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Chapter Introduction
Key Events of Development
10.1 Beginnings of the Embryo
10.2 Growth, Differentiation, and Form
10.3 From One Cell to Many: Making the Organism
Developmental Diversity
10.4 Developmental Patterns and
Evolutionary Relationships
10.5 Human Development
10.6 Birth Defects
Mechanisms of Cell Differentiation
10.7 Exploring the Mechanisms of Differentiation
10.8 The Genetic Equivalence of
Differentiating Cells
10.9 Determination and Differentiation
10.10 Cytoplasmic Determination
10.11 Cell-Cell Interactions
Chapter Highlights
Chapter Animations
Learning Outcomes
By the end of this chapter you will be able to:
A Describe the stages of embryonic development in
amphibians and humans.
B Describe how developmental patterns relate to
evolutionary relationships.
C Describe human embryonic development.
D Describe methods used to understand mechanisms
underlying differentiation.
E
Explain the genetic-equivalence hypothesis and describe
experiments performed to test it.
F
Explain determination and the role the cytoplasm has
in this process.
G Discuss examples of cell-cell interactions in differentiation.
Animal Growth and Development
 What characteristics
identify the different body
forms among these eggs?
 What biological processes
must occur before the
juvenile salmon resembles
the adult form?
This photo shows red salmon (Oncorhynchus
nerka) eggs in various stages of development.
Animal Growth and Development
• A newborn baby forms
through a series of
fascinating biological events.
• Biologists use the techniques
of cellular and molecular
biology to unravel the
mysteries—to reveal the
mechanisms that control
an embryo’s development.
This photo shows red salmon (Oncorhynchus
nerka) eggs in various stages of development.
Key Events in Development
10.1 Beginnings of the Embryo
• Development of most new animals begins with
fertilization, the union of a sperm and an egg.
• The sperm and egg,
are also called
gametes.
Note that the sperm are enlarged
x602, much more than the eggs. In
the chicken, frog, and fish, the ova
are surrounded by other materials
(shown in outline).
Key Events in Development
10.1 Beginnings of the Embryo (cont.)
• Animal sperm cells are usually very small and have
a flagellum that they use to swim toward an egg.
• The cytoplasm of the larger egg cells contains yolk,
an energy-rich mass of nutrients.
• Each gamete nucleus has one-half of the
chromosome set found in each of the parent’s cells.
Key Events in Development
10.1 Beginnings of the Embryo (cont.)
• In fertilization, the sperm and egg nuclei fuse,
reestablishing the full chromosome set of the
normal animal cell.
• A fertilized egg, or zygote, is the earliest stage
of the embryo.
A scanning electron micrograph
of contact between a purple sea
urchin (Arbacia puretulata)
sperm and egg, x20,000.
Key Events in Development
10.1 Beginnings of the Embryo (cont.)
• Before fertilization, the eggs of many animals are
metabolically inactive.
• Fertilization stimulates activation which turns on
the egg’s metabolism within seconds of egg-sperm
fusion.
• When the zygote begins to divide, it is as
metabolically active as most adult cells.
Key Events in Development
10.1 Beginnings of the Embryo (cont.)
• Activation has two other major effects:
1. a rapid change in the plasma membrane, which
blocks fertilization by a second sperm
2. a rearrangement of the zygote cytoplasm by
movements in the cytoskeleton that helps produce
differences among cells when they divide
After the unfertilized egg of the sea squirt, Styela clava, (a) is fertilized, polarity is
established (b) when pigments cover a portion of the cytoplasm, resulting in a yellowish
region and a gray, non-pigmented area. These regions are evident at the 2-cell stage
(c), where the yellowish lower hemisphere (which will become the muscle cells of the
tail) is distinct from the dark upper half of the embryo. At this stage, the embryo is
divided into right and left halves. Development continues through the 4-cell stage (d)
to the 32-cell blastula stage (e). The last photograph (f) shows the tadpolelike larva.
Key Events in Development
10.2 Growth, Differentiation, and Form
• Animal development includes growth, cell
specialization, and formation of tissues and organs.
• As the embryonic cells divide, some become
different from others, a process called
differentiation.
