Download The Role of Differential Gene Expression in Cell Differentiation

2007. 9. 12.
<Biology>
Development
(ch.19,20,21)
Sung Su Yea
Dept. of Biochemistry
College of Medicine, Inje Univ.
19
Differential Gene Expression in Development
• The Processes of Development
• The Role of Differential Gene Expression in Cell
Differentiation
• The Roles of Cytoplasmic Segregation and
Induction in Cell Determination
• The Role of Pattern Formation in Organ
Development
• The Role of Differential Gene Expression in
Establishing Body Segmentation
Figure 19.1 Stages of Development
19
The Processes of Development
• Development is a series of progressive changes in shape, form, and
function that occur during an organism’s life cycle.
• The earliest stage is called the embryonic stage.
• Embryos typically acquire food directly or indirectly from a parent.
• Embryonic stages precede birth, and development continues until death.
• Development consists of growth, differentiation, and morphogenesis.
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The Processes of Development
• Growth occurs by cell division and/or expansion.
• Repeated mitotic cell divisions increase cell number.
• An animal embryo may consist of thousands of cells before it becomes larger.
• Differentiation is the process in which cells become specialized structurally and
functionally.
• When the embryo is small, each cell has the potential to develop in many
different ways.
• As development proceeds, the possibilities available to individual cells narrow,
until each cell’s fate is determined and the cell has differentiated.
• Morphogenesis is the shaping of the multicellular body and its organs.
• Morphogenesis results from pattern formation, the organization of differentiated
tissues into specific structures.
• Animal cells can move, which is important during morphogenesis.
• Programmed cell death is essential in the orderly development.
• Staining early embryo cells can produce fate maps revealing which adult
structures derive from which part of the embryo.
Figure 19.2 Developmental Potential in Early Frog Embryos
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The Processes of Development
• Moving a section of cells from a region of an early frog embryo to another
region causes the cells to differentiate appropriately for the new—not the
old—location.
• Cells do not generally maintain this developmental plasticity.
• Later in development, transplanted tissue from an embryo develops into the
original type of tissue, regardless of its new location. At this point the cells are
said to be determined.
• Determination is the commitment of a cell to a particular fate and is
influenced by the extracellular environment and cell contents acting on the
cell’s genome.
• Determination is followed by differentiation.
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The Role of Differential Gene
Expression in Cell Differentiation
• Differentiation results from differential gene expression.
• The fertilized egg is a totipotent cell. That is, it can give rise to all other cell
types of the organism.
• As development proceeds, cells lose their totipotency and become determined,
and then differentiate.
• In some cases, differentiation is irreversible.
• Red blood cells lose their nuclei as they mature.
• Sometimes differentiation is reversible.
• In general, differentiation in plant cells can be reversed more easily than
differentiation in animal cells.
• A carrot root cell can be tricked into forming a whole new plant.
• The new plant is called a clone, because it is genetically identical to the original
plant.
• The ability to generate an entire plant from a single cell is invaluable to
biotechnology.
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The Role of Differential Gene
Expression in Cell Differentiation
• Nuclear transplant experiments have shown that somatic cells contain the
entire genome.
• When the nucleus of an unfertilized frog egg is replaced with the nucleus of a
somatic cell from an early frog zygote, normal early embryos develop.
• These experiments led to two important conclusions:
 No information is lost in the early stages of embryonic development (a
principle known as genomic equivalence).
 The cytoplasmic environment around a nucleus can modify its fate.
• In humans, the totipotency of early embryonic cells permits genetic screening
and in vitro fertilization:
 A single cell is removed from an 8-cell human embryo.
 The cell is tested for harmful genetic conditions.
 Each remaining cell, being totipotent, can be stimulated to divide and form
a normal embryo.
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The Role of Differential Gene
Expression in Cell Differentiation
• In 1997, Ian Wilmut and colleagues
starved sheep cells of nutrients,
which arrested them in the G1 phase.
• These cells were fused with
enucleated eggs from a different ewe
and stimulated to enter the S phase.
• The early embryos were transplanted
to the womb of a surrogate mother
ewe. One lamb, named Dolly,
survived to birth.
