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Chapter 42
Animal Development
Sections 1-6
Albia Dugger • Miami Dade College
42.1 Mind-Boggling Births
• In vitro fertilization (IVF) is an assisted reproductive
technology which combines egg and sperm outside the body
• Prior to IVF a woman is given hormones to encourage
maturation of multiple eggs and to prevent natural ovulation
• Each zygote undergoes mitotic divisions, forming a blastocyst
that is placed in a woman’s womb to develop to term
• Louise Brown, born in 1978, was the first “test tube baby”
conceived by IVF
Louise Brown with Husband and Son
Nadya Suleman with IVF Octuplets
42.2 Stages of
Reproduction and Development
• Animals as different as sea stars and sea otters pass through
the same stages in their developmental journey from a single,
fertilized egg to a multicelled adult
• In all sexually reproducing animals, a new individual begins
life as a zygote, the diploid cell that forms at fertilization
Sperm penetrates
an egg, the egg and
sperm nuclei fuse,
and a zygote forms.
Fertilization
Mitotic cell divisions yield
a ball of cells, a blastula.
Each cell gets a different
bit of the egg cytoplasm.
Cleavage
Cell rearrangements and
migrations form a gastrula,
an early embryo that has
primary tissue layers.
Gastrulation
Organs form as the result
of tissue interactions that
cause cells to move, change
shape, and commit suicide.
Organs grow in size,
take on mature form,
and gradually assume
specialized functions.
Organ Formation
Growth, Tissue
Specialization
Figure 42-2 p752
Processes of Development
• Fertilization
• Egg and sperm join to form a zygote
• Cleavage (blastula formation)
• Repeated mitotic divisions increase the number of cells
(blastomeres), not the volume
• Gastrulation
• Gastrula (early embryo) forms with two or three germ
layers (forerunners of tissues and organs)
Processes of Development
• Organ formation
• The neural tube and notochord characteristic of all
chordate embryos form early
• Growth and tissue specialization
• Many organs incorporate tissues derived from more than
one germ layer
• In some animals, a larva undergoes metamorphosis – a
drastic remodeling of tissues into the adult form
Three Primary Germ Layers
• Outermost layer (ectoderm) gives rise to nervous tissue and
to the outer layer of skin
• Middle layer (mesoderm) gives rise to muscles, connective
tissues, and the circulatory system.
• Inner layer (endoderm) gives rise to the respiratory tract and
gut linings
Table 42-1 p752
Life Cycle: Leopard Frog
gray
crescent
Figure 42-3b p753
blastocoel
blastula
Figure 42-3b p753
ectoderm
dorsal lip
future gut
cavity
yolk
plug
neural
plate
ectoderm
mesoderm
endoderm
Figure 42-3b p753
neural
tube
notochord
gut cavity
Figure 42-3b p753
Figure 42-3b p753
Figure 42-3b p753
Figure 42-3b p753
ANIMATED FIGURE: Leopard frog life cycle
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Take-Home Message: How does an adult
vertebrate develop from a zygote?
• A zygote undergoes cleavage, which increases the number of
cells. Cleavage ends with formation of a blastula.
• Rearrangement of blastula cells forms a three-layered
gastrula.
• After gastrulation, organs such as the nerve cord begin
forming.
• Continued growth and tissue specialization produce the adult
body.
ANIMATION: Early frog development
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42.3 From Zygote to Gastrula
• Localization of yolk and other material in the egg cytoplasm
and specific cleavage patterns affect early development
• Cytoplasmic localization
• Many cytoplasmic components in an unfertilized egg, are
localized in specific parts of the cytoplasm
• Hans Spemann showed that substances essential to
amphibian development are localized in the gray crescent
animal pole
pigmented
cortex
yolk-rich
cytoplasm
vegetal pole
sperm
penetrating
egg
gray
crescent
fertilized egg
Figure 42-4a p754
gray crescent
of salamander
zygote
First cleavage
plane; gray
crescent split
equally. The
blastomeres
are separated
experimentally.
Two normal larvae
develop from the two
blastomeres.
Figure 42-4b p754
gray crescent
of salamander
zygote
First cleavage
plane; gray
crescent
missed
entirely. The
blastomeres
are separated
experimentally.
A ball of
undifferentiated
cells forms.
Only one
normal larva
develops.
Figure 42-4c p754
ANIMATED FIGURE: Cytoplasmic
localization
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Cleavage –The Start of Multicellularity
• Cleavage divides a fertilized egg into a number of small cells
but does not increase its original volume
• Cleavage puts different parts of the egg cytoplasm into
different cells (blastomeres) which will make them behave
differently later in development
• Each species has a characteristic cleavage pattern
Main Cleavage Patterns
• Protostomes (bilateral invertebrates) undergo spiral
cleavage
• Dorsal lip from gastrulation forms the mouth
• Most deuterostomes (echinoderms and vertebrates) undergo
radial cleavage
• Dorsal lip from gastrulation forms the anus
• Mammals undergo rotational cleavage
Gastrulation
• Starting with gastrulation, cells migrate about and rearrange
themselves
• Experiments by Hilde Mangold showed how gastrulation is
regulated in vertebrates
• Transplanted cells of the dorsal lip of the blastula (descended
from the zygote’s gray crescent) induced gastrulation in
salamanders
Gastrulation in a Fruit Fly
A Dorsal lip excised from donor embryo,
grafted to novel site in another embryo.
Figure 42-6a p755
B Graft induces a second
site of inward migration.
Figure 42-6b p755
C The embryo
develops into a
“double” larva, with
two heads, two tails,
and two bodies
joined at the belly.