• As cells differentiate, they organize to form the
tissues and organs of a complete animal during a
period of development called morphogenesis.
The process of cellular differentiation
Key Events in Development
10.2 Growth, Differentiation, and Form (cont.)
• Each type of cell that differentiates during
development has a unique structure and function.
• Skin cells are tough, thin, flat cells that
are specialized to protect the body.
• Skeletal muscle cells
are filled with protein
fibers that enable
them to contract.
Epidermal cells, x99
Muscle cells, x320
Key Events in Development
10.2 Growth, Differentiation, and Form (cont.)
• Nerve cells have long, thin branches
that are specialized to transmit
information.
• The mature
human red blood
cell lacks a
nucleus and is
specialized to
transport oxygen.
Nerve cells, x3900
Red blood cells, x9000
Key Events in Development
10.2 Growth, Differentiation, and Form (cont.)
• Proteins are the keys to differentiation in
animal cells.
• Specific groups of genes are expressed in each type
of cell, leading to the production of specific proteins.
• Differences between cells in gene expression lead to
differences in cell form and function.
Key Events in Development
10.2 Growth, Differentiation, and Form (cont.)
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• After fertilization, the zygote divides into two cells.
• During this period of development, called cleavage,
the cells usually divide simultaneously, doubling in
number with each cycle.
Early cell divisions in these organisms are directed by the distribution and amount
of yolk in the fertilized egg (not to scale). The invertebrate sea star has a sparse,
evenly distributed yolk, similar to that of the vertebrate frog and mouse. In contrast,
the duck has a dense yolk limited to one area of the egg. From the 16- to 64-cell
stage, an embryo is referred to as a morula.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• By the end of cleavage, the embryo consists of a
mass of many cells called a blastula.
(cont.)
• The cells are usually all of the same general size
and appearance.
• The shape of the blastula depends on the structure
of the original egg and how its yolk is arranged.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• As some cells move from the surface to the
interior of the blastula the embryo becomes a
three-layered gastrula.
(cont.)
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• The three cell layers, called the primary germ
layers, will form all the body’s tissues.
(cont.)
– The outer layer, the ectoderm, will form the skin,
nervous system, and related structures.
– The middle layer, the mesoderm, will produce the
skeleton, muscles, heart, blood, and many other
internal organs.
– The inner layer, called the endoderm, is
usually a tube and will become the lining
of the digestive system.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• Development of a blastula into a gastrula, or
gastrulation, involves major changes.
(cont.)
• Morphogenesis includes:
– Coordinated movements of individual cells and
tissues
– Changing cell shapes or splitting of cell layers
– Formation of tissue masses by local cell division
– Even shaping of organs by genetically timed death
of some cells
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• In vertebrates the general shape, or body plan,
of the organism appears during gastrulation.
(cont.)
– The first mesoderm becomes the notochord, a
stiff rod that develops into part of the backbone.
– Notochord development establishes the anteriorposterior axis, a line running from head to tail
and the dorsal-ventral direction (from the back
to the belly).
– A large head, segmented backbone, and limbs
complete the vertebrate body plan.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
(cont.)
During the early stages
of development, the
structure of this body
plan is directed by a
genetic program.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
(cont.)
• Above the notochord, the dorsal ectoderm folds
up to become the neural tube which will form the
brain, spinal cord, and nerves.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
(cont.)
A vertebrate gastrula possessing a notochord (a) begins the process of
neurulation (formation of the neural tube) when the ectoderm begins to
fold (b). The edges of the U-shaped fold arch toward the midline of the
embryo (c), where the ectoderm fuses to form the neural tube (d). Note
the reorganization of the primary germ layers in the resulting embryo,
called a neurula.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• The tissue interactions that produce the
neural tube establish the foundation on which
the later stages of development are based.
(cont.)
In this cross section of a
vertebrate embryo, note the
approximate locations of the
three primary germ layers.
Key Events in Development
10.3 From One Cell to Many: Making the Organism
• Some animals, such as birds and mammals,
develop directly into young that are like the adult.
(cont.)
