Animal
Development
• The question of how a zygote becomes an animal
has been asked for centuries
• As recently as the 18th century, the prevailing
theory was called preformation
• Preformation is the idea that the egg or sperm
contains a miniature infant, or “homunculus,”
which becomes larger during development
WHAT DETERMINES
DEVELOPMENT
• Development is determined by the
zygote’s genome and differences
between embryonic cells
• Cell differentiation is the specialization
of cells in structure and function
• Morphogenesis is the process by which
an animal takes shape
Big ideas
Gametes (fertilizaiton) 
Zygote (cleavage) 
Blastula (gastrulation) 
Gastrula (neurulation) 
Organogenesis
Role of genes & protein concentration
gradients
• Induction: communication from an inducer
to a competent responder
•
•
•
•
•
•
• Fertilization
• 2 major events:
• Fertilization brings the haploid nuclei of sperm
and egg together, forming a diploid zygote
• The sperm’s contact with the egg’s surface
initiates metabolic reactions in the egg that
trigger the onset of embryonic development
• Most info comes from sea urchin studies
– External fertilization
– Problems of external fertilization:
• Dilution/protection of gametes in the enormous volume of the
ocean
• Correct species fertilization
• Blocking polyspermy
The Acrosomal Reaction
• The acrosomal reaction is triggered when
the sperm meets the egg
• This reaction releases hydrolytic enzymes
that digest material surrounding the egg
• Acrosomal process adheres to receptors on
vitelline layer (species specific)
– Sperm/egg membranes fuse, sperm nucleus
enters
– Na+ influx, depolarization
– Depolarization sets up fast block to polyspermy
Fast block polyspermy
Contact and fusion
of sperm and egg
membranes
Acrosomal
reaction
Sperm plasma
membrane
Contact
Basal body
(centriole)
Entry of sperm
nucleus
Sperm
nucleus
Cortical reaction
Acrosomal
process
Sperm
head
Actin
Acrosome
Jelly coat
Sperm-binding
receptors
Fertilization
envelope
Fused plasma
Cortical membranes
granule
Hydrolytic enzymes Perivitelline
space
Vitelline layer
Egg plasma
membrane
EGG CYTOPLASM
Cortical granule
membrane
The Cortical Reaction
• Fusion of egg and sperm also initiates the cortical
reaction
• This reaction induces a rise in Ca2+ in cytoplasm
that stimulates cortical granules to release their
contents outside the egg
• Cortical granules fuse w/ membrane
– Enzymes
– Polysaccharides
– Fertilization envelope formed = slow block to
polyspermy (follows repolarization)
• These changes cause formation of a fertilization
envelope that functions as a slow block to
polyspermy
Fast block polyspermy
500 µm
1 sec before
fertilization
10 sec after
fertilization
Point of
sperm
entry
20 sec
Spreading wave
of calcium ions
30 sec
Activation of the Egg
• The sharp rise in Ca2+ in the egg’s cytosol
increases the rates of cellular respiration and
protein synthesis by the egg cell
– Chemical signals from cortical rxn cause H+ to be
transported out --> increase in pH
• Nuclei fuse
• Egg/sperm differences
– Egg contains proteins, mRNA not found in sperm
– Ca2+ injection, temperature shock can cause artificial
activation
• With these rapid changes in metabolism, the egg
is said to be activated
Minutes
Seconds
LE 47-5
1
Binding of sperm to egg
2
3
4
Acrosomal reaction: plasma membrane
depolarization (fast block to polyspermy)
6
8
10
Increased intracellular calcium level
20
Cortical reaction begins (slow block to polyspermy)
30
40
50
1
Formation of fertilization envelope complete
2
Increased intracellular pH
3
4
5
Increased protein synthesis
10
20
30
40
60
90
Fusion of egg and sperm nuclei complete
Onset of DNA synthesis
First cell division
Fertilization in Mammals
• In mammalian fertilization, the cortical
reaction modifies the zona pellucida as
a slow block to polyspermy
LE 47-6
Follicle
cell
Zona
pellucida
Egg plasma
membrane
Acrosomal
