Principles of
Development
Chapter 8
Key Events in Development
 Development describes
the changes in an organism
from its earliest beginnings
through maturity.
 Search for commonalities.
Key Events in Development
 Specialization of cell types occurs as a
hierarchy of developmental decisions.
 Cell types arise from conditions created in preceding
stages.
 Interactions become increasingly restrictive.
 With each new stage:
 Each stage limits developmental fate.
 Cells lose option to become something different
 Said to be determined.
Key Events in Development
 The two basic processes responsible for this
progressive subdivision:
 Cytoplasmic localization
 Induction
Fertilization
 Fertilization is the initial event in development
in sexual reproduction.
 Union of male and female gametes
 Provides for recombination of paternal and
maternal genes.
 Restores the diploid number.
 Activates the egg to begin development.
Fertilization
 Oocyte Maturation
 Egg grows in size by accumulating yolk.
 Contains much mRNA, ribosomes, tRNA and
elements for protein synthesis.
 Morphogenetic determinants direct the activation
and repression of specific genes later in postfertilization development.
 Egg nucleus grows in size, bloated with RNA.
 Now called the germinal vesicle.
Fertilization
 Most of these preparations in the egg occur
during the prolonged prophase I.
 In mammals
 Oocyte now has a highly structured system.
 After fertilization it will support nutritional
requirements of the embryo and direct its
development through cleavage.
 After meiosis resumes, the egg is ready to fuse
its nucleus with the sperm nucleus.
Fertilization
 A century of
research has been
conducted on
marine
invertebrates.
 Especially sea
urchins
Contact Between Sperm &
Egg
 Broadcast spawners often
release a chemotactic
factor that attracts sperm
to eggs.
 Species specific
 Sperm enter the jelly
layer.
 Egg-recognition proteins
on the acrosomal process
bind to species-specific
sperm receptors on the
vitelline envelope.
Fertilization in Sea Urchins
 Prevention of
polyspermy – only one
sperm can enter.
 Fast block
 Depolarization of
membrane
 Slow block
 Cortical reaction
resulting in fertilization
membrane
Fertilization in Sea Urchins
 The cortical reaction follows the fusion of
thousands of enzyme-rich cortical granules with
the egg membrane.
 Cortical granules release contents between the
membrane and vitelline envelope.
 Creates an osmotic gradient
 Water rushes into space
 Elevates the envelope
 Lifts away all bound sperm except the one sperm
that has successfully fused with the egg plasma
membrane.
Fertilization in Sea Urchins
Fertilization in Sea Urchins
 One cortical granule
enzyme causes the
vitelline envelope to
harden.
 Now called the
fertilization
membrane.
 Block to polyspermy is
now complete.
 Similar process occurs
in mammals.
Fertilization in Sea Urchins
 The increased Ca2+
concentration in the
egg after the cortical
reaction results in an
increase in the rates
of cellular respiration
and protein
synthesis.
 The egg is
activated.
Fusion of Pronuclei
 After sperm and egg membranes fuse, the sperm loses
its flagellum.
 Fusion of male and female pronuclei forms a diploid
zygote nucleus.
Cleavage
 Cleavage – rapid
cell divisions
following fertilization.
 Very little growth
occurs.
 Each cell called a
blastomere.
 Morula – solid ball
of cells. First 5-7
divisions.
Polarity
 The eggs and zygotes of many animals (not mammals)
have a definite polarity.
 The polarity is defined by the distribution of yolk.
 The vegetal pole has the most yolk and the animal
pole has the least.
Body Axes
 The development of body axes
in frogs is influenced by the
polarity of the egg.
The polarity of the egg determines
the anterior-posterior axis before
fertilization.
At fertilization, the pigmented cortex slides
over the underlying cytoplasm toward the
point of sperm entry. This rotation (red
arrow) exposes a region of lighter-colored
cytoplasm, the gray crescent, which is a
marker of the dorsal side.
The first cleavage division bisects the
gray crescent. Once the anteriorposterior and dorsal-ventral axes are
defined, so is the left-right axis.
Amount of Yolk
 Different types of animals
have different amounts of
yolk in their eggs.
 Isolecithal – very little
yolk, even distribution.
 Mesolecithal – moderate
amount of yolk
concentrated at vegetal
pole.
 Telolecithal – Lots of yolk
at vegetal pole.
 Centrolecithal – lots of
yolk, centrally located.
Cleavage in Frogs
 Cleavage planes usually
follow a specific pattern
that is relative to the
animal and vegetal poles
of the zygote.
 Animal pole blastomeres
are smaller.
 Blastocoel in animal
hemisphere.
 Little yolk, cleavage
furrows complete.