• Other animals—such as frogs, sea stars, and
insects—first form a larva (plural: larvae), a feeding
individual that looks nothing like the adult.
• The larva later goes through metamorphosis, a
series of changes that transforms the larva into
an adult.
Development Diversity
10.4 Developmental Patterns and
Evolutionary Relationships
• The basic developmental pattern varies greatly
among animals.
• Developmental patterns are a clue to relationships
among living groups of animals.
• Differences can suggest a more distant relationship
or adaptation to different environments.
• Charles Darwin was among the first biologists to
compare developmental patterns to help determine
relationships among animal species.
Development Diversity
10.4 Developmental Patterns and
Evolutionary Relationships (cont.)
Note the similarities in
these developmental
stages of several
vertebrate animals
that suggest the close
relationship of animals
whose adult forms are
quite different.
Development Diversity
10.4 Developmental Patterns and
Evolutionary Relationships (cont.)
• Similar genes in many animals are responsible for
segmentation, division of the body into a number of
similar sections.
• The body-pattern genes were first discovered in
fruit flies that carried errors in these genes.
• Errors in these homeotic genes can transform one
organ into another.
Development Diversity
10.4 Developmental Patterns and
Evolutionary Relationships (cont.)
• Each homeotic gene acts in a different body
segment and contains one or more copies of a
180-base-pair sequence called a homeobox.
• The homeobox encodes a 60-amino-acid protein
called a homeodomain.
• This part of the protein binds to DNA, regulating the
transcription of important genes.
• Nearly identical gene sequences, named
Hox genes, were found in mice.
Homeotic genes (a) in the fruit fly and Hox genes (b) in the mouse are color-coded
to indicate which gene is active in which body part and to identify which genes
have similar DNA sequences in these two species. Note the similar arrangement of
genes on the chromosomes and its relationship to the embryo segments expressing
those genes.
Development Diversity
10.4 Developmental Patterns and
Evolutionary Relationships (cont.)
• Homeotic genes show that most animals share the
same basic genetic program for the body plan.
• These genes have changed very little during
evolution.
• Changes in homeotic genes often lead to
embryonic death or severe abnormalities.
Development Diversity
10.5 Human Development
• The development of humans and most other
mammals is unique.
• Their embryos develop within the mother where her
body provides a warm, protected environment.
• The mother’s blood circulation provides nutrition
and oxygen to the embryo and takes away wastes
and carbon dioxide.
Development Diversity
10.5 Human Development (cont.)
• About 5 days after fertilization, the embryo, called a
blastocyst, resembles the hollow blastula of other
animals.
• The blastocyst sinks into the
wall of the mother’s uterus
to develop and grow.
In this section of a human blastocyst,
the embryo develops from the inner cell
mass. The thicker part of the blastocyst
seen at the left.
Development Diversity
10.5 Human Development (cont.)
• Part of a thick mass of cells inside the blastocyst
forms the disk that becomes the embryo.
In this section of a human blastocyst,
the embryo develops from the inner cell
mass. The thicker part of the blastocyst
seen at the left.
Development Diversity
10.5 Human Development (cont.)
• The rest of the blastocyst develops into membranes
that surround, nourish, and protect the embryo.
• The amnion immediately surrounds the embryo.
• The chorion encloses all the other membranes and
forms from the blastocyst’s thin outer wall.
A developing human fetus in the uterus, the embryonic membranes, and the
placental connection. Part of the placenta is enlarged to show both fetal and
maternal circulations. The mother’s blood does not mix with that of the fetus;
exchange of materials occurs across the thin membranes that separate the
fetal capillaries from small pools of maternal blood.
Development Diversity
10.5 Human Development (cont.)
• As gastrulation begins, the chorion extends fingerlike
projections, or villi (singular: villus), into the lining of
the uterus.
• The chorionic villi and uterine lining form the
placenta, which exchanges nutrients, wastes,
oxygen, and carbon dioxide between mother and
embryo.
• Blood vessels in the umbilical cord connect the
embryo to the placenta, but the two blood supplies
remain completely separate
Development Diversity
10.5 Human Development (cont.)
• A human takes about 40 weeks to develop in
the uterus.