vesicle
Sperm
Cortical
basal
ganules
body Sperm
nucleus
EGG CYTOPLASM
Cleavage
• Fertilization is followed by cleavage, a period
of rapid cell division without growth
• Cleavage partitions the cytoplasm of one
large cell into many smaller cells called
blastomeres
LE 47-7
Fertilized egg
Four-cell stage
Morula
Blastula
• The eggs and zygotes of many animals,
except mammals, have a definite polarity
• The polarity is defined by distribution of yolk,
with the vegetal pole having the most yolk
• The development of body axes in frogs is
influenced by the egg’s polarity
LE 47-8
Point of
sperm entry
Animal
hemisphere
Vegetal
hemisphere
Point of
sperm
entry
Anterior
Right
Ventral
Gray
crescent
Vegetal pole
Future
dorsal
side of
tadpole
First
cleavage
Dorsal
Left
Posterior
Body axes
Animal pole
Establishing the axes
• Cleavage planes usually follow a pattern that
is relative to the zygote’s animal and vegetal
poles
LE 47-9
Zygote
0.25 mm
2-cell
stage
forming
4-cell
stage
forming
Eight-cell stage (viewed
from the animal pole)
8-cell
stage
0.25 mm
Animal pole
Blastula
(cross
section)
Blastocoel
Vegetal pole
Blastula (at least 128 cells)
• Meroblastic cleavage, incomplete division of
the egg, occurs in species with yolk-rich eggs,
such as reptiles and birds
LE 47-10
Fertilized egg
Disk of
cytoplasm
Zygote
Four-cell stage
Blastoderm
Cutaway view of
the blastoderm
Blastocoel
BLASTODERM
YOLK MASS
Epiblast
Hypoblast
• Holoblastic cleavage, complete division
of the egg, occurs in species whose
eggs have little or moderate amounts of
yolk, such as sea urchins and frogs
Gastrulation
• Gastrulation rearranges the cells of a blastula
into a three-layered embryo, called a
gastrula, which has a primitive gut
• The three layers produced by gastrulation
are called embryonic germ layers
– The ectoderm forms the outer layer
– The endoderm lines the digestive tract
– The mesoderm partly fills the space between the
endoderm and ectoderm
Video: Sea Urchin Embryonic Development
• The mechanics of gastrulation in a frog are
more complicated than in a sea urchinINVAGINATION
• OTHERS- INVOLUTION
LE 47-12
CROSS SECTION
SURFACE VIEW
Animal pole
Blastocoel
Vegetal pole
Dorsal lip
of blastopore
Dorsal
tip of
blastopore
Blastula
Blastocoel
shrinking
Archenteron
Ectoderm
Mesoderm
Endoderm
Blastocoel
remnant
Key
Future ectoderm
Future mesoderm
Future endoderm
Yolk plug Yolk plug
Gastrula
• Gastrulation in the chick and frog is
similar, with cells moving from the
embryo’s surface to an interior location
• During gastrulation, some epiblast cells
move toward the blastoderm’s midline
and then detach and move inward
toward the yolk. INVOLUTION
LE 47-13
Epiblast
Primitive
streak
Future
ectoderm
Endoderm
Migrating
cells
(mesoderm)
Hypoblast
YOLK
Organogenesis
• During organogenesis, various regions of the
germ layers develop into rudimentary organs
organs
• Early in vertebrate organogenesis, the
notochord forms from mesoderm, and
the neural plate forms from ectoderm
Video: Frog Embryo Development
LE 47-14a
Neural folds
LM
1 mm
Neural Neural
fold
plate
Notochord
Ectoderm
Mesoderm
Endoderm
Archenteron
Neural plate formation
LE 47-14b
Neural
fold
Neural plate
Neural crest
Outer layer
of ectoderm
Neural crest
Neural tube
Formation of the neural tube
The neural plate
soon curves inward,
forming the neural
tube
LE 47-14c
Somites
Eye
SEM
Neural tube
Notochord
Coelom
Archenteron
(digestive cavity)
Somites
Tail bud
1 mm
Neural
crest
Somite
•Mesoderm lateral
to the notochord
forms blocks
called somites
•Lateral to the
somites, the
mesoderm splits
to form the coelom
LE 47-15
Eye
Neural tube
Notochord
Forebrain
Somite
Heart
Coelom
Archenteron
Endoderm
Mesoderm
Lateral fold
Blood
vessels
Ectoderm
Somites
Yolk stalk
YOLK
Yolk sac
Form extraembryonic
membranes
Early organogenesis
Neural tube
Late organogenesis
•Many structures are derived from the three
embryonic germ layers during organogenesis
Developmental Adaptations of
Amniotes
• Embryos of birds, other reptiles, and mammals
develop in a fluid-filled sac in a shell or the uterus
• Organisms with these adaptations are called
amniotes
• In these organisms, the three germ layers also
give rise to the four membranes that surround the
embryo
LE 47-17
Amnion
Allantois
Embryo
Amniotic
cavity
with
amniotic
fluid
Albumen
Shell
Yolk
(nutrients)
Chorion
Yolk sac
Mammalian Development
• The eggs of placental mammals
– Are small and store few nutrients
– Exhibit holoblastic cleavage
– Show no obvious polarity
• Gastrulation and organogenesis resemble
the processes in birds and other reptiles
• Early cleavage is relatively slow in humans
and other mammals
• At completion of cleavage, the blastocyst forms
• The trophoblast, the outer epithelium of the
blastocyst, initiates implantation in the uterus, and
the blastocyst forms a flat disk of cells
• As implantation is completed, gastrulation begins
• The extraembryonic membranes begin to form
• By the end of gastrulation, the embryonic germ
layers have formed-ECTODERM, MESODERM
AND ENDODERM
LE 47-18a
Endometrium
(uterine lining)
Inner cell mass
Trophoblast
Blastocoel
Blastocyst
reaches uterus.
Maternal
blood
vessel
Expanding
region of
trophoblast
Epiblast
Hypoblast
Trophoblast
Blastocyst
implants.
LE 47-18b
Expanding
region of
trophoblast
Amniotic
cavity
Amnion
Epiblast
Hypoblast
Chorion (from
trophoblast
Yolk sac (from
hypoblast)
Extraembryonic
membranes start
to form and
gastrulation
begins.
Extraembryonic mesoderm cells
(from epiblast)
Allantois
Amnion
Chorion
Ectoderm
Mesoderm
Endoderm
Yolk sac
Extraembryonic
mesoderm
Gastrulation has produced a
three-layered embryo with four
extraembryonic membranes.
• The extraembryonic membranes in mammals
are homologous to those of birds and other
reptiles and develop in a similar way
Morphogenesis in animals
involves specific changes in cell
shape, position, and adhesion
• Morphogenesis is a major aspect of
development in plants and animals
• But only in animals does it involve the
movement of cells
The Cytoskeleton, Cell Motility,
and Convergent Extension
• Changes in cell shape usually involve
reorganization of the cytoskeleton
• Microtubules and microfilaments affect
formation of the neural tube
LE 47-19
Ectoderm
Neural
plate
• The cytoskeleton also drives cell migration, or cell
crawling, the active movement of cells
• In gastrulation, tissue invagination is caused by
changes in cell shape and migration
• Cell crawling is involved in convergent extension,
a morphogenetic movement in which cells of a
tissue become narrower and longer
LE 47-20
Roles of the Extracellular Matrix
and Cell Adhesion Molecules
• Fibers of the extracellular matrix may
function as tracks, directing migrating
cells along routes
• Several kinds of glycoproteins, including
fibronectin, promote cell migration by
providing molecular anchorage for
moving cells
LE 47-21
Direction of migration
50 µm
• Cell adhesion molecules contribute to cell
migration and stable tissue structure
• One class of cell-to-cell adhesion molecule is
the cadherins, which are important in
formation of the frog blastula
LE 47-22
Control embryo
Experimental embryo
The developmental fate of cells
depends on their history and on
inductive signals
• Coupled with morphogenetic changes,
development requires timely differentiation
of cells at specific locations
• Two general principles underlie
differentiation:
– During early cleavage divisions, embryonic cells
must become different from one another
– After cell asymmetries are set up, interactions
among embryonic cells influence their fate,
usually causing changes in gene expression
Fate Mapping
• Fate maps are general territorial
diagrams of embryonic development
• Classic studies using frogs indicated
that cell lineage in germ layers is
traceable to blastula cells
LE 47-23a
Epidermis
Epidermis
Central
nervous
system
Notochord
Mesoderm
Endoderm
Blastula
Fate map of a frog embryo
Neural tube stage
(transverse section)
• Techniques in later studies marked an
individual blastomere during cleavage
and followed it through development
LE 47-23b
Cell lineage analysis in a tunicate
Establishing Cellular
Asymmetries
• To understand how embryonic cells acquire
their fates, think about how basic axes of the
embryo are established
The Axes of the Basic Body
Plan
• In nonamniotic vertebrates, basic instructions
for establishing the body axes are set down
early, during oogenesis or fertilization
• In amniotes, local environmental differences
play the major role in establishing initial
differences between cells and, later, the body
axes
Restriction of Cellular Potency
• In many species that have cytoplasmic
determinants, only the zygote is
totipotent
• That is, only the zygote can develop into
all the cell types in the adult
• Unevenly distributed cytoplasmic
determinants in the egg cell help
establish the body axes
• These determinants set up differences
in blastomeres resulting from cleavage
LE 47-24
Right (experimental):
Left (control):
Fertilized eggs were
Fertilized
constricted by a
salamander eggs
thread so that the
were allowed to
first cleavage plane
divide normally,
restricted the gray
resulting in the
crescent to one
gray crescent
blastomere.
being evenly
divided between
the two blastomeres.
Gray
crescent
Gray
crescent
The two blastomeres were
then separated and
allowed to develop.
Normal
Belly
piece
Normal
• As embryonic development proceeds,
potency of cells becomes more limited
Cell Fate Determination and
Pattern Formation by Inductive
Signals
• After embryonic cell division creates
cells that differ from each other, the
cells begin to influence each other’s
fates by induction
The “Organizer” of Spemann
and Mangold
• Based on their famous experiment,
Spemann and Mangold concluded that
the blastopore’s dorsal lip is an
organizer of the embryo
• The organizer initiates inductions that
result in formation of the notochord,
neural tube, and other organs
LE 47-25a
Pigmented gastrula
(donor embryo)
Dorsal lip of
blastopore
Nonpigmented gastrula
(recipient embryo)
LE 47-25b
Primary embryo
Secondary (induced) embryo
Primary
structures:
Neural tube
Notochord
Secondary
structures:
Notochord (pigmented cells)
Neural tube (mostly nonpigmented cells)
Formation of the Vertebrate
Limb
• Inductive signals play a major role in pattern
formation, development of spatial
organization
• The molecular cues that control pattern
formation are called positional
information
• This information tells a cell where it is
with respect to the body axes
• It determines how the cell and its
descendents respond to future
molecular signals
• The wings and legs of chicks, like all
vertebrate limbs, begin as bumps of tissue
called limb buds
LE 47-26a
Anterior
AER
Limb bud
ZPA
Posterior
Apical
ectodermal
ridge
Organizer regions
50 µm
• The embryonic cells in a limb bud
respond to positional information
indicating location along three axes
LE 47-26b
Digits
Anterior
Ventral
Proximal
Dorsal
Wing of chick embryo
Distal
Posterior
• One limb-bud organizer region is the apical
ectodermal ridge (AER)
• The AER is thickened ectoderm at the bud’s
tip
• The second region is the zone of polarizing
activity (ZPA)
• The ZPA is mesodermal tissue under the
ectoderm where the posterior side of the
bud is attached to the body
• Tissue transplantation experiments support
the hypothesis that the ZPA produces an
inductive signal that conveys positional
information indicating “posterior”
LE 47-27
Anterior
Donor
limb
bud
New
ZPA
Host
limb
bud
ZPA
Posterior
• Signal molecules produced by inducing cells
influence gene expression in cells receiving
them
• Signal molecules lead to differentiation and
the development of particular structures