 Holoblastic cleavage
Cleavage in Birds
 Meroblastic
cleavage,
incomplete division
of the egg.
 Occurs in species
with yolk-rich eggs,
such as reptiles and
birds.
 Blastoderm – cap
of cells on top of
yolk.
Direct vs. Indirect
Development
 When lots of nourishing yolk is present, embryos
develop into a miniature adult.
 Direct development
 When little yolk is present, young develop into larval
stages that can feed.
 Indirect development
 Mammals have little yolk, but nourish the embryo via
the placenta.
Blastula
 A fluid filled cavity, the blastocoel, forms within
the embryo – a hollow ball of cells now called a
blastula.
Gastrulation
 The morphogenetic
process called
gastrulation
rearranges the cells of
a blastula into a threelayered (triploblastic)
embryo, called a
gastrula, that has a
primitive gut.
 Diploblastic
organisms have two
germ layers.
Gastrulation
 The three tissue layers produced by gastrulation
are called embryonic germ layers.
 The ectoderm forms the outer layer of the gastrula.
 Outer surfaces, neural tissue
 The endoderm lines the embryonic digestive tract.
 The mesoderm partly fills the space between the
endoderm and ectoderm.
 Muscles, reproductive system
Gastrulation – Sea Urchin
 Gastrulation in a sea urchin produces an
embryo with a primitive gut (archenteron) and
three germ layers.
 Blastopore – open end of gut, becomes anus
in deuterostomes.
Gastrulation - Frog
 Result – embryo with gut & 3 germ layers.
 More complicated:
 Yolk laden cells in vegetal hemisphere.
 Blastula wall more than one cell thick.
Gastrulation - Chick
 Gastrulation in the chick is affected by the large
amounts of yolk in the egg.
 Primitive streak – a groove on the surface along
the future anterior-posterior axis.
 Functionally equivalent to blastopore lip in frog.
Gastrulation - Chick
 Blastoderm consists of
two layers:
 Epiblast and
hypoblast
 Layers separated
by a blastocoel
 Epiblast forms
endoderm and
mesoderm.
 Cells on surface of
embryo form ectoderm.
Gastrulation - Mouse
 In mammals the blastula is called a
blastocyst.
 Inner cell mass will become the embryo while
trophoblast becomes part of the placenta.
 Notice that the gastrula is similar to that of the
chick.
Suites of Developmental Characters
 Two major groups of triploblastic animals:
 Protostomes
 Deuterostomes
 Differentiated by:




Spiral vs. radial cleavage
Regulative vs. mosaic cleavage
Blastopore becomes mouth vs. anus
Schizocoelous vs. enterocoelous coelom formation.
Deuterostome Development
 Deuterostomes include echinoderms (sea
urchins, sea stars etc) and chordates.
 Radial cleavage
Deuterostome Development
 Regulative development – the fate of a cell
depends on its interactions with neighbors, not
what piece of cytoplasm it has. A blastomere
isolated early in cleavage is able to from a
whole individual.
Deuterostome Development
 Deuterostome means second mouth.
 The blastopore becomes the anus and the
mouth develops as the second opening.
Deuterostome Development
 The coelom is a body cavity completely
surrounded by mesoderm.
 Mesoderm & coelom form simultaneously.
 In enterocoely, the coelom forms as
outpocketing of the gut.
Deuterostome Development
 Typical deuterostomes have coeloms that
develop by enterocoely.
 Vertebrates use a modified version of
schizocoely.
Protostome Development
 Protostomes include flatworms, annelids and
molluscs.
 Spiral cleavage
Protostome Development
 Mosaic
development – cell
fate is determined by
the components of
the cytoplasm found
in each blastomere.
 Morphogenetic
determinants.
 An isolated
blastomere can’t
develop.
Protostome Development
 Protostome means first mouth.
 Blastopore becomes the mouth.
 The second opening will become the anus.
Protostome Development
 In protostomes, a mesodermal band of tissue forms
before the coelom is formed.
 The mesoderm splits to form a coelom.
 Schizocoely
 Not all protostomes have a true coelom.
 Pseudocoelomates have a body cavity between
mesoderm and endoderm.
 Acoelomates have no body cavity at all other than the
gut.
Two Clades of Protostomes
 Lophotrochozoan protostomes include annelid
worms, molluscs, & some small phyla.
 Lophophore – horseshoe shaped feeding structure.
 Trochophore larva
 Feature all four protostome characteristics.
Two Clades of Protostomes
 The ecdysozoan protostomes include
arthropods, roundworms, and other taxa that
molt their exoskeletons.
 Ecdysis – shedding of the cuticle.
 Many do not show spiral cleavage.
Building a Body Plan
 An organism’s development is determined by
the genome of the zygote and also by
differences that arise between early
embryonic cells.
