Transcript C. Egg - Cloudfront.net

Ch. 42 - Animal Development
I. Fertilization
A. Union of sperm & egg to form
diploid zygote
B. Provides stimulus for egg to begin
development
C. Egg
1. Has 3 barriers to entry of sperm
a. Corona radiata - follicle cells
b. Zona pellucida - extracellular
matrix
c. Plasma membrane
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Fertilization
D. Sperm
1. Shape adapted to swimming
2. Flagellum for movement
3. Head contains:
a. Haploid nucleus
b. Acrosome
 Membrane enclosed-sac that
contains enzymes that help it
penetrate egg
4. Has middle piece with
many mitochondria to
help move flagellum
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E. Sperm Entry into Egg
1. Sperm squeeze through corona
radiata
2. Species-specific proteins on
sperm bind with receptors on zona
pellucida
3. Sperm acrosome releases
digestive enzymes
4. Sperm binds to egg plasma
membrane
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5. This triggers changes in egg:
a. Membrane depolarizes (fast
block); becomes impermeable to
other sperm. This prevents
polyspermy
b. Vesicles in egg, cortical granules,
release enzymes that turn zona
pellucida into an impenetrable
fertilization membrane (slow block)
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6. Formation of diploid zygote:
a. Microvilli extending from egg
plasma membrane bring entire
sperm into the egg
b. Nucleus releases its chromatin &
forms it into a sperm pronucleus
c. Egg chromosomes are in egg
pronucleus
d. Nuclear envelope envelopes both
pronuclei.
e. NOW there is a fertilized egg, or a
zygote.
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II. Cleavage
A. Rapid series of mitotic divisions
B. Produces a cluster of cells still
size of original egg
a. This is called a
morula
C. Nuclei cycle rapidly between DNA
replication, mitosis & cytokinesis
1. Transcription does not occur
a. Early mRNA comes from egg
2. Little protein synthesis occurs
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III. Blastulation
A. Newly formed cells (blastomere)
pump sodium ions into
extracellular spaces
1. Water follows by osmosis
2. Cells become arranged in a
sphere surrounding a fluid-filled
cavity called a blastocoel.
3. The embryo is now called a
blastula.
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Lancelet Early Development (little yolk)
B. Blastulation Depends on Yolk
1. Egg with Moderate Yolk (Frogs)
a. Cleavage is not equal because of
the presence of yolk, a dense
nutrient material. This affects
cleavage.
b. When yolk is present, the zygote &
embryo exhibit polarity. The animal
has two poles:
 Animal pole - small cells - gray
 Vegetal pole - larger cells due to
presence of more yolk - yellow
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Sea Urchin & Frog Cleavage
Little Yolk
Moderate Yolk
2. Egg with Much Yolk (Birds)
a. Yolk-filled eggs don’t particpate in
cleavage
b. The blastula is a layer of cells that
spreads out over the yolk.
c. The blastocoel is a space that separates
these cells from the yolk.
IV.Gastrulation
A. Sorts cells into distinct layers
1. Ectoderm - outer layer
a. Epidermis (skin); sense
organs; eye; nervous system
2. Endoderm - inner layer
a. Lining of digestive & other
major systems; some
endocrine glands
3. Mesoderm - middle layer
a. Notochord; skeleton;
muscles, circulatory system;
reproductive system; dermis
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B. Steps of Gastrulation:
1. Begins when small groove
appears on one side of blastula
2. Invagination begins where cells
are somewhat smaller
3. Continues until there is a second
layer of cells inside; almost
destroying the blastocoel.
4. Early gastrula has two layers:
ectoderm & endoderm
a. Endoderm borders a new
cavity called the archenteron
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Gastrulation
Blastocoel
Archenteron
Ectoderm
Endoderm
4. Gastrulation is not complete until all
three germ layers are produced.
a. Ectoderm covers all of surface
except for the yolk plug
b. Endoderm surrounds the new cavity,
the archenteron, which will become
the digestive tract
c. Mesoderm has now formed in
between these two layers
5. The opening of the cavity to the outside
is the blastopore
a. This will become the anus in human
6. The embryo is now called a gastrula
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Comparative Development of Mesoderm
a. Mesoderm forms by outpocketing of archenteron
b. Mesoderm forms as cells migrate between
ectoderm & endoderm
c. Mesoderm forms by invagination of cells between
upper ectoderm & lower endoderm
V. Organ Formation
A. Tissues & organs must appear
quickly (bones, heart, muscles)
B. After gastrulation, a structure
called the notochord appears
1. It forms from mesoderm just
above the archenteron
2. It is made of cartilage
3. It extends the length of the
embryo
4. Provides support for other
developing tissues
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Early Frog Embryo
C. Neurulation
1. The neural plate forms
a. This is a thickened region of the
ectoderm above the notochord
2. From it arises a pair of ridges
called neural folds
3. These folds roll up and form a tube
which sinks below the surface
a. This new tube is called the
neural tube
b. It will become the brain and
spinal cord
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Development of Neural Tube
in a Frog Embryo
Development of Neural Tube and Coelom
in a Frog Embryo
D. Other Organ Formation
1. Somites appear
a. These are blocks of mesoderm
that will give rise to vertebrae
& muscles of backbone
2. The notochord will eventually be
replaced by the backbone which
will surround the spinal cord
3. Limb buds begin to form as well as
a tail
4. The eyes begin to appear
5. A hollow space, called the coelom,
forms. This will be the body cavity.
