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Principles of Development
Chapter 8
Early Concepts: Preformation vs Epigenesis
The question of how a zygote becomes an animal has been asked for centuries.
As recently as the 18th century, the prevailing theory was a notion called preformation – the idea that the egg or sperm contains an embryo.A preformed miniature infant, or
“homunculus,” that simply becomes larger during development.
Early Concepts: Preformation vs Epigenesis
Kaspar Friederich Wolff (1759) demonstrated there was no preformed chick in the early egg.Undifferentiated granular material became
arranged into layers.The layers thickened, thinned, and folded to
produce the embryo.
Early Concepts: Preformation vs Epigenesis
Epigenesis is the concept that the fertilized egg contains building materials only, somehow assembled by an unknown directing force.
Although current ideas of development are essentially epigenetic in concept, far more is known about what directs growth and differentiation.
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
differentSaid to be determined.
Key Events in Development
The two basic processes responsible for this progressive subdivision:Cytoplasmic localization Induction
FertilizationFertilizationFertilization is the initial event in
development in sexual reproduction.Union of male and female gametesProvides for recombination of paternal and
maternal genes.Restores the diploid number.
Activates the egg to begin development.
FertilizationFertilizationOocyte 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 post-fertilization development.
Egg nucleus grows in size, bloated with RNA.Now called the germinal vesicle.
FertilizationMost 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.
FertilizationA 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 UrchinsPrevention of
polyspermy – only one sperm can enter.Fast block
Depolarization of membrane
Slow blockCortical reaction
resulting in fertilization membrane
Fertilization in Sea UrchinsThe 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 UrchinsOne 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 UrchinsThe 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 PronucleiAfter sperm and egg membranes fuse, the
sperm loses its flagellum.
Enlarged sperm nucleus is the male pronucleus and migrates inward to contact the female pronucleus.
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.
PolarityThe 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.
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 polarity of the egg determines the anterior-posterior axis before fertilization.
The first cleavage division bisects the gray crescent. Once the anterior-posterior and dorsal-ventral axes are defined, so is the left-right axis.
Amount of YolkDifferent 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 FrogsCleavage 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 BirdsMeroblastic
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.
BlastulaA fluid filled cavity, the blastocoel,
forms within the embryo – a hollow ball of cells now called a blastula.
GastrulationThe morphogenetic
process called gastrulation rearranges the cells of a blastula into a three-layered (triploblastic) embryo, called a gastrula, that has a primitive gut.Diploblastic
organisms have two germ layers.
GastrulationThe 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 UrchinGastrulation 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 - FrogResult – embryo with gut & 3 germ
layers.
More complicated:Yolk laden cells in vegetal hemisphere.Blastula wall more than one cell thick.
Gastrulation - ChickGastrulation 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 - ChickBlastoderm 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:ProtostomesDeuterostomes
Differentiated by:Spiral vs. radial cleavageRegulative vs. mosaic cleavageBlastopore becomes mouth vs. anusSchizocoelous 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 DevelopmentProtostomes include flatworms,
annelids and molluscs.Spiral cleavage
Protostome DevelopmentMosaic
development – cell fate is determined by the components of the cytoplasm found in each blastomere.Morphogenetic
determinants.An isolated
blastomere can’t develop.
Protostome DevelopmentProtostome means first mouth.
Blastopore becomes the mouth.
The second opening will become the anus.
Protostome DevelopmentIn 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 PlanAn 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 PlanUneven 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
SignalsOnce embryonic cell division creates cells
that differ from each other,The cells begin to influence each
other’s fates by induction.
InductionInduction 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 PlanCell 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 PlanThe sequence includes
Cell movementChanges in adhesionCell 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 GenesHox 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.
MorphogenesisMorphogenesis 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 MigrationThe 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-DevoEvolutionary 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-DevoInstead 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
tubeNotochordPharyngeal gill
pouchesPostanal tail
AmniotesThe 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.
AmniotesIn these three types of organisms, the
three germ layers also give rise to the four extraembryonic membranes that surround the developing embryo.
AmniotesAmnion – 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.
AmniotesAllantois - 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 DevelopmentMarsupials
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 DevelopmentThe eggs of placental mammals:
Are small and store few nutrients.Exhibit holoblastic cleavage.Show no obvious polarity.
Mammalian DevelopmentGastrulation 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 DevelopmentThe extraembryonic membranes in
mammals are homologous to those of birds and other reptiles and have similar functions.
Mammalian DevelopmentAmnion Surrounds embryo Secretes fluid in
which embryo floatsYolk sac Contains no yolk Source of stem cells that give rise to blood and lymphoid cellsStem cells migrate to into the developing embryo Allantois Not needed to store wastesContributes to the formation of the umbilical cordChorion Forms most of the placenta
OrganogenesisVarious regions of the three embryonic
germ layers develop into the rudiments of organs during the process of organogenesis.
OrganogenesisMany
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 nervesPigment cellsCartilageBoneGanglia of the autonomic systemMedulla of the adrenal glandParts 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 ArchesDuring gastrulation,
the archenteron forms as the primitive gut.
This endodermal cavity eventually produces:Digestive tractLining of pharynx and
lungsMost of the liver and
pancreasThyroid, parathyroid
glands and thymus
Derivatives of Endoderm: Digestive Tube and Survival of
Gill ArchesPharyngeal 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 function1st 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 HeartMost 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 HeartThe splitting, fusion and
migration of somites produce the:Axial skeletonDermis of dorsal skinMuscles of the back, body wall,
and limbsHeart
Lateral to the somites the mesoderm splits to form the coelom.
