Transcript Lecture Presentation to accompany Principles of Life

33
Animal Development
Chapter 33 Animal Development
Key Concepts
• 33.1 Fertilization Activates Development
• 33.2 Cleavage Repackages the Cytoplasm
of the Zygote
• 33.3 Gastrulation Creates Three Tissue
Layers
• 33.4 Neurulation Creates the Nervous
System
• 33.5 Extraembryonic Membranes Nourish
the Growing Embryo
Chapter 33 Opening Question
How does the Sonic hedgehog
pathway control development of
the vertebrate brain and eyes?
Concept 33.1 Fertilization Activates Development
Different contributions to the zygote:
• Sperm contributes DNA and a centriole, in
some species
• Ovum contributes DNA, organelles,
nutrients, transcription factors, and
mRNAs
Centrosome of ovum degrades during
oogenesis—sperm centriole becomes
zygote centrosome
Concept 33.1 Fertilization Activates Development
Cytoplasmic factors in the egg set up signal
cascades in the major steps of
development:
• Determination
• Differentiation
• Morphogenesis
• Growth
Concept 33.1 Fertilization Activates Development
In an unfertilized frog egg:
• Vegetal hemisphere—the lower half of
the egg, where nutrients are concentrated
• Animal hemisphere—the opposite end of
the egg, contains the haploid nucleus
Cytoplasmic movement after fertilization is
visible because of pigments.
Concept 33.1 Fertilization Activates Development
The animal hemisphere has two pigmented
regions:
• Cortical cytoplasm—heavily pigmented
• Underlying cytoplasm—diffusely
pigmented
The vegetal hemisphere is not pigmented.
Concept 33.1 Fertilization Activates Development
Egg cytoplasm is rearranged beginning with
fertilization.
Sperm enters the animal hemisphere—
cortical cytoplasm rotates toward site of
entry.
The gray crescent—a band of pigmented
cytoplasm opposite the site of sperm
entry— is important in development.
Figure 33.1 The Gray Crescent
Concept 33.1 Fertilization Activates Development
Movement of cytoplasm establishes bilateral
symmetry.
Animal and vegetal hemispheres define
anterior–posterior axis of embryo.
Site of sperm entry becomes the ventral
region and gray crescent becomes dorsal
region of embryo.
Concept 33.1 Fertilization Activates Development
The centriole from the sperm initiates
cytoplasmic reorganization.
The centriole causes microtubules in the
vegetal hemisphere to form a parallel array
to guide cytoplasm.
Microtubules also move organelles and
proteins.
Concept 33.1 Fertilization Activates Development
As cytoplasm, proteins, and organelles
move, developmental signals are
distributed.
b-catenin is a key transcription factor from
maternal mRNA, found throughout
cytoplasm.
It is necessary for development of three
embryonic germ layers.
Concept 33.1 Fertilization Activates Development
Glycogen kinase synthase-3 (GSK-3), also
in cytoplasm, phosphorylates and
degrades b-catenin.
GSK-3 inhibitor—found only in the vegetal
cortex of the egg
After fertilization the GSK-3 inhibitor moves
along microtubules to the gray crescent
and prevents degradation of b-catenin.
The result is a higher concentration of bcatenin in the dorsal region.
Figure 33.2 Cytoplasmic Factors Set Up Signaling Cascades (Part 1)
Figure 33.2 Cytoplasmic Factors Set Up Signaling Cascades (Part 2)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Cleavage—a rapid series of cell division,
but no cell growth. Embryo becomes a ball
of small cells.
Blastocoel—a central fluid-filled cavity that
forms in the ball.
The embryo becomes a blastula and its
cells are called blastomeres.
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Complete cleavage occurs in eggs with
little yolk.
Cleavage furrows divide the egg
completely—blastomeres are of similar
size.
However, in frogs, vegetal pole contains
more yolk, division is unequal and
daughter cells in animal pole are smaller.