 Different genes will be expressed in different
cells.
Building a Body Plan
 Uneven distribution of
substances in the egg
called cytoplasmic
determinants results in
some of these
differences.
 Position of cells in the
early embryo result in
differences as well.
 Induction
Restriction of Cellular Potency
 In many species that have cytoplasmic
determinants only the zygote is totipotent,
capable of developing into all the cell types
found in the adult.
Restriction of Cellular Potency
 Unevenly distributed cytoplasmic determinants
in the egg cell:
 Are important in establishing the body axes.
 Set up differences in blastomeres resulting from
cleavage.
Restriction of Cellular Potency
 As embryonic development proceeds, the
potency of cells becomes progressively more
limited in all species.
Cell Fate Determination and Pattern
Formation by Inductive Signals
 Once embryonic cell division creates cells that
differ from each other,
 The cells begin to influence each other’s
fates by induction.
Induction
 Induction is the
capacity of some
cells to cause other
cells to develop in a
certain way.
 Dorsal lip of the
blastopore induces
neural development.
 Primary organizer
Spemann-Mangold
Experiment
 Transplanting a
piece of dorsal
blastopore lip from a
salamander gastrula
to a ventral or lateral
position in another
gastrula developed
into a notochord &
somites and it
induced the host
ectoderm to form a
neural tube.
Building a Body Plan
 Cell differentiation – the specialization of cells
in their structure and function.
 Morphogenesis – the process by which an
animal takes shape and differentiated cells end
up in their appropriate locations.
Building a Body Plan
 The sequence includes
 Cell movement
 Changes in adhesion
 Cell proliferation
 There is no “hard-wired” master control panel
directing development.
 Sequence of local patterns in which one step in
development is a subunit of another.
 Each step in the developmental hierarchy is a
necessary preliminary for the next.
Hox Genes
 Hox genes control the
subdivision of embryos
into regions of different
developmental fates
along the
anteroposterior axis.
 Homologous in diverse
organisms.
 These are master genes
that control expression
of subordinate genes.
Formation of the Vertebrate
Limb
 Inductive signals play a major role in pattern
formation – the development of an animal’s
spatial organization.
Formation of the Vertebrate
Limb
 The molecular cues that control pattern
formation, called positional information:
 Tell a cell where it is with respect to the animal’s
body axes.
 Determine how the cell and its descendents respond
to future molecular signals.
Formation of the Vertebrate
Limb
 The wings and legs of chicks, like all vertebrate
limbs begin as bumps of tissue called limb
buds.
 The embryonic cells within a limb bud respond
to positional information indicating location
along three axes.
Formation of the Vertebrate
Limb
 One limb-bud organizer region is the apical
ectodermal ridge (AER).
 A thickened area of ectoderm at the tip of the bud.
 The second major limb-bud organizer region is
the zone of polarizing activity (ZPA).
 A block of mesodermal tissue located underneath the
ectoderm where the posterior side of the bud is
attached to the body.
Morphogenesis
 Morphogenesis is a major aspect of
development in both plants and animals but
only in animals does it involve the movement of
cells.
The Cytoskeleton, Cell Motility, and
Convergent Extension
 Changes in the shape of a cell usually involve
reorganization of the cytoskeleton.
Changes in Cell Shape
 The formation of the
neural tube is
affected by
microtubules and
microfilaments.
Cell Migration
 The cytoskeleton also drives cell migration, or
cell crawling.
 The active movement of cells from one place to
another.
 In gastrulation, tissue invagination is caused by
changes in both cell shape and cell migration.
Evo-Devo
 Evolutionary developmental biology evolution is a process in which organisms
become different as a result of changes in the
genetic control of development.
 Genes that control development are similar in
diverse groups of animals.
 Hox genes
Evo-Devo
 Instead of evolution proceeding by the gradual
accumulation of numerous small mutations,
could it proceed by relatively few mutations in a
few developmental genes?
 The induction of legs or eyes by a mutation in one
gene suggests that these and other organs can
develop as modules.
The Common Vertebrate Heritage
 Vertebrates share a
common ancestry
and a common
pattern of early
development.
 Vertebrate
hallmarks all
present briefly.
 Dorsal neural tube
 Notochord
 Pharyngeal gill
pouches
 Postanal tail
Amniotes
 The embryos of birds, reptiles, and mammals
develop within a fluid-filled sac that is contained
within a shell or the uterus.
 Organisms with these adaptations form a
monophyletic group called amniotes.
 Allows for embryo to develop away from water.
Amniotes
 In these three types of organisms, the three
germ layers also give rise to the four
extraembryonic membranes that surround
the developing embryo.
Amniotes
 Amnion – fluid filled
membranous sac
that encloses the
embryo. Protects
embryo from shock.