• The ultimate goal of sheep cloning is
to develop transgenic (genetically
modified) ewes that can produce
drugs in their milk.
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The Role of Differential Gene
Expression in Cell Differentiation
• Stem cells are undifferentiated, dividing cells that are found even in adults.
• A few examples are those found in bone marrow, skin, and intestine, tissues
which need frequent cell replacement.
• In the body, stem cells have a limited ability to differentiate, but by manipulating
the environment, stem cells can be made to differentiate.
• An example of a stem cell differentiation in response to altered environmental
signals in mice:
 Brain stem cells were transplanted to bone marrow, where they became
bone marrow stem cells and produced blood cells.
 The brain stem cells normally differentiate into nerve cells.
 The reverse experiment resulted in bone marrow cells that differentiated
into brain cells.
• Stem cells with the greatest totipotency are the cells of early embryos.
• Stem cells can be removed from embryos and grown in the lab. They can be
induced to differentiate using certain signal molecules.
• In the future, manipulation and customizing of embryonic cells in culture may
make new disease treatments possible.
Figure 19.6 The Potential Use of Embryonic Stem Cells in Medicine
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The Role of Differential Gene
Expression in Cell Differentiation
• If embryonic stem cells are used to form tissues for a transplantation, genetic
differences between the donor and recipient may result in rejection of the
transplant.
• A procedure called therapeutic cloning has been proposed to address this
problem.
• This process involves fusing a cell nucleus from the recipient with an
enucleated egg cell from a female donor.
• These cells can be induced to differentiate into the desired tissue for
transplantation without the risk of immune system rejection.
• There appears to be genome constancy or equivalence in all somatic cells.
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The Roles of Cytoplasmic Segregation
and Induction in Cell Determination
• To understand what determines differential gene expression, myoblasts,
precursors to muscle cells, have been studied.
• MyoD1 is the first gene switched on and it codes for a helix-loop-helix type
transcription factor (MyoD1), a DNA-binding protein.
• MyoD1 protein binds to the promoters of muscle-specific genes, switching
them on.
• It also binds to its own promoter, keeping itself in the myoblasts and their
descendants.
• Chemical signals are involved in cell differentiation.
• Two overall mechanisms for producing these signals have been found:
 Cytoplasmic segregation occurs when a factor is unequally distributed
in the zygote and ends up in some daughter cells but not others.
 Induction occurs when a factor is produced and secreted by some cells
to induce other cells to differentiate.
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The Roles of Cytoplasmic Segregation
and Induction in Cell Determination
• Polarity is an early event in development.
• Polarity includes the establishment of the
most obvious differentiation, such as
anterior–posterior ends and dorsal–ventral
surfaces.
• When the egg divides, the resulting cells
receive unequal amounts of materials that
were distributed unevenly in the cytoplasm
of the zygote.
• These differences in cytoplasmic makeup
account for some of the earliest
differentiation in embryos.
Figure 19.10 Induction during Vulval Development in C. elegans
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The Role of Pattern Formation
in Organ Development
• Apoptosis is programmed cell death.
• It is caused by the activation of “death” genes.
• Of the 1,090 somatic cells produced by C. elegans, 131 cells are programmed to die.
• The genes ced-4 and ced-3 appear to control this process.
• A third gene, ced-9, codes for an inhibitor of ced-4. Therefore, when cell death is
required, ced-3 and ced-4 are active and ced-9 is inactive.
• Early human embryos have webs between fingers and toes.
• Between day 41 and day 56, the cells of the webbing die,
freeing individual fingers and toes.
• The enzyme caspase stimulates apoptosis and is
homologous to ced-3.
• The protein in humans that inhibits apoptosis is bcl-2,
which is similar to the protein encoded by the C. elegans
gene ced-9.
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The Role of Pattern Formation
in Organ Development
• Certain cells in both plants and animals seem to “know” where they are within
the organism. This is called positional information.
• Positional information usually comes in the form of a signal created by the
concentration gradient of a morphogen.
• There are two requirements for a signal to be considered a morphogen.