Figure 33.3 Some Patterns of Cleavage (Part 1)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Incomplete cleavage occurs in eggs with a
lot of yolk when cleavage furrows do not
penetrate.
Discoidal cleavage is common in eggs with
a dense yolk—the embryo forms as a
blastodisc that sits on top of the yolk.
Figure 33.3 Some Patterns of Cleavage (Part 2)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Superficial cleavage is a type of
incomplete cleavage.
Example: In Drosophila, mitosis occurs
without cell division.
A syncytium, a cell with many nuclei, forms.
The plasma membrane grows inward
around nuclei and forms cells.
Figure 33.3 Some Patterns of Cleavage (Part 3)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Mammalian cleavage is slow. During the
fourth division, cells separate into two
groups:
• Inner cell mass—becomes the embryo—
cells are pluripotent and in culture are
embryonic stem cells (ESCs)
• Trophoblast—a sac that forms from the
outer cells—secretes fluid and creates the
blastocoel, with inner cell mass at one end
Embryo is now called a blastocyst.
Figure 33.4 A Human Blastocyst at Implantation (Part 1)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
When the blastocyst arrives in the uterus
the zygote hatches out of the zona
pellucida.
The trophoblast adheres to the
endometrium, or uterine lining.
Implantation occurs when the blastocyst
begins to burrow into the lining.
Figure 33.4 A Human Blastocyst at Implantation (Part 2)
Figure 33.4 A Human Blastocyst at Implantation (Part 3)
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Specific blastomeres rearrange during
development and form specific tissues and
organs.
Fate maps are produced by labeling
blastomeres to identify the tissues and
organs they generate.
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
Blastomeres become determined—
committed to specific development—at
different times.
In mosaic development each blastomere
contributes certain aspects to the adult
animal.
In regulative development, cells
compensate for any lost cells.
Figure 33.5 Fate Map of a Frog Blastula
Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote
If blastomeres separate into two groups,
each can produce an embryo.
Monozygotic twins come from the same
zygote and are identical.
Conjoined twins result from incomplete
separation of the inner cell mass.
Dizygotic twins, or “fraternal twins,” are from
two eggs fertilized by two sperm.
Concept 33.3 Gastrulation Creates Three Tissue Layers
The blastula is transformed into an embryo
during gastrulation, through movement of
cells.
The embryo has multiple tissue layers and
distinct body axes.
During gastrulation three germ layers
form—also called cell layers or tissue
layers
Concept 33.3 Gastrulation Creates Three Tissue Layers
• Endoderm—innermost layer; becomes
the lining of the digestive and respiratory
tracts, pancreas, and liver
• Ectoderm—outer germ layer; becomes
the nervous system, the eyes and ears,
and the skin
• Mesoderm—middle layer; contains cells
that migrate between the other layers;
forms organs, blood vessels, muscle, and
bones
Concept 33.3 Gastrulation Creates Three Tissue Layers
During gastrulation:
• Vegetal hemisphere flattens as cells
change shape
• Vegetal pole bulges inward, invaginates;
cells become endoderm and form the
archenteron, or gut
• Some cells migrate into the central cavity
and become mesenchyme—cells of the
mesoderm layer
Concept 33.3 Gastrulation Creates Three Tissue Layers
Filopodia form and adhere to the ectoderm;
pull the archenteron by contracting
The mouth forms where the archenteron
meets the ectoderm.
The blastopore is the opening of the
invagination of the vegetal pole and
becomes the anus.
Figure 33.6 Gastrulation in Sea Urchins (Part 1)
Figure 33.6 Gastrulation in Sea Urchins (Part 2)
Concept 33.3 Gastrulation Creates Three Tissue Layers
In frogs, gastrulation begins when bottle
cells form in the gray crescent.
Involution occurs as bottle cells move
inward and create the dorsal lip.
Cells from the animal hemisphere move
toward the site of involution—epiboly.
