Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 38
Reproduction and Development
(Sections 38.9 - 38.11)
Albia Dugger • Miami Dade College
38.9 Overview of Animal Development
• All sexually reproducing animals begin life as a zygote, the
diploid cell that forms at fertilization
• The same development steps and processes occur in all
vertebrates – evidence of their common ancestry
5 Stages of Vertebrate Development
• Fertilization
• Sperm penetrates an egg, the egg and sperm nuclei fuse,
and a zygote forms
• Cleavage
• Mitotic cell divisions yield a ball of cells (blastula); each
cell gets a different bit of the egg cytoplasm
• Gastrulation
• Cell rearrangements and migrations form a gastrula, an
early embryo that has primary tissue layers
5 Stages of Vertebrate Development
• Organ formation
• Organs form as the result of tissue interactions that cause
cells to move, change shape, and commit suicide
• Growth and tissue specialization
• Organs grow in size, take on mature form, and gradually
assume specialized functions
Overview: Frog Development
transformation to
adult nearly complete
adult, three
years old
Sexual reproduction
(gamete formation,
external fertilization)
tadpole
larva (tadpole)
organ
formation
cleavage
eggs
and
sperm
zygote
Fig. 38.13, p. 642
Overview: Frog Development
transformation to
adult nearly complete
adult, three
years old
Sexual reproduction
(gamete formation,
external fertilization)
tadpole
larva (tadpole)
organ
formation
eggs
and
sperm
cleavage
zygote
Stepped Art
Fig. 38.13, p. 642
Details of Frog Development (1)
• Cleavage divides a zygote’s cytoplasm into smaller
blastomeres
• Number of cells increases, but the zygote’s original volume
remains unchanged
• cleavage
• Mitotic division of an animal cell
Details of Frog Development (1)
Details of Frog Development (1)
gray
crescent
Here we show the first three divisions of cleavage, a process that
carves up a zygote’s cytoplasm. In this species, cleavage results in a
blastula, a ball of cells with a fluid-filled cavity.
1
Fig. 38.13.1, p. 643
Details of Frog Development (2)
• In this species, cleavage results in a blastula, a ball of cells
with a fluid-filled cavity (blastocoel)
• Tight junctions hold cells of the blastula together
• blastula
• Hollow ball of cells that forms as a result of cleavage
Details of Frog Development (2)
Details of Frog Development (2)
blastocoel
blastula
Cleavage is over when the
blastula forms.
2
Fig. 38.13.2, p. 643
Details of Frog Development (3)
• The blastula becomes a three-layered gastrula by the
process of gastrulation: Cells at the dorsal lip migrate inward
and start rearranging themselves
• gastrula
• Three-layered developmental stage formed by gastrulation
• gastrulation
• Cell movements that produce a three-layered gastrula
Germ Layers
• A gastrula consists of three primary tissue layers (germ
layers)
• Three germ layers give rise to the same types of tissues and
organs in all vertebrates – evidence of a shared ancestry
• germ layer
• One of three primary layers in an early embryo
Three Embryonic Germ Layers
• ectoderm
• Outermost tissue layer of an animal embryo
• endoderm
• Innermost tissue layer of an animal embryo
• mesoderm
• Middle tissue layer of a three-layered animal embryo
Details of Frog Development (3)
Details of Frog Development (3)
ectoderm
dorsal lip
future gut
cavity
yolk
plug
neural
plate
ectoderm
mesoderm
endoderm
The blastula becomes a three-layered gastrula—a process called
gastrulation.
At the dorsal lip (a fold of ectoderm above the first opening that appears in the
blastula) cells migrate inward and start rearranging themselves.
3
Fig. 38.13.3, p. 643
Details of Frog Development (4)
• Organs begin to form as a primitive gut cavity opens up
• A neural tube, then a notochord and other organs, form from
the primary tissue layers
• Many organs incorporate tissues derived from more than one
germ layer
Details of Frog Development (4)
Details of Frog Development (4)
neural
tube
notochord
gut cavity
Organs begin to form as a primitive gut cavity
opens up. A neural tube, then a notochord and other
organs, form from the primary tissue layers.