 It must directly affect target cells.
 Different concentrations of the signal must cause different effects.
• A vertebrate limb develops from a round bud.
• The cells that become the bones and muscles of the limb must receive
positional information, then organize to shape properly.
• A group of cells at the posterior base of the bud makes a morphogen called
BMP2, whose gradient determines the anterior–posterior axis of the limb.
• Cells that get the highest dose of BMP2 make the thumb, and the smallest dose
results in the little finger.
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The Role of Differential Gene Expression
in Establishing Body Segmentation
• The adult Drosophila has different types of body segments:
 The head is composed of several fused segments. There are three different
thoracic segments, and eight abdominal segments.
 The 13 seemingly identical segments of the Drosophila larva correspond to
the specialized adult segments.
 The process of differentiation begins with establishing the polarity of the
embryo.
• In Drosophila, unequal distribution of morphogens helps establish the basic
coordinates.
• The morphogen molecules are products of specific maternal effect genes
distributed by the mother to the eggs.
• The maternal effect genes are transcribed in the ovarian cells that surround the
developing egg.
• It is their influence that determines the dorsal–ventral and anterior–posterior
axes of the embryo.
• Two maternal effect genes are bicoid, which controls anterior larval
development, and nanos, which controls posterior larval development.
Figure 19.14 Bicoid and Nanos Protein Gradients Provide Positional Information
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The Role of Differential Gene Expression
in Establishing Body Segmentation
• Segmentation genes influence the number, boundaries, and polarity of the
body segments.
• Three classes of segmentation genes act one after another:
 Gap genes organize large areas along the anterior–posterior axis.
 Pair rule genes divide the embryo into units of two segments each.
 Segment polarity genes determine the boundaries of anterior-posterior
segments.
• Finally, homeotic genes are expressed along the length of the body and tell the
segments what to become.
Figure 19.15 A Gene Cascade Controls Pattern Formation in the Drosophila Embryo
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The Role of Differential Gene Expression
in Establishing Body Segmentation
• Development is the result of a sequence of changes, each one triggering the next.
• The unfertilized egg has stored mRNA that supports protein synthesis during early
development.
• Cytoplasmic segregation of the stored mRNA provides positional information.
• mRNA for Bicoid protein is localized at the end of the egg destined to become the
anterior end of the fly.
• The Bicoid and Nanos proteins regulate the expression of the gap genes.
• Bicoid affects transcription, Nanos affects translation.
• High Bicoid at the anterior turns on a gap gene (hunchback), while simultaneously
turning off another gap gene (Krüppel).
• Nanos at the posterior reduces hunchback.
• The gap genes control the expression of pair rule genes.
• The pair rule gene products control the segmentation polarity genes.
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The Role of Differential Gene Expression
in Establishing Body Segmentation
• The homeotic genes specify the properties of each segment. Mutations in
these genes produce changes in segment identity.
• One homeotic gene mutant (Antennapedia) causes legs to grow in the place of
antennae. Bithorax causes an extra pair of wings to grow.
• Antennapedia and bithorax are mutations of adjacent gene clusters.
• The genes in these clusters are arranged on the chromosome in the same order
as the segments they determine.
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The Role of Differential Gene Expression
in Establishing Body Segmentation
• A 180-base-pair DNA sequence that is common to the Antennapedia and
bithorax homeotic genes is called the homeobox.
• It codes for a 60-amino-acid sequence called the homeodomain, which binds
DNA.
• The homeodomain has the helix-turn-helix motif.
• Each homeodomain recognizes a specific DNA sequence.
• Homeotic genes code for transcription factors.
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Animal Development: From Genes to Organism
• Development Begins with Fertilization
• Cleavage: Repackaging the Cytoplasm
• Gastrulation: Producing the Body Plan
• Neurulation: Initiating the Nervous System
• Extraembryonic Membranes
• Human Development
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Development Begins with Fertilization
• Fertilization is the union of a haploid sperm and egg to form a diploid zygote.
• The entry of a sperm into an egg activates the egg metabolically and
initiates the rapid series of divisions that produces the multicellular embryo.