At end of gastrulation, embryo has three
germ layers and dorsal-ventral and
anterior-posterior organization
Figure 33.7 Gastrulation in the Frog Embryo (Part 1)
Figure 33.7 Gastrulation in the Frog Embryo (Part 2)
Figure 33.7 Gastrulation in the Frog Embryo (Part 3)
Concept 33.3 Gastrulation Creates Three Tissue Layers
Hans Spemann experimented with bisecting
fertilized eggs.
Results differed depending on how the eggs
were bisected.
He found that cytoplasmic factors, such as
those in the gray crescent, are necessary
for normal development.
Figure 33.8 Gastrulation and the Gray Crescent
Concept 33.3 Gastrulation Creates Three Tissue Layers
Spemann conducted transplant studies and
showed that the fates of embryonic stem
cells are determined during stages of
gastrulation.
His study with Mangold showed that the
dorsal lip tissue is capable of inducing
embryo formation.
Figure 33.9 The Dorsal Lip Induces Embryonic Organization (Part 1)
Figure 33.9 The Dorsal Lip Induces Embryonic Organization (Part 2)
Concept 33.3 Gastrulation Creates Three Tissue Layers
Dorsal lip tissue is known as the primary
embryonic organizer, or the organizer.
The transcription factor b-catenin is possibly
the key inductive signal.
It has been shown that the presence of bcatenin is necessary and sufficient to form
the organizer.
Concept 33.3 Gastrulation Creates Three Tissue Layers
The organizer changes its activity in order to
induce different structures.
Growth factors in adjacent cells can be
inhibited by organizer cells.
Specific antagonists to growth factors are
produced at different times to influence
patterns of differentiation.
Figure 33.10 Differentiation Can Be Due to Inhibition of Transcription (Part 1)
Figure 33.10 Differentiation Can Be Due to Inhibition of Transcription (Part 2)
Concept 33.3 Gastrulation Creates Three Tissue Layers
Reptiles and birds:
Gastrulation occurs in a flat disk of cells
called the blastodisc.
Some cells enter a fluid space between the
blastodisc and the yolk and form the
hypoblast—a continuous layer that will
contribute to extraembryonic membranes.
Overlying cells form the epiblast—
becomes the embryo.
Concept 33.3 Gastrulation Creates Three Tissue Layers
Epiblast cells move toward the midline and
form a ridge called the primitive streak.
The primitive groove develops along the
primitive streak—cells migrate through it
and become endoderm and mesoderm.
Hensen’s node is the equivalent of the
amphibian dorsal lip and contains many
signaling molecules.
Figure 33.11 Gastrulation in Birds (Part 1)
Figure 33.11 Gastrulation in Birds (Part 2)
Concept 33.3 Gastrulation Creates Three Tissue Layers
Gastrulation patterns are similar in amniotes
but placental mammals lack yolk.
The inner cell mass splits to form the
epiblast (upper layer) and hypoblast (lower
layer).
The embryo forms from the epiblast and the
placenta, formed by extraembryonic
membranes, develops from the hypoblast.
Concept 33.4 Neurulation Creates the Nervous System
Gastrulation produces an embryo with three
germ layers that will influence each other
during development.
During organogenesis, organs and organ
systems develop simultaneously.
Neurulation is the initiation of the nervous
system—occurs in early organogenesis.
Concept 33.4 Neurulation Creates the Nervous System
Steps in neurulation:
• The ectoderm lying over the notochord
thickens and forms the neural plate
• Edges of the neural plate fold and a deep
groove forms
• The folds fuse, forming the neural tube
and a layer of ectoderm
Figure 33.12 Neurulation in a Vertebrate (Part 1)
Figure 33.12 Neurulation in a Vertebrate (Part 2)
Concept 33.4 Neurulation Creates the Nervous System
Signaling molecules such as Noggin,
Chordin, and Sonic hedgehog (Shh) are
released from the notochord and guide
differentiation of the neural tube.
Neural crest cells dissociate from the
neural tube and migrate outward.
They lead development of connections
between the brain and spinal cord and the
rest of the body.