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Vertebrate Embryo, Cross Section
VI. Developmental Processes
A. Development requires:
1. Growth
2. Cellular differentiation
a. When cells become specialized
3. Morphogenesis
a. Produces shape & form of body
b. Includes pattern formation –
♣How tissues & organs are
arranged in body
♣Apoptosis – cell death
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B. Apoptosis
1. Animals have suicide genes that
code for proteins that will kill the
cell that produces them
a. Used to destroy cells between
developing fingers and toes in
humans
b. Also used in development of
nervous & immune systems
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C. Embryonic Polarity (frogs)
1. Dorsal-ventral
a. Determined by yolk
2. Site of sperm fusion (ANTERIOR)
a. Crescent shaped gray area
forms on egg opposite of sperm
entry. Called the gray crescent.
● First cleavage should pass
through this dividing it equally
♣ If it does and then the cells
are separated, both will
develop into tadpoles
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Polarity of Embryos
● If first cleavage does NOT pass
through crescent:
 the cell without it will not grow
3. Totipotent
a. When first 2 cells have the full
potential to develop normally
b. Occurs in sea stars & most
vertebrates
♣ Works up to 8-cell stage in
mammals
c. In mollusks & segmented worms
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after 3rd cleavage, not totipotent
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Polarity of Embryos
VII. Embryonic Induction
A. Process by which one embryonic tissue
can influence the development of
another
1. Frog gastrula
a. Gray crescent becomes dorsal lip
of blastopore
b. Moves inward to form mesoderm
in roof of archenteron
c. Detaches & forms notochord
d. Induces ectoderm lying above
to fold inward & form the nervous
system
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2. Hans Spemann’s experiment
a. Transplanted dorsal lip from
normal position in blastula of a
salamander to the belly-region of
another blastula
 Gastrulation occurs in 2 places
 2 nervous systems formed
 2 nearly complete embryos
were created; joined ventrally
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Hans Spemann’s experiment
Hans Spemann’s experimental results
3. Lens Induction
a. Eyes form as lateral outpockets
(optic vesicles) from brain tissue
 When optic vesicles contact
epidermis on head, epidermal
cells form a thick plate of cells
 Plate of cells sinks inward
 Plate later detaches & becomes
a lens
 Optic vesicle transforms into the
optic cup, will eventually
becomes the retina
 Lens induces formation of cornea
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Eye - Lens Induction
b. Warren Lewis’ experiment:
 Removed optic vesicle from
head of an embryo &
transplanted it to the trunk
 Vesicle grew and developed
 It induced the formation of a
lens in the trunk
 No lens appeared in the head
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VIII. Human Embryonic Development
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A. Pregnancy = gestation
1. Time from implantation to
birth (parturition) is 266 days
(approximately 9 months)
2. Divided into two periods:
a. Embryonic development
First 2 months (8-9 weeks)
Major organs are formed
b. Fetal development
Months 3 through 9
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B. Extraembryonic Membranes in Humans
1. Develop from extensions of the
germ layers
a. Chorion
Forms fetal half of placenta
b. Amnion
Sac around embryo & fetus;
contains fluid to cushion & protect
c. Allantois
Blood vessels become umbilical cord
d. Yolk sac
Lacks yolk; first site of blood
formation
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Extraembryonic Membranes
Collects nitrogen wastes
Contains amniotic
fluid to bathe &
cushion embryo
Provides nourishment
Carries on gas exchange
Human Development Before Implantation
Fertilization occurs in
upper 1/3 of oviduct
~ Day 5: blastocyst formed. The
outer trophoblast will help embryo
to implant in uterus. Inner cell
mass becomes embryo.
Trophoblast
begins to
secrete HCG
which helps
to maintain
corpus
luteum & the
endometrium
Human Embryonic Development
Human Embryo at Fifth Week
Human Embryo at Eight Weeks
By the 8th week of development, the embryo is recognizable
as human. About 38 mm long at this point.
Anatomy of Placenta
Human placenta functions in gas,
nutrient & waste exchange. It is
fully formed by the 10th week of
development.
It then starts to produce estrogen &
progesterone which help to
maintain pregnancy.
Maternal & fetal blood do not mix.
Just an exchange of materials
occurs. Harmful chemicals in mom
can enter the fetus.
Umbilical cord stretches between
placenta & the fetus.
Fetal Development
By end of third month, genitals appear.
By about six months a fine, downy hair called
lanugo covers the fetus. Skin is wrinkled making
fetus look “old”.
By 24 weeks (~6 months) the fetus has a chance
of surviving if born early.
Dramatic weight gain during the last three months
increases chances of survival.
Stages of Birth
When fetal brain is sufficiently mature, the
hypothalamus starts a series of events that
lead to the production of oxytocin.
This helps the uterus to contract and expel the
fetus.
Birth involves three stages:
1. Cervix dilates & the amnion ruptures
2. Baby is born
3. Placenta is delivered
Stages of Birth