• In many species, the point of entry of the sperm creates an asymmetry in
the radially symmetrical egg.
• This asymmetry enables the bilateral body plan to emerge from the radial
symmetry of the egg.
• Nearly all the cytoplasm of the zygote is from the egg.
• The egg cytoplasm is rich in nutrients, ribosomes, mitochondria, and mRNAs.
• The sperm’s mitochondria degenerate, so all mitochondria in the zygote
come from the egg.
• In many species the sperm contributes a centriole, which becomes the
centrosome of the zygote.
• This produces the mitotic spindles for subsequent cell division.
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Development Begins with Fertilization
• In mammals, certain development genes are active only if they come from a
sperm; others are active only if they come from an egg.
• This phenomenon is called genomic imprinting.
• When cell division occurs, the cytoplasm of the zygote is not distributed
equally among the daughter cells.
• The uneven distribution of cytoplasmic elements results in signal
transduction cascades that orchestrate the three steps of development:
determination, differentiation, and morphogenesis.
• The dense nutrients accumulate in the vegetal hemisphere, which has no
pigment.
• The haploid nucleus of the egg is located in the animal hemisphere. The
outermost (cortical) cytoplasm of this hemisphere is darkly pigmented.
• Specific sperm-binding sites ensure that the sperm always enters the egg
at the animal hemisphere.
• After fertilization, the cortical cytoplasm rotates toward the site of sperm
entry and exposes a less pigmented band opposite the point of sperm entry.
This band is called the gray crescent.
Figure 20.2 The Gray Crescent
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Development Begins with Fertilization
• Organelles and proteins move from the vegetal hemisphere to the gray
crescent.
• b-catenin, a transcription factor, and GSK-3, a protein kinase are found
throughout the cytoplasm, but an inhibitor of GSK-3 is segregated in the
vegetal pole.
• After sperm entry, the inhibitor moves to the gray crescent and prevents
degradation of b-catenin.
• The concentration of b-catenin is higher on the dorsal side than on the
ventral side of the embryo.
• b-catenin is a key player in cell–cell signaling cascade that begins the
process of determination.
Figure 20.3 Cytoplasmic Factors Set Up Signaling Cascades
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Cleavage: Repackaging the Cytoplasm
• Cleavage is the rapid series of mitotic cell divisions that follows fertilization.
• In most animals, there is little cell growth or gene expression.
• The embryo becomes a solid ball of small cells called a morula.
• The ball eventually forms a fluid-filled cavity called a blastocoel, and the
embryo is then called a blastula. The individual cells are blastomeres.
• The pattern of cleavage is influenced by two factors: the amount of yolk
and the formation of mitotic spindles.
• Yolk is the nutrient material stored in an egg. Yolk impedes the formation
of a cleavage furrow.
• In embryos with little or no yolk, all daughter cells tend to be of similar size,
as in the sea urchin.
Figure 20.4 Patterns of Cleavage in Four Model Organisms
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Cleavage: Repackaging the Cytoplasm
• When yolk quantity is larger, asymmetry of cell size is observed.
• In the frog egg, the vegetal hemisphere ends up with fewer but larger cells
than the animal hemisphere.
• Both sea urchins and frogs have complete cleavage.
• In eggs with a lot of yolk, such as the chicken egg, cleavage is incomplete.
The cleavage furrows do not penetrate the yolk.
• The embryo forms a disc of cells, called the blastodisc, on top of the yolk.
• This type of incomplete cleavage is called discoidal cleavage and is
common in birds, reptiles, and fish.
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Cleavage: Repackaging the Cytoplasm
• Another type of incomplete cleavage, called superficial cleavage, occurs in
Drosophila and other insects.
• The yolk is in the center of insect eggs. In early development, mitosis occurs but
not cytokinesis. The nuclei migrate to the periphery of the egg, and the plasma
membrane grows inward, partitioning the nuclei into individual cells.
• Orientation of the mitotic spindles determine the cleavage planes and
arrangements of daughter cells.
• If the mitotic spindles form at right angles or parallel to the animal–vegetal axis,
a radial cleavage pattern results.