– After the beginning of the eighth week, the
embryo is called a fetus.
– After 3 months, or the first trimester, most of the
organs have begun to form, and the skeleton can
be seen in ultrasound images.
– Most of the last 3 months, or third trimester,
is a period of rapid growth and maturation of
organ systems.
At 4 weeks after
fertilization, the human
embryo, like all
vertebrates, has a tail.
These features are
even more distinct at
12 weeks.
At 6 weeks, retinal
pigments mark the
eye, and the head is
growing rapidly.
By 14 weeks the fetus is
moving, making facial
expressions, and thumbsucking.
By 8 weeks (c), the
embryo looks human and
has fingers and toes.
At 16 weeks these
movements are more
pronounced.
Development Diversity
10.6 Birth Defects
• Some birth defects are caused by defective genes
and others by environmental factors acting on
normal or abnormal developmental genes.
• Polydactyly, the condition of having extra fingers
or toes, is caused by an altered gene.
Development Diversity
10.6 Birth Defects (cont.)
• Neural-tube defects occur when part of the neural
tube does not close completely.
– In spina bifida, the posterior end of the neural
tube fails to close, and even the body wall
remains open.
– In anencephaly, the anterior part of the tube fails
to close and the exposed brain degenerates and
the top of the skull fails to form.
• Both genes and environmental factors affect neuraltube development.
Development Diversity
10.6 Birth Defects (cont.)
Neural tube closing at 23 days of development (a) leads to normal
development (b). Failure of the anterior end of the neural tube to close
results in anencephaly (c). Failure of the posterior end of the neural
tube to close leads to spina bifida (d). Because the muscles of the legs
are normally controlled by nerves that extend from the posterior end of
the spinal cord, many people with spina bifida are unable to walk (e).
Mechanisms of Cell Differentiation
10.7 Exploring the Mechanisms of Differentiation
• Just describing the development of an embryo
cannot tell us what cellular and molecular
processes control this orderly series of events.
• To find an explanation, a scientist must propose a
testable hypothesis and then design an experiment
to test it.
Mechanisms of Cell Differentiation
10.7 Exploring the Mechanisms of Differentiation (cont.)
• Early experiments with embryos involved
removing certain cells or moved tissues to
new locations.
• A later method involved replacing the nucleus
of an unfertilized egg with the nucleus of a
differentiated cell.
Mechanisms of Cell Differentiation
10.7 Exploring the Mechanisms of Differentiation (cont.)
• Molecular methods now help determine which
genes are active in a particular cell.
• In a method called DNA-RNA hybridization, tagged
DNA molecules are used as probes to detect RNA
with a matching nucleotide sequence.
In DNA-RNA hybridization,
single-stranded DNA is
prepared by heating or
chemical treatment, and a
chemical tag is added. In
cells with complementary
RNA, tagged DNA
hybridizes. The chemical
tag reveals the location of
the complementary RNA.
Mechanisms of Cell Differentiation
10.8 The Genetic Equivalence of Differentiating Cells
• The selective-gene-loss hypothesis proposes
that cells lose some unused genes when it
differentiates.
• The genetic-equivalence hypothesis states that all
cells contain the same genes, but some genes
become inactive during differentiation.
Two possible explanations for differentiation
Mechanisms of Cell Differentiation
10.8 The Genetic Equivalence of Differentiating Cells
(cont.)
• In 1952, Robert Briggs and Thomas King
injected the nuclei of differentiated cells from leopard
frogs (Rana pipiens) into unfertilized frog eggs.
• A nucleus from a blastula, an early stage of embryo
development, supported development of the egg all
the way to becoming a tadpole.
• Development stopped soon after gastrulation when a
nucleus from a skin cell was used showing that the
more differentiated cell still had all of the genes
needed for early development.
Cells from an early
developmental stage retain
the ability to direct formation
of a complete tadpole (a),
while the differentiated skin
cells (b) do not.
Mechanisms of Cell Differentiation
10.8 The Genetic Equivalence of Differentiating Cells
(cont.)
• John Gurdon’s experiments with South African
clawed frogs (Xenopus laevis) showed that an egg
with the nucleus of a differentiated tadpole cell could
develop into a reproducing adult frog.
