Animal Development 33. Chapter 33 Animal Development Key Concepts 33.1 Fertilization Activates Development 33.2 Cleavage Repackages the Cytoplasm of the.

Animal Development

33

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


Cytoplasmic factors in the egg set up signal cascades in the major steps of development:

• Determination

• Differentiation

• Morphogenesis

• Growth


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.


The animal hemisphere has two pigmented regions:

• Cortical cytoplasm—heavily pigmented

• Underlying cytoplasm—diffusely pigmented

The vegetal hemisphere is not pigmented.


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


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.


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.


As cytoplasm, proteins, and organelles move, developmental signals are distributed.

-catenin is a key transcription factor from maternal mRNA, found throughout cytoplasm.

It is necessary for development of three embryonic germ layers.


Glycogen kinase synthase-3 (GSK-3), also in cytoplasm, phosphorylates and degrades -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 -catenin.

The result is a higher concentration of -catenin 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.


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)


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.



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.



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)


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.




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.


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


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


• 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


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


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)


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)




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


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)


Dorsal lip tissue is known as the primary embryonic organizer, or the organizer.

The transcription factor -catenin is possibly the key inductive signal.

It has been shown that the presence of -catenin is necessary and sufficient to form the organizer.


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)


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.


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)


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.


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)


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.


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.


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)



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



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.


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)


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.


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.


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)


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.


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

Animal Development 33. Chapter 33 Animal Development Key Concepts 33.1 Fertilization Activates Development 33.2 Cleavage Repackages the Cytoplasm of the.

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