• If the mitotic spindles are at oblique angles to the animal–vegetal axis, the
pattern has a twist, and is called spiral cleavage.
• In mammals, the first cell division is parallel to the animal–vegetal axis and the
second cell division occurs at right angles.
• This pattern of cleaves is referred to as rotational cleavage and is unique to
mammals.
• Cleavage in mammals is slow, with divisions occurring 12 to 24 hours apart.
• The cell divisions are not synchronous, so the number of cells in the embryo
does not follow the regular progression (2, 4, 8, 16, 32, etc.) typical of other
species.
Figure 20.5 The Mammalian Zygote Becomes a Blastocyst
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Cleavage: Repackaging the Cytoplasm
• Unlike other animals, gene expression plays a role during mammalian
cleavage.
• At the 8-cell stage of a mammal embryo, the cells form tight junctions and a
compact mass.
• At the transition from the 16-cell to 32-cell stage, the cells separate into
two masses.
• The inner cell mass develops into the embryo; the outer cells become the
trophoblast, which becomes part of the placenta.
• The trophoblast cells secrete fluid which forms the blastocoel. The embryo is
called a blastocyst.
• Fertilization in mammals occurs in the upper oviduct; cleavage occurs as the
zygote travels down the oviduct.
• When the blastocyst arrives in the uterus, the trophoblast adheres to the
uterine wall (the endometrium), which begins the process of implantation.
• Early implantation in the oviduct wall is prevented by the zona pellucida.
• In the uterus, the blastocyst hatches out of the zona pellucida, and
implantation can occur.
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Cleavage: Repackaging the Cytoplasm
• In all animals, cleavage results in the repackaging of the egg cytoplasm
into the cells of the blastula. The cells get different amounts of nutrients
and cytoplasmic determinants.
• In the next stage, the cells of the blastula begin to move and differentiate.
• The cells can be labeled with dyes to determine what tissues and organs
develop from each. Fate maps of the blastula are the result.
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Cleavage: Repackaging the Cytoplasm
• Blastomeres become determined, or committed to a specific fate, at
different times in different animals.
• Roundworm and clam blastomeres are already determined at the 8-cell
stage.
• If one cell is removed, a portion of the embryo fails to develop normally.
This is called mosaic development.
• Other animals have regulative development. If some cells are lost during
cleavage, other cells can compensate.
• If blastomeres are separated in an early stage, two embryos can result.
• Since the two embryos came from the same zygote, they are
monozygotic twins, or genetically identical twins.
• Non-identical twins are the result of two separate eggs fertilized by two
separate sperm and are not genetically identical.
Figure 20.7 Twinning in Humans
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Gastrulation: Producing the Body Plan
• Gastrulation is the process in which a blastula is transformed into an embryo
with three tissue layers and body axes.
• During gastrulation, three germ layers form:
 The inner germ layer is the endoderm and gives rise to the digestive
tract, circulatory tract, and respiratory tract.
 The outer layer, the ectoderm, gives rise to the epidermis and nervous
system.
 The middle layer, the mesoderm, contributes to bone, muscle, liver,
heart, and blood vessels.
• The sea urchin blastula is a simple, hollow ball of cells.
• When gastrulation starts, the cells around the vegetal hemisphere flatten.
The region invaginates into the blastocoel.
• Some cells migrate away from the invaginating region and become primary
mesenchyme cells.
• The invagination becomes the primitive gut or archenteron, and the
mesenchyme cells become mesododerm.
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Gastrulation: Producing the Body Plan
• Secondary mesenchyme cells break free from the tip of the archenteron.
• The secondary mesenchyme cells are attached to the archenteron and
send out extensions to the overlying ectoderm. The extensions contract,
pulling the archenteron inward.
• The region where the archenteron contacts the far side of the sphere
becomes the mouth.
• The anus forms at the region around the origin of the invagination, called
the blastopore.
Figure 20.8 Gastrulation in Sea Urchins
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Gastrulation: Producing the Body Plan
• Amphibian gastrulation begins when cells in the gray crescent change
shape and bulge inward. These cells are called bottle cells.
