Animal Development:
From Genes to Organism
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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.
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Development Begins with Fertilization
• 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.
Figure 20.1 Sperm and Egg Differ Greatly in Size
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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.
• Zygotes constructed in the laboratory with two
male or two female nuclei fail to develop properly.
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Development Begins with Fertilization
• The entry of the sperm into the egg establishes
the polarity of the embryo.
• 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.
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Development Begins with Fertilization
• In frogs, pigments in the egg cytoplasm allow easy
study of development.
• 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.
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Cleavage: Repackaging the Cytoplasm
• 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 (Part 1)
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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.
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Cleavage: Repackaging the Cytoplasm
• 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.
Figure 20.4 Patterns of Cleavage in Four Model Organisms (Part 2)
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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.
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Cleavage: Repackaging the Cytoplasm
• 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.
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Cleavage: Repackaging the Cytoplasm
• 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 (Part 1)
Figure 20.5 The Mammalian Zygote Becomes a Blastocyst (Part 2)
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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.
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Cleavage: Repackaging the Cytoplasm
• 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.
Figure 20.6 Fate Map of a Frog Blastula
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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.
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Cleavage: Repackaging the Cytoplasm
• 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.
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Gastrulation: Producing the Body Plan
• 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 (Part 1)
Figure 20.9 Gastrulation in the Frog Embryo (Part 2)
Figure 20.9 Gastrulation in the Frog Embryo (Part 3)
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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.
Figure 20.10 Spemann’s Experiment
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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.
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Gastrulation: Producing the Body Plan
• 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.
Figure 20.11 The Dorsal Lip Induces Embryonic Organization
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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.
Figure 20.12 Molecular Mechanisms of the Primary Embryonic Organizer
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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.
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Gastrulation: Producing the Body Plan
• 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 (Part 1)
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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.
Figure 20.13 Gastrulation in Birds (Part 2)
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Gastrulation: Producing the Body Plan
• 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.
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Neurulation: Initiating the Nervous System
• 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.
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Neurulation: Initiating 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.
• The anterior end of the neural tube becomes the
brain.
Figure 20.15 Neurulation in the Frog Embryo (Part 1)
Figure 20.15 Neurulation in the Frog Embryo (Part 2)
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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).
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Neurulation: Initiating the Nervous System
• 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.
Figure 20.17 Hox Genes Control Body Segmentation (Part 1)
Figure 20.17 Hox Genes Control Body Segmentation (Part 2)
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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.
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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.
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Extraembryonic Membranes
• 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
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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.
Figure 20.19 The Mammalian Placenta
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Extraembryonic Membranes
• 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 20.20 Chorionic Villus Sampling
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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.
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Human Development
• 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.
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Human Development
• 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
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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.
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Human Development
• 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.