4
Fig. 38.13.4, p. 643
Details of Frog Development (5)
• In frogs, and some other animals, a larva undergoes
metamorphosis: a remodeling of tissues into an adult form
• The tadpole is a swimming larva with segmented muscles and
notochord extending into a tail
• During metamorphosis, the frog grows limbs, and the tadpole
tail is absorbed
Details of Frog Development (5)
Tadpole
Metamorphosis
Sexually
mature, fourlegged adult
frog
ANIMATION: Leopard frog life cycle
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38.10 Early Marching Orders
• Egg cytoplasm includes yolk proteins, mRNA transcripts,
tRNAs and ribosomes, and other proteins
• Some cytoplasmic components are not distributed evenly, but
localized in one particular region or another
• cytoplasmic localization
• Accumulation of different materials in different regions of
the egg cytoplasm
Cytoplasmic Localization
• In a yolk-rich egg, the vegetal pole has most of the yolk and
the animal pole has little
• In some amphibian eggs, pigment molecules accumulate in
the cell cortex, close to the animal pole
• After fertilization, a gray crescent forms, where substances
essential to development are localized
Experiment: Cytoplasmic Localization
• At fertilization,
cytoplasm shifts, and
exposes a gray
crescent opposite the
sperm’s entry point
• First cleavage normally
distributes half of the
gray crescent to each
descendant cell
Experiment:
Cytoplasmic
Localization
animal pole
pigmented
cortex
yolk-rich
cytoplasm
vegetal pole
sperm
penetrating
egg
gray
crescent
fertilized egg
A Many amphibian eggs have a dark pigment
concentrated in cytoplasm near the animal
pole. At fertilization, the cytoplasm shifts, and
exposes a gray crescent-shaped region just
opposite the sperm’s entry point. The first
cleavage normally distributes half of the gray
crescent to each descendant cell.
Fig. 38.14a, p. 644
ANIMATION: Cytoplasmic localization
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Experiment: Cytoplasmic Localization
• In one experiment, the
first two cells formed by
normal cleavage were
physically separated
from each other
• Each cell developed
into a normal larva
Experiment:
Cytoplasmic
Localization
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.
B In one experiment, the first two cells
formed by normal cleavage were physically separated from each other. Each
cell developed into a normal larva.
Fig. 38.14b, p. 644
Experiment: Cytoplasmic Localization
• In another experiment,
one descendant cell
received all the gray
crescent, and
developed normally
• The other gave rise to
an undifferentiated ball
of cells
Experiment:
Cytoplasmic
Localization
gray crescent
of salamander
zygote
First cleavage
plane; gray
crescent missed
entirely. The
blastomeres
are separated
experimentally.
A ball of
Only one
undifferentiated normal larva
cells forms.
develops.
C In another experiment, a zygote was
manipulated so one descendant cell
received all the gray crescent. This cell
developed normally. The other gave rise
to an undifferentiated ball of cells.
Fig. 38.14c, p. 644
Cleavage: The Start of Multicellularity
• During cleavage, a furrow appears on the cell surface and
defines the plane of the cut
• The plane of division is not random – it dictates what types
and proportions of materials a blastomere will get
• Each species has a characteristic cleavage pattern
From Blastula to Gastrula
• At gastrulation, certain cells at the embryo’s surface move
inward through an opening on the surface
• Cells in the dorsal (upper) lip of the opening are descended
from a zygote’s gray crescent
• Gastrulation is caused by signals from dorsal lip cells
Gastrulation in a Fruit Fly
• The opening cells move in through will become the fly’s
mouth; descendants of stained cells will form mesoderm
Gastrulation in a Fruit Fly
Fig. 38.15a, p. 645
Gastrulation in a Fruit Fly
Fig. 38.15b, p. 645
Gastrulation in a Fruit Fly
Fig. 38.15c, p. 645
Gastrulation in a Fruit Fly
Fig. 38.15d, p. 645
Experiment: Dorsal Lip Transplant
• Dorsal lip of a salamander embryo was transplanted to a
different site in another embryo – a second set of body parts
started to form
Experiment: Dorsal Lip Transplant
A Dorsal lip excised from donor embryo,
grafted to novel site in another embryo.
Fig. 38.16a, p. 645
Experiment: Dorsal Lip Transplant
B Graft induces a second
site of inward migration.
Fig. 38.16b, p. 645
Dorsal Lip Transplant (cont.)
• The embryo develops into a “double” larva, with two heads,
two tails, and two bodies joined at the belly
Dorsal Lip Transplant (cont.)
C The embryo
develops into a
“double” larva,
with two heads,
two tails, and two
bodies joined at
the belly.
