnancy and Birth
Pregnancy and Diabetes
 Hormones cause mother to develop
increased insulin resistance – can
lead to gestational diabetes
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Effect of Age on Body Systems
Aging
 Stages of Life:
 Effect of Age on Body Systems:
 Infancy, Childhood, Adolescence,
Adulthood
 Skin becomes thinner and less elastic.
 Decline in bone density.
 Weight gain occurs because metabolism
decreases and inactivity increases.
 Females undergo menopause.
 Aging—progressive changes that
contribute to an increased risk of
infirmity, disease, and death.



Genetic in origin—aging has a genetic basis.
Whole-body process—decline in hormonal
system affects many organs.
Extrinsic factors—poor health habits and
exercise program.
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Important concepts
 Know how the sperm fertilizes the oocyte –
what layers does it need to go through to do
it.
 Three phases of embryo development
 Know what stage of embryonic development
implants in the uterus
 Know what the three layers of the embryo are
 The main tissues that the three layers give
rise to
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 Biologically successful old age begins
with the health habits developed when
we are younger.
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Important concepts
 What hormone does the placenta produce
that prevents ovulation and menustration
 What is the role of the placenta
 What are the four extraembryonic
membranes and their functions
 Does the blood from the mother and fetus
mix?
 What is gestational diabetes
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Definitions

Fertilization, zygote, embryo, stem cells,
Zona pellicuda, acrosome, corona radiata,
differentiate, differentiation, blastocyst,
placenta
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