• The dorsal lip of the blastopore forms here. Successive sheets of cells
move over the lip into the blastocoel in the process of involution.
• The first cells form the archenteron. The following cells form the
mesoderm.
• Cells from the surface animal hemisphere migrate toward the blastopore,
a process called epiboly.
• Gastrulation is complete when three germ layers have been established.
Figure 20.9 Gastrulation in the Frog Embryo
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Gastrulation: Producing the Body Plan
• Experiments by Spemann and
Mangold in the 1920s revealed
much about amphibian
development.
• Spemann constricted salamander
embryos with a single human
baby hair.
• Bisection with a shared gray
crescent produced twins, but if
just one side received a gray
crescent, only that side developed.
• Spemann hypothesized that
cytoplasmic detrminants in the
gray crescent are necessary for
gastrulation.
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Gastrulation: Producing the Body Plan
• The next experiments involved transplanting gastrula tissues onto other
gastrulas.
• In transplants in early gastrulas, the transplanted pieces developed into
tissue that were appropriate for the location where they were placed. The
fates of the cells had not yet been determined.
• If late gastrulas were used, the fates were determined, and transplants did
not develop into the same tissue.
• Next, they transplanted the dorsal lip of the blastopore onto the belly area of
another gastrula.
• A second whole embryo developed.
• Spemann and Mangold called the dorsal lip the primary embryonic organizer.
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Gastrulation: Producing the Body Plan
• The role of b-catenin in gastrulation
has been verified using molecular
biology technology.
• When the production of b-catenin is
depleted by injection of antisense RNA
into the egg, no gastrulation proceeds.
• If b-catenin is experimentally
overexpressed in another region of the
embryo, it can induce a second axis of
embryo formation.
• The protein b-catenin appears to play
critical roles in generating signals that
induce primary embryonic organizer
activity.
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Gastrulation: Producing the Body Plan
• There are a number of known genes necessary for normal left–right
organization of the body.
• If one of these genes is knocked out, it can randomize the left–right
organization of the internal organs.
• The complete details of this mechanism are not fully known.
• It appears that a left–right differential distribution of some of the of some
of the transcription factors triggers a mechanism that acts very early
during gastrulation.
• Bird and reptile embryos have modified gastrulation to adapt to huge yolk
sizes.
• Cleavage forms a blastodisc composed of an epiblast, which will form the
embryo, and a hypoblast, which gives rise to the extraembryonic
membranes.
• The blastocoel is the fluid-filled space between the epiblast and the
hypoblast.
Figure 20.13 Gastrulation in Birds
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Gastrulation: Producing the Body Plan
• Gastrulation begins when cells move toward the midline of the epiblast,
forming a ridge called the primitive streak.
• A primitive groove forms along the primitive streak.
• The primitive groove becomes the blastopore. Cells migrate through it and
become endoderm and mesoderm.
• At the forward region of the groove is Hensen’s node, which is equivalent to
the dorsal lip of the amphibian blastopore.
• Cells that pass over Hensen’s node become determined by the time they
reach their destination.
• Mammal eggs have no yolk.
• The inner cell mass of the blastocyst splits into an epiblast and hypoblast with
a fluid-filled cavity in between.
• The embryo forms from the epiblast; the extraembryonic membranes form
from the hypoblast.
• The epiblast also splits off a layer of cells that form the amnion. The amnion
grows around the developing embryo.
• Gastrulation is similar to that in birds; a primitive groove forms and cells
migrate through it to become endoderm and mesoderm.
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Neurulation: Initiating the Nervous System
• Gastrulation produces an embryo with three germ layers.
• Organogenesis occurs next and involves the formation of organs and organ
systems.
• Neurulation occurs early in organogenesis and begins the formation of the
nervous system in vertebrates.
• The first cells to pass over the dorsal lip become the endodermal lining of the
digestive tract.
• The second group of cells become mesoderm. The dorsal mesoderm closest to
the midline (chordomesoderm) becomes the notochord.
• The notochord is connective tissue and is eventually replaced by the vetebral
column.
• The chordomesoderm induces the overlying ectoderm to begin forming the
nervous system.