Concept 33.4 Neurulation Creates the Nervous System
The central nervous system develops from
the neural tube of an embryo.
The anterior part of the neural tube
develops into the hindbrain, midbrain,
and forebrain.
The rest of the neural tube becomes the
spinal cord.
Concept 33.4 Neurulation Creates the Nervous System
The embryonic hindbrain and midbrain
become structures that are collectively
known as the brainstem.
They govern physiological functions such as
breathing and heartbeat.
The hindbrain also produces the
cerebellum, which governs motor control
and cognitive functions.
Figure 33.13 Development of the Central Nervous System (Part 1)
Figure 33.13 Development of the Central Nervous System (Part 2)
Concept 33.4 Neurulation Creates the Nervous System
The embryonic forebrain develops into the
cerebral hemispheres—the major
information processing areas.
Forebrain also becomes the underlying
areas:
• Thalamus—major relay station for sensory
information
• Hypothalamus—regulates internal
environment
• Pituitary—hormone function
Figure 33.13 Development of the Central Nervous System (Part 3)
Concept 33.4 Neurulation Creates the Nervous System
Body segmentation develops during
neurulation.
Somites form from mesoderm on either
side of the neural tube.
They produce cells that become the
vertebrae, ribs, muscles, and lower skin
layer.
Transcription factors such as Shh from the
notochord direct the development.
Figure 33.14 Body Segmentation (Part 1)
Figure 33.14 Body Segmentation (Part 2)
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
The amniote egg, with its contained water
supply, frees development from requiring
an external water supply.
Extraembryonic membranes surround
embryos in amniote eggs.
They function in nutrition, gas exchange,
and waste removal.
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
In the chick, four membranes form:
• Yolk sac—encloses yolk within the egg
and passes nutrients to the embryo
• Allantoic membrane—a sac for waste
storage
• Amnion—secretes fluid for protection
• Chorion—reduces water loss and
exchanges gases
Figure 33.15 The Extraembryonic Membranes of Amniotes (Part 1)
Figure 33.15 The Extraembryonic Membranes of Amniotes (Part 2)
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
The yolk sac forms first by extension of the
hypoblast and some mesoderm.
The yolk sac encloses the yolk and forms a
tube continuous with the embryonic gut.
The allantoic membrane grows from
extraembryonic endoderm and
mesoderm—forms the allantois, a sac for
storing wastes.
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
Ectoderm and mesoderm fuse to form two
membranes—the inner amnion and the
outer chorion.
The amnion protects the embryo and
secretes fluid into the amniotic cavity.
The chorion forms a continuous membrane
that limits water loss and exchanges
gases.
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
In placental mammals, the mesodermal
tissues interact with trophoblast tissues to
form the chorion.
The placenta forms from the chorion and
uterine wall—exchanges nutrients, gases,
and wastes.
The amnion surrounds the embryo and is
filled with amniotic fluid.
Figure 33.16 The Mammalian Placenta (Part 1)
Figure 33.16 The Mammalian Placenta (Part 2)
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
Human gestation is divided into trimesters of
about 12 weeks each.
In the first trimester the embryo is very
sensitive to damage from radiation, drugs,
and chemicals.
Gastrulation occurs, tissues differentiate,
and the placenta forms.
By the end of the first trimester, most organs
have started to form and the embryo
becomes a fetus.
Concept 33.5 Extraembryonic Membranes Nourish the Growing
Embryo
During the second and third trimesters the
fetus grows rapidly.
Toward the end of the third trimester the
organ systems mature.
Birth occurs when the last of its critical
organs—the lungs—matures.
Answer to Opening Question
Shh induces part of the anterior neural tube
to differentiate into forebrain structures.
Shh inhibits Pax6, a transcription factor
essential to forming eye fields.
Shh expressed in the midline splits the eye
field into two regions.
If not expressed, Pax6 is not inhibited and
one eye field develops.
Figure 33.17 Environmentally Induced Holoprosencephaly