• The results support the hypothesis that all cells
in an individual are genetically equivalent, but
differentiation does restrict the expression of
some genes.
Mechanisms of Cell Differentiation
10.8 The Genetic Equivalence of Differentiating Cells
(cont.)
This graph shows that
as cells become more
fully differentiated, their
nuclei are less able to
direct development to
the swimming-tadpole
stage.
Mechanisms of Cell Differentiation
10.9 Determination and Differentiation
• Determination is the process by which a cell
commits to a particular course of development.
– In some animal embryos, determination occurs
independently in each cell.
– In other species, cells communicate which affects
each other’s differentiation.
Mechanisms of Cell Differentiation
10.9 Determination and Differentiation (cont.)
• Experiments with two-cell
embryos of a snail and of
a frog demonstrate two
extremes of determination.
• The two frog cells were
separated and allowed to
develop separately, each
producing a complete
tadpole.
Mechanisms of Cell Differentiation
10.9 Determination and Differentiation (cont.)
• The two snail cells did not
produce normal larvae.
• The smaller cell
produced only ectoderm,
and the larger cell made
a mass of mesoderm
and endoderm.
Mechanisms of Cell Differentiation
10.9 Determination and Differentiation (cont.)
• In snails, some proteins and other molecules are
distributed unevenly in the egg cell.
• Cleavage tends to leave each cell with different
cytoplasmic components that later influence gene
expression.
Regulatory molecules are not equally distributed
in the cytoplasm of an egg. As the egg divides,
different molecules may be incorporated in each
offspring cell, activating different genes.
Mechanisms of Cell Differentiation
10.10 Cytoplasmic Determination
• In a snail, a lobe of cytoplasm forms near one pole
of the zygote before it divides.
• When this lobe is removed, the first cleavage
produces two cells of equal size that develop into an
abnormal larva with no heart or intestine.
• This evidence supports the idea that the large cell
becomes mesoderm and endoderm because of
something it receives in the lobe.
Mechanisms of Cell Differentiation
10.10 Cytoplasmic Determination (cont.)
• In the embryos of tunicates (sea squirts), the lobes
(pigments) include RNA and proteins.
• Differentiation in tunicates seems to begin with
movements of the zygote’s cytoplasm that carry
pigment granules and regulatory molecules into
different regions of the cell.
• Cleavage distributes these molecules to
different cells.
Mechanisms of Cell Differentiation
10.10 Cytoplasmic Determination (cont.)
Cytoplasmic determination and larval tail-muscle differentiation is evident in the tunicate
Styela. Yellow crescent pigment persists in the tail-muscle cells. When cells are
separated and allowed to divide, those containing the crescent cytoplasm express
myosin (muscle protein) genes. The others do not.
Mechanisms of Cell Differentiation
10.10 Cytoplasmic Determination (cont.)
• RNA and protein molecules are also distributed
unevenly in fruit-fly eggs.
• Many experiments have supported the hypothesis
that RNA and proteins in egg cytoplasm help control
differentiation by regulating gene expression.
Mechanisms of Cell Differentiation
10.11 Cell-Cell Interactions
• Differentiation is more flexible in vertebrate embryos
than in invertebrates.
• Hans Spemann and Hilde Mangold hypothesized
that a signal from the notochord of a salamander
embryo shifts the neighboring dorsal ectoderm cells
from skin to neural-tube differentiation.
• The process of one embryonic cell influencing
another is called embryonic induction.
Spemann and Mangold’s first transplant experiments
Mechanisms of Cell Differentiation
10.11 Cell-Cell Interactions (cont.)
• Spemann and Mangold’s next experiment
showed that the notochord is the source of
the inducing signal.
Future notochord tissue
of an early gastrula was
removed and
transplanted into a host
embryo as shown.
The transplanted tissue
caused a second point
of infolding and a
second notochord to
appear in the host
embryo, inducing a
second neural tube.
This transplant
produced two embryos
joined at the abdomen.
