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Chapter 47
Animal Development
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Development is determined by the zygote’s
genome and differences between embryonic cells
• Cell differentiation is the specialization of cells in
structure and function
• Cytoplasmic determinants, the uneven distribution
of maternal substances within the unfertilized egg,
cause early embryonic cells to differentiate in
some species.
• Morphogenesis is the process by which an animal
takes shape
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 47.1: After fertilization, embryonic development
proceeds through cleavage, gastrulation, and organogenesis
• Important events regulating development occur
during fertilization and the three stages that build
the animal’s body
– Cleavage: cell division creates a hollow ball of
cells called the blastula.
– Gastrulation: produces a 3-layered embryo
called the gastrula.
– Organogenesis: generates rudimentary organs
from which adult structures grow.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fertilization
• Fertilization brings the haploid (1 set of
chromosome) nuclei of sperm and egg together,
forming a diploid (2 sets of chromosomes) zygote
• The sperm’s contact with the egg’s surface
initiates metabolic reactions in the egg that trigger
the onset of embryonic development (“activates
the egg”)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 47-3
Contact and fusion
of sperm and egg
membranes
Acrosomal
reaction
Sperm plasma
membrane
Contact
Basal body
(centriole)
Entry of sperm
nucleus
Sperm
nucleus
Cortical reaction
Acrosomal
process
Sperm
head
Actin
Acrosome
Jelly coat
Sperm-binding
receptors
Fertilization
envelope
Fused plasma
Cortical membranes
granule
Hydrolytic enzymes Perivitelline
space
Vitelline layer
Egg plasma
membrane
EGG CYTOPLASM
Cortical granule
membrane
The Acrosomal Reaction
• The acrosomal reaction is triggered when the
sperm meets the egg.
• This process begins when a specialized vesicle at
the tip of the sperm called the acrosome,
discharges hydrolytic enzymes.
– This reaction releases hydrolytic enzymes that
digest material surrounding the egg
– This allows the acrosomal process (elongated
sperm structure) to penetrate the jelly coat.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Molecules of a protein on the tip of the acrosomal
process adhere to molecules of a specific receptor
proteins on the egg’s surface.
• This gamete contact and/or fusion depolarizes the
egg cell membrane and sets up a fast block to
polyspermy (multiple sperm).
– Depolarization occurs when the ion channels
open on the egg’s plasma membrane allowing
sodium ions to flow into the egg cell and change
the membrane potential.
– Depolarization occurs within 1-3 seconds after a
sperm binds to an egg.
• Fusion of egg and sperm also initiates the cortical
reaction
• This reaction induces a rise in Ca2+ from the egg’s
ER into the egg’s cytosol. This causes cortical
granules in the egg to fuse with the plasma
membrane and discharge their contents. This leads
to swelling of the perivitelline space, hardening of
the vitelline layer, and clippling of sperm binding
receptors.
• These changes cause formation of a fertilization
envelope that functions as a longer-term slow block
to polyspermy.
– Does occur in vertebrates (fishes and animals)
Activation of the Egg
• The sharp rise in Ca2+ in the egg’s cytosol
increases the rates of cellular respiration and
protein synthesis by the egg cell
• With these rapid changes in metabolism, the egg
is said to be activated
• Sperm cells do not contribute any materials
required for activation. The unfertilized eggs of
many species can be artificially activated by the
injection of Ca2+ or by a variety of mildly injurious
treatments, such as temperature shock.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Minutes
Seconds
LE 47-5
1
Binding of sperm to egg
2
3
4
Acrosomal reaction: plasma membrane
depolarization (fast block to polyspermy)
6
8
10
Increased intracellular calcium level
20
Cortical reaction begins (slow block to polyspermy)
30
40
50
1
Formation of fertilization envelope complete
2
Increased intracellular pH
3
4
5
Increased protein synthesis
10
20
30
40
60
90
Fusion of egg and sperm nuclei complete
Onset of DNA synthesis
First cell division
Fertilization in Mammals
• Fertilization in other species share the same timing
as the sea urchin in the previous slide. However,
timing differs with species.
– Sea urchins meiosis is already completed when
the egg is released from the female.
– In humans, the unfertilized egg stays at
metaphase of meiosis II. Meiosis is NOT
completed until they are fertilized in the female
reproductive tract.
• Fertilization is generally internal.
Fertilization in Mammals continued
• Secretions in the mammalian female trace alter
certain molecules on the surface of sperm cells
and also increase sperm motility.
• The mammalian egg is cloaked by follicle cells
released along with the egg during ovulation. The
sperm must migrated through this layer of follicle
cells before it reaches the zona pellucida.
• In mammalian fertilization, the cortical reaction
modifies the zona pellucida as a slow block to
polyspermy
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 47-6
2. This binding induces
the acrosomal
reaction, in which the
sperm released
hydrolytic enzymes
into the zona pellucida.
1. Sperm migrated
through the coat of
follicle cells and binds
to receptor molecules
in the zona pellucida of
the egg.