• Neurulation begins with thickening of the ectoderm above the notochord to form
the neural plate.
• Edges of the neural plate thicken to form ridges. Between the ridges a grove
forms and deepens.
• The ridges fuse, forming a cylinder—the neural tube.
Figure 20.15 Neurulation in the Frog Embryo
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Neurulation: Initiating the Nervous System
• In humans, failure of the neural tube to close completely at the posterior
end results in spina bifida.
• If the tube fails to close at the anterior end, the result is anencephaly, in
which the forebrain does not develop.
• Neural tube defects can be reduced if pregnant women receive adequate
folic acid (a B vitamin).
• Body segmentation develops during neurulation.
• Blocks of mesoderm called somites form on both sides of the neural tube.
• Somites produce cells that form the vertebrae, ribs, and muscles of the
trunk and limbs. They also guide the organization of the peripheral nerves.
• When the neural tube closes, cells called neural crest cells break loose;
they migrate inward between the epidermis and the somites and under the
somites.
• The neural crest cells give rise to many structures, including peripheral
nerves, which connect to the spinal cord.
Figure 20.16 The Development of Body Segmentation
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Neurulation: Initiating the Nervous System
• As development progresses, the body
segments differentiate.
• Differentiation on the anterior–
posterior axis is controlled by
homeotic genes.
• Four families of genes, called
homeobox or Hox genes, control
differentiation along the body axis in
mice.
• Each family consists of 10 genes and
resides on a different chromosome.
• Temporal and spatial expression of
these genes follows the same pattern
as their linear order on their
chromosomes.
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Neurulation: Initiating the Nervous System
• Other genes give dorsal–ventral position information.
• Sonic hedgehog is an example of a dorsal–ventral gene that is
expressed in the notochord and induces cells in the overlying neural
tube to become ventral spinal cord cells.
• Another family of homeobox genes, the Pax genes, are important in
nervous system and somite development.
• Pax3 is expressed in neural tube cells that will become dorsal spinal
cord cells.
• Pax3 and sonic hedgehog interact to determine dorsal–ventral
differentiation of the spinal cord.
20
Extraembryonic Membranes
• Extraembryonic membranes originate from the germ layers of the embryo
and function in nutrition, gas exchange, and waste removal.
• In the chicken, the yolk sac is the first to form, by extension of the
endodermal tissue of the hypoblast.
• It constricts at the top to create a tube that is continuous with the gut of
the embryo.
• Yolk is digested by the endodermal cells of the yolk sac, and the nutrients
are transported through blood vessels lining the outer surface of the yolk
sac.
• The allantoic membrane, an outgrowth of the extraembryonic ectoderm,
forms the allantois, a sac for storage of metabolic wastes.
• Ectoderm and mesoderm combine and extend beyond the embryo to form
the amnion and the chorion.
• The amnion surrounds the embryo, forming a cavity. The amnion secretes
fluid into the cavity that provides protection for the embryo.
• The chorion form a continous membrane just under the eggshell. It limits
water loss and functions in gas exchange.
Figure 20.18 The Extraembryonic Membranes
20
Extraembryonic Membranes
• In mammals, the first extraembryonic membrane to form is the
trophoblast.
• When the blastocyst hatches from the zona pellucida, the trophoblast
cells attach to the uterine wall, This is the beginning of implantation.
• The trophoblast becomes part of the uterine wall, and sends out villi to
increase surface area and contact with maternal blood.
• The hypoblast cells extend to form the chorion. The chorion and other
tissues produce the placenta.
• The epiblast produces the amnion. Allantoic tissues form the umbilical
cord.
• Cells from the embryo that are in the amniotic fluid can be sampled and
tested for defects. The test is called amniocentesis.
• Problems such as Down syndrome, cystic fibrosis, and Tay Sachs
disease can be detected using this technique.
• A newer technique is chorionic villus sampling which makes earlier
detection possible.
Figure 20.19 The Mammalian Placenta
Figure 20.20 Chorionic Villus Sampling
20
Human Development
• The events of human gestation (pregnancy) are divided into three trimesters.
