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Fertilization
Animal Development
Fertilization
In all sexually-reproducing animals, the first
step is fertilization – union of male and
female gametes
Chapter 53
Fertilization itself consists of three events:
-Sperm penetration and membrane fusion
-Egg activation
-Fusion of nuclei
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Sperm penetration and membrane fusion
-Protective layers of egg include the jelly
layer and vitelline envelope in sea urchins,
and the zona pellucida in mammals
-The acrosome of sperm contains digestive
enzymes that enable the sperm to tunnel its
way through to the egg’s cell membrane
-Membrane fusion permit sperm nucleus
to enter directly into egg’s cytoplasm
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Fertilization
Egg activation
-Membrane fusion triggers egg activation by
the release of Ca2+ which initiates changes
in the egg
-A block to polyspermy occurs
-Changes in egg’s membrane potential
-Alteration of egg’s exterior coats
-Enzymes from cortical granules
remove sperm receptors
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Fertilization
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Fertilization
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Fertilization (Cont.)
Egg activation
-Sperm penetration has three other effects
1. Triggers the egg to complete meiosis
2. Triggers a cytoplasmic rearrangement
3. Causes a sharp increase in protein
synthesis and metabolic activity in
general
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Fertilization
Fertilization
Cleavage
Fusion of nuclei
-The haploid sperm and haploid egg nuclei
migrate toward each other along a microtubule based aster
-They then fuse, forming the diploid
nucleus of the zygote
Cleavage is the rapid division of the zygote
into a larger and larger number of smaller
and smaller cells called blastomeres
-It is not accompanied by an increase in the
overall size of the embryo
In many animals, the two embryo ends are:
-Animal pole = Forms external tissues
-Vegetal pole = Forms internal tissues
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Cleavage
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Cleavage Patterns
The outermost blastomeres in the ball of cells
become joined by tight junctions
Innermost blastomeres pump Na+ into the
intracellular spaces
-Create osmotic gradient, which draws
water
The result is a hollow ball of cells, the
blastula, containing a fluid-filled cavity, the
blastocoel
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Cleavage Patterns (Cont.)
Cleavage patterns are highly diverse
-Influenced by amount of yolk in the egg
Cleavage patterns are highly diverse
-Influenced by amount of yolk in the egg
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Cleavage Patterns
Eggs with moderate to little yolk undergo
holoblastic (complete) cleavage
-In sea urchins, a symmetrical blastula is
produced, surrounding spherical blastocoel
-In amphibians, an asymmetrical blastula is
produced, with a displaced blastocoel
-Because egg contains much more yolk
in one hemisphere than the other
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Cleavage Patterns
Cleavage Patterns
Cleavage Patterns
Eggs with large amounts of yolk undergo
meroblastic (incomplete) cleavage
-In eggs of reptiles and birds, the clear
cytoplasm is concentrated at one pole called
the blastodisc
-Cleavage is restricted to this area
-Resulting embryo is not spherical
Mammalian eggs contain very little yolk, and
so undergo holoblastic cleavage
-Form a blastocyst, which is composed of:
-Trophoblast = Outer layer of cells
-Contributes to the placenta
-Blastocoel = Central fluid-filled cavity
-Inner cell mass = Located at one pole
-Forms the developing embryo
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Cleavage Patterns
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Fate of Blastomeres
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Gastrulation
Gastrulation
In mammals, early blastomeres do not appear
to be committed to a particular fate
-The earliest patterning events occur at the
eight-cell stage
-Outer surfaces of blastomeres flatten
against each other in a process called
compaction
-Produces polarized blastomeres,
which then divide asymmetrically
Gastrulation is a process involving a complex
series of cell shape changes and cell
movements that occurs in the blastula
-It establishes the basic body plan and
creates the three primary germ layers
-Ectoderm – Exterior
-Mesoderm – Middle
-Endoderm – Inner
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Gastrulation
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Cells move during gastrulation using a variety
of cell shape changes
-Cells that are tightly attached to each other
via junctions will move as cell sheets
-Invagination – Cell sheet dents inward
-Involution – Cell sheet rolls inward
-Delamination – Cell sheet splits in two
-Ingression – Cells break away from cell
sheet and migrate as individual cells 26
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Gastrulation Patterns
Also vary according to the amount of yolk
Gastrulation in sea urchins
-Begins with formation of vegetal plate and
ingression of primary mesenchyme cells
(future mesoderm cells) into blastocoel
-Remaining cells of vegetal plate invaginate
into blastocoel forming the endoderm
-Archenteron (future digestive gut)
-Cells staying at surface form ectoderm 27
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Gastrulation Patterns
Gastrulation in frogs
-Cells from the animal pole involute over the
dorsal lip of blastopore into the blastocoel
-Cells eventually press against far wall
-Eliminate blastocoel, producing the
archenteron with yolk plug
-These movements create two layers
-Outer ectoderm and inner endoderm
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-Mesoderm forms later in between
Gastrulation Patterns
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Gastrulation in birds
-Avian blastula consists of a disc of cells,
the blastoderm, sitting atop large yolk mass
-First, blastoderm delaminates into two
layers, with blastocoel cavity in between
-The upper layer produces all 3 germ layers
-Cells that migrate through primitive
streak form endoderm or mesoderm
-Cells that remain form ectoderm
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Gastrulation Patterns
Gastrulation Patterns
Gastrulation in mammals
-Proceeds similarly to that in birds
-Embryo develops from inner cell mass
-ICM flattens and delaminates into 2 layers
-A primitive streak forms
-Cell movements through it give rise to
the three primary germ layers
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Gastrulation Patterns (Cont.)