Fig. 38.16c, p. 645
ANIMATION: Embryonic induction
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Specialized Cells, Tissues, and Organs
• All cells in an embryo have the same genes
• Selective gene expression causes different cell lineages in
the embryo to express different subsets of genes
• Selective gene expression is the key to cell differentiation –
the process by which cell lineages become specialized in
composition, structure, and function
Cell Differentiation
• An adult human has about 200 differentiated cell types
• Example: Cells of one lineage turn on genes for crystallin,
a transparent protein that forms the lens of the eye – no
other cells in the body make crystallin
• A differentiated cell still retains the entire genome
• It is possible to clone an adult animal (a genetic copy)
from one of its differentiated cells
Cell Communication in Development
• Long-range intercellular signals (morphogens) diffuse out
from certain embryonic cells and form a concentration
gradient in the embryo that controls differentiation
• morphogen
• Chemical encoded by a master gene; diffuses out from its
source and affects development
• Effects on target cells depend on its concentration
Cell Communication in Development
• Other chemical signals only operate at close range
• Example: Cells of a salamander gastrula’s dorsal lip cause
adjacent cells to migrate inward and become mesoderm
• embryonic induction
• Embryonic cells produce signals that alter the behavior of
neighboring cells
Cell Movements and Apoptosis
• Long-range and short-range signals regulate development of
tissues and organs
• Organs begin to form as cells migrate, entire sheets of tissue
fold and bend, and specific cells die on cue
Cell Movements in the Brain
• Neurons form in the
center of the brain, then
creep along extensions
of glial cells or axons of
other neurons until they
reach their final position
• Once in place, they
send out axons
Cell
Movements in
the Brain
A Cells migrate. This graphic
shows one embryonic neuron
(orange) at successive times
as it migrates along a glial cell
(yellow). Its adhesion proteins
stick to glial cell proteins.
Fig. 38.17a, p. 646
Neural Tube Formation
• Sheets of cells expand and fold to form the neural tube
1. Gastrulation produces a sheet of ectodermal cells
2. Cells at the embryo’s midline elongate and neighboring
cells become wedge-shaped, forming a neural groove
3. Edges of the groove move inward, and flaps of tissue fold
and meet at the midline, forming the neural tube
• The neural tube later develops into the brain and spinal cord
Neural Tube Formation
Neural Tube Formation
Gastrulation
produces a sheet of
ectodermal cells.
1
A neural groove
forms as microtubules
constrict or lengthen
in different cells,
making the cells
change shape.
2
Edges of the
groove meet and
detach from the
main sheet, forming
the neural tube.
neural groove
3
neural tube
B Cells change shape. Here, shape changes in ectodermal
cells form a neural tube.
Fig. 38.17b, p. 646
Apoptosis
• Signals from certain cells activate self-destruction in target
cells (apoptosis), which helps sculpt body parts
• Apoptosis causes a tadpole to lose its tail and it separates the
digits of the developing human hand
• apoptosis
• Mechanism of cell suicide
Apoptosis
• As a human hand develops, cells in the webs of skin between
digits die
ANIMATION: Formation of human fingers
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Pattern Formation
• Pattern formation is the process by which certain body parts
form in a specific place
• Example: Signals from AER (apical ectodermal ridge) at the
tips of limb buds induce mesoderm beneath it to form a limb
• pattern formation
• Formation of body parts in specific locations
Experiment: AER and Limb Formation
• AER of a limb bud tells mesoderm under it to form a limb
• If AER from a chick’s wing bud is removed, wing development
stops
• Earlier positional cues determined what the mesoderm will
become – mesoderm from a chick’s hindlimb implanted under
wing AER forms a leg
Experiment: AER and Limb Formation
Experiment: AER and Limb Formation
mesoderm of chick
embryo forelimb
AER (region of signalsending ectoderm)
A Experiment 1:
Remove wing
bud’s AER
AER
removed
B Experiment 2:
Graft a bit of leg
mesoderm under
the AER of a wing
mesoderm wing
from leg
no limb
forms
leg forms
Fig. 38.18, p. 647
Experiment: AER and Limb Formation
mesoderm of chick
embryo forelimb
AER (region of signalsending ectoderm)
A Experiment 1:
Remove wing
bud’s AER
AER
removed
B Experiment 2:
Graft a bit of leg
mesoderm under
the AER of a wing
mesoderm wing
from leg
no limb
forms
leg forms
Stepped Art
Fig. 38.18, p. 647
Evolution and Development
• Where and when particular genes are expressed determines
how an animal body develops:
• Localized molecules in an unfertilized egg induce
expression of master genes in the zygote
• Products of master genes form gradients in the embryo
• Depending on where they fall within these gradients, cells
activate or suppress other genes
Evolution and Development (cont.)
• Positional information set up by concentration gradients of
products of master genes affects expression of homeotic
genes, which regulate development of specific body parts
• All animals have similar homeotic genes
• homeotic gene
• Type of master gene; its expression controls formation of
specific body parts during development
Evolution and Development (cont.)
• Evolution of body plans are influenced by physical constraints
(such as surface-to-volume ratio) and existing body
framework (such as four limbs)
• Interactions among master genes also restrain evolution,
since a major change in any one probably would be lethal
• Mutations led to a variety of forms among animal lineages by
modifying existing developmental pathways, rather than
entirely new genetic innovations
Key Concepts
• Principles of Development
• The same processes regulate development of all animals
• Division of the single-celled zygote distributes different
materials to different cells
• These cells go on to express different master genes that
regulate formation of body parts in particular places
ANIMATION: Early frog development
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