Mechanisms of Cell Differentiation
10.11 Cell-Cell Interactions (cont.)
• Later experiments showed that the notochord
communicates with the ectoderm by releasing a
substance, not necessarily through direct contact.
• Two inducing proteins that the notochord produces
are called chordin and noggin.
• The chordin and noggin proteins each interferes
with the action of another protein that controls the
production of a family of other proteins.
• These latter proteins regulate the transcription of
many specific genes involved in the development of
nerve cells.
Summary
• The development of most animals begins when gametes come
together at fertilization, creating the zygote.
• The cell divisions of cleavage produce a multicellular blastula,
and the first signs of cell differentiation are seen as the blastula
transforms into a gastrula.
• Gastrulation establishes the three primary germ layers
(ectoderm, mesoderm, and endoderm).
• Further differentiation and morphogenesis produce the tissues
and organs of the larva and adult.
• Developmental biologists focus their experiments on
hypotheses about the control of cell determination,
differentiation, and morphogenesis.
• Developmental differences and similarities provide one
kind of evidence of evolutionary relationships among types
of animals.
Summary (cont.)
• In flies and mice, genes that control morphogenesis have
similar DNA structure and organization, showing the shared
evolutionary ancestry of animals.
• Experimental analysis of cell determination and differentiation
in embryos depends on microsurgical manipulations, cell and
tissue culture, and molecular-biology techniques.
• All of an animal’s cells contain a complete set of genes.
• Determination and differentiation involve selection of the genes
that are expressed in each cell.
• Cytoplasmic determination can distribute gene-regulation
factors to different cells during cleavage, determining the cells’
fates.
• Interactions between cells may influence cell determination
throughout development.
• Cells and tissues exchange molecular signals, stimulating
further specialization of structure and function.
Reviewing Key Terms
Match the term on the left with the correct description.
___
differentiation
c
___
determination
a
___
notochord
d
___
amnion
e
a. process in which a cell commits to
a particular development pathway
b. a DNA sequence that specifies
proteins which regulate
differentiation
___
homeobox
b
c. process in which cells become
specialized
___
gastrula
f
d. a stiff rod that develops into part
of the backbone
e. membrane that encloses the
embryo of a reptile, bird, or
mammal
f.
the two-layered, cup shaped
embryonic stage
Reviewing Ideas
1. What happens to an egg immediately after
it is fertilized? Explain.
Fertilization turns on the egg’s metabolism. This
activation usually occurs within seconds of eggsperm fusion. Cell respiration increases, and
soon new proteins are made, using messenger
RNA molecules already present in the
cytoplasm. When the zygote begins to divide, it
is as metabolically active as most adult cells.
Reviewing Ideas
2. What is spina bifida and when does it occur?
Neural-tube defects such as spina bifida occur
when part of the neural tube does not close
completely. In spina bifida, the posterior end of
the neural tube fails to close, and even the body
wall remains open.
Using Concepts
3. How do developmental patterns provide
evidence of evolutionary relationships?
Developmental patterns are a clue to relationships
among living groups of animals. Even when adult
animals are very different, embryonic similarities
reflect relatedness. Differences can suggest a
more distant relationship or adaptation to different
environments.
Using Concepts
4. What is DNA-RNA hybridization and how is it
used to explore cell differentiation?
Scientists can make large quantities of a particular
gene’s DNA and use chemicals to separate the
DNA’s two strands. Then they attach dye molecules
to the DNA to make it visible. These tagged DNA
molecules are used as probes to detect RNA with a
matching nucleotide sequence. Scientists treat
embryos with DNA probes and examine them with
microscopes. Cells that have transcribed the gene
contain mRNA that matches the probe’s sequence.
The dye bound to the probe marks these cells.
Synthesize
5. How is the development of humans and most
other mammals unique?
Their embryos develop within the mother in a warm,
protected environment. Her blood circulation
provides nutrition and oxygen to the embryo and
takes away wastes and carbon dioxide.
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Chapter Animations
The process of cellular differentiation
Two possible explanations for differentiation
Spemann and Mangold’s first
transplant experiments
The process of cellular differentiation
Two possible explanations for differentiation
Spemann and Mangold’s first transplant experiments
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