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Extraembryonic Membranes
Extraembryonic Membranes
As an adaptation to life on dry land, amniotic
species developed several extraembryonic
membranes
-Nourish and protect the developing embryo
1. Amnion = Encloses amniotic fluid
2. Chorion = Located near eggshell in birds
-Contributes to the placenta in mammals
3. Yolk sac = Food source in bird embryos
-Found in mammals, but it is not nutritive
4. Allantois = Unites with chorion in birds,
forming a structure used for gas exchange
-In mammals, it contributes blood vessels
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to the developing umbilical cord
These membranes are formed from
embryonic cells
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Extraembryonic Membranes
(Cont.)
Extraembryonic Membranes
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Organogenesis
Organogenesis is the formation of organs in
their proper locations
-Occurs by interaction of cells within and
between the three germ layers
-Thus, it follows rapidly on the heels
of gastrulation
-Indeed, in many animals it begins
before gastrulation is complete
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Organogenesis
Organogenesis in Drosophila
To a large degree, a cell’s location in the
developing embryo determines its fate
At some stage, every cell’s ultimate fate
becomes fixed – cell determination
A cell’s fate can be established in two ways:
1. Inheritance of cytoplasmic determinants
2. Interactions with neighboring cells
-Cell induction
Salivary gland development
-The sex combs reduced (scr) gene is a
homeotic gene in the Antennapedia
complex
-Prior to organogenesis, it is expressed
in an anterior band of cells
-At the same time, Decapentaplegic
protein (Dpp) is released from dorsal cells
-Forms a gradient in the dorsal-ventral 47
direction
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Organogenesis in Drosophila
Salivary gland development
-Dpp inhibits formation of salivary gland
rudiments
-Thus, during organogenesis, salivary
glands develop in areas where Scr is
expressed and Dpp is absent
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Organogenesis in Vertebrates
Development of Neural Tube
Organogenesis in vertebrates begins with the
formation of two structures unique to
chordates
-Notochord
-Dorsal nerve cord
-Its development is called neurulation
The notochord forms from mesoderm
-Region of dorsal ectodermal cells situated
above notochord thickens to form the
neural plate
-Cells of the neural plate fold together to
form a long hollow cylinder, the neural
tube
-Will become brain and spinal cord
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Generation of Somites
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Mesoderm sheets on either side of notochord
separate into rounded regions called
somitomeres
-These separate into segmented blocks
called somites
-Form in an anterior-posterior wave with
a regular periodicity
-Ultimately give rise to skeleton,
muscles and connective tissues
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Generation of Somites
Neural Crest Cells
Mesoderm in the head region remains
connected as somitomeres
-Form muscles of the face, jaws and throat
Some body organs develop within a strip of
mesoderm lateral to each row of somites
-Remainder of mesoderm moves out to
surround the endoderm completely
-Mesoderm separates into two layers
-Coelom forms in between
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Neurulation occurs in all chordates
However, in vertebrates it is accompanied by
an additional step
-Just before the neural groove closes to
form the neural tube, its edges pinch off,
forming a small cluster of cells called the
neural crest
-These cells migrate to colonize many
different regions of developing embryo
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Neural Crest Cells
Neural Crest Cells
Neural crest cells migrate in three pathways
-Cranial neural crest cells are anterior cells
that migrate into the head and neck
-Trunk neural crest cells are posterior cells
that migrate in two pathways
-Ventral pathway cells differentiate into
sensory neurons and Schwann cells
-Lateral pathway cells differentiate into
melanocytes of the skin
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A mutation in a gene that promotes survival of
neural crest cells produces white spotting
on ventral surfaces of human babies & mice
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Neural Crest Cells
Vertebrate Axis Formation
Many of the unique vertebrate adaptations
that contribute to their varied ecological
roles involve neural crest derivatives
-For example gill chambers provided a
greatly improved means of gas exchange
-Allowed transition from filter feeding to
active predation (higher metabolic rate)
-Other changes = Better prey detection, and
rapid response to sensory information
Hans Spemann & Hilde Mangold transplanted
cells of the dorsal lip of one embryo into the
future belly region of another
-Some of the embryos developed two
notochords: a normal dorsal one, and a
second one along the belly
-Moreover, a complete set of dorsal axial
structures formed at the ventral
transplantation site in most embryos
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Vertebrate Axis Formation
(Cont.)