 Yolk sac – stores
yolk and pre-dates
the amniotes by
millions of years.
Amniotes
 Allantois - storage of metabolic wastes during
development.
 Chorion - lies beneath the eggshell and
encloses the embryo and other extraembryonic
membrane.
 As embryo grows, the need for oxygen increases.
 Allantois and chorion fuse to form a respiratory
surface, the chorioallantoic membrane.
 Evolution of the shelled amniotic egg made
internal fertilization a requirement.
The Mammalian Placenta and Early
Mammalian Development
 Most mammalian embryos do not develop
within an egg shell.
 Develop within the mother’s body.
 Most retained in the mother’s body.
 Monotremes
 Primitive mammals that lay eggs.
 Large yolky eggs resembling bird eggs.
 Duck-billed platypus and spiny anteater.
The Mammalian Placenta and Early
Mammalian Development
 Marsupials
 Embryos born at an early stage of development and
continue development in abdominal pouch of mother.
 Placental Mammals
 Represent 94% of the class Mammalia.
 Evolution of the placenta required:
 Reconstruction of extraembryonic membranes.
 Modification of oviduct - expanded region formed a
uterus.
Mammalian Development
 The eggs of placental mammals:
 Are small and store few nutrients.
 Exhibit holoblastic cleavage.
 Show no obvious polarity.
Mammalian Development
 Gastrulation and organogenesis resemble the
processes in birds and other reptiles.
Mammalian Development
 Early embryonic development
in a human proceeds through
four stages:
 Blastocyst reaches uterus.
 Blastocyst implants.
 Extraembryonic membranes
start to form and gastrulation
begins.
 Gastrulation has produced a
3-layered embryo.
Mammalian Development
 The extraembryonic membranes in mammals are
homologous to those of birds and other reptiles
and have similar functions.
Mammalian Development
 Amnion
 Surrounds embryo
 Secretes fluid in
which embryo floats
 Yolk sac
 Contains no yolk
 Source of stem cells
that give rise to blood
and lymphoid cells
 Stem cells migrate to
into the developing
embryo
 Allantois
 Not needed to store
wastes
 Contributes to the
formation of the
umbilical cord
 Chorion
 Forms most of the
placenta
Organogenesis
 Various regions of the three embryonic germ
layers develop into the rudiments of organs
during the process of organogenesis.
Organogenesis
 Many different
structures are
derived from
the three
embryonic
germ layers
during
organogenesis.
Derivatives of Ectoderm: Nervous
System and Nerve Growth
 Just above the notochord
(mesoderm), the ectoderm
thickens to form a neural
plate.
 Edges of the neural plate
fold up to create an
elongated, hollow neural
tube.
 Anterior end of neural
tube enlarges to form the
brain and cranial nerves.
 Posterior end forms the
spinal cord and spinal
motor nerves.
Derivatives of Ectoderm: Nervous
System and Nerve Growth
 Neural crest cells pinch off from the neural
tube.
 Give rise to
 Portions of cranial nerves
 Pigment cells
 Cartilage
 Bone
 Ganglia of the autonomic system
 Medulla of the adrenal gland
 Parts of other endocrine glands
 Neural crest cells are unique to vertebrates.
 Important in evolution of the vertebrate head and
jaws.
Derivatives of Endoderm: Digestive
Tube and Survival of Gill Arches
 During gastrulation, the
archenteron forms as the
primitive gut.
 This endodermal cavity
eventually produces:
 Digestive tract
 Lining of pharynx and
lungs
 Most of the liver and
pancreas
 Thyroid, parathyroid
glands and thymus
Derivatives of Endoderm: Digestive
Tube and Survival of Gill Arches
 Pharyngeal pouches are derivatives of the
digestive tract.
 Arise in early embryonic development of all
vertebrates.
 During development, endodermally-lined pharyngeal
pouches interact with overlying ectoderm to form gill
arches.
 In fish, gill arches develop into gills.
 In terrestrial vertebrates:
 No respiratory function
 1st arch and endoderm-lined pouch form upper
and lower jaws, and inner ear.
 2nd, 3rd, and 4th gill pouches form tonsils,
parathyroid gland and thymus.
Derivatives of Mesoderm: Support,
Movement and the Beating Heart
 Most muscles arise
from mesoderm
along each side of
the neural tube.
 The mesoderm
divides into a linear
series of somites (38
in humans).
Derivatives of Mesoderm: Support,
Movement and the Beating Heart
 The splitting, fusion and
migration of somites produce
the:
 Axial skeleton
 Dermis of dorsal skin
 Muscles of the back, body wall, and
limbs
 Heart
 Lateral to the somites the
mesoderm splits to form the
coelom.