Figure 42-6c p755
ANIMATED FIGURE: Embryonic induction
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Take-Home Message: What are the effects of
cytoplasmic localization and cleavage?
• Enzymes, mRNAs, yolk, and other materials are localized in
specific parts of the cytoplasm of unfertilized eggs. This
cytoplasmic localization establishes polarity (an orientation of
the fertilized cell) in the egg and thus influences early
development.
• Cleavage divides a fertilized egg into a number of small cells
but does not increase its original volume. The cells—
blastomeres—inherit different parcels of cytoplasm that make
them behave differently, starting at gastrulation.
42.4 How Specialized
Tissues and Organs Form
• After gastrulation, cells become specialized as their
movement and interaction begin to shape tissues and organs
• Cell differentiation
• Process by which cell lineages become specialized
• Lays the groundwork for formation of specialized tissues
and organs
• Based on selective gene expression
• Signaling molecules contribute to differentiation (cell
signaling)
Responses to Morphogens
• Morphogens
• Signaling molecules encoded by master genes
• Diffuse from a source and form a concentration gradient
throughout the embryo
• Have different effects depending on their concentration in
each region
A Morphogen
• Bicoid protein of fruit flies is an example of a morphogen
• The bicoid gene is a maternal effect gene expressed during
egg production – its product influences development
• Bicoid mRNA accumulates at one end of the egg—an
example of cytoplasmic localization
• A gradient of bicoid protein (a transcription factor) determines
the front-to-back axis of the zygote
Embryonic Induction
• Gastrulation occurs when certain cells of the blastula make
and release short-range signals that cause nearby cells to
move about, either singly or as a cohesive group
• By the process of embryonic induction, cells of one
embryonic tissue alter the behavior of cells in an adjacent
tissue
• Example: Cells of a salamander gastrula’s dorsal lip induce
adjacent cells to migrate inward and become mesoderm
Organ Formation
• After gastrulation, vertebrate organ formation begins with the
neural tube
• Neural tube development is induced by signals from the
notochord, which formed earlier from mesoderm
• Development begins when ectodermal cells overlying the
notochord elongate, forming a thick neural plate
neural plate (ectoderm)
notochord (mesoderm)
neural groove
neural tube
Figure 42-7 p756
ANIMATED FIGURE: Neural tube formation
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Cell Migrations
• Cell migrations are an essential part of development
• Cells travel by inching along in an amoeba-like fashion
• Actin microfilaments cause a portion of the cell to protrude
forward, and adhesion proteins anchor it
• Cells may move in response to a concentration gradient of
some chemical signal or it may follow a “trail” of molecules
that its adhesion proteins recognize
How Cells Migrate
Apoptosis
• Programmed cell death (apoptosis) helps shape body parts
• An internal or external signal sets reactions in motion that
result in the activation of self-destructive enzymes
• Example: Apoptosis eliminates the webbing between digits of
a developing human hand
Apoptosis
Take-Home Message: What processes
differentiate cells, tissues, and organs?
• All cells in an embryo have the same genes, but they express
different subsets of the genome. Selective gene expression is
the basis of cell differentiation. It results in cell lineages with
characteristic structures and functions.
• Cytoplasmic localization results in concentration gradients of
signaling proteins called morphogens. Morphogens activate
sets of master genes, the products of which cause embryonic
cells to form tissues and organs in specific places.
• Migration, shape changes, and death of cells shape
developing organs.
42.5 An Evolutionary View of Development
• Similarities in developmental pathways among animals are
evidence of common ancestry
• Cytoplasmic localization in the egg induces expression of
localized master genes
• Concentration gradients of master gene products cause
embryonic cells to form tissues and organs at certain
locations
A General Model for Animal Development
• Where and when particular genes are expressed determines
how an animal body develops
• Positional information established by concentration gradients
of master gene products affects expression of homeotic
genes, which regulate development of specific body parts
Developmental Constraints
and Modifications
• Physical constraints
• Surface-to-volume ratio
• Architectural constraints
• Existing body frameworks, such as four limbs
• Phylogenetic constraints
• Master genes determine basic body form
Lethal Murtations
• Mutations that alter effects of master genes are often lethal
• Example: Development of somites
• Mesoderm on either side of the neural tube divides into
blocks of cells that will develop into bones and muscles
• A complex pathway involving many genes governs somite
formation – any mutation that disrupts this pathway so that
somites do not form is lethal during development
Lethal Mutation Affecting Somites
Take-Home Message: Why are developmental
processes similar among animal groups?
• In all animals, cytoplasmic localization affects expression of
sets of master genes shared by most animal groups. The
products of these genes cause embryonic cells to form
tissues and organs at certain locations.
• Once a developmental pathway evolves, drastic changes to
genes that govern this pathway are generally lethal.
42.6 Overview of Human Development
• Humans begin life as a single cell and go through a series of
prenatal developmental stages
• Second week: Blastocyst is embedded in the mother’s
uterus, where it develops
• Embryonic period (first 8 weeks): All organs form
• Fetal period (9 weeks to birth): Organs grow and
specialize
• Postnatal growth (after birth): Organ growth and maturation
continues through adolescence to adulthood
Table 42-2 p759
Prenatal and Postnatal Changes
8-week
embryo
12-week
embryo
newborn
2 years
5 years
13 years
(puberty)
22 years
Take-Home Message: How does human
development proceed?
• Humans are placental mammals, so offspring develop in the
mother’s uterus.
• By the end of the second week, the blastocyst is embedded in
the uterus.
• By the end of the eighth week, the embryo has all typical
human organs.
• Most of a pregnancy is taken up with the fetal period, during
which organs grow and begin to function.