3. Breakdown of the
zona pellucida by
these enzymes allows
the sperm to reach the
plasma membrane of
the egg. Membrane
proteins of the sperm
bind to the receptors
on the egg membrane,
and the two
membranes fuse
4. The nucleus and
other components of
the sperm cell enter
the egg.
Follicle
cell
Zona
pellucida
Egg plasma
membrane
Acrosomal
vesicle
EGG CYTOPLASM
Sperm
Cortical
basal
ganules
body Sperm
nucleus
5. Enzymes released
during the cortical
reaction harden the
zona pellucida, which
now functions as a
block to polyspermy.
Fertilization in Mammals continued
• After the egg and sperm membrane fuse, the
whole sperm, tail and all is taken into the egg.
• The egg lacks a centrosome. The basal body of
the sperm’s flagella now acts as the centrosome
and wraps itself around the centriole.
• This will allows mitotic spindles to form for the first
cell division.
• Fertilization is much slower in mammals. The first
cell division occurs 12-36 hours after sperm
binding in mammals.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Cleavage cell division creates a hollow ball of cells
called the blastula.
• Fertilization is followed by cleavage, a period of
rapid cell division without growth
• Cells undergo S and M phases of the cell cycle but
skip Gap 1 and Gap 2. Little or now protein
synthesis occurs.
– The embryo does not enlarge during this
period of development.
• Cleavage partitions the cytoplasm of one large cell
into many smaller cells called blastomeres. Each
with its own nucleus.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 47-7
Fertilized egg
Four-cell stage
Morula
Blastula
•
First 5-7 divisions from a cluster of cells known as the morula
•
A fluid-filled cavity called the blastocoel begins to form within the morula
and is fully formed in the blastula, a hollow ball of cells.
•
During cleavage, different regions of the cytoplasm end up in separate
blastomeres. These regions may contain different cytoplasmic
determinants, in many species this partitioning sets the stage for
subsequent developmental events.
Gastrulation
• Gastrulation rearranges the cells of a blastula into
a three-layered embryo, called a gastrula, which
has a primitive gut.
• Varies from one animal to another, the process is
driven by change in cell motility, changes in cell
shape, and changes in cellular adhesion to other
cells and to molecules of the extracellular matrix.
• This results in the three cell layers.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The three layers produced by gastrulation are called
embryonic germ layers
– The ectoderm forms the outer layer
– The endoderm lines the digestive tract
– The mesoderm partly fills the space between the
endoderm and ectoderm
• Eventually, these three cell layers develop into all
the tissues and organs of the adult animal.
Video: Sea Urchin Embryonic Development
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Organogenesis
• During organogenesis, various regions of the
germ layers develop into rudimentary organs
• Early in vertebrate organogenesis, the notochord
forms from mesoderm, and the neural plate forms
from ectoderm
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Mesoderm lateral to the notochord forms blocks
called somites
• Lateral to the somites, the mesoderm splits to form
the coelom
• The neural plate soon curves inward, forming the
neural tube
• Many structures are derived from the three
embryonic germ layers during organogenesis
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Developmental Adaptations of Amniotes
• Because all vertebrate embryos required an
aqueous environment for development, embryos
of birds, other reptiles, and mammals develop in a
fluid-filled sac in a shell (birds & reptiles) or the
uterus (marsupials & eutherian)
• Organisms with these adaptations are called
amniotes
• In these organisms, the three germ layers also
give rise to the four membranes that surround the
embryo
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 47-17
Amnion
Allantois
Embryo
Amniotic
cavity
with
amniotic
fluid
Albumen
Shell
Yolk
(nutrients)
Chorion
Yolk sac
Mammalian Development
• Fertilization takes place in the oviduct, and the
progresses as the embryo completes its journey
down the oviduct to the uterus.
• The eggs of placental mammals
– Are small and store few nutrients
– Exhibit holoblastic cleavage (complete cell division of egg,
having little or moderate amount of yolk)
• Gastrulation and organogenesis resemble the
processes in birds and other reptiles
• Early cleavage is relatively slow in humans and other
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• At completion of cleavage, the blastocyst forms
• The trophoblast, the outer epithelium of the
blastocyst, initiates implantation in the uterus, and
the blastocyst forms a flat disk of cells
• As implantation is completed, gastrulation begins
• The extraembryonic membranes begin to form
• By the end of gastrulation, the embryonic germ
layers have formed
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
LE 47-18a
Endometrium
(uterine lining)
Inner cell mass
Trophoblast
Blastocoel
Blastocyst
reaches uterus.
Maternal
blood
vessel
Expanding
region of
trophoblast
Epiblast
Hypoblast
Trophoblast
Blastocyst
implants.
LE 47-18b
Expanding
region of
trophoblast
Amniotic
cavity
Amnion
Epiblast
Hypoblast
Chorion (from
trophoblast
Yolk sac (from
hypoblast)
Extraembryonic
membranes start
to form and
gastrulation
begins.
Extraembryonic mesoderm cells
(from epiblast)
Allantois
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic
mesoderm
Gastrulation has produced a
three-layered embryo with four
extraembryonic membranes.
• The extraembryonic membranes in mammals are
homologous to those of birds and other reptiles
and develop in a similar way
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings