Animal Development

Overview: A Body-Building Plan
It is difficult to imagine that each of us began life as a single cell (fertilized egg) called a zygote.
A human embryo at about 6–8 weeks after conception shows development of distinctive features.

How did this complex embryo develop from a single fertilized egg?
1 mm

Development is determined by the zygote’s genome and molecules in the egg cytoplasm called Cytoplasmic determinants.
Cell differentiation is the specialization of cells in structure and function.
Morphogenesis is the process by which an animal takes shape / form.
Model organisms are species that are representative of a larger group and easily studied. Classic embryological studies use the sea urchin, frog, chick, and the nematode C. elegans.

After fertilization, embryonic development proceeds through cleavage, gastrulation, and organogenesis
Important events regulating development occur during fertilization and the three stages that build the animal’s body
Cleavage: cell division creates a hollow ball of cells called a blastula
Gastrulation: cells are rearranged into a three-layered gastrula
Organogenesis: the three germ layers interact and move to give rise to organs.

Fertilization: sperm + egg = zygote
n + n = 2n
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:
Acrosomal Reaction
Cortical Reaction

The Acrosomal Reaction
The acrosomal reaction is triggered when the sperm meets the egg.
The acrosome at the tip of the sperm releases hydrolytic enzymes that digest material surrounding the egg.
Gamete contact and/or fusion depolarizes the egg cell membrane and sets up a fast block to polyspermy.

The acrosomal and cortical reactions during sea urchin fertilization
Basal body
(centriole)
Sperm
head
Sperm-binding
receptors
Acrosome
Jelly coat
Vitelline layer
Egg plasma
membrane
Hydrolytic enzymes
Acrosomal
process
Actin
filament
Sperm
nucleus
Sperm plasma
membrane
Fused
plasma
membranes
Fertilization
envelope
Cortical
granule
Perivitelline
space
EGG CYTOPLASM

The Cortical Reaction
Fusion of egg and sperm also initiates the cortical reaction:
This reaction induces a rise in Ca2+ that stimulates cortical granules to release their contents outside the egg.
These changes cause formation of a fertilization envelope that functions as a slow block to polyspermy.

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.
With these rapid changes in metabolism, the egg is said to be activated.
The sperm nucleus merges with the egg nucleus and cell division begins.

Fertilization in Mammals
Fertilization in mammals and other terrestrial animals is internal.
In mammalian fertilization, the cortical reaction modifies the zona pellucida, the extracellular matrix of the egg, as a slow block to polyspermy.
In mammals the first cell division occurs 12–36 hours after sperm binding.
The diploid nucleus forms after this first division of the zygote.

Fertilization in mammals
Follicle cell
Zona pellucida
Cortical
granules
Sperm
nucleus
Sperm
basal body

Cleavage = Rapid Mitosis / No Mass change
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.
The blastula is a ball of cells with a fluid-filled cavity called a blastocoel.

Cleavage in an echinoderm embryo
(a) Fertilized egg
(b) Four-cell stage
(c) Early blastula
(d) Later blastula

The eggs and zygotes of many animals, except mammals, have a definite polarity.
The polarity is defined by distribution of yolk (stored nutrients).
The vegetal pole has more yolk; the animal pole has less yolk.

The three body axes are established by the egg’s polarity and by a cortical rotation following binding of the sperm.
Cortical rotation exposes a gray crescent opposite to the point of sperm entry.

The body axes and their establishment in an amphibian
(a) The three axes of the fully
developed embryo
(b) Establishing the axes
Pigmented
cortex
Right
First
cleavage
Dorsal
Left
Posterior
Ventral
Anterior
Gray
crescent
Future
dorsal
side
Vegetal
hemisphere
Vegetal pole - yolk
Animal pole
Animal
hemisphere
Point of
sperm
nucleus
entry

Cleavage planes usually follow a pattern that is relative to the zygote’s animal and vegetal poles.
Cell division is slowed by yolk. Yolk can cause uneven cell division at the poles.
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.
Meroblastic cleavage, incomplete division of the egg, occurs in species with yolk-rich eggs, such as reptiles and birds.

Cleavage in a frog embryo
Blastula
(cross
section)
Blastocoel
Animal pole
4-cell
stage
forming
2-cell
stage
forming
Zygote
8-cell
stage
Vegetal
pole:
yolk
0.25 mm
0.25 mm

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.

The blastula consists of a single layer of cells surrounding the blastocoel.
Mesenchyme cells migrate from the vegetal pole into the blastocoel.
The vegetal plate forms from the remaining cells of the vegetal pole and buckles inward through invagination.
The newly formed cavity is called the archenteron.
This opens through the blastopore, which will become the anus.
Gastrulation in the sea urchin embryo:

Gastrulation in a sea urchin embryo
Future ectoderm
Key
Future endoderm
Digestive tube
(endoderm)
Mouth
Ectoderm
Mesenchyme
(mesoderm
forms future
skeleton)
Anus (from blastopore)
Future mesoderm
Blastocoel
Archenteron - cavity
Blastopore
Blastopore
Mesenchyme
cells
Blastocoel
Blastocoel
Mesenchyme
cells
Vegetal Pole
Invagination
Vegetal
plate
Vegetal
pole
Animal
pole
Filopodia
pulling
archenteron
tip
50 µm

The frog blastula is many cell layers thick. Cells of the dorsal lip originate in the gray crescent and invaginate to create the archenteron.
Cells continue to move from the embryo surface into the embryo by involution. These cells become the endoderm and mesoderm.
The blastopore encircles a yolk plug when gastrulation is completed.
The surface of the embryo is now ectoderm, the innermost layer is endoderm, and the middle layer is mesoderm.
Gastrulation in the frog

Gastrulation in a frog embryo
Future ectoderm
Key
Future endoderm
Future mesoderm
SURFACE VIEW
Animal pole
Vegetal pole
Early
gastrula
Blastopore
Blastocoel
Dorsal lip
of blasto-
pore
CROSS SECTION
Dorsal lip
of blastopore
Late
gastrula
Blastocoel
shrinking
Archenteron
Blastocoel
remnant
Archenteron
Blastopore
Blastopore
Yolk plug
Ectoderm
Mesoderm
Endoderm

The embryo forms from a blastoderm and sits on top of a large yolk mass.
During gastrulation, the upper layer of the blastoderm (epiblast) moves toward the midline of the blastoderm and then into the embryo toward the yolk.
The midline thickens and is called the primitive streak.
The movement of different epiblast cells gives rise to the endoderm, mesoderm, and ectoderm.
Gastrulation in the chick

Gastrulation in a chick embryo
Endoderm
Future
ectoderm
Migrating
cells
(mesoderm)
Hypoblast
Dorsal
Fertilized egg
Blastocoel
YOLK
Anterior
Right
Ventral
Posterior
Left
Epiblast
Primitive streak
Embryo
Yolk
Primitive
streak

Organogenesis
During organogenesis, various regions of the germ layers develop into rudimentary organs.
The frog is used as a model for organogenesis.
Early in vertebrate organogenesis, the notochord forms from mesoderm, and the neural plate forms from ectoderm.

Early organogenesis in a frog embryo
Neural folds
Tail bud
Neural tube
(b) Neural tube formation
Neural
fold
Neural plate
Neural
fold
Neural
plate
Neural crest
cells
Neural crest
cells
Outer layer
of ectoderm
Mesoderm
Notochord
Archenteron
Ectoderm
Endoderm
(a) Neural plate formation
(c) Somites
Neural tube
Coelom
Notochord
1 mm
1 mm
SEM
Somite
Neural
crest
cells
Archenteron
(digestive
cavity)
Somites
Eye

The neural plate soon curves inward, forming the neural tube. The neural tube will become the central nervous system = brain and spinal cord.
Neural crest cells develop along the neural tube of vertebrates and form various parts of the embryo: nerves, parts of teeth, skull bones ...
Mesoderm lateral to the notochord forms blocks called somites.
Lateral to the somites, the mesoderm splits to form the coelom.

Organogenesis in a chick embryo is similar to that in a frog
Endoderm
(a) Early organogenesis
Neural tube
Coelom
Notochord
These layers
form extraembryonic
membranes
YOLK
Heart
Eye
Neural tube
Somite
Archenteron
Mesoderm
Ectoderm
Lateral fold
Yolk stalk
Yolk sac
(b) Late organogenesis
Somites
Forebrain
Blood
vessels

Adult derivatives of the three embryonic germ layers in vertebrates
ECTODERM
MESODERM
ENDODERM
Epidermis of skin and its
derivatives (including sweat
glands, hair follicles)
Epithelial lining of mouth
and anus
Cornea and lens of eye
Nervous system
Sensory receptors in
epidermis
Adrenal medulla
Tooth enamel
Epithelium of pineal and
pituitary glands
Notochord
Skeletal system
Muscular system
Muscular layer of
stomach and intestine
Excretory system
Circulatory and lymphatic
systems
Reproductive system
(except germ cells)
Dermis of skin
Lining of body cavity
Adrenal cortex
Epithelial lining of
digestive tract
Epithelial lining of
respiratory system
Lining of urethra, urinary
bladder, and reproductive
system
Liver
Pancreas
Thymus
Thyroid and parathyroid
glands

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.
Amniotes develop extra-embryonic membranes to support the embryo.

During amniote development, four extraembryonic membranes form around the embryo:
The chorion outermost membrane / functions in gas exchange.
The amnion encloses the amniotic fluid.
The yolk sac encloses the yolk.
The allantois disposes of nitrogenous waste products and contributes to gas exchange.
Amniote ExtraEmbryonic Membranes

ExtraEmbryonic Membranes in birds and other reptiles:
Embryo
Amnion
Amniotic
cavity
with
amniotic
fluid
Shell
Chorion
Yolk sac
Yolk
(nutrients)
Allantois
Albumen

Mammalian Development
The eggs of placental mammals
Are small yolk 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.
A group of cells called the inner cell mass develops into the embryo and forms the extraembryonic membranes.
The trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the inner cell mass of the blastocyst forms a flat disk of cells.
As implantation is completed, gastrulation begins.

Early embryonic development of a human
Blastocoel
Trophoblast
Uterus
Endometrial
epithelium
(uterine lining)
Inner cell mass

Early embryonic development of a human
Trophoblast
Hypoblast
Maternal
blood
vessel
Expanding
region of
trophoblast
Epiblast

The epiblast cells invaginate through a primitive streak to form mesoderm and endoderm.
The placenta is formed from the trophoblast, mesodermal cells from the epiblast, and adjacent endometrial tissue.
The placenta allows for the exchange of materials between the mother and embryo.
By the end of gastrulation, the embryonic germ layers have formed. The extraembryonic membranes in mammals are homologous to those of birds and other reptiles and develop in a similar way.

Early embryonic development of a human
Yolk sac (from
hypoblast)
Hypoblast
Expanding
region of
trophoblast
Amniotic
cavity
Epiblast
Extraembryonic
mesoderm cells
(from epiblast)
Chorion (from
trophoblast)

Early embryonic development of a human
Yolk sac
Mesoderm
Amnion
Chorion
Ectoderm
Extraembryonic
mesoderm
Atlantois
Endoderm

Four stages in early embryonic development of a human
Yolk sac
Mesoderm
Amnion
Chorion
Ectoderm
Extraembryonic
mesoderm
Trophoblast
Endoderm
Hypoblast
Expanding
region of
trophoblast
Epiblast
Maternal
blood
vessel
Allantois
Trophoblast
Hypoblast
Endometrial
epithelium
(uterine lining)
Inner cell mass
Blastocoel
Uterus
Epiblast
Amniotic
cavity
Expanding
region of
trophoblast
Yolk sac (from
hypoblast)
Chorion (from
trophoblast)
Extraembryonic
mesoderm cells
(from epiblast)

Morphogenesis in animals involves specific changes in cell shape, position, and adhesion
Morphogenesis is a major aspect of development in plants and animals.
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.

Change in cell shape during morphogenesis
Neural tube
Actin filaments
Microtubules
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.

Role of Cell Adhesion Molecules and the Extracellular Matrix
Cell adhesion molecules located on cell surfaces 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.

Cadherin is required for development of the blastula
Control embryo
Embryo without EP cadherin
0.25 mm
0.25 mm
RESULTS

The developmental fate of cells depends on their history and on inductive signals
Cells in a multicellular organism share the same genome.
Differences in cell types is the result of differentiation, the expression of different genes = differential gene expression.

1. During early cleavage divisions, embryonic cells must become different from one another.
If the egg’s cytoplasm is heterogenous, dividing cells vary in the cytoplasmic determinants they contain.
2. After cell asymmetries are set up, interactions among embryonic cells influence their fate, usually causing changes in gene expression
This mechanism is called induction, and is mediated by diffusible chemicals or cell-cell interactions.
Two general principles underlie differentiation

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.
To understand how embryonic cells acquire their fates, think about how basic axes of the embryo are established.

Fate Mapping for two chordates
Epidermis
(b) Cell lineage analysis in a tunicate
(a) Fate map of a frog embryo
Epidermis
Blastula
Neural tube stage
(transverse section)
Central
nervous
system
Notochord
Mesoderm
Endoderm
64-cell embryos
Larvae
Blastomeres
injected with dye

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 the body axes.
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.
As embryonic development proceeds, potency of cells becomes more limited.
After embryonic cell division creates cells that differ from each other, the cells begin to influence each other’s fates by induction signals.

How does distribution of the gray crescent affect the development potential of the two daughter cells?
Thread
Gray
crescent
Experimental egg
(side view)
Gray
crescent
Control egg
(dorsal view)
EXPERIMENT
Normal
Belly piece
Normal
RESULTS

The Dorsal Lip = “Organizer” of Spemann and Mangold
Based on their famous experiment, Hans Spemann and Hilde Mangold concluded that the blastopore’s dorsal lip is an organizer of the embryo.
The Spemann organizer initiates inductions that result in formation of the notochord, neural tube, and other organs.

Can the dorsal lip of the blastopore induce cells in another part of the amphibian embryo to change their developmental fate?
Primary structures:
Neural tube
Dorsal lip of
blastopore
Secondary
(induced) embryo
Notochord
Pigmented gastrula
(donor embryo)
EXPERIMENT
Primary embryo
RESULTS
Nonpigmented gastrula
(recipient embryo)
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.
The embryonic cells in a limb bud respond to positional information indicating location along three axes
Proximal-distal axis
Anterior-posterior axis
Dorsal-ventral axis

Vertebrate limb development
(a) Organizer regions
Apical
ectodermal
ridge (AER)
Digits
Limb buds
(b) Wing of chick embryo
Posterior
Anterior
Limb bud
AER
ZPA
50 µm
Anterior
2
3
4
Posterior
Ventral
Distal
Dorsal
Proximal

Signal molecules produced by inducing cells influence gene expression in cells receiving them.
Signal molecules lead to differentiation and the development of particular structures.
Hox genes also play roles during limb pattern formation.

Review
Sperm-egg fusion and
depolarization
of egg membrane (fast block to
polyspermy)
Cortical granule release
(cortical reaction)
Formation of fertilization envelope
(slow block to polyspermy)

Review:
Cleavage
frog
embryo
Blastocoel
Animal pole
2-cell
stage
forming
8-cell
stage
Blastula
Vegetal pole:
yolk

Review: Gastrulation / Early Embryonic Development
Sea urchin
Frog
Chick/bird

Review: Early Organogenesis
Neural tube
Coelom
Notochord
Coelom
Notochord
Neural tube

Review: Fate Map of Frog Embryo
Species:
Stage:

You should now be able to:
Describe the acrosomal reaction.
Describe the cortical reaction.
Distinguish among meroblastic cleavage and holoblastic cleavage.
Compare the formation of a blastula and gastrulation in a sea urchin, a frog, and a chick.
List and explain the functions of the extraembryonic membranes.