• The first trimester begins with fertilization. Implantation takes place 6 days later.
• Then gastrulation takes place, the placenta forms, and tissues and organs begin to form.
• The heart first beats at 4 weeks and limbs form at 8 weeks.
• The embryo is particularly vulnerable to radiation, drugs, and chemicals during the first
trimester.
• Hormonal changes can cause major responses in the mother, including morning sickness.
• During the second trimester the fetus grows rapidly to about 600g.
• Fingers, toes, and facial features become well formed.
• Fetal movements are first felt by the mother early in the second trimester.
• By the end of the second trimester, the fetus may suck its thumb.
• The fetus and the mother continue to grow during the third trimester.
• Throughout the third trimester, the fetus remains susceptible to environmental factors such
as malnutrition, alcohol consumption, and cigarette smoking.
• Kidneys produce urine, the liver stores glycogen, and the brain undergoes cycles of sleep
and waking.
Figure 20.21 Stages of Human Development
20
Human Development
• Development does not end at birth.
• The organization of the nervous system exhibits a great deal of plasticity
in the early years, as patterns of connections between neurons develop.
• For example, a child born with misaligned eyes will use mostly one eye.
• The connections to the brain from this eye will become stronger, while
the connections to the other eye will become weak.
• This can be changed if the alignment is corrected within the first three
years.
• A current area of research into this developmental plasticity in the
nervous system examines the role of learning in stimulating the
production and differentiation of new neurons in the brain.
21
Development and Evolutionary Change
• Introduction
• Evolution and Development
• Regulatory Genes and Modularity: Modifying
Morphology
• Plant Development and Evolution
• Environmental Influences on Developmental
Patterns
• Learning: A Modification of Development
21
Regulatory Genes and Modularity:
Modifying Morphology
• Developing embryos exhibit modularity—they are made up of self-contained
units that can be changed independently of the other units, or modules, that
compose the organism.
• There are two ways in which changes in genes that regulate development can
lead to important morphological changes:
 Mutations in genes that regulate developmental processes
 Changes in the time or place of expression of developmental regulatory
genes
• The evolution of webbed feet in ducks provides an example of an altered
spatial expression pattern of a regulatory gene.
• A gene encoding a protein called bone morphogenetic protein 4 (BMP4) is
expressed in the spaces between the developing bones of the toes and
instructs the cells in those spaces to undergo apoptosis, destroying the
webbing between the toes.
• Ducks express a BMP inhibitor protein called Gremlin in their webbing cells.
• This protein prevents the BMP4 protein from signaling for cell death in the
webbing, resulting in a webbed foot.
21
Environmental Influences
on Developmental Patterns
• It is now known that the development of many organisms is very sensitive
to environmental conditions.
• A single genotype may encode a range of phenotypes under different
environmental conditions.
• Signals from the environment can be divided into two major types:
 Environmental signals that are accurate predictors of future conditions.
It is expected that the developmental processes of organisms respond
adaptively to these signals.
 Environmental signals that are poorly correlated with future conditions.
Organisms are unlikely to respond to these signals.
• Developing organisms respond to signals such as day length, temperature,
and precipitation in such a way that the adults they become are adapted
to the predicted conditions.
Figure 21.9 Development of Eyespots in Bicyclus anynana Responds to Temperature
21
Environmental Influences
on Developmental Patterns
• Some organisms need help from another species to complete their
development.
• For example, house mice raised in microbe-free environments do not have
the bacteria that normally colonize their gut.
• These gut bacteria induce gene expression in the mouse intestine, which is
essential for normal capillary development.
• Despite this uncertainty, if the changes have occurred frequently during the
evolution of a species, developmental plasticity may allow individuals to
respond to them.
• The presence or absence of active predators is an example of one of these
uncertain environmental signals.
• Organisms cannot be expected to have evolved appropriate responses to
environmental signals that they have not encountered before.
• This is an important problem because human societies have changed the
environment in so many ways.
• One way humans change the environment is through the release of new
chemical compounds.
• Understanding how chemicals affect development is important because it
may help in the development of less harmful substitutes.