Vertebrate Axis Formation
Organizers
An organizer is a cluster of cells that release
diffusible signal molecules, which convey
positional information to other cells
-The closer a cell is to an organizer, the
higher the concentration of the signal
molecule (morphogen) it experiences
-Different morphogen concentrations
stimulate development of different organs
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Organizers
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Creation of the Spemann organizer
-In frogs, as in fruit flies, the process starts
during oogenesis in the mother
-Maternally-encoded dorsal determinants
are localized at the vegetal pole of the
unfertilized egg
-At fertilization, rearrangements in the
cytoplasm
cause this determinant
to shift to the future dorsal side of the egg
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Organizers
The maternally-encoded dorsal determinants
are mRNAs for proteins that function in the
intracellular Wnt signaling pathway
-Wnt genes encode a large family of cellsignaling proteins
-Affect the development of a number of
structures in both vertebrates and
invertebrates
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Organizers
Organizers
Function of the Spemann organizer
-Dorsal lip cells do not directly activate
dorsal development
-Instead, dorsal mesoderm development
is a result of the inhibition of ventral
development
The bone morphogenetic protein 4 (BMP4)
is expressed in all marginal zone cells (the
prospective mesoderm) of a frog embryo
-BMP4 is a morphogen that at high levels
specifies ventral mesoderm cell fates
The Spemann organizer functions by
secreting BMP4 antagonists
-Bind to BMP4 and prevents its binding to
its receptor
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Organizers
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Induction
Evidence indicates that organizers are
present in all vertebrates
-In chicks, a group of cells anterior to the
primitive streak called Hensen’s node
functions like the Spemann organizer
-Secrete molecules that inhibit ventral
development
-Same as those in frog embryos
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Induction
Primary induction occurs between the three
primary germ layers
-Example: Differentiation of the central
nervous system during neurulation
Secondary induction occurs between
tissues that have already been specified to
develop along a particular pathway
-Example: Development of the lens of the
vertebrate eye
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Human Development
First Trimester
First Trimester
Human development from fertilization to birth
takes an average of 266 days, or about 9
months
-This time is commonly divided into three
periods called trimesters
First month
-The zygote undergoes its first cleavage
about 30 hr after fertilization
-By the time the embryo reaches the
uterus, 6-7 days after fertilization, it has
differentiated into a blastocyst
-Trophoblast cells digest their way
into the endometrium in the
process known as
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implantation
First month
-During the second week, the developing
chorion and mother’s endometrium engage
to form the placenta
-Mom and baby’s blood come into close
proximity, but do not mix
-Gases are exchanged, however
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First Trimester
First month
-One hormone released by the placenta is
human chorionic gonadotropin (hCG)
-Maintains mother’s corpus luteum
-Gastrulation occurs in the second week
-Neurulation occurs in the third week
-Organogenesis begins in the fourth week
-Embryo is 5 mm in length
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First Trimester
First Trimester
Second month
-Miniature limbs assume adult shape
-Major organs within abdominal cavity
become evident
-Embryo grows to about 25 mm in length
-Weighs about 1 gm, and looks distinctly
human
Third month
-The ninth week marks the transition from
embryo to fetus
-Nervous system develops
-Limbs start to move
-Secretion of hCG by the placenta declines,
and so corpus luteum degenerates
-Placenta takes over hormone secretion
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Second Trimester
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Third Trimester
The basic body plan develops further
-Bones actively enlarge in fourth month
-Rapid fetal heartbeat can be heard by a
stethoscope
Estrogen stimulates mother’s uterus to
release prostaglandins, and produce
more oxytocin receptors
-Prostaglandins begin uterine contractions
-Sensory feedback from uterus stimulates
oxytocin release from posterior pituitary
-Oxytocin and prostaglandins further
stimulate uterine contractions
Weight of the fetus doubles several times
Most of the major nerve tracts in the brain
are formed
-Brain continues to develop and produce
neurons for months after birth
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Birth
A period of growth and organ maturation
By the end of the sixth month, fetus is over
30 cm long, and weighs 600 gm
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Birth
Nursing
Strong contractions, aided by the mother’s
voluntary pushing, expel the fetus
-Now called a newborn baby, or neonate
Milk production (lactation) occurs in alveoli
of mammary glands when stimulated by
the anterior pituitary hormone prolactin
-Milk is secreted into alveolar ducts
During pregnancy, progesterone stimulates
development of mammary alveoli
-And estrogen stimulates development of
alveolar ducts
After birth, continuing uterine contractions
expel the placenta and associated
membranes
-Collectively called the afterbirth
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Nursing
After birth, anterior pituitary secretes prolactin
-Sensory impulses associated with baby’s
suckling trigger the posterior pituitary to
release oxytocin
-Stimulates contraction of smooth
muscles surrounding alveolar ducts
-Milk is ejected (milk let-down reflex)
The first milk produced after birth, colostrum,
is rich in nutrients & maternal antibodies 94
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Postnatal Development
Growth of the infant continues rapidly after
birth
-Babies typically double their birth weight
within 2 months
Different components grow at different rates
-Allometric growth
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